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2022-23First EditionFebruary 2006 Phalguna 1927ReprintedOctober 2006 Kartika 1928November 2007 Kartika 1929January 2009 Pausa 1930January 2010 Magha 1931January 2011 Pausa 1932January 2012 Magha 1933December 2012 Pausa 1934December 2013 Agrahayana 1935December 2014 Pausa 1936May 2016 Vaishakha 1938March 2017 Phalguna 1938December 2017 Magha 1939January 2019 Magha 1940September 2019 Bhadrapada 1941February 2021 Magha 1942November 2021 Agrahayana 1943PD 260T RSPŠ National Council of EducationalResearch and Training, 2006` 230.00ALL RIGHTS RESERVEDq No part of this publication may be reproduced, stored in a retrieval system ortransmitted, in any form or by any means, electronic, mechanical, photocopying,recording or otherwise without the prior permission of the publisher.q This book is sold subject to the condition that it shall not, by way of trade, be lent, resold,hired out or otherwise disposed of without the publisherâs consent, in any formof binding or cover other than that in which it is published.q The correct price of this publication is the price printed on this page, Any revisedprice indicated by a rubber stamp or by a sticker or by any other means is incorrectand should be unacceptable.Publication TeamHead, Publication : Anup Kumar RajputDivisionChief Editor : Shveta UppalChief Production : Arun ChitkaraOfficerChief Business : Vipin DewanManagerAssistant Editor : Shashi ChadhaProduction Assistant : Om PrakashCover and LayoutShweta RaoIllustrationsLalit MauryaOFFICES OF THE PUBLICATIONDIVISION, NCERTNCERT CampusSri Aurobindo MargNew Delhi 110 016 Phone : 011-26562708108, 100 Feet RoadHosdakere Halli ExtensionBanashankari III StageBangaluru 560 085 Phone : 080-26725740Navjivan Trust BuildingP.O.NavjivanAhmedabad 380 014 Phone : 079-27541446CWC CampusOpp. Dhankal Bus StopPanihatiKolkata 700 114 Phone : 033-25530454CWC ComplexMaligaonGuwahati 781 021 Phone : 0361-2674869Printed on 80 GSM paper with NCERTwatermarkPublished at the Publication Division by theSecretary, National Council of EducationalResearch and Training, Sri Aurobindo Marg,New Delhi 110 016 and printed at EsskayPress Pvt. Ltd. 220, Patparganj IndustrialArea, Delhi - 110 092ISBN 81-7450-496-611080 â BIOLOGYTextbook for Class XI2022-23FOREWORDThe National Curriculum Framework (NCF) 2005, recommends that childrenâs life atschool must be linked to their life outside the school. This principle marks a departurefrom the legacy of bookish learning which continues to shape our system and causesa gap between the school, home and community. The syllabi and textbooks developedon the basis of NCF signify an attempt to implement this basic idea. They also attemptto discourage rote learning and the maintenance of sharp boundaries between differentsubject areas. We hope these measures will take us significantly further in the directionof a child-centred system of education outlined in the National Policy onEducation (1986).The success of this effort depends on the steps that school principals and teacherswill take to encourage children to reflect on their own learning and to pursueimaginative activities and questions. We must recognise that, given space, time andfreedom, children generate new knowledge by engaging with the information passedon to them by adults. Treating the prescribed textbook as the sole basis of examinationis one of the key reasons why other resources and sites of learning are ignored.Inculcating creativity and initiative is possible if we perceive and treat children asparticipants in learning, not as receivers of a fixed body of knowledge.These aims imply considerable change in school routines and mode of functioning.Flexibility in the daily time-table is as necessary as rigour in implementing the annualcalendar so that the required number of teaching days are actually devoted to teaching.The methods used for teaching and evaluation will also determine how effective thistextbook proves for making childrenâs life at school a happy experience, rather than asource of stress or boredom. Syllabus designers have tried to address the problem ofcurricular burden by restructuring and reorienting knowledge at different stages withgreater consideration for child psychology and the time available for teaching. Thetextbook attempts to enhance this endeavour by giving higher priority and space toopportunities for contemplation and wondering, discussion in small groups, andactivities requiring hands-on experience.The National Council of Educational Research and Training (NCERT) appreciatesthe hard work done by the textbook development committee responsible for this book.We wish to thank the Chairperson of the advisory group in science and mathematics,Professor J.V. Narlikar and the Chief Advisor for this book, Professor K. Muralidhar,Department of Zoology, University of Delhi, Delhi for guiding the work of this committee.2022-23Several teachers contributed to the development of this textbook. We are grateful totheir principals for making this possible. We are indebted to the institutions andorganisations which have generously permitted us to draw upon their resources,material and personnel. We are especially grateful to the members of the NationalMonitoring Committee, appointed by the Department of Secondary and HigherEducation, Ministry of Human Resource Development under the Chairpersonship ofProfessor Mrinal Miri and Professor G.P. Deshpande, for their valuable time andcontribution.As an organisation committed to systemic reform and continuous improvementin the quality of its products, NCERT welcomes comments and suggestions whichwill enable us to undertake further revision and refinement.DirectorNew Delhi National Council of Educational20 December 2005 Research and Training(iv)2022-23CHAIRPERSON, ADVISORY GROUP FOR TEXTBOOKS IN SCIENCE AND MATHEMATICSJ.V. Narlikar, Emeritus Professor, Chairman, Advisory Committee, Inter University Centre for Astronomyand Astrophysics (IUCAA), Pune University, PuneCHIEF ADVISORK. Muralidhar, Professor, Department of Zoology, University of Delhi, DelhiMEMBERSAjit Kumar Kavathekar, Reader (Botany), Sri Venkateswara College, University of Delhi, DelhiB.B.P. Gupta, Professor, Department of Zoology, North-Eastern Hill University, ShillongC.V. Shimray, Lecturer, Department of Education in Science and Mathematics, NCERTDinesh Kumar, Reader, Department of Education in Science and Mathematics, NCERTJ.S. Gill, Professor, Department of Education in Science and Mathematics, NCERTK. Sarath Chandran, Reader (Zoology), Sri Venkateswara College, University of Delhi, DelhiNalini Nigam, Reader (Botany), Ramjas College, University of Delhi, DelhiPratima Gaur, Professor, Department of Zoology, University of Allahabad, AllahabadRatnam Kaul Wattal, Reader (Botany), Zakir Hussain College, University of Delhi, DelhiR.K. Seth, UGC Scientist C, Department of Zoology, University of Delhi, DelhiR.P. Singh, Lecturer (Biology), Rajkiya Pratibha Vikas Vidyalaya, Kishan Ganj, DelhiSangeeta Sharma, PGT (Biology), Kendriya Vidyalaya, JNU, New DelhiSavithri Singh, Principal, Acharya Narendra Dev College, University of Delhi; Former Fellow, Centrefor Science Education and Communication, University of Delhi, DelhiS.C. Jain, Professor, Department of Education in Science and Mathematics, NCERTSunaina Sharma, Lecturer (Biology), Rajkiya Pratibha Vikas Vidyalaya, Dwarka, New DelhiTejinder Chawla, PGT (Biology), Guru Harkrishan Public School, Vasant Vihar, New DelhiT.N. Lakhanpal, Professor (Retd.), Department of Bio Sciences, Himachal Pradesh University, ShimlaU.K. Nanda, Professor, Regional Institute of Education, BhubaneshwarMEMBER-COORDINATORB.K. Tripathi, Reader, Department of Education in Science and Mathematics, NCERT.TEXTBOOK DEVELOPMENT COMMITTEE2022-23ACKNOWLEDGEMENTSNational Council of Educational Research and Training (NCERT) gratefully acknowledgesthe contribution of the individuals and organisations involved in the development of theBiology textbook for Class XI. The Council is grateful to Arvind Gupte, Principal (Retd.),Government Collegiate Education Services, Madhya Pradesh; Shailaja Hittalmani,Associate Professor (Genetics), University of Agricultural Sciences, Bangalore;K.R. Shivanna, Professor (Retd.), Department of Botany, University of Delhi, Delhi; R.S.Bedwal, Professor, Department of Zoology, University of Rajasthan, Jaipur; P.S.Srivastava, Professor, Department of Biotechnology, Hamdard University, New Delhi andPramila Shivanna, former Teacher, D.A.V. School, Delhi, for their valuable suggestions.The Council is also thankful to V.K. Bhasin, Professor and Head, Department of Zoology,University of Delhi, Delhi; P.P. Bakre, Professor and Head, Department of Zoology,University of Rajasthan, Jaipur and Savithri Singh, Principal, Acharya Narendra DevCollege, New Delhi for their support. The Council is also grateful to B.K. Gupta, Scientist,Central Zoo Authority, New Delhi for providing pictures of zoological parks andSameer Singh for the pictures on the front and back cover. All the other photographsused in the book provided by Savithri Singh and taken at either at NCERT, IARI Campusor Acharya Narendra Dev College is gratefully acknowledged.NCERT sincerely acknowledges the contributions of the members who participated inthe review of the manuscripts â M.K. Tiwari, PGT (Biology), Kendriya Vidyalaya, Mandsaur,Madhya Pradesh; Maria Gracias Fernandes, PGT (Biology), G.V.M.S. Higher Secondary,Ponda, Goa; A.K. Ganguly, PGT (Biology), Jawahar Navodaya Vidyalaya, Roshnabad,Haridwar; Shivani Goswami, PGT (Biology), The Motherâs International School, New Delhiand B.N. Pandey, Principal, Ordinance Factory Sr. Sec. School, Dehradun.The Council is highly thankful to M. Chandra, Professor and Head, DESM; HukumSingh, Professor, DESM, NCERT for their valuable support throughout the making ofthis book. The contributions of V.V. Anand, Professor (Retd), Regional Institute ofEducation (RIE), Mysuru; A.K. Mohapatra, Professor, RIE, Bhubaneswar; Abhay Kumar,Assistant Professor, CIET, NCERT; G.V. Gopal, Professor, RIE, Mysuru; Ishwant Kaur,AHM, DMS, Ajmer; Sunita Farkya, Professor, DESM, NCERT; Pushplata Verma, AssistantProfessor, DESM, NCERT; C. Padmaja, Professor, RIE, Mysuru and Jaydeep Mandal,Professor, RIE, Bhopal in the review of this textbook in 2017-18 are acknowledged.The Council also gratefully acknowledges the contribution of Deepak Kapoor,Incharge, Computer Station; Mohd. Khalid Raza and Arvind Sharma, DTP operators;Saswati Banerjee and Hari Darshan Lodhi, Copy Editor; Archana Srivastava, ProofReader and APC office and administrative staff of DESM, NCERT.The efforts of the Publication Department, NCERT in bringing out this publicationare also appreciated.2022-23A NOTE FOR THE TEACHERS AND STUDENTSBiology is the science of life. It is the story of life on earth. It is the science of life forms andliving processes. Biological systems, often appear to challenge physical laws that govern thebehaviour of matter and energy in our world. Historically, biological knowledge was ancillaryto knowledge of human body and its function. The latter as we know, is the basis of medicalpractice. However, parts of biological knowledge developed independent of human application.Fundamental questions about origin of life, the origin and growth of biodiversity, the evolutionof flora and fauna of different habitats, etc., caught the imagination of biologists.The very description of living organisms, be it from morphological perspective, physiologicalperspective, taxonomical perspective, etc., engaged scientists to such an extent that for sheerconvenience, if not for anything else, the subject matter got artificially divided into the subdisciplinesof botany and zoology and later into even microbiology. Meanwhile, physical sciencesmade heavy inroads into biology, and established biochemistry and biophysics as new subdisciplinesof biology. Mendelâs work and its rediscovery in the early twentieth century led tothe promotion of study of genetics. The discovery of the double-helical structure of DNA andthe deciphering of three dimensional structures of many macromolecules led to theestablishment of and phenomenal growth in the dominating area of molecular biology. In asense, functional disciplines laying emphasis on mechanisms underlying living processes,received more attention, support, intellectual and social recognition. Biology, unfortunately,got divided into classical and modern biology. To the majority of practising biologists, pursuitof biological research became more empirical rather than a curiosity and hypothesis drivenintellectual exercise as is the case with theoretical physics, experimental physics, structuralchemistry and material science. Fortunately and quietly, general unifying principles of biologywere also being discovered, rediscovered and emphasised. The work of Mayr, Dobhzhansky,Haldane, Perutz, Khorana, Morgan, Darlington, Fisher and many others brought respect andseriousness to both classical and molecular biological disciplines. Ecology and Systems biologygot established as unifying biological disciplines. Every area of biology began developinginterface with not only other areas of biology but also other disciplines of science andmathematics. Pretty soon, the boundaries became porous. They are now on the verge ofdisappearing altogether. Progress in human biology, biomedical sciences, especially thestructure, functioning and evolution of human brain brought in respect, awe and philosophicalinsights to biology. Biology even stepped out of laboratories, museums and natural parks andraised social, economic and cultural issues capturing the imagination of general public andhence political attention. Educationists did not lag behind and realised that biology should betaught as an interdisciplinary and integrating science at all stages of educational trainingespecially at school and undergraduate levels. A new synthesis of all areas of basic andapplied areas of biology is the need of the hour. Biology has come of age. It has an independentset of concepts which are universal just like physics and chemistry and mathematics.The present volume is the first time presentation of the integrated biology for the schoollevel children. One of the lacunae in biology teaching and study is the absence of integration2022-23with other disciplinary knowledge of physics, chemistry etc. Further many processes in plants,animals and microbes are similar when looked from physico-chemical perspective. Cell biologyhas brought out the unifying common cellular level activities underlying apparently diversephenomena across plants, animals and microbes. Similarly, molecular science (e.g.biochemistry or molecular biology) has revealed the similar molecular mechanisms in allthese apparently diverse organisms like plants, animals and microbes. Phenomena likerespiration, metabolism, energy utlisation, growth, reproduction and development can bediscussed in a unifying manner rather than as separate unrelated processes in plants andanimals. An attempt has been made to unify such diverse disciplines in the book. Theintegration achieved however, is partial and not complete. Hopefully along with changes inthe teaching and learning context, to be brought out in the next few years, the next edition ofthis book will reveal more integration of botany, zoology and microbiology and truly reflect thetrue nature of biology â the future science of man by man and for man.This new textbook of Biology for class XI is a completely rewritten book in view of thesyllabus revision and restructuring. It is also in accordance with the spirit of the NationalCurriculum Framework (2005) guidelines. The subject matter is presented under twenty-twochapters which are grouped under five thematic units. Each unit has a brief write up precedingthe unit highlighting the essence of the chapters to follow under that unit. Each unit also hasa biographical sketch of a prominent scientist in that area. Each chapter has, on the firstpage, a detailed table of contents giving sub-headings within the chapter. Decimal systemusing arabic numerals has been employed to indicate these sub-headings. At the end of eachchapter a brief summary is provided. This brings to the notice of the student, what she/he issupposed to have learnt by studying the chapter. A set of questions is also provided at theconclusion of each chapter. These questions are essentially to enable the student to testherself/himself as to how much she/he has understood the subject matter. There are questionswhich are purely of information recall type; there are questions which need analytical thinkingto answer and hence test true understanding; there are questions which are problems tosolve and finally there are questions which need analysis and speculation as there is no oneto answer to such questions. This tests the critical understanding of the subject matter in themind of the student.Special emphasis has been given on the narrative style, illustrations, activity exercises,clarity of expression, coverage of topics within the available time in school. A large number ofextremely talented and dedicated people including practising teachers helped in bringing outthis beautiful book. Our main purpose was to make sure that school level biology is not aburden for students and teachers. We sincerely wish that teaching biology and learning biologywould become an enjoyable activity.Professor K. MuralidharDepartment of ZoologyUniversity of Delhi2022-23CONTENTSFOREWORD iiiA NOTE FOR THE TEACHERS AND STUDENTS viiUNIT IDIVERSITY IN THE LIVING WORLD 1-62Chapter 1 : The Living World 3Chapter 2 : Biological Classification 16Chapter 3 : Plant Kingdom 29Chapter 4 : Animal Kingdom 46UNIT IISTRUCTURAL ORGANISATION IN PLANTS AND ANIMALS 63-122Chapter 5 : Morphology of Flowering Plants 65Chapter 6 : Anatomy of Flowering Plants 84Chapter 7 : Structural Organisation in Animals 100UNIT IIICELL : STRUCTURE AND FUNCTIONS 123-172Chapter 8 : Cell : The Unit of Life 125Chapter 9 : Biomolecules 142Chapter 10 : Cell Cycle and Cell Division 1622022-23UNIT IVPLANT PHYSIOLOGY 173-254Chapter 11 : Transport in Plants 175Chapter 12 : Mineral Nutrition 194Chapter 13 : Photosynthesis in Higher Plants 206Chapter 14 : Respiration in Plants 226Chapter 15 : Plant Growth and Development 239UNIT VHUMAN PHYSIOLOGY 255-343Chapter 16 : Digestion and Absorption 257Chapter 17 : Breathing and Exchange of Gases 268Chapter 18 : Body Fluids and Circulation 278Chapter 19 : Excretory Products and their Elimination 290Chapter 20 : Locomotion and Movement 302Chapter 21 : Neural Control and Coordination 315Chapter 22 : Chemical Coordination and Integration 3312022-23Biology is the science of life forms and living processes. The living worldcomprises an amazing diversity of living organisms. Early man couldeasily perceive the difference between inanimate matter and livingorganisms. Early man deified some of the inanimate matter (wind, sea,fire etc.) and some among the animals and plants. A common feature ofall such forms of inanimate and animate objects was the sense of aweor fear that they evoked. The description of living organisms includinghuman beings began much later in human history. Societies whichindulged in anthropocentric view of biology could register limitedprogress in biological knowledge. Systematic and monumentaldescription of life forms brought in, out of necessity, detailed systemsof identification, nomenclature and classification. The biggest spin offof such studies was the recognition of the sharing of similarities amongliving organisms both horizontally and vertically. That all present dayliving organisms are related to each other and also to all organismsthat ever lived on this earth, was a revelation which humbled man andled to cultural movements for conservation of biodiversity. In thefollowing chapters of this unit, you will get a description, includingclassification, of animals and plants from a taxonomistâs perspective.DIVERSITY IN THE LIVING WORLDChapter 1The Living WorldChapter 2Biological ClassificationChapter 3Plant KingdomChapter 4Animal KingdomUNIT 12022-23Born on 5 July 1904, in Kempten, Germany, ERNST MAYR, theHarvard University evolutionary biologist who has been calledâThe Darwin of the 20th centuryâ, was one of the 100 greatestscientists of all time. Mayr joined Harvardâs Faculty of Artsand Sciences in 1953 and retired in 1975, assuming the titleAlexander Agassiz Professor of Zoology Emeritus. Throughouthis nearly 80-year career, his research spanned ornithology,taxonomy, zoogeography, evolution, systematics, and thehistory and philosophy of biology. He almost single-handedlymade the origin of species diversity the central question ofevolutionary biology that it is today. He also pioneered thecurrently accepted definition of a biological species. Mayr wasawarded the three prizes widely regarded as the triple crown ofbiology: the Balzan Prize in 1983, the International Prize forBiology in 1994, and the Crafoord Prize in 1999. Mayr died atthe age of 100 in the year 2004.Ernst Mayr(1904 â 2004)2022-23How wonderful is the living world ! The wide range of living types isamazing. The extraordinary habitats in which we find living organisms,be it cold mountains, deciduous forests, oceans, fresh water lakes, desertsor hot springs, leave us speechless. The beauty of a galloping horse, ofthe migrating birds, the valley of flowers or the attacking shark evokesawe and a deep sense of wonder. The ecological conflict and cooperationamong members of a population and among populations of a communityor even the molecular traffic inside a cell make us deeply reflect on â whatindeed is life? This question has two implicit questions within it. The firstis a technical one and seeks answer to what living is as opposed to thenon-living, and the second is a philosophical one, and seeks answer towhat the purpose of life is. As scientists, we shall not attempt answeringthe second question. We will try to reflect on â what is living?1.1 WHAT IS âLIVINGâ?When we try to define âlivingâ, we conventionally look for distinctivecharacteristics exhibited by living organisms. Growth, reproduction, abilityto sense environment and mount a suitable response come to our mindimmediately as unique features of living organisms. One can add a fewmore features like metabolism, ability to self-replicate, self-organise,interact and emergence to this list. Let us try to understand each of these.All living organisms grow. Increase in mass and increase in numberof individuals are twin characteristics of growth. A multicellular organismTHE LIVING WORLDCHAPTER 11.1 What is âLivingâ?1.2 Diversity in theLiving World1.3 TaxonomicCategories1.4 TaxonomicalAids2022-234 BIOLOGYgrows by cell division. In plants, this growth by cell division occurscontinuously throughout their life span. In animals, this growth is seenonly up to a certain age. However, cell division occurs in certain tissues toreplace lost cells. Unicellular organisms grow by cell division. One caneasily observe this in in vitro cultures by simply counting the number ofcells under the microscope. In majority of higher animals and plants,growth and reproduction are mutually exclusive events. One mustremember that increase in body mass is considered as growth. Non-livingobjects also grow if we take increase in body mass as a criterion for growth.Mountains, boulders and sand mounds do grow. However, this kind ofgrowth exhibited by non-living objects is by accumulation of material onthe surface. In living organisms, growth is from inside. Growth, therefore,cannot be taken as a defining property of living organisms. Conditionsunder which it can be observed in all living organisms have to be explainedand then we understand that it is a characteristic of living systems. Adead organism does not grow.Reproduction, likewise, is a characteristic of living organisms.In multicellular organisms, reproduction refers to the production ofprogeny possessing features more or less similar to those of parents.Invariably and implicitly we refer to sexual reproduction. Organismsreproduce by asexual means also. Fungi multiply and spread easily dueto the millions of asexual spores they produce. In lower organisms likeyeast and hydra, we observe budding. In Planaria (flat worms), we observetrue regeneration, i.e., a fragmented organism regenerates the lost part ofits body and becomes, a new organism. The fungi, the filamentous algae,the protonema of mosses, all easily multiply by fragmentation. When itcomes to unicellular organisms like bacteria, unicellular algae or Amoeba,reproduction is synonymous with growth, i.e., increase in number of cells.We have already defined growth as equivalent to increase in cell numberor mass. Hence, we notice that in single-celled organisms, we are not veryclear about the usage of these two terms â growth and reproduction.Further, there are many organisms which do not reproduce (mules, sterileworker bees, infertile human couples, etc). Hence, reproduction also cannotbe an all-inclusive defining characteristic of living organisms. Of course,no non-living object is capable of reproducing or replicating by itself.Another characteristic of life is metabolism. All living organismsare made of chemicals. These chemicals, small and big, belonging tovarious classes, sizes, functions, etc., are constantly being made andchanged into some other biomolecules. These conversions are chemicalreactions or metabolic reactions. There are thousands of metabolicreactions occurring simultaneously inside all living organisms, be they2022-23THE LIVING WORLD 5unicellular or multicellular. All plants, animals, fungi and microbes exhibitmetabolism. The sum total of all the chemical reactions occurring in ourbody is metabolism. No non-living object exhibits metabolism. Metabolicreactions can be demonstrated outside the body in cell-free systems. Anisolated metabolic reaction(s) outside the body of an organism, performedin a test tube is neither living nor non-living. Hence, while metabolism isa defining feature of all living organisms without exception, isolatedmetabolic reactions in vitro are not living things but surely living reactions.Hence, cellular organisation of the body is the defining feature oflife forms.Perhaps, the most obvious and technically complicated feature of allliving organisms is this ability to sense their surroundings or environmentand respond to these environmental stimuli which could be physical,chemical or biological. We sense our environment through our senseorgans. Plants respond to external factors like light, water, temperature,other organisms, pollutants, etc. All organisms, from the prokaryotes tothe most complex eukaryotes can sense and respond to environmentalcues. Photoperiod affects reproduction in seasonal breeders, both plantsand animals. All organisms handle chemicals entering their bodies. Allorganisms therefore, are âawareâ of their surroundings. Human being isthe only organism who is aware of himself, i.e., has self-consciousness.Consciousness therefore, becomes the defining property of livingorganisms.When it comes to human beings, it is all the more difficult to definethe living state. We observe patients lying in coma in hospitals virtuallysupported by machines which replace heart and lungs. The patient isotherwise brain-dead. The patient has no self-consciousness. Are suchpatients who never come back to normal life, living or non-living?In higher classes, you will come to know that all living phenomenaare due to underlying interactions. Properties of tissues are not presentin the constituent cells but arise as a result of interactions among theconstituent cells. Similarly, properties of cellular organelles are not presentin the molecular constituents of the organelle but arise as a result ofinteractions among the molecular components comprising the organelle.These interactions result in emergent properties at a higher level oforganisation. This phenomenon is true in the hierarchy of organisationalcomplexity at all levels. Therefore, we can say that living organisms areself-replicating, evolving and self-regulating interactive systems capableof responding to external stimuli. Biology is the story of life on earth.Biology is the story of evolution of living organisms on earth. All livingorganisms â present, past and future, are linked to one another by thesharing of the common genetic material, but to varying degrees.2022-236 BIOLOGY1.2 DIVERSITY IN THE LIVING WORLDIf you look around you will see a large variety of living organisms, be itpotted plants, insects, birds, your pets or other animals and plants. Thereare also several organisms that you cannot see with your naked eye butthey are all around you. If you were to increase the area that you makeobservations in, the range and variety of organisms that you see wouldincrease. Obviously, if you were to visit a dense forest, you would probablysee a much greater number and kinds of living organisms in it. Eachdifferent kind of plant, animal or organism that you see, represents aspecies. The number of species that are known and described rangebetween 1.7-1.8 million. This refers to biodiversity or the number andtypes of organisms present on earth. We should remember here that aswe explore new areas, and even old ones, new organisms are continuouslybeing identified.As stated earlier, there are millions of plants and animals in the world;we know the plants and animals in our own area by their local names.These local names would vary from place to place, even within a country.Probably you would recognise the confusion that would be created if wedid not find ways and means to talk to each other, to refer to organismswe are talking about.Hence, there is a need to standardise the naming of living organismssuch that a particular organism is known by the same name all over theworld. This process is called nomenclature. Obviously, nomenclature ornaming is only possible when the organism is described correctly and weknow to what organism the name is attached to. This is identification.In order to facilitate the study, number of scientists have establishedprocedures to assign a scientific name to each known organism. This isacceptable to biologists all over the world. For plants, scientific names arebased on agreed principles and criteria, which are provided in InternationalCode for Botanical Nomenclature (ICBN). You may ask, how are animalsnamed? Animal taxonomists have evolved International Code of ZoologicalNomenclature (ICZN). The scientific names ensure that each organismhas only one name. Description of any organism should enable the people(in any part of the world) to arrive at the same name. They also ensurethat such a name has not been used for any other known organism.Biologists follow universally accepted principles to provide scientificnames to known organisms. Each name has two components â theGeneric name and the specific epithet. This system of providing a namewith two components is called Binomial nomenclature. This namingsystem given by Carolus Linnaeus is being practised by biologists allover the world. This naming system using a two word format was foundconvenient. Let us take the example of mango to understand the way of2022-23THE LIVING WORLD 7providing scientific names better. The scientific name of mango is writtenas Mangifera indica. Let us see how it is a binomial name. In this nameMangifera represents the genus while indica, is a particular species, or aspecific epithet. Other universal rules of nomenclature are as follows:1. Biological names are generally in Latin and written in italics.They are Latinised or derived from Latin irrespective of theirorigin.2. The first word in a biological name represents the genus whilethe second component denotes the specific epithet.3. Both the words in a biological name, when handwritten, areseparately underlined, or printed in italics to indicate their Latinorigin.4. The first word denoting the genus starts with a capital letterwhile the specific epithet starts with a small letter. It can beillustrated with the example of Mangifera indica.Name of the author appears after the specific epithet, i.e., at the end ofthe biological name and is written in an abbreviated form, e.g., Mangiferaindica Linn. It indicates that this species was first described by Linnaeus.Since it is nearly impossible to study all the living organisms, it isnecessary to devise some means to make this possible. This process isclassification. Classification is the process by which anything is groupedinto convenient categories based on some easily observable characters.For example, we easily recognise groups such as plants or animals ordogs, cats or insects. The moment we use any of these terms, we associatecertain characters with the organism in that group. What image do yousee when you think of a dog ? Obviously, each one of us will see âdogsâand not âcatsâ. Now, if we were to think of âAlsatiansâ we know what we aretalking about. Similarly, suppose we were to say âmammalsâ, you would,of course, think of animals with external ears and body hair. Likewise, inplants, if we try to talk of âWheatâ, the picture in each of our minds will beof wheat plants, not of rice or any other plant. Hence, all these - âDogsâ,âCatsâ, âMammalsâ, âWheatâ, âRiceâ, âPlantsâ, âAnimalsâ, etc., are convenientcategories we use to study organisms. The scientific term for thesecategories is taxa. Here you must recognise that taxa can indicatecategories at very different levels. âPlantsâ â also form a taxa. âWheatâ isalso a taxa. Similarly, âanimalsâ, âmammalsâ, âdogsâ are all taxa â but youknow that a dog is a mammal and mammals are animals. Therefore,âanimalsâ, âmammalsâ and âdogsâ represent taxa at different levels.Hence, based on characteristics, all living organisms can be classifiedinto different taxa. This process of classification is taxonomy. Externaland internal structure, along with the structure of cell, development2022-238 BIOLOGYprocess and ecological information of organisms are essential and formthe basis of modern taxonomic studies.Hence, characterisation, identification, classification and nomenclatureare the processes that are basic to taxonomy.Taxonomy is not something new. Human beings have always beeninterested in knowing more and more about the various kinds oforganisms, particularly with reference to their own use. In early days,human beings needed to find sources for their basic needs of food, clothingand shelter. Hence, the earliest classifications were based on the âusesâ ofvarious organisms.Human beings were, since long, not only interested in knowing moreabout different kinds of organisms and their diversities, but also therelationships among them. This branch of study was referred to assystematics. The word systematics is derived from the Latin wordâsystemaâ which means systematic arrangement of organisms. Linnaeusused Systema Naturae as the title of his publication. The scope ofsystematics was later enlarged to include identification, nomenclatureand classification. Systematics takes into account evolutionaryrelationships between organisms.1.3 TAXONOMIC CATEGORIESClassification is not a single step process but involves hierarchy of stepsin which each step represents a rank or category. Since the category is apart of overall taxonomic arrangement, it is called the taxonomic categoryand all categories together constitute the taxonomic hierarchy. Eachcategory, referred to as a unit of classification, in fact, represents a rankand is commonly termed as taxon (pl.: taxa).Taxonomic categories and hierarchy can be illustrated by an example.Insects represent a group of organisms sharing common features likethree pairs of jointed legs. It means insects are recognisable concreteobjects which can be classified, and thus were given a rank or category.Can you name other such groups of organisms? Remember, groupsrepresent category. Category further denotes rank. Each rank or taxon,in fact, represents a unit of classification. These taxonomic groups/categories are distinct biological entities and not merely morphologicalaggregates.Taxonomical studies of all known organisms have led to thedevelopment of common categories such as kingdom, phylum or division(for plants), class, order, family, genus and species. All organisms,including those in the plant and animal kingdoms have species as thelowest category. Now the question you may ask is, how to place an2022-23THE LIVING WORLD 9organism in various categories? The basic requirement is the knowledgeof characters of an individual or group of organisms. This helps inidentifying similarities and dissimilarities among the individuals of thesame kind of organisms as well as of other kinds of organisms.1.3.1 SpeciesTaxonomic studies consider a group of individual organisms withfundamental similarities as a species. One should be able to distinguishone species from the other closely related species based on the distinctmorphological differences. Let us consider Mangifera indica, Solanumtuberosum (potato) and Panthera leo (lion). All the three names, indica,tuberosum and leo, represent the specific epithets, while the first wordsMangifera, Solanum and Panthera are genera and represents anotherhigher level of taxon or category. Each genus may have one or more thanone specific epithets representing different organisms, but havingmorphological similarities. For example, Panthera has another specificepithet called tigris and Solanum includes species like nigrum andmelongena. Human beings belong to the species sapiens which isgrouped in the genus Homo. The scientific name thus, for human being,is written as Homo sapiens.1.3.2 GenusGenus comprises a group of related species which has more charactersin common in comparison to species of other genera. We can say thatgenera are aggregates of closely related species. For example, potato andbrinjal are two different species but both belong to the genus Solanum.Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with severalcommon features, are all species of the genus Panthera. This genus differsfrom another genus Felis which includes cats.1.3.3 FamilyThe next category, Family, has a group of related genera with still lessnumber of similarities as compared to genus and species. Families arecharacterised on the basis of both vegetative and reproductive features ofplant species. Among plants for example, three different genera Solanum,Petunia and Datura are placed in the family Solanaceae. Among animalsfor example, genus Panthera, comprising lion, tiger, leopard is put alongwith genus, Felis (cats) in the family Felidae. Similarly, if you observe thefeatures of a cat and a dog, you will find some similarities and somedifferences as well. They are separated into two different families â Felidaeand Canidae, respectively.2022-2310 BIOLOGY1.3.4 OrderYou have seen earlier that categories like species, genus andfamilies are based on a number of similar characters. Generally,order and other higher taxonomic categories are identified basedon the aggregates of characters. Order being a higher category,is the assemblage of families which exhibit a few similarcharacters. The similar characters are less in number ascompared to different genera included in a family. Plant familieslike Convolvulaceae, Solanaceae are included in the orderPolymoniales mainly based on the floral characters. The animalorder, Carnivora, includes families like Felidae and Canidae.1.3.5 ClassThis category includes related orders. For example, order Primatacomprising monkey, gorilla and gibbon is placed in classMammalia along with order Carnivora that includes animals liketiger, cat and dog. Class Mammalia has other orders also.1.3.6 PhylumClasses comprising animals like fishes, amphibians, reptiles, birdsalong with mammals constitute the next higher category calledPhylum. All these, based on the common features like presenceof notochord and dorsal hollow neural system, are included inphylum Chordata. In case of plants, classes with a few similarcharacters are assigned to a higher category called Division.1.3.7 KingdomAll animals belonging to various phyla are assigned to thehighest category called Kingdom Animalia in the classificationsystem of animals. The Kingdom Plantae, on the other hand, isdistinct, and comprises all plants from various divisions.Henceforth, we will refer to these two groups as animal andplant kingdoms.The taxonomic categories from species to kingdom have beenshown in ascending order starting with species in Figure 1.1.These are broad categories. However, taxonomists have alsodeveloped sub-categories in this hierarchy to facilitate moresound and scientific placement of various taxa.Look at the hierarchy in Figure 1.1. Can you recall the basisof arrangement? Say, for example, as we go higher from speciesto kingdom, the number of common characteristics goes onFigure 1.1 Taxonomicc a t e g o r i e ss h o w i n ghierarchialarrangementin ascendingorder2022-23THE LIVING WORLD 111.4 TAXONOMICAL AIDSTaxonomic studies of various species of plants, animals and otherorganisms are useful in agriculture, forestry, industry and in general inknowing our bio-resources and their diversity. These studies wouldrequire correct classification and identification of organisms. Identificationof organisms requires intensive laboratory and field studies. The collectionof actual specimens of plant and animal species is essential and is theprime source of taxonomic studies. These are also fundamental to studiesand essential for training in systematics. It is used for classification of anorganism, and the information gathered is also stored along with thespecimens. In some cases the specimen is preserved for future studies.Biologists have established certain procedures and techniques to storeand preserve the information as well as the specimens. Some of these areexplained to help you understand the usage of these aids.1.4.1 HerbariumHerbarium is a store house of collected plant specimens that are dried,pressed and preserved on sheets. Further, these sheets are arrangeddecreasing. Lower the taxa, more are the characteristics that the memberswithin the taxon share. Higher the category, greater is the difficulty ofdetermining the relationship to other taxa at the same level. Hence, theproblem of classification becomes more complex.Table 1.1 indicates the taxonomic categories to which some commonorganisms like housefly, man, mango and wheat belong.Common Biological Genus Family Order Class Phylum/Name Name DivisionMan Homo sapiens Homo Hominidae Primata Mammalia ChordataHousefly Musca Musca Muscidae Diptera Insecta ArthropodadomesticaMango Mangifera Mangifera Anacardiaceae Sapindales Dicotyledonae AngiospermaeindicaWheat Triticum Triticum Poaceae Poales Monocotyledonae AngiospermaeaestivumTABLE 1.1 Organisms with their Taxonomic Categories2022-2312 BIOLOGYaccording to a universally accepted system of classification. Thesespecimens, along with their descriptions on herbarium sheets, become astore house or repository for future use (Figure 1.2). The herbarium sheetsalso carry a label providing information about date and place of collection,English, local and botanical names, family, collectorâs name, etc. Herbariaalso serve as quick referral systems in taxonomical studies.1.4.2 Botanical GardensThese specialised gardens have collections of living plants for reference.Plant species in these gardens are grown for identification purposes andeach plant is labelled indicating its botanical/scientific name and its family.The famous botanical gardens are at Kew (England), Indian BotanicalGarden, Howrah (India) and at National Botanical Research Institute,Lucknow (India).1.4.3 MuseumBiological museums are generally set up in educational institutes suchas schools and colleges. Museums have collections of preserved plantand animal specimens for study and reference. Specimens are preservedin the containers or jars in preservative solutions. Plant and animalspecimens may also be preserved as dry specimens. Insects are preservedin insect boxes after collecting, killing and pinning. Larger animals likebirds and mammals are usually stuffed and preserved. Museums oftenhave collections of skeletons of animals too.Figure 1.2 Herbarium showing stored specimens2022-23THE LIVING WORLD 131.4.4 Zoological ParksThese are the places where wild animals are kept in protected environmentsunder human care and which enable us to learn about their food habitsand behaviour. All animals in a zoo are provided, as far as possible, theconditions similar to their natural habitats. Children love visiting theseparks, commonly called Zoos (Figure 1.3).Figure 1.3 Pictures showing animals in different zoological parks of India1.4.5 KeyKey is another taxonomical aid used for identification of plants and animalsbased on the similarities and dissimilarities. The keys are based on thecontrasting characters generally in a pair called couplet. It representsthe choice made between two opposite options. This results in acceptanceof only one and rejection of the other. Each statement in the key is calleda lead. Separate taxonomic keys are required for each taxonomic categorysuch as family, genus and species for identification purposes. Keys aregenerally analytical in nature.2022-2314 BIOLOGYSUMMARYThe living world is rich in variety. Millions of plants and animals have beenidentified and described but a large number still remains unknown. The veryrange of organisms in terms of size, colour, habitat, physiological andmorphological features make us seek the defining characteristics of livingorganisms. In order to facilitate the study of kinds and diversity of organisms,biologists have evolved certain rules and principles for identification, nomenclatureand classification of organisms. The branch of knowledge dealing with these aspectsis referred to as taxonomy. The taxonomic studies of various species of plantsand animals are useful in agriculture, forestry, industry and in general for knowingour bio-resources and their diversity. The basics of taxonomy like identification,naming and classification of organisms are universally evolved under internationalcodes. Based on the resemblances and distinct differences, each organism isidentified and assigned a correct scientific/biological name comprising two wordsas per the binomial system of nomenclature. An organism represents/occupies aplace or position in the system of classification. There are many categories/ranksand are generally referred to as taxonomic categories or taxa. All the categoriesconstitute a taxonomic hierarchy.Taxonomists have developed a variety of taxonomic aids to facilitateidentification, naming and classification of organisms. These studies are carriedout from the actual specimens which are collected from the field and preserved asreferrals in the form of herbaria, museums and in botanical gardens and zoologicalparks. It requires special techniques for collection and preservation of specimensin herbaria and museums. Live specimens, on the other hand, of plants andanimals, are found in botanical gardens or in zoological parks. Taxonomists alsoprepare and disseminate information through manuals and monographs forfurther taxonomic studies. Taxonomic keys are tools that help in identificationbased on characteristics.Flora, manuals, monographs and catalogues are some other meansof recording descriptions. They also help in correct identification. Floracontains the actual account of habitat and distribution of plants of agiven area. These provide the index to the plant species found in aparticular area. Manuals are useful in providing information foridentification of names of species found in an area. Monographs containinformation on any one taxon.2022-23THE LIVING WORLD 15EXERCISES1. Why are living organisms classified?2. Why are the classification systems changing every now and then?3. What different criteria would you choose to classify people that you meet often?4. What do we learn from identification of individuals and populations?5. Given below is the scientific name of Mango. Identify the correctly written name.Mangifera IndicaMangifera indica6. Define a taxon. Give some examples of taxa at different hierarchical levels.7. Can you identify the correct sequence of taxonomical categories?(a) Species Order Phylum Kingdom(b) Genus Species Order Kingdom(c) Species Genus Order Phylum8. Try to collect all the currently accepted meanings for the word âspeciesâ. Discusswith your teacher the meaning of species in case of higher plants and animalson one hand, and bacteria on the other hand.9. Define and understand the following terms:(i) Phylum (ii) Class (iii) Family (iv) Order (v) Genus10. How is a key helpful in the identification and classification of an organism?11. Illustrate the taxonomical hierarchy with suitable examples of a plant and ananimal.2022-2316 BIOLOGYSince the dawn of civilisation, there have been many attempts to classifyliving organisms. It was done instinctively not using criteria that werescientific but borne out of a need to use organisms for our own use â forfood, shelter and clothing. Aristotle was the earliest to attempt a morescientific basis for classification. He used simple morphological charactersto classify plants into trees, shrubs and herbs. He also divided animalsinto two groups, those which had red blood and those that did not.In Linnaeus' time a Two Kingdom system of classification withPlantae and Animalia kingdoms was developed that included allplants and animals respectively. This system did not distinguish betweenthe eukaryotes and prokaryotes, unicellular and multicellular organismsand photosynthetic (green algae) and non-photosynthetic (fungi)organisms. Classification of organisms into plants and animals was easilydone and was easy to understand, but, a large number of organismsdid not fall into either category. Hence the two kingdom classificationused for a long time was found inadequate. Besides, gross morphologya need was also felt for including other characteristics like cell structure,nature of wall, mode of nutrition, habitat, methods of reproduction,evolutionary relationships, etc. Classification systems for the livingorganisms have hence, undergone several changes over the time.Though plant and animal kingdoms have been a constant under alldifferent systems, the understanding of what groups/organisms beincluded under these kingdoms have been changing; the number andnature of other kingdoms have also been understood differently bydifferent scientists over the time.BIOLOGICAL CLASSIFICATIONCHAPTER 22.1 Kingdom Monera2.2 Kingdom Protista2.3 Kingdom Fungi2.4 Kingdom Plantae2.5 KingdomAnimalia2.6 Viruses, Viroidsand Lichens2022-23BIOLOGICAL CLASSIFICATION 17R.H. Whittaker (1969) proposed a Five Kingdom Classification. Thekingdoms defined by him were named Monera, Protista, Fungi, Plantaeand Animalia. The main criteria for classification used by him include cellstructure, body organisation, mode of nutrition, reproduction andphylogenetic relationships. Table 2.1 gives a comparative account of differentcharacteristics of the five kingdoms.The three-domain system has also been proposed that divides the KingdomMonera into two domains, leaving the remaining eukaryotic kingdoms in thethird domain and thereby a six kingdom classification. You will learn aboutthis system in detail at higher classes.Let us look at this five kingdom classification to understand the issuesand considerations that influenced the classification system. Earlierclassification systems included bacteria, blue green algae, fungi, mosses,ferns, gymnosperms and the angiosperms under âPlantsâ. The characterthat unified this whole kingdom was that all the organisms included had acell wall in their cells. This placed together groups which widely differed inother characteristics. It brought together the prokaryotic bacteria and theblue green algae ( cyanobacteria) with other groups which were eukaryotic.It also grouped together the unicellular organisms and the multicellularones, say, for example, Chlamydomonas and Spirogyra were placed togetherunder algae. The classification did not differentiate between the heterotrophicgroup â fungi, and the autotrophic green plants, though they also showeda characteristic difference in their walls composition â the fungi had chitinFive KingdomsCharactersCell typeCell wallNuclearmembraneBodyorganisationMode ofnutritionMoneraProkaryoticNoncellulosic(Polysaccharide+ amino acid)AbsentCellularAutotrophic(chemosyntheticandphotosynthetic)and Heterotrophic(saprophytic/parasitic)ProtistaEukaryoticPresent insomePresentCellularAutotrophic(Photosynthetic)andHeterotrophicFungiEukaryoticPresentwith chitinPresentMulticeullar/loose tissueHeterotrophic(Saprophytic/Parasitic)PlantaeEukaryoticPresent(cellulose)PresentTissue/organAutotrophic(Photosynthetic)AnimaliaEukaryoticAbsentPresentTissue/organ/organ systemHeterotrophic( H o l o z o i c /Saprophyticetc.)TABLE 2.1 Characteristics of the Five Kingdoms2022-2318 BIOLOGYin their walls while the green plants had a cellulosic cell wall. When suchcharacteristics were considered, the fungi were placed in a separatekingdom â Kingdom Fungi. All prokaryotic organisms were groupedtogether under Kingdom Monera and the unicellular eukaryotic organismswere placed in Kingdom Protista. Kingdom Protista has brought togetherChlamydomonas, Chlorella (earlier placed in Algae within Plants and bothhaving cell walls) with Paramoecium and Amoeba (which were earlier placedin the animal kingdom which lack cell wall). It has put together organismswhich, in earlier classifications, were placed in different kingdoms. Thishappened because the criteria for classification changed. This kind ofchanges will take place in future too depending on the improvement in ourunderstanding of characteristics and evolutionary relationships. Over time,an attempt has been made to evolve a classification system which reflectsnot only the morphological, physiological and reproductive similarities,but is also phylogenetic, i.e., is based on evolutionary relationships.In this chapter we will study characteristics of Kingdoms Monera,Protista and Fungi of the Whittaker system of classification. The KingdomsPlantae and Animalia, commonly referred to as plant and animalkingdoms, respectively, will be dealt separately in chapters 3 and 4.Spore FlagellumCocci BacilliSpirillaVibrioFigure 2.1 Bacteria of different shapes2.1 KINGDOM MONERABacteria are the sole members of the Kingdom Monera. They are the mostabundant micro-organisms. Bacteria occur almost everywhere. Hundredsof bacteria are present in a handful of soil. They also live in extreme habitatssuch as hot springs, deserts, snow and deep oceans where very few otherlife forms can survive. Many of them live in or on other organisms asparasites.Bacteria are grouped under four categories based on their shape: thespherical Coccus (pl.: cocci), the rod-shaped Bacillus (pl.: bacilli), thecomma-shaped Vibrium (pl.: vibrio) and the spiral Spirillum (pl.: spirilla)(Figure 2.1).2022-23BIOLOGICAL CLASSIFICATION 19Though the bacterial structure is very simple, they are very complexin behaviour. Compared to many other organisms, bacteria as a groupshow the most extensive metabolic diversity. Some of the bacteria areautotrophic, i.e., they synthesise their own food from inorganic substrates.They may be photosynthetic autotrophic or chemosynthetic autotrophic.The vast majority of bacteria are heterotrophs, i.e., they depend on otherorganisms or on dead organic matter for food.2.1.1 ArchaebacteriaThese bacteria are special since they live in some of the most harsh habitatssuch as extreme salty areas (halophiles), hot springs (thermoacidophiles)and marshy areas (methanogens). Archaebacteria differ from other bacteriain having a different cell wall structure and this feature is responsible fortheir survival in extreme conditions. Methanogens are present in the gutof several ruminant animals such as cows and buffaloes and they areresponsible for the production of methane (biogas) from the dung of theseanimals.Figure 2.2 A filamentous blue-greenalgae â Nostoc2.1.2 EubacteriaThere are thousands of different eubacteria or âtruebacteriaâ. They are characterised by the presence of arigid cell wall, and if motile, a flagellum. Thecyanobacteria (also referred to as blue-green algae)have chlorophyll a similar to green plants and arephotosynthetic autotrophs (Figure 2.2). Thecyanobacteria are unicellular, colonial or filamentous,freshwater/marine or terrestrial algae. The coloniesare generally surrounded by gelatinous sheath. Theyoften form blooms in polluted water bodies. Some ofthese organisms can fix atmospheric nitrogen inspecialised cells called heterocysts, e.g., Nostoc andAnabaena. Chemosynthetic autotrophic bacteriaoxidise various inorganic substances such asnitrates, nitrites and ammonia and use the releasedenergy for their ATP production. They play a great rolein recycling nutrients like nitrogen, phosphorous,iron and sulphur.Heterotrophic bacteria are most abundant innature. The majority are important decomposers.Many of them have a significant impact on humanaffairs. They are helpful in making curd from milk,production of antibiotics, fixing nitrogen in legume2022-2320 BIOLOGYroots, etc. Some are pathogens causing damageto human beings, crops, farm animals and pets.Cholera, typhoid, tetanus, citrus canker are wellknown diseases caused by different bacteria.Bacteria reproduce mainly by fission (Figure2.3). Sometimes, under unfavourable conditions,they produce spores. They also reproduce by asort of sexual reproduction by adopting aprimitive type of DNA transfer from one bacteriumto the other.The Mycoplasma are organisms thatcompletely lack a cell wall. They are the smallestliving cells known and can survive without oxygen. Many mycoplasmaare pathogenic in animals and plants.2.2 KINGDOM PROTISTAAll single-celled eukaryotes are placed under Protista, but the boundariesof this kingdom are not well defined. What may be âa photosyntheticprotistanâ to one biologist may be âa plantâ to another. In this book weinclude Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds andProtozoans under Protista. Members of Protista are primarily aquatic.This kingdom forms a link with the others dealing with plants, animalsand fungi. Being eukaryotes, the protistan cell body contains a well definednucleus and other membrane-bound organelles. Some have flagella orcilia. Protists reproduce asexually and sexually by a process involvingcell fusion and zygote formation.2.2.1 ChrysophytesThis group includes diatoms and golden algae (desmids). They are foundin fresh water as well as in marine environments. They are microscopicand float passively in water currents (plankton). Most of them arephotosynthetic. In diatoms the cell walls form two thin overlapping shells,which fit together as in a soap box. The walls are embedded with silicaand thus the walls are indestructible. Thus, diatoms have left behindlarge amount of cell wall deposits in their habitat; this accumulation overbillions of years is referred to as âdiatomaceous earthâ. Being gritty thissoil is used in polishing, filtration of oils and syrups. Diatoms are thechief âproducersâ in the oceans.Figure 2.3 A dividing bacterium2022-23BIOLOGICAL CLASSIFICATION 212.2.2 DinoflagellatesThese organisms are mostly marine and photosynthetic.They appear yellow, green, brown, blue or red dependingon the main pigments present in their cells. The cell wallhas stiff cellulose plates on the outer surface. Most ofthem have two flagella; one lies longitudinally and theother transversely in a furrow between the wall plates.Very often, red dinoflagellates (Example: Gonyaulax)undergo such rapid multiplication that they make thesea appear red (red tides). Toxins released by such largenumbers may even kill other marine animals such asfishes.2.2.3 EuglenoidsMajority of them are fresh water organisms found instagnant water. Instead of a cell wall, they have a proteinrich layer called pellicle which makes their body flexible.They have two flagella, a short and a long one. Thoughthey are photosynthetic in the presence of sunlight, whendeprived of sunlight they behave like heterotrophs bypredating on other smaller organisms. Interestingly, thepigments of euglenoids are identical to those present inhigher plants. Example: Euglena (Figure 2.4b).2.2.4 Slime MouldsSlime moulds are saprophytic protists. The body movesalong decaying twigs and leaves engulfing organicmaterial. Under suitable conditions, they form anaggregation called plasmodium which may grow andspread over several feet. During unfavourable conditions,the plasmodium differentiates and forms fruiting bodiesbearing spores at their tips. The spores possess true walls.They are extremely resistant and survive for many years,even under adverse conditions. The spores are dispersedby air currents.2.2.5 ProtozoansAll protozoans are heterotrophs and live as predators orparasites. They are believed to be primitive relatives ofanimals. There are four major groups of protozoans.Amoeboid protozoans: These organisms live in freshwater, sea water or moist soil. They move and captureFigure 2.4 (a) Dinoflagellates(b) Euglena(c) Slime mould(d) Paramoecium(d)(a)(c)(b)2022-2322 BIOLOGYtheir prey by putting out pseudopodia (false feet) as in Amoeba. Marineforms have silica shells on their surface. Some of them such as Entamoebaare parasites.Flagellated protozoans: The members of this group are either free-livingor parasitic. They have flagella. The parasitic forms cause diaseases suchas sleeping sickness. Example: Trypanosoma.Ciliated protozoans: These are aquatic, actively moving organisms becauseof the presence of thousands of cilia. They have a cavity (gullet) that opensto the outside of the cell surface. The coordinated movement of rows ofcilia causes the water laden with food to be steered into the gullet. Example:Paramoecium (Figure 2.4d).Sporozoans: This includes diverse organisms that have an infectiousspore-like stage in their life cycle. The most notorious is Plasmodium(malarial parasite) which causes malaria, a disease which has a staggeringeffect on human population.2.3 KINGDOM FUNGIThe fungi constitute a unique kingdom of heterotrophic organisms. Theyshow a great diversity in morphology and habitat. You must have seenfungi on a moist bread and rotten fruits. The common mushroom you eatand toadstools are also fungi. White spots seen on mustard leaves are dueto a parasitic fungus. Some unicellular fungi, e.g., yeast are used to makebread and beer. Other fungi cause diseases in plants and animals; wheatrust-causing Puccinia is an important example. Some are the source ofantibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water,soil and on animals and plants. They prefer to grow in warm and humidplaces. Have you ever wondered why we keep food in the refrigerator ? Yes,it is to prevent food from going bad due to bacterial or fungal infections.With the exception of yeasts which are unicellular, fungi arefilamentous. Their bodies consist of long, slender thread-like structurescalled hyphae. The network of hyphae is known as mycelium. Some hyphaeare continuous tubes filled with multinucleated cytoplasm â these arecalled coenocytic hyphae. Others have septae or cross walls in theirhyphae. The cell walls of fungi are composed of chitin and polysaccharides.Most fungi are heterotrophic and absorb soluble organic matter fromdead substrates and hence are called saprophytes. Those that dependon living plants and animals are called parasites. They can also live assymbionts â in association with algae as lichens and with roots of higherplants as mycorrhiza.Reproduction in fungi can take place by vegetative means âfragmentation, fission and budding. Asexual reproduction is by spores2022-23BIOLOGICAL CLASSIFICATION 23called conidia or sporangiospores or zoospores, and sexual reproductionis by oospores, ascospores and basidiospores. The various spores areproduced in distinct structures called fruiting bodies. The sexual cycleinvolves the following three steps:(i) Fusion of protoplasms between two motile or non-motile gametescalled plasmogamy.(ii) Fusion of two nuclei called karyogamy.(iii) Meiosis in zygote resulting in haploid spores.When a fungus reproduces sexually, two haploidhyphae of compatible mating types come together andfuse. In some fungi the fusion of two haploid cellsimmediately results in diploid cells (2n). However, in otherfungi (ascomycetes and basidiomycetes), an interveningdikaryotic stage (n + n, i.e., two nuclei per cell) occurs;such a condition is called a dikaryon and the phase iscalled dikaryophase of fungus. Later, the parental nucleifuse and the cells become diploid. The fungi form fruitingbodies in which reduction division occurs, leading toformation of haploid spores.The morphology of the mycelium, mode of sporeformation and fruiting bodies form the basis for thedivision of the kingdom into various classes.2.3.1 PhycomycetesMembers of phycomycetes are found in aquatic habitatsand on decaying wood in moist and damp places or asobligate parasites on plants. The mycelium is aseptateand coenocytic. Asexual reproduction takes place byzoospores (motile) or by aplanospores (non-motile). Thesespores are endogenously produced in sporangium. Azygospore is formed by fusion of two gametes. Thesegametes are similar in morphology (isogamous) ordissimilar (anisogamous or oogamous). Some commonexamples are Mucor (Figure 2.5a), Rhizopus (the breadmould mentioned earlier) and Albugo (the parasitic fungion mustard).2.3.2 AscomycetesCommonly known as sac-fungi, the ascomycetes are mostlymulticellular, e.g., Penicillium, or rarely unicellular, e.g., yeast(Saccharomyces). They are saprophytic, decomposers,parasitic or coprophilous (growing on dung). MyceliumFigure 2.5 Fungi: (a) Mucor(b) Aspergillus (c) Agaricus(c)(a)(b)2022-2324 BIOLOGYis branched and septate. The asexual spores are conidia producedexogenously on the special mycelium called conidiophores. Conidia ongermination produce mycelium. Sexual spores are called ascosporeswhich are produced endogenously in sac like asci (singular ascus). Theseasci are arranged in different types of fruiting bodies called ascocarps.Some examples are Aspergillus (Figure 2.5b), Claviceps and Neurospora.Neurospora is used extensively in biochemical and genetic work. Manymembers like morels and truffles are edible and are considered delicacies.2.3.3 BasidiomycetesCommonly known forms of basidiomycetes are mushrooms, bracket fungior puffballs. They grow in soil, on logs and tree stumps and in livingplant bodies as parasites, e.g., rusts and smuts. The mycelium is branchedand septate. The asexual spores are generally not found, but vegetativereproduction by fragmentation is common. The sex organs are absent,but plasmogamy is brought about by fusion of two vegetative or somaticcells of different strains or genotypes. The resultant structure is dikaryoticwhich ultimately gives rise to basidium. Karyogamy and meiosis takeplace in the basidium producing four basidiospores. The basidiosporesare exogenously produced on the basidium (pl.: basidia). The basidia arearranged in fruiting bodies called basidiocarps. Some common membersare Agaricus (mushroom) (Figure 2.5c), Ustilago (smut) and Puccinia (rustfungus).2.3.4 DeuteromycetesCommonly known as imperfect fungi because only the asexual orvegetative phases of these fungi are known. When the sexual forms ofthese fungi were discovered they were moved into classes they rightlybelong to. It is also possible that the asexual and vegetative stage havebeen given one name (and placed under deuteromycetes) and the sexualstage another (and placed under another class). Later when the linkageswere established, the fungi were correctly identified and moved out ofdeuteromycetes. Once perfect (sexual) stages of members ofdueteromycetes were discovered they were often moved to ascomycetesand basidiomycetes. The deuteromycetes reproduce only by asexual sporesknown as conidia. The mycelium is septate and branched. Some membersare saprophytes or parasites while a large number of them aredecomposers of litter and help in mineral cycling. Some examples areAlternaria, Colletotrichum and Trichoderma.2022-23BIOLOGICAL CLASSIFICATION 252.4 KINGDOM PLANTAEKingdom Plantae includes all eukaryotic chlorophyll-containingorganisms commonly called plants. A few members are partiallyheterotrophic such as the insectivorous plants or parasites. Bladderwortand Venus fly trap are examples of insectivorous plants and Cuscuta is aparasite. The plant cells have an eukaryotic structure with prominentchloroplasts and cell wall mainly made of cellulose. You will study theeukaryotic cell structure in detail in Chapter 8. Plantae includes algae,bryophytes, pteridophytes, gymnosperms and angiosperms.Life cycle of plants has two distinct phases â the diploid sporophyticand the haploid gametophytic â that alternate with each other. The lengthsof the haploid and diploid phases, and whether these phases are freeâliving or dependent on others, vary among different groups in plants.This phenomenon is called alternation of generation. You will studyfurther details of this kingdom in Chapter 3.2.5 KINGDOM ANIMALIAThis kingdom is characterised by heterotrophic eukaryotic organismsthat are multicellular and their cells lack cell walls. They directly orindirectly depend on plants for food. They digest their food in an internalcavity and store food reserves as glycogen or fat. Their mode of nutritionis holozoic â by ingestion of food. They follow a definite growth patternand grow into adults that have a definite shape and size. Higher formsshow elaborate sensory and neuromotor mechanism. Most of them arecapable of locomotion.The sexual reproduction is by copulation of male and female followedby embryological development. Salient features of various phyla aredescribed in Chapter 4.2.6 VIRUSES, VIROIDS, PRIONS AND LICHENSIn the five kingdom classification of Whittaker there is no mention of lichensand some acellular organisms like viruses, viroids and prions. These arebriefly introduced here.All of us who have suffered the ill effects of common cold or âfluâ knowwhat effects viruses can have on us, even if we do not associate it with ourcondition. Viruses did not find a place in classification since they are notconsidered truly âlivingâ, if we understand living as those organisms thathave a cell structure. The viruses are non-cellular organisms that arecharacterised by having an inert crystalline structure outside the living cell.2022-2326 BIOLOGYOnce they infect a cell they take over the machinery of the host cell to replicatethemselves, killing the host. Would you call viruses living or non-living?Virus means venom or poisonous fluid. Dmitri Ivanowsky (1892)recognised certain microbes as causal organism of the mosaic disease oftobacco (Figure 2.6a). These were found to be smaller than bacteriabecause they passed through bacteria-proof filters. M.W. Beijerinek(1898) demonstrated that the extract of the infected plants of tobaccocould cause infection in healthy plants and named the new pathogenâvirusâ and called the fluid as Contagium vivum fluidum (infectious livingfluid). W.M. Stanley (1935) showed that viruses could be crystallisedand crystals consist largely of proteins. They are inert outside their specifichost cell. Viruses are obligate parasites.In addition to proteins, viruses also contain genetic material, that couldbe either RNA or DNA. No virus contains both RNA and DNA. A virus isa nucleoprotein and the genetic material is infectious. In general, virusesthat infect plants have single stranded RNA and viruses that infect animalshave either single or double stranded RNA or double stranded DNA.Bacterial viruses or bacteriophages (viruses that infect the bacteria) areusually double stranded DNA viruses (Figure 2.6b). The protein coatcalled capsid made of small subunits called capsomeres, protects thenucleic acid. These capsomeres are arranged in helical or polyhedralgeometric forms. Viruses cause diseases like mumps, small pox, herpesand influenza. AIDS in humans is also caused by a virus. In plants, thesymptoms can be mosaic formation, leaf rolling and curling, yellowingand vein clearing, dwarfing and stunted growth.RNA Capsid(a)SheathHeadTail fibresCollar(b)Figure 2.6 (a) Tobacco Mosaic Virus (TMV) (b) Bacteriophage2022-23BIOLOGICAL CLASSIFICATION 27SUMMARYBiological classification of plants and animals was first proposed by Aristotle on thebasis of simple morphological characters. Linnaeus later classified all living organismsinto two kingdoms â Plantae and Animalia. Whittaker proposed an elaborate fivekingdom classification â Monera, Protista, Fungi, Plantae and Animalia. The maincriteria of the five kingdom classification were cell structure, body organisation,mode of nutrition and reproduction, and phylogenetic relationships.In the five kingdom classification, bacteria are included in Kingdom Monera.Bacteria are cosmopolitan in distribution. These organisms show the most extensivemetabolic diversity. Bacteria may be autotrophic or heterotrophic in their mode ofnutrition. Kingdom Protista includes all single-celled eukaryotes such asChrysophytes, Dinoflagellates, Euglenoids, Slime-moulds and Protozoans. Protistshave defined nucleus and other membrane bound organelles. They reproduceboth asexually and sexually. Members of Kingdom Fungi show a great diversityin structures and habitat. Most fungi are saprophytic in their mode of nutrition.They show asexual and sexual reproduction. Phycomycetes, Ascomycetes,Basidiomycetes and Deuteromycetes are the four classes under this kingdom.The plantae includes all eukaryotic chlorophyll-containing organisms. Algae,bryophytes, pteridophytes, gymnosperms and angiosperms are included in thisgroup. The life cycle of plants exhibit alternation of generations â gametophyticand sporophytic generations. The heterotrophic eukaryotic, multicellularorganisms lacking a cell wall are included in the Kingdom Animalia. The mode ofnutrition of these organisms is holozoic. They reproduce mostly by the sexualmode. Some acellular organisms like viruses and viroids as well as the lichens arenot included in the five kingdom system of classification.Viroids : In 1971, T.O. Diener discovered a new infectious agent thatwas smaller than viruses and caused potato spindle tuber disease. It wasfound to be a free RNA; it lacked the protein coat that is found in viruses,hence the name viroid. The RNA of the viroid was of low molecular weight.Prions : In modern medicine certain infectious neurological diseaseswere found to be transmitted by an agent consisting of abnormally foldedprotein. The agent was similar in size to viruses. These agents were calledprions. The most notable diseases caused by prions are bovine spongiformencephalopathy (BSE) commonly called mad cow disease in cattle andits analogous variant CrâJacob disease (CJD) in humans.Lichens : Lichens are symbiotic associations i.e. mutually usefulassociations, between algae and fungi. The algal component is known asphycobiont and fungal component as mycobiont, which are autotrophicand heterotrophic, respectively. Algae prepare food for fungi and fungiprovide shelter and absorb mineral nutrients and water for its partner.So close is their association that if one saw a lichen in nature one wouldnever imagine that they had two different organisms within them. Lichensare very good pollution indicators â they do not grow in polluted areas.2022-2328 BIOLOGYEXERCISES1. Discuss how classification systems have undergone several changes over aperiod of time?2. State two economically important uses of:(a) heterotrophic bacteria(b) archaebacteria3. What is the nature of cell-walls in diatoms?4. Find out what do the terms âalgal bloomâ and âred-tidesâ signify.5. How are viroids different from viruses?6. Describe briefly the four major groups of Protozoa.7. Plants are autotrophic. Can you think of some plants that are partiallyheterotrophic?8. What do the terms phycobiont and mycobiont signify?9. Give a comparative account of the classes of Kingdom Fungi under the following:(i) mode of nutrition(ii) mode of reproduction10. What are the characteristic features of Euglenoids?11. Give a brief account of viruses with respect to their structure and nature ofgenetic material. Also name four common viral diseases.12. Organise a discussion in your class on the topic â Are viruses living or nonliving?2022-23PLANT KINGDOM 29In the previous chapter, we looked at the broad classification of livingorganisms under the system proposed by Whittaker (1969) wherein hesuggested the Five Kingdom classification viz. Monera, Protista, Fungi,Animalia and Plantae. In this chapter, we will deal in detail with furtherclassification within Kingdom Plantae popularly known as the âplantkingdomâ.We must stress here that our understanding of the plant kingdomhas changed over time. Fungi, and members of the Monera and Protistahaving cell walls have now been excluded from Plantae though earlierclassifications placed them in the same kingdom. So, the cyanobacteriathat are also referred to as blue green algae are not âalgaeâ any more. Inthis chapter, we will describe Algae, Bryophytes, Pteridophytes,Gymnosperms and Angiosperms under Plantae .Let us also look at classification within angiosperms to understandsome of the concerns that influenced the classification systems. Theearliest systems of classification used only gross superficial morphologicalcharacters such as habit, colour, number and shape of leaves, etc. Theywere based mainly on vegetative characters or on the androeciumstructure (system given by Linnaeus). Such systems were artificial; theyseparated the closely related species since they were based on a fewcharacteristics. Also, the artificial systems gave equal weightage tovegetative and sexual characteristics; this is not acceptable since we knowthat often the vegetative characters are more easily affected byenvironment. As against this, natural classification systems developed,which were based on natural affinities among the organisms and consider,PLANT KINGDOMCHAPTER 33.1 Algae3.2 Bryophytes3.3 Pteridophytes3.4 Gymnosperms3.5 Angiosperms3.6 Plant Life Cyclesand Alternationof Generations2022-2330 BIOLOGYnot only the external features, but also internal features, like ultrastructure,anatomy, embryology and phytochemistry. Such aclassification for flowering plants was given by George Bentham andJoseph Dalton Hooker.At present phylogenetic classification systems based onevolutionary relationships between the various organisms are acceptable.This assumes that organisms belonging to the same taxa have a commonancestor. We now use information from many other sources too to helpresolve difficulties in classification. These become more important whenthere is no supporting fossil evidence. Numerical Taxonomy which isnow easily carried out using computers is based on all observablecharacteristics. Number and codes are assigned to all the characters andthe data are then processed. In this way each character is given equalimportance and at the same time hundreds of characters can beconsidered. Cytotaxonomy that is based on cytological information likechromosome number, structure, behaviour and chemotaxonomy thatuses the chemical constituents of the plant to resolve confusions, are alsoused by taxonomists these days.3.1 ALGAEAlgae are chlorophyll-bearing, simple, thalloid, autotrophic and largelyaquatic (both fresh water and marine) organisms. They occur in avariety of other habitats: moist stones, soils and wood. Some of themalso occur in association with fungi (lichen) and animals (e.g., on slothbear).The form and size of algae is highly variable, ranging from colonialforms like Volvox and the filamentous forms like Ulothrix and Spirogyra(Figure 3.1). A few of the marine forms such as kelps, form massive plantbodies.The algae reproduce by vegetative, asexual and sexual methods.Vegetative reproduction is by fragmentation. Each fragment develops intoa thallus. Asexual reproduction is by the production of different types ofspores, the most common being the zoospores. They are flagellated(motile) and on germination gives rise to new plants. Sexual reproductiontakes place through fusion of two gametes. These gametes can beflagellated and similar in size (as in Ulothrix) or non-flagellated (non-motile)but similar in size (as in Spirogyra). Such reproduction is calledisogamous. Fusion of two gametes dissimilar in size, as in species ofEudorina is termed as anisogamous. Fusion between one large, nonmotile(static) female gamete and a smaller, motile male gamete is termedoogamous, e.g., Volvox, Fucus.2022-23PLANT KINGDOM 31Figure 3.1 Algae : (a) Green algae (i) Volvox (ii) Ulothrix(b) Brown algae (i) Laminaria (ii) Fucus (iii) Dictyota(c) Red algae (i) Porphyra (ii) Polysiphonia2022-2332 BIOLOGYAlgae are useful to man in a variety of ways. At least a half of the totalcarbon dioxide fixation on earth is carried out by algae throughphotosynthesis. Being photosynthetic they increase the level of dissolvedoxygen in their immediate environment. They are of paramountimportance as primary producers of energy-rich compounds which formthe basis of the food cycles of all aquatic animals. Many species of Porphyra,Laminaria and Sargassum are among the 70 species of marine algaeused as food. Certain marine brown and red algae produce large amountsof hydrocolloids (water holding substances), e.g., algin (brown algae) andcarrageen (red algae) which are used commercially. Agar, one of thecommercial products obtained from Gelidium and Gracilaria are used togrow microbes and in preparations of ice-creams and jellies. Chlorella aunicellular alga rich in proteins is used as food supplement even by spacetravellers. The algae are divided into three main classes: Chlorophyceae,Phaeophyceae and Rhodophyceae.3.1.1 ChlorophyceaeThe members of chlorophyceae are commonly called green algae. Theplant body may be unicellular, colonial or filamentous. They are usuallygrass green due to the dominance of pigments chlorophyll a and b. Thepigments are localised in definite chloroplasts. The chloroplasts may bediscoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped indifferent species. Most of the members have one or more storage bodiescalled pyrenoids located in the chloroplasts. Pyrenoids contain proteinbesides starch. Some algae may store food in the form of oil droplets.Green algae usually have a rigid cell wall made of an inner layer of celluloseand an outer layer of pectose.Vegetative reproduction usually takes place by fragmentation or byformation of different types of spores. Asexual reproduction is byflagellated zoospores produced in zoosporangia. The sexual reproductionshows considerable variation in the type and formation of sex cells and itmay be isogamous, anisogamous or oogamous. Some commonly foundgreen algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara(Figure 3.1a).3.1.2 PhaeophyceaeThe members of phaeophyceae or brown algae are found primarily inmarine habitats. They show great variation in size and form. They rangefrom simple branched, filamentous forms (Ectocarpus) to profuselybranched forms as represented by kelps, which may reach a height of100 metres. They possess chlorophyll a, c, carotenoids and xanthophylls.They vary in colour from olive green to various shades of brown dependingupon the amount of the xanthophyll pigment, fucoxanthin present in2022-23PLANT KINGDOM 33them. Food is stored as complex carbohydrates, which may be in theform of laminarin or mannitol. The vegetative cells have a cellulosic wallusually covered on the outside by a gelatinous coating of algin. Theprotoplast contains, in addition to plastids, a centrally located vacuoleand nucleus. The plant body is usually attached to the substratum by aholdfast, and has a stalk, the stipe and leaf like photosynthetic organ âthe frond. Vegetative reproduction takes place by fragmentation. Asexualreproduction in most brown algae is by biflagellate zoospores that arepear-shaped and have two unequal laterally attached flagella.Sexual reproduction may be isogamous, anisogamous or oogamous.Union of gametes may take place in water or within the oogonium(oogamous species). The gametes are pyriform (pear-shaped) and beartwo laterally attached flagella. The common forms are Ectocarpus, Dictyota,Laminaria, Sargassum and Fucus (Figure 3.1b).3.1.3 RhodophyceaeThe members of rhodophyceae are commonly called red algae because ofthe predominance of the red pigment, r-phycoerythrin in their body. Majorityof the red algae are marine with greater concentrations found in the warmerareas. They occur in both well-lighted regions close to the surface of waterand also at great depths in oceans where relatively little light penetrates.The red thalli of most of the red algae are multicellular. Some of themhave complex body organisation. The food is stored as floridean starchwhich is very similar to amylopectin and glycogen in structure.The red algae usually reproduce vegetatively by fragmentation. Theyreproduce asexually by non-motile spores and sexually by non-motileTABLE 3.1 Divisions of Algae and their Main CharacteristicsClasses Common Major Stored Cell Wall Flagellar HabitatName Pigments Food Number andPosition ofInsertionsChlorophyceae Green Chlorophyll Starch Cellulose 2-8, equal, Fresh water,algae a, b apical brackish water,salt waterPhaeophyceae Brown Chlorophyll Mannitol, Cellulose 2, unequal, Fresh wateralgae a, c, laminarin and algin lateral (rare) brackishfucoxanthin water, saltwaterRhodophyceae Red Chlorophyll Floridean Cellulose, Absent Fresh wateralgae a, d, starch pectin and (some),phycoerythrin poly brackishsulphate water, saltesters water (most)2022-2334 BIOLOGYgametes. Sexual reproduction is oogamous and accompanied by complexpost fertilisation developments. The common members are: Polysiphonia,Porphyra (Figure 3.1c), Gracilaria and Gelidium.3.2 BRYOPHYTESBryophytes include the various mosses and liverworts that are foundcommonly growing in moist shaded areas in the hills (Figure 3.2).Archegoniophore(a) (b)(c)(d)AntheridiophoreCapsuleAntheridialbranch BranchesArchegonialbranchSetaSporophyteGametophyteLeavesMain axisRhizoidsGemma cupRhizoidsGemma cupRhizoidsFigure 3.2 Bryophytes: A liverwort â Marchantia (a) Female thallus (b) Male thallusMosses â (c) Funaria, gametophyte and sporophyte (d) Sphagnumgametophyte2022-23PLANT KINGDOM 35Bryophytes are also called amphibians of the plant kingdom becausethese plants can live in soil but are dependent on water for sexualreproduction. They usually occur in damp, humid and shaded localities.They play an important role in plant succession on bare rocks/soil.The plant body of bryophytes is more differentiated than that of algae.It is thallus-like and prostrate or erect, and attached to the substratumby unicellular or multicellular rhizoids. They lack true roots, stem orleaves. They may possess root-like, leaf-like or stem-like structures. Themain plant body of the bryophyte is haploid. It produces gametes, henceis called a gametophyte. The sex organs in bryophytes are multicellular.The male sex organ is called antheridium. They produce biflagellateantherozoids. The female sex organ called archegonium is flask-shapedand produces a single egg. The antherozoids are released into water wherethey come in contact with archegonium. An antherozoid fuses with theegg to produce the zygote. Zygotes do not undergo reduction divisionimmediately. They produce a multicellular body called a sporophyte.The sporophyte is not free-living but attached to the photosyntheticgametophyte and derives nourishment from it. Some cells of thesporophyte undergo reduction division (meiosis) to produce haploidspores. These spores germinate to produce gametophyte.Bryophytes in general are of little economic importance but somemosses provide food for herbaceous mammals, birds and other animals.Species of Sphagnum, a moss, provide peat that have long been used asfuel, and as packing material for trans-shipment of living material becauseof their capacity to hold water. Mosses along with lichens are the firstorganisms to colonise rocks and hence, are of great ecological importance.They decompose rocks making the substrate suitable for the growth ofhigher plants. Since mosses form dense mats on the soil, they reduce theimpact of falling rain and prevent soil erosion. The bryophytes are dividedinto liverworts and mosses.3.2.1 LiverwortsThe liverworts grow usually in moist, shady habitats such as banks ofstreams, marshy ground, damp soil, bark of trees and deep in the woods.The plant body of a liverwort is thalloid, e.g., Marchantia. The thallus isdorsiventral and closely appressed to the substrate. The leafy membershave tiny leaf-like appendages in two rows on the stem-like structures.Asexual reproduction in liverworts takes place by fragmentation ofthalli, or by the formation of specialised structures called gemmae(sing. gemma). Gemmae are green, multicellular, asexual buds, whichdevelop in small receptacles called gemma cups located on the thalli.The gemmae become detached from the parent body and germinate toform new individuals. During sexual reproduction, male and female sex2022-2336 BIOLOGYorgans are produced either on the same or on different thalli. Thesporophyte is differentiated into a foot, seta and capsule. After meiosis,spores are produced within the capsule. These spores germinate to formfree-living gametophytes.3.2.2 MossesThe predominant stage of the life cycle of a moss is the gametophyte whichconsists of two stages. The first stage is the protonema stage, whichdevelops directly from a spore. It is a creeping, green, branched andfrequently filamentous stage. The second stage is the leafy stage, whichdevelops from the secondary protonema as a lateral bud. They consist ofupright, slender axes bearing spirally arranged leaves. They are attachedto the soil through multicellular and branched rhizoids. This stage bearsthe sex organs.Vegetative reproduction in mosses is by fragmentation and buddingin the secondary protonema. In sexual reproduction, the sex organsantheridia and archegonia are produced at the apex of the leafy shoots.After fertilisation, the zygote develops into a sporophyte, consisting of afoot, seta and capsule. The sporophyte in mosses is more elaborate thanthat in liverworts. The capsule contains spores. Spores are formed aftermeiosis. The mosses have an elaborate mechanism of spore dispersal.Common examples of mosses are Funaria, Polytrichum and Sphagnum(Figure 3.2).3.3 PTERIDOPHYTESThe Pteridophytes include horsetails and ferns. Pteridophytes are usedfor medicinal purposes and as soil-binders. They are also frequently grownas ornamentals. Evolutionarily, they are the first terrestrial plants topossess vascular tissues â xylem and phloem. You shall study more aboutthese tissues in Chapter 6. The pteridophytes are found in cool, damp,shady places though some may flourish well in sandy-soil conditions.You may recall that in bryophytes the dominant phase in the lifecycle is the gametophytic plant body. However, in pteridophytes, themain plant body is a sporophyte which is differentiated into true root,stem and leaves (Figure 3.3). These organs possess well-differentiatedvascular tissues. The leaves in pteridophyta are small (microphylls) asin Selaginella or large (macrophylls) as in ferns. The sporophytes bearsporangia that are subtended by leaf-like appendages calledsporophylls. In some cases sporophylls may form distinct compactstructures called strobili or cones (Selaginella, Equisetum). Thesporangia produce spores by meiosis in spore mother cells. The sporesgerminate to give rise to inconspicuous, small but multicellular,2022-23PLANT KINGDOM 37Figure 3.3 Pteridophytes : (a) Selaginella (b) Equisetum (c) Fern (d) SalviniaStrobilusNodeInternodeBranchRhizome(b)(c)(d)2022-2338 BIOLOGYfree-living, mostly photosynthetic thalloid gametophytes calledprothallus. These gametophytes require cool, damp, shady places togrow. Because of this specific restricted requirement and the need forwater for fertilisation, the spread of living pteridophytes is limited andrestricted to narrow geographical regions. The gametophytes bear maleand female sex organs called antheridia and archegonia, respectively.Water is required for transfer of antherozoids â the male gametes releasedfrom the antheridia, to the mouth of archegonium. Fusion of male gametewith the egg present in the archegonium result in the formation of zygote.Zygote thereafter produces a multicellular well-differentiated sporophytewhich is the dominant phase of the pteridophytes. In majority of thepteridophytes all the spores are of similar kinds; such plants are calledhomosporous. Genera like Selaginella and Salvinia which producetwo kinds of spores, macro (large) and micro (small) spores, are knownas heterosporous. The megaspores and microspores germinate and giverise to female and male gametophytes, respectively. The femalegametophytes in these plants are retained on the parent sporophytesfor variable periods. The development of the zygotes into young embryostake place within the female gametophytes. This event is a precursor tothe seed habit considered an important step in evolution.The pteridophytes are further classified into four classes: Psilopsida(Psilotum); Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum)and Pteropsida (Dryopteris, Pteris, Adiantum).3.4 GYMNOSPERMSThe gymnosperms (gymnos : naked, sperma : seeds) are plants in whichthe ovules are not enclosed by any ovary wall and remain exposed, bothbefore and after fertilisation. The seeds that develop post-fertilisation, arenot covered, i.e., are naked. Gymnosperms include medium-sized treesor tall trees and shrubs (Figure 3.4). One of the gymnosperms, the giantredwood tree Sequoia is one of the tallest tree species. The roots aregenerally tap roots. Roots in some genera have fungal association in theform of mycorrhiza (Pinus), while in some others (Cycas) small specialisedroots called coralloid roots are associated with N2- fixing cyanobacteria.The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leavesmay be simple or compound. In Cycas the pinnate leaves persist for a fewyears. The leaves in gymnosperms are well-adapted to withstand extremesof temperature, humidity and wind. In conifers, the needle-like leavesreduce the surface area. Their thick cuticle and sunken stomata alsohelp to reduce water loss.2022-23PLANT KINGDOM 39The gymnosperms are heterosporous; they producehaploid microspores and megaspores. The two kinds ofspores are produced within sporangia that are borneon sporophylls which are arranged spirally along an axisto form lax or compact strobili or cones. The strobilibearing microsporophylls and microsporangia arecalled microsporangiate or male strobili. Themicrospores develop into a male gametophyticgeneration which is highly reduced and is confined toonly a limited number of cells. This reducedgametophyte is called a pollen grain. The developmentof pollen grains take place within the microsporangia.The cones bearing megasporophylls with ovules ormegasporangia are called macrosporangiate or femalestrobili. The male or female cones or strobili may beborne on the same tree (Pinus). However, in cycas malecones and megasporophylls are borne on different trees.The megaspore mother cell is differentiated from one ofthe cells of the nucellus. The nucellus is protected byenvelopes and the composite structure is called anovule. The ovules are borne on megasporophylls whichmay be clustered to form the female cones. Themegaspore mother cell divides meiotically to form fourmegaspores. One of the megaspores enclosed within themegasporangium develops into a multicellular femalegametophyte that bears two or more archegonia orfemale sex organs. The multicellular female gametophyteis also retained within megasporangium.Unlike bryophytes and pteridophytes, ingymnosperms the male and the female gametophytesdo not have an independent free-living existence. Theyremain within the sporangia retained on thesporophytes. The pollen grain is released from themicrosporangium. They are carried in air currents andcome in contact with the opening of the ovules borneon megasporophylls. The pollen tube carrying themale gametes grows towards archegonia in the ovulesand discharge their contents near the mouth of thearchegonia. Following fertilisation, zygote developsinto an embryo and the ovules into seeds. These seedsare not covered.(c)Figure 3.4 Gymnosperms: (a) Cycas(b) Pinus (c) GinkgoDwarf ShootLong ShootSeeds(b)(a)2022-2340 BIOLOGY3.5 ANGIOSPERMSUnlike the gymnosperms where the ovules are naked, in the angiospermsor flowering plants, the pollen grains and ovules are developed inspecialised structures called flowers. In angiosperms, the seeds areenclosed in fruits. The angiosperms are an exceptionally large group ofplants occurring in wide range of habitats. They range in size from thesmallest Wolffia to tall trees of Eucalyptus (over 100 metres). They provideus with food, fodder, fuel, medicines and several other commerciallyimportant products. They are divided into two classes : the dicotyledonsand the monocotyledons (Figure 3.5). The dicotyledons arecharacterised by seeds having two cotyledons, reticulate venations inleaves, and tetramerous or pentamerous flowers, i.e., having four or fivemembers in each floral whorls. The monocotyledons on the other handare characterised by single cotyledonous seeds, parallel venation inleaves, and trimerous flowers having three members in each floral whorls.The male sex organ in a flower is the stamen. Each stamen consists of aslender filament with an anther at the tip. Within the anthers, the pollenmother cell divide by meiosis to produce microspores which maturesinto pollen grains. The female sex organ in a flower is the pistil. Pistilconsists of a swollen ovary at its base, a long slender style and stigma.Inside the ovary, ovules are present. Generally each ovule has amegaspore mother cell that undergoes meiosis to form four haploidmegaspores. Three of them degenerate and one divide to form the embryosac. Each embryo-sac has a three-celled egg apparatus â one egg celland two synergids, three antipodal cells and two polar nuclei. The polar(a) (b)Figure 3.5 Angiosperms : (a) A dicotyledon (b) A monocotyledon2022-23PLANT KINGDOM 41Figure 3.6 Life cycle of an angiospermnuclei eventually fuse to produce a diploid secondary nucleus. Pollengrain, after dispersal from the anthers, are carried by wind or various otheragencies to the stigma of a pistil. This is termed as pollination. The pollengrains germinate on the stigma and the resulting pollen tubes grow throughthe tissues of stigma and style and reach the ovule. The pollen tubes enterthe embryo-sac where two male gametes are discharged. One of the malegametes fuses with the egg cell (syngamy) to form a zygote. The other malegamete fuses with the diploid secondary nucleus to produce the triploidprimary endosperm nucleus (PEN). Because of the occurrence of twofusions i.e., syngamy and triple fusion, this event is termed as doublefertilisation, an event unique to angiosperms. The zygote develops intoan embryo (with one or two cotyledons) and the PEN develops intoendosperm which provides nourishment to the developing embryo. Thesynergids and antipodals degenerate after fertilisation. During these eventsthe ovules develop into seeds and the ovaries develop into fruit. The lifecycle of an angiosperm is shown in Figure 3.6.2022-2342 BIOLOGY3.6 PLANT LIFE CYCLES AND ALTERNATION OFGENERATIONSIn plants, both haploid and diploid cells can divide bymitosis. This ability leads to the formation of differentplant bodies - haploid and diploid. The haploid plantbody produces gametes by mitosis. This plant bodyrepresents a gametophyte. Following fertilisation thezygote also divides by mitosis to produce a diploidsporophytic plant body. Haploid spores are producedby this plant body by meiosis. These in turn, divide bymitosis to form a haploid plant body once again. Thus,during the life cycle of any sexually reproducing plant,there is an alternation of generations between gameteproducing haploid gametophyte and spore producingdiploid sporophyte.However, different plant groups, as well as individualsrepresenting them, differ in the following patterns:1. Sporophytic generation is represented only by theone-celled zygote. There are no free-livingsporophytes. Meiosis in the zygote results in theformation of haploid spores. The haploid sporesdivide mitotically and form the gametophyte. Thedominant, photosynthetic phase in such plants isthe free-living gametophyte. This kind of life cycleis termed as haplontic. Many algae such as Volvox,Spirogyra and some species of Chlamydomonasrepresent this pattern (Figure 3.7 a).2. On the other extreme, is the type wherein the diploidsporophyte is the dominant, photosynthetic,independent phase of the plant. The gametophyticphase is represented by the single to few-celledhaploid gametophyte. This kind of life cycle istermed as diplontic. An alga, Fucus sp., representsthis pattern (Fig. 3.7b). In addition, all seed bearingplants i.e., gymnosperms and angiosperms, followthis pattern with some variations, wherein, thegametophytic phase is few to multi-celled.3. Bryophytes and pteridophytes, interestingly, exhibitan intermediate condition (Haplo-diplontic); bothphases are multicellular. However, they differ in theirdominant phases.SyngamyZygote(2n)Spores(n)HaplonticABGametogenesisMeiosisGametophyte(n)(a)BAHaplo-diplonticSpores(n)MeiosisGametophyte(n)SyngamyZygote(2n)GametogenesisSporophyte(2n)(c)Figure 3.7 Life cycle patterns : (a) Haplontic(b) Diplontic (c) Haplo-diplontic2022-23PLANT KINGDOM 43SUMMARYPlant kingdom includes algae, bryophytes, pteridophytes, gymnosperms andangiosperms. Algae are chlorophyll-bearing simple, thalloid, autotrophic andlargely aquatic organisms. Depending on the type of pigment possesed and thetype of stored food, algae are classfied into three classes, namely Chlorophyceae,Phaeophyceae and Rhodophyceae. Algae usually reproduce vegetatively byfragmentation, asexually by formation of different types of spores and sexually byformation of gametes which may show isogamy, anisogamy or oogamy.Bryophytes are plants which can live in soil but are dependent on water forsexual reproduction. Their plant body is more differentiated than that of algae. Itis thallus-like and prostrate or erect and attached to the substratum by rhizoids.They possess root-like, leaf-like and stem-like structures. The bryophytes aredivided into liverworts and mosses. The plant body of liverworts is thalloid anddorsiventral whereas mosses have upright, slender axes bearing spirally arrangedleaves. The main plant body of a bryophyte is gamete-producing and is called agametophyte. It bears the male sex organs called antheridia and female sex organscalled archegonia. The male and female gametes produced fuse to form zygotewhich produces a multicellular body called a sporophyte. It produces haploidspores. The spores germinate to form gametophytes.In pteridophytes the main plant is a sporophyte which is differentiated intotrue root, stem and leaves. These organs possess well-differentiated vasculartissues. The sporophytes bear sporangia which produce spores. The sporesgerminate to form gametophytes which require cool, damp places to grow. Thegametophytes bear male and female sex organs called antheridia and archegonia,respectively. Water is required for transfer of male gametes to archegonium wherezygote is formed after fertilisation. The zygote produces a sporophyte.A dominant, independent, photosynthetic, thalloid or erect phase isrepresented by a haploid gametophyte and it alternates with the shortlivedmulticelluler sporophyte totally or partially dependent on thegametophyte for its anchorage and nutrition. All bryophytes representthis pattern.The diploid sporophyte is represented by a dominant, independent,photosynthetic, vascular plant body. It alternates with multicellular,saprophytic/autotrophic, independent but short-lived haploidgametophyte. Such a pattern is known as haplo-diplontic life cycle. Allpteridophytes exhibit this pattern (Figure 3.7 c).Interestingly, while most algal genera are haplontic, some of themsuch as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. Fucus, analga is diplontic.2022-2344 BIOLOGYThe gymnosperms are the plants in which ovules are not enclosed by anyovary wall. After fertilisation the seeds remain exposed and therefore these plantsare called naked-seeded plants. The gymnosperms produce microspores andmegaspores which are produced in microsporangia and megasporangia borne onthe sporophylls. The sporophylls â microsporophylls and megasporophylls â arearranged spirally on axis to form male and female cones, respectively. The pollengrain germinates and pollen tube releases the male gamete into the ovule, where itfuses with the egg cell in archegonia. Following fertilisation, the zygote developsinto embryo and the ovules into seeds.In angiosperms, the male sex organs (stamen) and female sex organs (pistil)are borne in a flower. Each stamen consists of a filament and an anther. The antherproduces pollen grains (male gametophyte) after meiosis. The pistil consists of anovary enclosing one to many ovules. Within the ovule is the female gametophyteor embryo sac which contains the egg cell. The pollen tube enters the embryo-sacwhere two male gametes are discharged. One male gamete fuses with egg cell(syngamy) and other fuses with diploid secondary nucleus (triple fusion). Thisphenomenon of two fusions is called double fertilisation and is unique toangiosperms. The angiosperms are divided into two classes â the dicotyledonsand the monocotyledons.During the life cycle of any sexually reproducing plant, there is alternation ofgenerations between gamete producing haploid gametophyte and spore producingdiploid sporophyte. However, different plant groups as well as individuals mayshow different patterns of life cycles â haplontic, diplontic or intermediate.EXERCISES1. What is the basis of classification of algae?2. When and where does reduction division take place in the life cycle of a liverwort,a moss, a fern, a gymnosperm and an angiosperm?3. Name three groups of plants that bear archegonia. Briefly describe the life cycleof any one of them.4. Mention the ploidy of the following: protonemal cell of a moss; primary endospermnucleus in dicot, leaf cell of a moss; prothallus cell of a ferm; gemma cell inMarchantia; meristem cell of monocot, ovum of a liverwort, and zygote of a fern.5. Write a note on economic importance of algae and gymnosperms.6. Both gymnosperms and angiosperms bear seeds, then why are they classifiedseparately?7. What is heterospory? Briefly comment on its significance. Give two examples.2022-23PLANT KINGDOM 458. Explain briefly the following terms with suitable examples:-(i) protonema(ii) antheridium(iii) archegonium(iv) diplontic(v) sporophyll(vi) isogamy9. Differentiate between the following:-(i) red algae and brown algae(ii) liverworts and moss(iii) homosporous and heterosporous pteridophyte(iv) syngamy and triple fusion10. How would you distinguish monocots from dicots?11. Match the following (column I with column II)Column I Column II(a) Chlamydomonas (i) Moss(b) Cycas (ii) Pteridophyte(c) Selaginella (iii) Algae(d) Sphagnum (iv) Gymnosperm12. Describe the important characteristics of gymnosperms.2022-2346 BIOLOGYWhen you look around, you will observe different animals with differentstructures and forms. As over a million species of animals have beendescribed till now, the need for classification becomes all the moreimportant. The classification also helps in assigning a systematic positionto newly described species.4.1 BASIS OF CLASSIFICATIONInspite of differences in structure and form of different animals, there arefundamental features common to various individuals in relation to thearrangement of cells, body symmetry, nature of coelom, patterns ofdigestive, circulatory or reproductive systems. These features are usedas the basis of animal classification and some of them are discussed here.4.1.1 Levels of OrganisationThough all members of Animalia are multicellular, all of them do notexhibit the same pattern of organisation of cells. For example, in sponges,the cells are arranged as loose cell aggregates, i.e., they exhibit cellularlevel of organisation. Some division of labour (activities) occur amongthe cells. In coelenterates, the arrangement of cells is more complex. Herethe cells performing the same function are arranged into tissues, hence iscalled tissue level of organisation. A still higher level of organisation, i.e.,organ level is exhibited by members of Platyhelminthes and other higherphyla where tissues are grouped together to form organs, each specialisedfor a particular function. In animals like Annelids, Arthropods, Molluscs,ANIMAL KINGDOMCHAPTER 44.1 Basis ofClassification4.2 Classification ofAnimals2022-23ANIMAL KINGDOM 47 47 ANIMAL KINGDOMEchinoderms and Chordates, organs haveassociated to form functional systems, eachsystem concerned with a specific physiologicalfunction. This pattern is called organ systemlevel of organisation. Organ systems in differentgroups of animals exhibit various patterns ofcomplexities. For example, the digestive systemin Platyhelminthes has only a single openingto the outside of the body that serves as bothmouth and anus, and is hence calledincomplete. A complete digestive system hastwo openings, mouth and anus. Similarly, thecirculatory system may be of two types:(i) open type in which the blood is pumpedout of the heart and the cells and tissues aredirectly bathed in it and(ii) closed type in which the blood is circulatedthrough a series of vessels of varying diameters(arteries, veins and capillaries).4.1.2 SymmetryAnimals can be categorised on the basis of theirsymmetry. Sponges are mostly asymmetrical,i.e., any plane that passes through the centredoes not divide them into equal halves. Whenany plane passing through the central axis ofthe body divides the organism into two identicalhalves, it is called radial symmetry.Coelenterates, ctenophores and echinodermshave this kind of body plan (Figure 4.1a).Animals like annelids, arthropods, etc., wherethe body can be divided into identical left andright halves in only one plane, exhibit bilateralsymmetry (Figure 4.1b).4.1.3 Diploblastic and TriploblasticOrganisationAnimals in which the cells are arranged in twoembryonic layers, an external ectoderm andan internal endoderm, are called diploblasticanimals, e.g., coelenterates. An undifferentiatedlayer, mesoglea, is present in between theectoderm and the endoderm (Figure 4.2a).Figure 4.2 Showing germinal layers :(a) Diploblastic (b) Triploblastic(a) (b)Ectoder mMesogleaEndoder mMesoder mFigure 4.1 (b) Bilateral symmetryFigure 4.1 (a) Radial symmetry2022-2348 BIOLOGY4.1.4 CoelomPresence or absence of a cavity between the bodywall and the gut wall is very important inclassification. The body cavity, which is linedby mesoderm is called coelom. Animalspossessing coelom are called coelomates, e.g.,annelids, molluscs, arthropods, echinoderms,hemichordates and chordates (Figure 4.3a). Insome animals, the body cavity is not lined bymesoderm, instead, the mesoderm is present asscattered pouches in between the ectoderm andendoderm. Such a body cavity is calledpseudocoelom and the animals possessing themare called pseudocoelomates, e.g.,aschelminthes (Figure 4.3b). The animals inwhich the body cavity is absent are calledacoelomates, e.g., platyhelminthes (Figure 4.3c).Figure 4.3 Diagrammatic sectional view of :(a) Coelomate (b) Pseudocoelomate(c) AcoelomateThose animals in which the developing embryo has a third germinal layer,mesoderm, in between the ectoderm and endoderm, are calledtriploblastic animals (platyhelminthes to chordates, Figure 4.2b).4.1.5 SegmentationIn some animals, the body is externally and internally divided intosegments with a serial repetition of at least some organs. For example, inearthworm, the body shows this pattern called metameric segmentationand the phenomenon is known as metamerism.4.1.6 NotochordNotochord is a mesodermally derived rod-like structure formed on thedorsal side during embryonic development in some animals. Animals withnotochord are called chordates and those animals which do not form thisstructure are called non-chordates, e.g., porifera to echinoderms.4.2 CLASSIFICATION OF ANIMALSThe broad classification of Animalia based on common fundamentalfeatures as mentioned in the preceding sections is given in Figure 4.4.2022-23ANIMAL KINGDOM 49 49 ANIMAL KINGDOMThe important characteristic features of thedifferent phyla are described.4.2.1 Phylum â PoriferaMembers of this phylum are commonly knownas sponges. They are generally marine and mostlyasymmetrical animals (Figure 4.5). These areprimitive multicellular animals and have cellularlevel of organisation. Sponges have a watertransport or canal system. Water enters throughminute pores (ostia) in the body wall into a centralcavity, spongocoel, from where it goes outthrough the osculum. This pathway of watertransport is helpful in food gathering, respiratoryexchange and removal of waste. Choanocytesor collar cells line the spongocoel and the canals.Digestion is intracellular. The body is supportedby a skeleton made up of spicules or sponginfibres. Sexes are not separate (hermaphrodite),i.e., eggs and sperms are produced by the sameindividual. Sponges reproduce asexually byfragmentation and sexually by formation ofgametes. Fertilisation is internal and developmentis indirect having a larval stage which ismorphologically distinct from the adult.*Echinodermata exhibits radial or bilateral symmetry depending on the stage.Figure 4.4 Broad classification of Kingdom Animalia based on common fundamental features(a)(b)(c)Figure 4.5 Examples of Porifera : (a) Sycon(b) Euspongia (c) Spongilla2022-2350 BIOLOGYcnidoblasts or cnidocytes (which contain the stinging capsules ornematocysts) present on the tentacles and the body. Cnidoblasts are usedfor anchorage, defense and for the capture of prey (Figure 4.7). Cnidariansexhibit tissue level of organisation and are diploblastic. They have a centralgastro-vascular cavity with a single opening, mouth on hypostome.Digestion is extracellular and intracellular. Some of the cnidarians, e.g.,corals have a skeleton composed of calcium carbonate. Cnidarians exhibittwo basic body forms called polyp and medusa (Figure 4.6). The formeris a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, thelatter is umbrella-shaped and free-swimming like Aurelia or jelly fish.Those cnidarians which exist in both forms exhibit alternation ofgeneration (Metagenesis), i.e., polyps produce medusae asexually andmedusae form the polyps sexually (e.g., Obelia).Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone),Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).Figure 4.7Diagrammatic view ofCnidoblastFigure 4.6 Examples of Coelenterata indicating outline of their body form :(a) Aurelia (Medusa) (b) Adamsia (Polyp)(a) (b)Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia(Bath sponge).4.2.2 Phylum â Coelenterata (Cnidaria)They are aquatic, mostly marine, sessile or free-swimming, radiallysymmetrical animals (Figure 4.6). The name cnidaria is derived from the2022-23ANIMAL KINGDOM 51 51 ANIMAL KINGDOM4.2.3 Phylum â CtenophoraCtenophores, commonly known as sea walnuts or comb jelliesare exclusively marine, radially symmetrical, diploblasticorganisms with tissue level of organisation. The body bearseight external rows of ciliated comb plates, which help inlocomotion (Figure 4.8). Digestion is both extracellular andintracellular. Bioluminescence (the property of a livingorganism to emit light) is well-marked in ctenophores. Sexesare not separate. Reproduction takes place only by sexualmeans. Fertilisation is external with indirect development.Examples: Pleurobrachia and Ctenoplana.4.2.4 Phylum â PlatyhelminthesThey have dorso-ventrally flattened body, hence are calledflatworms (Figure 4.9). These are mostly endoparasites foundin animals including human beings. Flatworms are bilaterallysymmetrical, triploblastic and acoelomate animals with organlevel of organisation. Hooks and suckers are present in theparasitic forms. Some of them absorb nutrients from the hostdirectly through their body surface. Specialised cells calledflame cells help in osmoregulation and excretion. Sexes are notseparate. Fertilisation is internal and development is throughmany larval stages. Some members like Planaria possess highregeneration capacity.Examples: Taenia (Tapeworm), Fasciola (Liver fluke).Figure 4.8 Example ofCtenophora(Pleurobrachia)(a) (b)Figure 4.9 Examples of Platyhelminthes : (a) Tape worm (b) Liver fluke2022-2352 BIOLOGY4.2.5 Phylum â AschelminthesThe body of the aschelminthes is circular incross-section, hence, the name roundworms(Figure 4.10). They may be freeliving, aquaticand terrestrial or parasitic in plants and animals.Roundworms have organ-system level of bodyorganisation. They are bilaterally symmetrical,triploblastic and pseudocoelomate animals.Alimentary canal is complete with a welldevelopedmuscular pharynx. An excretorytube removes body wastes from the body cavitythrough the excretory pore. Sexes are separate(dioecious), i.e., males and females are distinct.Often females are longer than males. Fertilisationis internal and development may be direct (theyoung ones resemble the adult) or indirect.Examples : Ascaris (Roundworm), Wuchereria(Filaria worm), Ancylostoma (Hookworm).4.2.6 Phylum â AnnelidaThey may be aquatic (marine and fresh water) orterrestrial; free-living, and sometimes parasitic.They exhibit organ-system level of bodyorganisation and bilateral symmetry. They aretriploblastic, metamerically segmented andcoelomate animals. Their body surface isdistinctly marked out into segments ormetameres and, hence, the phylum nameAnnelida (Latin, annulus : little ring) (Figure 4.11).They possess longitudinal and circular muscleswhich help in locomotion. Aquatic annelids likeNereis possess lateral appendages, parapodia,which help in swimming. A closed circulatorysystem is present. Nephridia (sing. nephridium)help in osmoregulation and excretion. Neuralsystem consists of paired ganglia (sing. ganglion)connected by lateral nerves to a double ventralnerve cord. Nereis, an aquatic form, is dioecious,but earthworms and leeches are monoecious.Reproduction is sexual.Examples : Nereis, Pheretima (Earthworm) andHirudinaria (Blood sucking leech).Figure 4.11 Examples of Annelida : (a) Nereis(b) HirudinariaMale FemaleFigure 4.10 Example ofAschelminthes:Roundworm2022-23ANIMAL KINGDOM 53 53 ANIMAL KINGDOM4.2.7 Phylum â ArthropodaThis is the largest phylum of Animalia whichincludes insects. Over two-thirds of all namedspecies on earth are arthropods (Figure 4.12).They have organ-system level of organisation.They are bilaterally symmetrical, triploblastic,segmented and coelomate animals. The bodyof arthropods is covered by chitinousexoskeleton. The body consists of head, thoraxand abdomen. They have jointed appendages(arthros-joint, poda-appendages). Respiratoryorgans are gills, book gills, book lungs ortracheal system. Circulatory system is of opentype. Sensory organs like antennae, eyes(compound and simple), statocysts orbalancing organs are present. Excretion takesplace through malpighian tubules. They aremostly dioecious. Fertilisation is usuallyinternal. They are mostly oviparous.Development may be direct or indirect.Examples: Economically important insects âApis (Honey bee), Bombyx (Silkworm), Laccifer(Lac insect)Vectors â Anopheles, Culex and Aedes(Mosquitoes)Gregarious pest â Locusta (Locust)Living fossil â Limulus (King crab).4.2.8 Phylum â MolluscaThis is the second largest animal phylum(Figure 4.13). Molluscs are terrestrial or aquatic(marine or fresh water) having an organ-systemlevel of organisation. They are bilaterallysymmetrical, triploblastic and coelomateanimals. Body is covered by a calcareous shelland is unsegmented with a distinct head,muscular foot and visceral hump. A soft andspongy layer of skin forms a mantle over thevisceral hump. The space between the humpand the mantle is called the mantle cavity inwhich feather like gills are present. They haverespiratory and excretory functions. Theanterior head region has sensory tentacles. Themouth contains a file-like rasping organ forfeeding, called radula.Figure 4.12 Examples of Arthropoda :(a) Locust (b) Butterfly(c) Scorpion (d) Prawn(a)(c)(b)(d)Figure 4.13 Examples of Mollusca :(a) Pila (b) Octopus(b)(a)2022-2354 BIOLOGYThey are usually dioecious and oviparous with indirectdevelopment.Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia(Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Seahare),Dentalium (Tusk shell) and Chaetopleura (Chiton).4.2.9 Phylum â EchinodermataThese animals have an endoskeleton of calcareous ossiclesand, hence, the name Echinodermata (Spiny bodied, Figure4.14). All are marine with organ-system level of organisation.The adult echinoderms are radially symmetrical but larvaeare bilaterally symmetrical. They are triploblastic andcoelomate animals. Digestive system is complete with mouthon the lower (ventral) side and anus on the upper (dorsal)side. The most distinctive feature of echinoderms is thepresence of water vascular system which helps inlocomotion, capture and transport of food and respiration.An excretory system is absent. Sexes are separate.Reproduction is sexual. Fertilisation is usually external.Development is indirect with free-swimming larva.Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon(Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).4.2.10 Phylum â HemichordataHemichordata was earlier considered as a sub-phylum underphylum Chordata. But now it is placed as a separate phylumunder non-chordata. Hemichordates have a rudimentarystructure in the collar region called stomochord, a structuresimilar to notochord.This phylum consists of a small group of worm-likemarine animals with organ-system level of organisation. Theyare bilaterally symmetrical, triploblastic and coelomateanimals. The body is cylindrical and is composed of ananterior proboscis, a collar and a long trunk (Figure 4.15).Circulatory system is of open type. Respiration takes placethrough gills. Excretory organ is proboscis gland. Sexes areseparate. Fertilisation is external. Development is indirect.Examples: Balanoglossus and Saccoglossus.4.2.11 Phylum â ChordataAnimals belonging to phylum Chordata are fundamentallycharacterised by the presence of a notochord, a dorsalFigure 4.14 Examples ofEchinodermata :(a) Asterias(b) Ophiura(a)(b)Figure 4.15 BalanoglossusProboscisCollarTrunk2022-23ANIMAL KINGDOM 55 55 ANIMAL KINGDOMhollow nerve cord and paired pharyngealgill slits (Figure 4.16). These are bilaterallysymmetrical, triploblastic, coelomate withorgan-system level of organisation. Theypossess a post anal tail and a closed circulatorysystem.Table 4.1 presents a comparison of salientfeatures of chordates and non-chordates.Phylum Chordata is divided into threesubphyla: Urochordata or Tunicata,Cephalochordata and Vertebrata.Subphyla Urochordata andCephalochordata are often referred to asprotochordates (Figure 4.17) and areexclusively marine. In Urochordata, notochordis present only in larval tail, while inCephalochordata, it extends from head to tailregion and is persistent throughout their life.Examples: Urochordata â Ascidia, Salpa,Doliolum; Cephalochordata â Branchiostoma(Amphioxus or Lancelet).The members of subphylum Vertebratapossess notochord during the embryonicperiod. The notochord is replaced by acartilaginous or bony vertebral column in theadult. Thus all vertebrates are chordates butall chordates are not vertebrates. Besides thebasic chordate characters, vertebrates have aventral muscular heart with two, three or fourchambers, kidneys for excretion andosmoregulation and paired appendages whichmay be fins or limbs.Nerve cord Notochor dPost-anal partGill slitsFigure 4.16 Chordata characteristicsFigure 4.17 AscidiaTABLE 4.1 Comparison of Chordates and Non-chordatesS.No. Chordates Non-chordates1. Notochord present. Notochord absent.2. Central nervous system is dorsal, Central nervous system is ventral, solidhollow and single. and double.3. Pharynx perforated by gill slits. Gill slits are absent.4. Heart is ventral. Heart is dorsal (if present).5. A post-anal part (tail) is present. Post-anal tail is absent.2022-2356 BIOLOGYFigure 4.18 A jawless vertebrate - PetromyzonFigure 4.19 Example of Cartilaginous fishes :(a) Scoliodon (b) Pristis(a)(b)4.2.11.1 Class â CyclostomataAll living members of the class Cyclostomata areectoparasites on some fishes. They have anelongated body bearing 6-15 pairs of gill slitsfor respiration. Cyclostomes have a sucking andcircular mouth without jaws (Fig. 4.18). Theirbody is devoid of scales and paired fins.Cranium and vertebral column arecartilaginous. Circulation is of closed type.Cyclostomes are marine but migrate forspawning to fresh water. After spawning, withina few days, they die. Their larvae, aftermetamorphosis, return to the ocean.Examples: Petromyzon (Lamprey) and Myxine(Hagfish).4.2.11.2 Class â ChondrichthyesThey are marine animals with streamlined bodyand have cartilaginous endoskeleton(Figure 4.19). Mouth is located ventrally.Notochord is persistent throughout life. Gillslits are separate and without operculum (gillcover). The skin is tough, containing minuteplacoid scales. Teeth are modified placoidscales which are backwardly directed. Theirjaws are very powerful. These animals arepredaceous. Due to the absence of air bladder,they have to swim constantly to avoid sinking.VertebrataDivisionAgnatha(lacks jaw)Class1. CyclostomataGnathostomata(bears jaw)Super ClassPisces(bear fins)Tetrapoda(bear limbs)Class1. Amphibia2. Reptilia3. Aves4. MammalsClass1. Chondrichthyes2. OsteichthyesThe subphylum Vertebrata is further divided as follows:2022-23ANIMAL KINGDOM 57 57 ANIMAL KINGDOMHeart is two-chambered (one auricle and one ventricle).Some of them have electric organs (e.g., Torpedo) andsome possess poison sting (e.g., Trygon). They arecold-blooded (poikilothermous) animals, i.e., they lackthe capacity to regulate their body temperature. Sexesare separate. In males pelvic fins bear claspers. Theyhave internal fertilisation and many of them areviviparous.Examples: Scoliodon (Dog fish), Pristis (Saw fish),Carcharodon (Great white shark), Trygon (Sting ray).4.2.11.3 Class â OsteichthyesIt includes both marine and fresh water fishes with bonyendoskeleton. Their body is streamlined. Mouth ismostly terminal (Figure 4.20). They have four pairs ofgills which are covered by an operculum on each side.Skin is covered with cycloid/ctenoid scales. Air bladderis present which regulates buoyancy. Heart is twochambered(one auricle and one ventricle). They arecold-blooded animals. Sexes are separate. Fertilisationis usually external. They are mostly oviparous anddevelopment is direct.Examples: Marine â Exocoetus (Flying fish),Hippocampus (Sea horse); Freshwater â Labeo (Rohu),Catla (Katla), Clarias (Magur); Aquarium â Betta(Fighting fish), Pterophyllum (Angel fish).4.2.11.4 Class â AmphibiaAs the name indicates (Gr., Amphi : dual, bios, life),amphibians can live in aquatic as well as terrestrialhabitats (Figure 4.21). Most of them have two pairs oflimbs. Body is divisible into head and trunk. Tail maybe present in some. The amphibian skin is moist(without scales). The eyes have eyelids. A tympanumrepresents the ear. Alimentary canal, urinary andreproductive tracts open into a common chamber calledcloaca which opens to the exterior. Respiration is bygills, lungs and through skin. The heart is threechambered(two auricles and one ventricle). These arecold-blooded animals. Sexes are separate. Fertilisationis external. They are oviparous and developmentis indirect.Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog),Salamandra (Salamander), Ichthyophis (Limblessamphibia).Figure 4.21 Examples of Amphibia :(a) Salamandra(b) Rana(a)(b)Figure 4.20 Examples of Bony fishes :(a) Hippocampus (b) Catla(a) (b)2022-2358 BIOLOGY4.2.11.5 Class â ReptiliaThe class name refers to their creeping or crawling mode of locomotion(Latin, repere or reptum, to creep or crawl). They are mostly terrestrialanimals and their body is covered by dry and cornified skin, epidermalscales or scutes (Fig. 4.22). They do not have external ear openings.Tympanum represents ear. Limbs, when present, are two pairs. Heart isusually three-chambered, but four-chambered in crocodiles. Reptiles arepoikilotherms. Snakes and lizards shed their scales as skin cast. Sexesare separate. Fertilisation is internal. They are oviparous and developmentis direct.Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard),Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator).Hemidactylus (Wall lizard), Poisonous snakes â Naja (Cobra), Bangarus(Krait), Vipera (Viper).4.2.11.6 Class â AvesThe characteristic features of Aves (birds) are the presence of feathersand most of them can fly except flightless birds (e.g., Ostrich). They possessbeak (Figure 4.23). The forelimbs are modified into wings. The hind limbsgenerally have scales and are modified for walking, swimming or claspingthe tree branches. Skin is dry without glands except the oil gland at thebase of the tail. Endoskeleton is fully ossified (bony) and the long bonesare hollow with air cavities (pneumatic). The digestive tract of birds hasadditional chambers, the crop and gizzard. Heart is completely fourchambered.They are warm-blooded (homoiothermous) animals, i.e.,they are able to maintain a constant body temperature. Respiration is byFigure 4.22 Reptiles: (a) Chameleon (b) Crocodilus (c) Chelone (d) Naja(a) (b) (c) (d)2022-23ANIMAL KINGDOM 59 59 ANIMAL KINGDOMlungs. Air sacs connected to lungs supplement respiration. Sexes areseparate. Fertilisation is internal. They are oviparous and development isdirect.Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio(Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).4.2.11.7 Class â MammaliaThey are found in a variety of habitats â polar ice caps, deserts, mountains,forests, grasslands and dark caves. Some of them have adapted to fly orlive in water. The most unique mammalian characteristic is the presenceof milk producing glands (mammary glands) by which the young onesare nourished. They have two pairs of limbs, adapted for walking, running,climbing, burrowing, swimming or flying (Figure 4.24). The skin of(a)Figure 4.23 Some birds : (a) Neophron (b) Struthio (c) Psittacula (d) Pavo(b) (c) (d)Figure 4.24 Some mammals : (a) Ornithorhynchus (b) Macropus (c) Pteropus (d) Balaenoptera(a)(b)(c)(d)2022-2360 BIOLOGYmammals is unique in possessing hair. External ears or pinnae arepresent. Different types of teeth are present in the jaw. Heart is fourchambered.They are homoiothermous. Respiration is by lungs. Sexesare separate and fertilisation is internal. They are viviparous with fewexceptions and development is direct.Examples: Oviparous-Ornithorhynchus (Platypus); Viviparous -Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca(Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant),Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale),Panthera tigris (Tiger), Panthera leo (Lion).The salient distinguishing features of all phyla under animal kingdomis comprehensively given in the Table 4.2.Level ofOrganisationCellularTissueTissueOrgan &OrgansystemOrgansystemOrgansystemOrgansystemOrgansystemOrgansystemOrgansystemOrgansystemSymmetryVariousRadialRadialBilateralBilateralBilateralBilateralBilateralRadialBilateralBilateralCoelomAbsentAbsentAbsentAbsentPseudocoelomateCoelomateCoelomateCoelomateCoelomateCoelomateCoelomateSegmentationAbsentAbsentAbsentAbsentAbsentPresentPresentAbsentAbsentAbsentPresentDigestiveSystemAbsentIncompleteIncompleteIncompleteCompleteCompleteCompleteCompleteCompleteCompleteCompleteCirculatorySystemAbsentAbsentAbsentAbsentAbsentPresentPresentPresentPresentPresentPresentRespiratorySystemAbsentAbsentAbsentAbsentAbsentAbsentPresentPresentPresentPresentPresentDistinctiveFeaturesBody with poresand canals in walls.Cnidoblastspresent.Comb plates forlocomotion.Flat body, suckers.Often wormshaped,elongated.Body segmentationlike rings.Exoskeleton of cuticle,jointed appendages.External skeletonof shell usuallypresent.Water vascularsystem, radialsymmetry.Worm-like withproboscis, collarand trunk.Notochord, dorsalhollow nerve cord,gill slits withlimbs or fins.PhylumPoriferaCoelenterata(Cnidaria)CtenophoraPlatyhelminthesAschelminthesAnnelidaArthropodaMolluscaEchinodermataHemichordataChordataTABLE 4.2 Salient Features of Different Phyla in the Animal Kingdom2022-23ANIMAL KINGDOM 61 61 ANIMAL KINGDOMSUMMARYThe basic fundamental features such as level of organisation, symmetry, cellorganisation, coelom, segmentation, notochord, etc., have enabled us to broadlyclassify the animal kingdom. Besides the fundamental features, there are manyother distinctive characters which are specific for each phyla or class.Porifera includes multicellular animals which exhibit cellular level oforganisation and have characteristic flagellated choanocytes. The coelenterateshave tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating.The ctenophores are marine animals with comb plates. The platyhelminths haveflat body and exhibit bilateral symmetry. The parasitic forms show distinct suckersand hooks. Aschelminthes are pseudocoelomates and include parasitic as well asnon-parasitic roundworms.Annelids are metamerically segmented animals with a true coelom. Thearthropods are the most abundant group of animals characterised by the presenceof jointed appendages. The molluscs have a soft body surrounded by an externalcalcareous shell. The body is covered with external skeleton made of chitin. Theechinoderms possess a spiny skin. Their most distinctive feature is the presenceof water vascular system. The hemichordates are a small group of worm-like marineanimals. They have a cylindrical body with proboscis, collar and trunk.Phylum Chordata includes animals which possess a notochord eitherthroughout or during early embryonic life. Other common features observed inthe chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits.Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possessjaws (Gnathostomata). Agnatha is represented by the class, Cyclostomata. Theyare the most primitive chordates and are ectoparasites on fishes. Gnathostomatahas two super classes, Pisces and Tetrapoda. Classes Chondrichthyes andOsteichthyes bear fins for locomotion and are grouped under Pisces. TheChondrichthyes are fishes with cartilaginous endoskeleton and are marine. Classes,Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thusgrouped under Tetrapoda. The amphibians have adapted to live both on land andwater. Reptiles are characterised by the presence of dry and cornified skin. Limbsare absent in snakes. Fishes, amphibians and reptiles are poikilothermous (coldblooded).Aves are warm-blooded animals with feathers on their bodies andforelimbs modified into wings for flying. Hind limbs are adapted for walking,swimming, perching or clasping. The unique features of mammals are the presenceof mammary glands and hairs on the skin. They commonly exhibit viviparity.2022-2362 BIOLOGYEXERCISES1. What are the difficulties that you would face in classification of animals, if commonfundamental features are not taken into account?2. If you are given a specimen, what are the steps that you would follow to classifyit?3. How useful is the study of the nature of body cavity and coelom in theclassification of animals?4. Distinguish between intracellular and extracellular digestion?5. What is the difference between direct and indirect development?6. What are the peculiar features that you find in parasitic platyhelminthes?7. What are the reasons that you can think of for the arthropods to constitute thelargest group of the animal kingdom?8. Water vascular system is the characteristic of which group of the following:(a) Porifera (b) Ctenophora (c) Echinodermata (d) Chordata9. âAll vertebrates are chordates but all chordates are not vertebratesâ. Justify thestatement.10. How important is the presence of air bladder in Pisces?11. What are the modifications that are observed in birds that help them fly?12. Could the number of eggs or young ones produced by an oviparous and viviparousmother be equal? Why?13. Segmentation in the body is first observed in which of the following:(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda14. Match the following:(a) Operculum (i) Ctenophora(b) Parapodia (ii) Mollusca(c) Scales (iii) Porifera(d) Comb plates (iv) Reptilia(e) Radula (v) Annelida(f ) Hairs (vi) Cyclostomata and Chondrichthyes(g) Choanocytes (vii) Mammalia(h) Gill slits (viii) Osteichthyes15. Prepare a list of some animals that are found parasitic on human beings.2022-23UNIT 2The description of the diverse forms of life on earth was made only byobservation â through naked eyes or later through magnifying lensesand microscopes. This description is mainly of gross structural features,both external and internal. In addition, observable and perceivableliving phenomena were also recorded as part of this description. Beforeexperimental biology or more specifically, physiology, was establishedas a part of biology, naturalists described only biology. Hence, biologyremained as a natural history for a long time. The description, by itself,was amazing in terms of detail. While the initial reaction of a studentcould be boredom, one should keep in mind that the detailed description,was utilised in the later day reductionist biology where living processesdrew more attention from scientists than the description of life formsand their structure. Hence, this description became meaningful andhelpful in framing research questions in physiology or evolutionarybiology. In the following chapters of this unit, the structural organisationof plants and animals, including the structural basis of physiologial orbehavioural phenomena, is described. For convenience, this descriptionof morphological and anatomical features is presented separately forplants and animals.STRUCTURAL ORGANISATIONIN PLANTS AND ANIMALSChapter 5Morphology ofFlowering PlantsChapter 6Anatomy of FloweringPlantsChapter 7Structural Organisation inAnimals2022-23KATHERINE ESAU was born in Ukraine in 1898. She studiedagriculture in Russia and Germany and received her doctoratein 1931 in United States. She reported in her early publicationsthat the curly top virus spreads through a plant via the foodconductingor phloem tissue. Dr Esauâs Plant Anatomy publishedin 1954 took a dynamic, developmental approach designed toenhance oneâs understanding of plant structure and anenormous impact worldwide, literally bringing about a revivalof the discipline. The Anatomy of Seed Plants by Katherine Esauwas published in 1960. It was referred to as Websterâs of plantbiology â it is encyclopediac. In 1957 she was elected to theNational Academy of Sciences, becoming the sixth woman toreceive that honour. In addition to this prestigious award, shereceived the National Medal of Science from President GeorgeBush in 1989.When Katherine Esau died in the year 1997, Peter Raven,director of Anatomy and Morphology, Missouri BotanicalGarden, remembered that she âabsolutely dominatedâ the fieldof plant biology even at the age of 99.Katherine Esau(1898 â 1997)2022-23The wide range in the structure of higher plants will never fail to fascinateus. Even though the angiosperms show such a large diversity in externalstructure or morphology, they are all characterised by presence of roots,stems, leaves, flowers and fruits.In chapters 2 and 3, we talked about classification of plants basedon morphological and other characteristics. For any successful attemptat classification and at understanding any higher plant (or for thatmatter any living organism) we need to know standard technical termsand standard definitions. We also need to know about the possiblevariations in different parts, found as adaptations of the plants to theirenvironment, e.g., adaptions to various habitats, for protection,climbing, storage, etc.If you pull out any weed you will see that all of them have roots, stemsand leaves. They may be bearing flowers and fruits. The undergroundpart of the flowering plant is the root system while the portion above theground forms the shoot system (Figure 5.1).5.1 THE ROOTIn majority of the dicotyledonous plants, the direct elongation of the radicleleads to the formation of primary root which grows inside the soil.It bears lateral roots of several orders that are referred to as secondary,tertiary, etc. roots. The primary roots and its branches constitute theMORPHOLOGY OF FLOWERING PLANTSCHAPTER 55.1 The Root5.2 The Stem5.3 The Leaf5.4 The Inflorescence5.5 The Flower5.6 The Fruit5.7 The Seed5.8 Semi-technicalDescription of aTypicalFlowering Plant5.9 Description ofSome ImportantFamilies2022-2366 BIOLOGYFlowerShootsystemRootsystemFruitBudStemLeafNodeInternodePrimaryrootSecondaryroot{Figure 5.2 Different types of roots : (a) Tap (b) Fibrous (c) Adventitious(b) (c)Figure 5.1 Parts of a flowering plantFibrous roots Adventitious rootsLaterals(a)Main roottap root system, as seen in the mustardplant (Figure 5.2a). In monocotyledonousplants, the primary root is short lived andis replaced by a large number of roots.These roots originate from the base of thestem and constitute the fibrous rootsystem, as seen in the wheat plant (Figure5.2b). In some plants, like grass,Monstera and the banyan tree, roots arisefrom parts of the plant other than theradicle and are called adventitious roots(Figure 5.2c). The main functions of theroot system are absorption of water andminerals from the soil, providing a properanchorage to the plant parts, storingreserve food material and synthesis ofplant growth regulators.2022-23MORPHOLOGY OF FLOWERING PLANTS 675.1.1 Regions of the RootThe root is covered at the apex by a thimble-likestructure called the root cap (Figure 5.3). Itprotects the tender apex of the root as it makesits way through the soil. A few millimetres abovethe root cap is the region of meristematicactivity. The cells of this region are very small,thin-walled and with dense protoplasm. Theydivide repeatedly. The cells proximal to thisregion undergo rapid elongation andenlargement and are responsible for the growthof the root in length. This region is called theregion of elongation. The cells of the elongationzone gradually differentiate and mature. Hence,this zone, proximal to region of elongation, iscalled the region of maturation. From thisregion some of the epidermal cells form very fineand delicate, thread-like structures called roothairs. These root hairs absorb water andminerals from the soil.5.1.2 Modifications of RootRoots in some plants change their shape andstructure and become modified to performfunctions other than absorption andconduction of water and minerals. They aremodified for support, storage of food andrespiration (Figure 5.4 and 5.5). Tap roots ofcarrot, turnip and adventitious roots of sweetpotato, get swollen and store food. Can you givesome more such examples? Have you everwondered what those hanging structures thatsupport a banyan tree are? These are calledprop roots. Similarly, the stems of maize andsugarcane have supporting roots coming outof the lower nodes of the stem. These are calledstilt roots. In some plants such as Rhizophoragrowing in swampy areas, many roots come outof the ground and grow vertically upwards.Such roots, called pneumatophores, help toget oxygen for respiration (Figure 5.5b).Figure 5.3 The regions of the root-tipFigure 5.4 Modification of root for support:Banyan tree2022-2368 BIOLOGY5.2 THE STEMWhat are the features that distinguish a stem from a root? The stem is theascending part of the axis bearing branches, leaves, flowers and fruits. Itdevelops from the plumule of the embryo of a germinating seed. The stembears nodes and internodes. The region of the stem where leaves areborn are called nodes while internodes are the portions between two nodes.The stem bears buds, which may be terminal or axillary. Stem is generallygreen when young and later often become woody and dark brown.The main function of the stem is spreading out branches bearingleaves, flowers and fruits. It conducts water, minerals and photosynthates.Some stems perform the function of storage of food, support, protectionand of vegetative propagation.5.2.1 Modifications of StemThe stem may not always be typically like what they are expected to be.They are modified to perform different functions (Figure 5.6). Undergroundstems of potato, ginger, turmeric, zaminkand, Colocasia are modified tostore food in them. They also act as organs of perennation to tide overconditions unfavourable for growth. Stem tendrils which develop fromaxillary buds, are slender and spirally coiled and help plants to climbsuch as in gourds (cucumber, pumpkins, watermelon) and grapevines.Axillary buds of stems may also get modified into woody, straight andpointed thorns. Thorns are found in many plants such as Citrus,Bougainvillea. They protect plants from browsing animals. Some plantsof arid regions modify their stems into flattened (Opuntia), or fleshycylindrical (Euphorbia) structures. They contain chlorophyll and carryFigure 5.5 Modification of root for : (a) storage (b) respiration: pneumatophore inRhizophora( a ) (b)Turnip Carrot SweetpotatoAsparagus2022-23MORPHOLOGY OF FLOWERING PLANTS 69Figure 5.6 Modifications of stem for : (a) storage (b) support (c) protection(d) spread and vegetative propagationout photosynthesis. Underground stems of some plants such as grassand strawberry, etc., spread to new niches and when older parts die newplants are formed. In plants like mint and jasmine a slender lateral brancharises from the base of the main axis and after growing aerially for sometime arch downwards to touch the ground. A lateral branch with shortinternodes and each node bearing a rosette of leaves and a tuft of roots isfound in aquatic plants like Pistia and Eichhornia. In banana, pineappleand Chrysanthemum, the lateral branches originate from the basal andunderground portion of the main stem, grow horizontally beneath thesoil and then come out obliquely upward giving rise to leafy shoots.5.3 THE LEAFThe leaf is a lateral, generally flattened structure borne on the stem. Itdevelops at the node and bears a bud in its axil. The axillary bud laterdevelops into a branch. Leaves originate from shoot apical meristems andare arranged in an acropetal order. They are the most important vegetativeorgans for photosynthesis.A typical leaf consists of three main parts: leaf base, petiole and lamina(Figure 5.7 a). The leaf is attached to the stem by the leaf base and may(a)(b)(c) (d)Axillary budmodifiedinto tendrilRoots arisingfrom nodesStem modifiedinto spineGingerZaminkandPotatoBougainvillea sp.Oxalis sp.2022-2370 BIOLOGYbear two lateral small leaf like structures calledstipules. In monocotyledons, the leaf base expandsinto a sheath covering the stem partially or wholly.In some leguminous plants the leafbase maybecome swollen, which is called the pulvinus. Thepetiole help hold the blade to light. Long thin flexiblepetioles allow leaf blades to flutter in wind, therebycooling the leaf and bringing fresh air to leaf surface.The lamina or the leaf blade is the green expandedpart of the leaf with veins and veinlets. There is,usually, a middle prominent vein, which is knownas the midrib. Veins provide rigidity to the leaf bladeand act as channels of transport for water, mineralsand food materials. The shape, margin, apex, surfaceand extent of incision of lamina varies in differentleaves.5.3.1 VenationThe arrangement of veins and the veinlets in thelamina of leaf is termed as venation. When theveinlets form a network, the venation is termed asreticulate (Figure 5.7 b). When the veins runparallel to each other within a lamina, the venationis termed as parallel (Figure 5.7 c). Leaves ofdicotyledonous plants generally possess reticulatevenation, while parallel venation is the characteristicof most monocotyledons.5.3.2 Types of LeavesA leaf is said to be simple, when its lamina is entireor when incised, the incisions do not touch themidrib. When the incisions of the lamina reach upto the midrib breaking it into a number of leaflets,the leaf is called compound. A bud is presentin the axil of petiole in both simple and compoundleaves, but not in the axil of leaflets of the compoundleaf.The compound leaves may be of two types(Figure 5.8). In a pinnately compound leaf anumber of leaflets are present on a common axis,the rachis, which represents the midrib of the leafas in neem.Figure 5.7 Structure of a leaf :(a) Parts of a leaf(b) Reticulate venation(c) Parallel venation(b) (c)(b) Silk Cotton(a)LaminaPetioleStipuleLeafbaseAxillarybud(a) NeemFigure 5.8 Compound leaves :(a) pinnately compound leaf(b) palmately compound leafRachis2022-23MORPHOLOGY OF FLOWERING PLANTS 71In palmately compound leaves, theleaflets are attached at a common point, i.e.,at the tip of petiole, as in silk cotton.5.3.3 PhyllotaxyPhyllotaxy is the pattern of arrangement ofleaves on the stem or branch. This is usuallyof three types â alternate, opposite andwhorled (Figure 5.9). In alternate type ofphyllotaxy, a single leaf arises at each nodein alternate manner, as in china rose,mustard and sun flower plants. In oppositetype, a pair of leaves arise at each node andlie opposite to each other as in Calotropisand guava plants. If more than two leavesarise at a node and form a whorl, it is calledwhorled, as in Alstonia.5.3.4 Modifications of LeavesLeaves are often modified to performfunctions other than photosynthesis. Theyare converted into tendrils for climbing asin peas or into spines for defence as in cacti(Figure 5.10 a, b). The fleshy leaves of onionand garlic store food (Figure 5.10c). In someplants such as Australian acacia, the leavesare small and short-lived. The petioles inthese plants expand, become green andsynthesise food. Leaves of certaininsectivorous plants such as pitcher plant,venus-fly trap are also modified leaves.5.4 THE INFLORESCENCEA flower is a modified shoot wherein the shootapical meristem changes to floral meristem.Internodes do not elongate and the axis getscondensed. The apex produces differentkinds of floral appendages laterally atsuccessive nodes instead of leaves. When ashoot tip transforms into a flower, it is alwayssolitary. The arrangement of flowers on theFigure 5.10 Modifications of leaf for :(a) support: tendril (b) protection:spines (c) storage: fleshy leaves(c) OnionFleshyleavesLeaftendril(a) Pea(b) CactusLeavesmodifiedinto spinesFigure 5.9 Different types of phyllotaxy :(a) Alternate (b) Opposite(c) Whorled(c) Alstonia(a) China rose(b) Guava2022-2372 BIOLOGYfloral axis is termed as inflorescence. Dependingon whether the apex gets developed into a flower orcontinues to grow, two major types of inflorescencesare defined â racemose and cymose. In racemosetype of inflorescences the main axis continues togrow, the flowers are borne laterally in an acropetalsuccession (Figure 5.11).In cymose type of inflorescence the main axisterminates in a flower, hence is limited in growth.Theflowers are borne in a basipetal order (Figure 5.12).5.5 THE FLOWERThe flower is the reproductive unit in theangiosperms. It is meant for sexual reproduction.A typical flower has four different kinds of whorlsarranged successively on the swollen end of thestalk or pedicel, called thalamus or receptacle.These are calyx, corolla, androecium andgynoecium. Calyx and corolla are accessory organs,while androecium and gynoecium are reproductiveorgans. In some flowers like lily, the calyx andcorolla are not distinct and are termed as perianth.When a flower has both androecium andgynoecium, it is bisexual. A flower having eitheronly stamens or only carpels is unisexual.In symmetry, the flower may beactinomorphic (radial symmetry) orzygomorphic (bilateral symmetry). When a flowercan be divided into two equal radial halves in anyradial plane passing through the centre, it is saidto be actinomorphic, e.g., mustard, datura, chilli.When it can be divided into two similar halves onlyin one particular vertical plane, it is zygomorphic,e.g., pea, gulmohur, bean, Cassia. A flower isasymmetric (irregular) if it cannot be divided intotwo similar halves by any vertical plane passingthrough the centre, as in canna.A flower may be trimerous, tetramerous orpentamerous when the floral appendages are inmultiple of 3, 4 or 5, respectively. Flowerswith bracts-reduced leaf found at the base of thepedicel- are called bracteate and those withoutbracts, ebracteate.Figure 5.12 Cymose inflorescenceFigure 5.11 Racemose inflorescence2022-23MORPHOLOGY OF FLOWERING PLANTS 73Based on the position of calyx, corolla and androecium in respect ofthe ovary on thalamus, the flowers are described as hypogynous,perigynous and epigynous (Figure 5.13). In the hypogynous flower thegynoecium occupies the highest position while the other parts are situatedbelow it. The ovary in such flowers is said to be superior, e.g., mustard,china rose and brinjal. If gynoecium is situated in the centre and otherparts of the flower are located on the rim of the thalamus almost at thesame level, it is called perigynous. The ovary here is said to be halfinferior, e.g., plum, rose, peach. In epigynous flowers, the margin ofthalamus grows upward enclosing the ovary completely and getting fusedwith it, the other parts of flower arise above the ovary. Hence, the ovary issaid to be inferior as in flowers of guava and cucumber, and the rayflorets of sunflower.5.5.1 Parts of a FlowerEach flower normally has four floral whorls, viz., calyx, corolla,androecium and gynoecium (Figure 5.14).5.5.1.1 CalyxThe calyx is the outermost whorl of the flower and the members are calledsepals. Generally, sepals are green, leaf like and protect the flower in thebud stage. The calyx may be gamosepalous (sepals united) orpolysepalous (sepals free).5.5.1.2 CorollaCorolla is composed of petals. Petals are usually brightly coloured toattract insects for pollination. Like calyx, corolla may also beFigure 5.13 Position of floral parts on thalamus : (a) Hypogynous (b) and (c)Perigynous (d) Epigynous(a) (b) (c) (d)2022-2374 BIOLOGYgamopetalous (petals united) or polypetalous (petals free). The shapeand colour of corolla vary greatly in plants. Corolla may be tubular, bellshaped,funnel-shaped or wheel-shaped.Aestivation: The mode of arrangement of sepals or petals in floral budwith respect to the other members of the same whorl is known asaestivation. The main types of aestivation are valvate, twisted, imbricateand vexillary (Figure 5.15). When sepals or petals in a whorl just touchone another at the margin, without overlapping, as in Calotropis, it issaid to be valvate. If one margin of the appendage overlaps that of thenext one and so on as in china rose, ladyâs finger and cotton, it is calledtwisted. If the margins of sepals or petals overlap one another but not inany particular direction as in Cassia and gulmohur, the aestivation iscalled imbricate. In pea and bean flowers, there are five petals, the largest(standard) overlaps the two lateral petals (wings) which in turn overlapthe two smallest anterior petals (keel); this type of aestivation is knownas vexillary or papilionaceous.PedicelCalyxCorollaAndroeciumGynoeciumFigure 5.14 Parts of a flowerFigure 5.15 Types of aestivation in corolla : (a) Valvate (b) Twisted (c) Imbricate (d) Vexillary(a) (b) (c) (d)2022-23MORPHOLOGY OF FLOWERING PLANTS 755.5.1.3 AndroeciumAndroecium is composed of stamens. Each stamen whichrepresents the male reproductive organ consists of a stalk or afilament and an anther. Each anther is usually bilobed and eachlobe has two chambers, the pollen-sacs. The pollen grains areproduced in pollen-sacs. A sterile stamen is called staminode.Stamens of flower may be united with other members such aspetals or among themselves. When stamens are attached to thepetals, they are epipetalous as in brinjal, or epiphyllous whenattached to the perianth as in the flowers of lily. The stamens in aflower may either remain free (polyandrous) or may be united invarying degrees. The stamens may be united into one bunch orone bundle (monoadelphous) as in china rose, or two bundles(diadelphous) as in pea, or into more than two bundles(polyadelphous) as in citrus. There may be a variation in the lengthof filaments within a flower, as in Salvia and mustard.5.5.1.4 GynoeciumGynoecium is the female reproductive part of the flower and is madeup of one or more carpels. A carpel consists of three parts namelystigma, style and ovary. Ovary is the enlarged basal part, on whichlies the elongated tube, the style. The style connects the ovary to thestigma. The stigma is usually at the tip of the style and is thereceptive surface for pollen grains. Each ovary bears one or moreovules attached to a flattened, cushion-like placenta. When morethan one carpel is present, they may be free (as in lotus and rose)and are called apocarpous. They are termed syncarpous whencarpels are fused, as in mustard and tomato. After fertilisation, theovules develop into seeds and the ovary matures into a fruit.Placentation: The arrangement of ovules within the ovary is knownas placentation. The placentation are of different types namely,marginal, axile, parietal, basal, central and free central (Figure 5.16).In marginal placentation the placenta forms a ridge along theventral suture of the ovary and the ovules are borne on this ridgeforming two rows, as in pea. When the placenta is axial and theovules are attached to it in a multilocular ovary, the placentaion issaid to be axile, as in china rose, tomato and lemon. In parietalplacentation, the ovules develop on the inner wall of the ovary oron peripheral part. Ovary is one-chambered but it becomes twochambereddue to the formation of the false septum, e.g., mustardand Argemone. When the ovules are borne on central axis andsepta are absent, as in Dianthus and Primrose the placentation isFigure 5.16 Types ofplacentation :(a) Marginal(b) Axile(c) Parietal(d) Free central(e) Basal(a)(e)(b)(d)(c)(a)2022-2376 BIOLOGYcalled free central. In basal placentation, the placenta develops at thebase of ovary and a single ovule is attached to it, as in sunflower, marigold.5.6 THE FRUITThe fruit is a characteristic feature of the flowering plants. It is a matureor ripened ovary, developed after fertilisation. If a fruit is formed withoutfertilisation of the ovary, it is called a parthenocarpic fruit.Generally, the fruit consists of a wall or pericarp and seeds. Thepericarp may be dry or fleshy. When pericarp is thick and fleshy, it isdifferentiated into the outer epicarp, the middle mesocarp and the innerendocarp.In mango and coconut, the fruit is known as a drupe (Figure 5.17).They develop from monocarpellary superior ovaries and are one seeded.In mango the pericarp is well differentiated into an outer thin epicarp, amiddle fleshy edible mesocarp and an inner stony hard endocarp. Incoconut which is also a drupe, the mesocarp is fibrous.5.7 THE SEEDThe ovules after fertilisation, develop into seeds. A seed is made up of aseed coat and an embryo. The embryo is made up of a radicle, an embryonalaxis and one (as in wheat, maize) or two cotyledons (as in gram and pea).5.7.1 Structure of a Dicotyledonous SeedThe outermost covering of a seed is the seed coat. The seed coat has twolayers, the outer testa and the inner tegmen. The hilum is a scar on theseed coat through which the developing seeds were attached to the fruit.Above the hilum is a small pore called the micropyle. Within the seedFigure 5.17 Parts of a fruit : (a) Mango (b) Coconut(a) (b)2022-23MORPHOLOGY OF FLOWERING PLANTS 77coat is the embryo, consisting of anembryonal axis and two cotyledons. Thecotyledons are often fleshy and full of reservefood materials. At the two ends of theembryonal axis are present the radicle andthe plumule (Figure 5.18). In some seedssuch as castor the endosperm formed as aresult of double fertilisation, is a food storingtissue and called endospermic seeds. Inplants such as bean, gram and pea, theendosperm is not present in mature seedsand such seeds are called nonendospermous.Seed coatHilumMicropyleCotyledonPlumuleRadicleFigure 5.18 Structure of dicotyledonous seedFigure 5.19 Structure of a monocotyledonous seedSeed coat & fruit-wallAleurone layerEndospermScutellumColeoptilePlumuleRadicleColeorhizaEndospermEmbryo5.7.2 Structure of Monocotyledonous SeedGenerally, monocotyledonous seeds are endospermic but some as inorchids are non-endospermic. In the seeds of cereals such as maize theseed coat is membranous and generally fused with the fruit wall. Theendosperm is bulky and stores food. The outer covering of endospermseparates the embryo by a proteinous layer called aleurone layer. Theembryo is small and situated in a groove at one end of the endosperm. Itconsists of one large and shield shaped cotyledon known as scutellumand a short axis with a plumule and a radicle. The plumule and radicleare enclosed in sheaths which are called coleoptile and coleorhizarespectively (Figure 5.19).2022-2378 BIOLOGY5.8 SEMI-TECHNICAL DESCRIPTION OF A TYPICALFLOWERING PLANTVarious morphological features are used to describe aflowering plant. The description has to be brief, in a simpleand scientific language and presented in a propersequence. The plant is described beginning with its habit,vegetative characters â roots, stem and leaves and thenfloral characters inflorescence and flower parts. Afterdescribing various parts of plant, a floral diagram and afloral formula are presented. The floral formula isrepresented by some symbols. In the floral formula, Brstands for bracteate K stands for calyx , C for corolla, P forperianth, A for androecium and G for Gynoecium, G forsuperior ovary and G for inferior ovary, for male, forfemale, for bisexual plants, à for actinomorphic andfor zygomorphic nature of flower. Fusion is indicated byenclosing the figure within bracket and adhesion by a linedrawn above the symbols of the floral parts. A floraldiagram provides information about the number of partsof a flower, their arrangement and the relation they havewith one another (Figure 5.20). The position of the motheraxis with respect to the flower is represented by a dot onthe top of the floral diagram. Calyx, corolla, androeciumand gynoecium are drawn in successive whorls, calyx beingthe outermost and the gynoecium being in the centre.Floral formula also shows cohesion and adhesion withinparts of whorls and between whorls. The floral diagramand floral formula in Figure 5.20 represents the mustardplant (Family: Brassicaceae).5.9 DESCRIPTION OF SOME IMPORTANT FAMILIES5.9.1 FabaceaeThis family was earlier called Papilionoideae, a subfamilyof family Leguminosae. It is distributed all over the world(Figure 5.21).Vegetative CharactersTrees, shrubs, herbs; root with root nodulesStem: erect or climberLeaves: alternate, pinnately compound or simple; leaf base,pulvinate; stipulate; venation reticulate.Figure 5.20 Floral diagram withfloral formulaà K2+2 C4 A2+4 G(2)2022-23MORPHOLOGY OF FLOWERING PLANTS 79(b)(c)(a) (d)(e) (f)Figure 5.21 Pisum sativum (pea) plant : (a) Flowering twig (b) Flower (c) Petals(d) Reproductive parts (e) L.S.carpel (f) Floral diagramFloral charactersInflorescence: racemoseFlower: bisexual, zygomorphicCalyx: sepals five, gamosepalous; valvate/imbricate aestivationCorolla: petals five, polypetalous, papilionaceous, consisting of a posteriorstandard, two lateral wings, two anterior ones forming a keel (enclosingstamens and pistil), vexillary aestivationAndroecium: ten, diadelphous, anther dithecousGynoecium: ovary superior, mono carpellary, unilocular with manyovules, style singleFruit: legume; seed: one to many, non-endospermicFloral Formula: % K(5) C1+2+(2) A(9)+1 G1Economic importanceMany plants belonging to the family are sources of pulses (gram, arhar,sem, moong, soyabean; edible oil (soyabean, groundnut); dye (Indigofera);fibres (sunhemp); fodder (Sesbania, Trifolium), ornamentals (lupin, sweetpea); medicine (muliathi).5.9.2 SolanaceaeIt is a large family, commonly called as the âpotato familyâ. It is widelydistributed in tropics, subtropics and even temperate zones (Figure 5.22).Vegetative CharactersPlants mostly herbs, shrubs and rarely small treesStem: herbaceous rarely woody, aerial; erect, cylindrical, branched, solid2022-2380 BIOLOGYor hollow, hairy or glabrous, underground stem in potato (Solanumtuberosum)Leaves: alternate, simple, rarely pinnately compound, exstipulate;venation reticulateFloral CharactersInflorescence : Solitary, axillary or cymose as in SolanumFlower: bisexual, actinomorphicCalyx: sepals five, united, persistent, valvate aestivationCorolla: petals five, united; valvate aestivationAndroecium: stamens five, epipetalousGynoecium: bicarpellary obligately placed, syncarpous; ovary superior,bilocular, placenta swollen with many ovules, axileFruits: berry or capsuleSeeds: many, endospermousFloral Formula: à Economic ImportanceMany plants belonging to this family are source of food (tomato, brinjal,potato), spice (chilli); medicine (belladonna, ashwagandha); fumigatory(tobacco); ornamentals (petunia).(b)(a)(c)(d)(e) (f)Figure 5.22 Solanum nigrum (makoi) plant : (a) Flowering twig (b) Flower(c) L.S. of flower (d) Stamens (e) Carpel (f) Floral diagram2022-23MORPHOLOGY OF FLOWERING PLANTS 815.9.3 LiliaceaeCommonly called the âLily familyâ is a characteristic representative ofmonocotyledonous plants. It is distributed world wide (Figure 5.23).Vegetative characters: Perennial herbs with underground bulbs/corms/rhizomesLeaves mostly basal, alternate, linear, exstipulate with parallel venationFloral charactersInflorescence: solitary / cymose; often umbellate clustersFlower: bisexual; actinomorphicPerianth tepal six (3+3), often united into tube; valvate aestivationAndroecium: stamen six, 3+3, epitepalousGynoecium: tricarpellary, syncarpous, ovary superior, trilocular withmany ovules; axile placentationFruit: capsule, rarely berrySeed: endospermousFloral Formula: Br à P(3+3) A3+3 G(3)Economic ImportanceMany plants belonging to this family are good ornamentals (tulip,Gloriosa), source of medicine (Aloe), vegetables (Asparagus), andcolchicine (Colchicum autumnale).Figure 5.23 Allium cepa (onion) plant : (a) Plant (b) Inflorescence (c) Flower(d) Floral diagram(d)(b) (c)(a)2022-2382 BIOLOGYSUMMARYFlowering plants exhibit enormous variation in shape, size, structure, mode ofnutrition, life span, habit and habitat. They have well developed root and shootsystems. Root system is either tap root or fibrous. Generally, dicotyledonous plantshave tap roots while monocotyledonous plants have fibrous roots. The roots insome plants get modified for storage of food, mechanical support and respiration.The shoot system is differentiated into stem, leaves, flowers and fruits. Themorphological features of stems like the presence of nodes and internodes,multicellular hair and positively phototropic nature help to differentiate the stemsfrom roots. Stems also get modified to perform diverse functions such as storageof food, vegetative propagation and protection under different conditions. Leaf is alateral outgrowth of stem developed exogeneously at the node. These are green incolour to perform the function of photosynthesis. Leaves exhibit marked variationsin their shape, size, margin, apex and extent of incisions of leaf blade (lamina).Like other parts of plants, the leaves also get modified into other structures suchas tendrils, spines for climbing and protection respectively.The flower is a modified shoot, meant for sexual reproduction. The flowers arearranged in different types of inflorescences. They exhibit enormous variation instructure, symmetry, position of ovary in relation to other parts, arrangement ofpetals, sepals, ovules etc. After fertilisation, the ovary is modified into fruits andovules into seeds. Seeds either may be monocotyledonous or dicotyledonous. Theyvary in shape, size and period of viability. The floral characteristics form the basisof classification and identification of flowering plants. This can be illustratedthrough semi-technical descriptions of families. Hence, a flowering plant isdescribed in a definite sequence by using scientific terms. The floral features arerepresented in the summarised form as floral diagrams and floral formula.EXERCISES1. What is meant by modification of root? What type of modification of root is foundin the:(a) Banyan tree (b) Turnip (c) Mangrove trees2. Justify the following statements on the basis of external features:(i) Underground parts of a plant are not always roots.(ii) Flower is a modified shoot.3. How is a pinnately compound leaf different from a palmately compound leaf?4. Explain with suitable examples the different types of phyllotaxy.2022-23MORPHOLOGY OF FLOWERING PLANTS 835. Define the following terms:(a) aestivation (b) placentation (c) actinomorphic(d) zygomorphic (e) superior ovary (f) perigynous flower(g) epipetalous stamen6. Differentiate between(a) Racemose and cymose inflorescence(b) Fibrous root and adventitious root(c) Apocarpous and syncarpous ovary7. Draw the labelled diagram of the following:(i) gram seed (ii) V.S. of maize seed8. Describe modifications of stem with suitable examples.9. Take one flower each of the families Fabaceae and Solanaceae and write itssemi-technical description. Also draw their floral diagram after studying them.10. Describe the various types of placentations found in flowering plants.11. What is a flower? Describe the parts of a typical angiosperm flower.12. How do the various leaf modifications help plants?13. Define the term inflorescence. Explain the basis for the different typesinflorescence in flowering plants.14. Write the floral formula of a actinomorphic, bisexual, hypogynous flower withfive united sepals, five free petals, five free stamens and two united carpleswith superior ovary and axile placentation.15. Describe the arrangement of floral members in relation to their insertion onthalamus.2022-2384 BIOLOGYYou can very easily see the structural similarities and variations in theexternal morphology of the larger living organism, both plants andanimals. Similarly, if we were to study the internal structure, one alsofinds several similarities as well as differences. This chapter introducesyou to the internal structure and functional organisation of higher plants.Study of internal structure of plants is called anatomy. Plants have cellsas the basic unit, cells are organised into tissues and in turn the tissuesare organised into organs. Different organs in a plant show differences intheir internal structure. Within angiosperms, the monocots and dicotsare also seen to be anatomically different. Internal structures also showadaptations to diverse environments.6.1 THE TISSUESA tissue is a group of cells having a common origin and usually performinga common function. A plant is made up of different kinds of tissues. Tissuesare classified into two main groups, namely, meristematic and permanenttissues based on whether the cells being formed are capable of dividingor not.6.1.1 Meristematic TissuesGrowth in plants is largely restricted to specialised regions of active cell divisioncalled meristems (Gk. meristos: divided). Plants have different kinds ofmeristems. The meristems which occur at the tips of roots and shoots andproduce primary tissues are called apical meristems (Figure 6.1).ANATOMY OF FLOWERING PLANTSCHAPTER 66.1 The Tissues6.2 The TissueSystem6.3 Anatomy ofDicotyledonousandMonocotyledonousPlants6.4 SecondaryGrowth2022-23ANATOMY OF FLOWERING PLANTS 85Root apical meristem occupies the tip of a root while the shoot apicalmeristem occupies the distant most region of the stem axis. During theformation of leaves and elongation of stem, some cells âleft behindâ fromshoot apical meristem, constitute the axillary bud. Such buds are presentin the axils of leaves and are capable of forming a branch or a flower. Themeristem which occurs between mature tissues is known as intercalarymeristem. They occur in grasses and regenerate parts removed by thegrazing herbivores. Both apical meristems and intercalary meristems areprimary meristems because they appear early in life of a plant andcontribute to the formation of the primary plant body.The meristem that occurs in the mature regions of roots and shoots ofmany plants, particularly those that produce woody axis and appearlater than primary meristem is called the secondary or lateral meristem.They are cylindrical meristems. Fascicular vascular cambium,interfascicular cambium and cork-cambium are examples of lateralmeristems. These are responsible for producing the secondary tissues.Following divisions of cells in both primary and as well as secondarymeristems, the newly formed cells become structurally and functionallyspecialised and lose the ability to divide. Such cells are termed permanentor mature cells and constitute the permanent tissues. During theformation of the primary plant body, specific regions of the apical meristemproduce dermal tissues, ground tissues and vascular tissues.Central cylinderCortexProtodermInitials of centralcylinderand cortexInitials ofroot capRoot capRoot apicalmeristemLeaf primordiumShoot apicalMeristematic zoneAxillary budDifferentiatingvascular tissueFigure 6.1 Apical meristem: (a) Root (b) Shoot2022-2386 BIOLOGY6.1.2 Permanent TissuesThe cells of the permanent tissues do not generallydivide further. Permanent tissues having all cellssimilar in structure and function are called simpletissues. Permanent tissues having many differenttypes of cells are called complex tissues.6.1.2.1 Simple TissuesA simple tissue is made of only one type of cells.The various simple tissues in plants areparenchyma, collenchyma and sclerenchyma(Figure 6.2). Parenchyma forms the majorcomponent within organs. The cells of theparenchyma are generally isodiametric. Theymay be spherical, oval, round, polygonal orelongated in shape. Their walls are thin and madeup of cellulose. They may either be closely packedor have small intercellular spaces. Theparenchyma performs various functions likephotosynthesis, storage, secretion.The collenchyma occurs in layers below theepidermis in most of the dicotyledonous plants. It isfound either as a homogeneous layer or in patches.It consists of cells which are much thickened at thecorners due to a deposition of cellulose,hemicellulose and pectin. Collenchymatous cellsmay be oval, spherical or polygonal and oftencontain chloroplasts. These cells assimilate foodwhen they contain chloroplasts. Intercellular spacesare absent. They provide mechanical support to thegrowing parts of the plant such as young stem andpetiole of a leaf.Sclerenchyma consists of long, narrow cellswith thick and lignified cell walls having a few ornumerous pits. They are usually dead and withoutprotoplasts. On the basis of variation in form,structure, origin and development, sclerenchymamay be either fibres or sclereids. The fibres arethick-walled, elongated and pointed cells,generally occuring in groups, in various parts ofthe plant. The sclereids are spherical, oval orcylindrical, highly thickened dead cells with veryIntercelluar spaceFigure 6.2 Simple tissues :(a) Parenchyma(b) Collenchyma(c) SclerenchymaA fibreA sclereid(c)LumenThickcell wallLumenPitsThickcell wall(b)Thickened cornersProtoplasmVacuoleCell wall(a)2022-23ANATOMY OF FLOWERING PLANTS 87narrow cavities (lumen). These are commonly found in the fruitwalls of nuts; pulp of fruits like guava, pear and sapota; seedcoats of legumes and leaves of tea. Sclerenchyma providesmechanical support to organs.6.1.2.2 Complex TissuesThe complex tissues are made of more than one type of cellsand these work together as a unit. Xylem and phloem constitutethe complex tissues in plants (Figure 6.3).Xylem functions as a conducting tissue for water andminerals from roots to the stem and leaves. It also providesmechanical strength to the plant parts. It is composed of fourdifferent kinds of elements, namely, tracheids, vessels, xylemfibres and xylem parenchyma. Gymnosperms lack vessels intheir xylem. Tracheids are elongated or tube like cells withthick and lignified walls and tapering ends. These are dead andare without protoplasm. The inner layers of the cell walls havethickenings which vary in form. In flowering plants, tracheidsand vessels are the main water transporting elements. Vessel isa long cylindrical tube-like structure made up of many cellscalled vessel members, each with lignified walls and a largecentral cavity. The vessel cells are also devoid of protoplasm.Vessel members are interconnected through perforations in theircommon walls. The presence of vessels is a characteristic featureof angiosperms. Xylem fibres have highly thickened walls andobliterated central lumens. These may either be septate oraseptate. Xylem parenchyma cells are living and thin-walled,and their cell walls are made up of cellulose. They store foodmaterials in the form of starch or fat, and other substances liketannins. The radial conduction of water takes place by the rayparenchymatous cells.Primary xylem is of two types â protoxylem and metaxylem.The first formed primary xylem elements are called protoxylemand the later formed primary xylem is called metaxylem. Instems, the protoxylem lies towards the centre (pith) and themetaxylem lies towards the periphery of the organ. This typeof primary xylem is called endarch. In roots, the protoxylemlies towards periphery and metaxylem lies towards the centre.Such arrangement of primary xylem is called exarch.Phloem transports food materials, usually from leaves toother parts of the plant. Phloem in angiosperms is composedof sieve tube elements, companion cells, phloem parenchymaPhloemparenchymaCompanioncell(b)Sieve poreSieve tubeelementFigure 6.3 (a) Xylem(b) Phloem(a)TracheidVessels2022-2388 BIOLOGYand phloem fibres. Gymnosperms have albuminous cells and sieve cells.They lack sieve tubes and companion cells. Sieve tube elements arealso long, tube-like structures, arranged longitudinally and areassociated with the companion cells. Their end walls are perforated in asieve-like manner to form the sieve plates. A mature sieve elementpossesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.The functions of sieve tubes are controlled by the nucleus of companioncells. The companion cells are specialised parenchymatous cells, whichare closely associated with sieve tube elements. The sieve tube elementsand companion cells are connected by pit fields present between theircommon longitudinal walls. The companion cells help in maintaining thepressure gradient in the sieve tubes. Phloem parenchyma is made upof elongated, tapering cylindrical cells which have dense cytoplasm andnucleus. The cell wall is composed of cellulose and has pits through whichplasmodesmatal connections exist between the cells. The phloemparenchyma stores food material and other substances like resins, latexand mucilage. Phloem parenchyma is absent in most of themonocotyledons. Phloem fibres (bast fibres) are made up ofsclerenchymatous cells. These are generally absent in the primary phloembut are found in the secondary phloem. These are much elongated,unbranched and have pointed, needle like apices. The cell wall of phloemfibres is quite thick. At maturity, these fibres lose their protoplasm andbecome dead. Phloem fibres of jute, flax and hemp are used commercially.The first formed primary phloem consists of narrow sieve tubes and isreferred to as protophloem and the later formed phloem has bigger sievetubes and is referred to as metaphloem.6.2 THE TISSUE SYSTEMWe were discussing types of tissues based on the types of cells present.Let us now consider how tissues vary depending on their location in theplant body. Their structure and function would also be dependent onlocation. On the basis of their structure and location, there are three typesof tissue systems. These are the epidermal tissue system, the ground orfundamental tissue system and the vascular or conducting tissue system.6.2.1 Epidermal Tissue SystemThe epidermal tissue system forms the outer-most covering of the wholeplant body and comprises epidermal cells, stomata and the epidermalappendages â the trichomes and hairs. The epidermis is the outermostlayer of the primary plant body. It is made up of elongated, compactly2022-23ANATOMY OF FLOWERING PLANTS 89The cells of epidermis bear a number of hairs. The root hairs areunicellular elongations of the epidermal cells and help absorb water andminerals from the soil. On the stem the epidermal hairs are calledtrichomes. The trichomes in the shoot system are usually multicellular.They may be branched or unbranched and soft or stiff. They may evenbe secretory. The trichomes help in preventing water loss due totranspiration.6.2.2 The Ground Tissue SystemAll tissues except epidermis and vascular bundles constitute the groundtissue. It consists of simple tissues such as parenchyma, collenchymaand sclerenchyma. Parenchymatous cells are usually present in cortex,pericycle, pith and medullary rays, in the primary stems and roots. Inleaves, the ground tissue consists of thin-walled chloroplast containingcells and is called mesophyll.arranged cells, which form a continuous layer. Epidermis is usually singlelayered.Epidermal cells are parenchymatous with a small amount ofcytoplasm lining the cell wall and a large vacuole. The outside of theepidermis is often covered with a waxy thick layer called the cuticle whichprevents the loss of water. Cuticle is absent in roots. Stomata are structurespresent in the epidermis of leaves. Stomata regulate the process oftranspiration and gaseous exchange. Each stoma is composed of two beanshapedcells known as guard cells which enclose stomatal pore. In grasses,the guard cells are dumb-bell shaped. The outer walls of guard cells (awayfrom the stomatal pore) are thin and the inner walls (towards the stomatalpore) are highly thickened. The guard cells possess chloroplasts andregulate the opening and closing of stomata. Sometimes, a few epidermalcells, in the vicinity of the guard cells become specialised in their shape andsize and are known as subsidiary cells. The stomatal aperture, guardcells and the surrounding subsidiary cells are together called stomatalapparatus (Figure 6.4).Figure 6.4 Diagrammatic representation: (a) stomata with bean-shaped guard cells(b) stomata with dumb-bell shaped guard cellEpidermal cellsSubsidiary cellsGuard cellsStomatalporeChloroplast2022-2390 BIOLOGY6.2.3 The Vascular Tissue SystemThe vascular system consists of complex tissues,the phloem and the xylem.The xylem andphloem together constitute vascular bundles(Figure 6.5). In dicotyledonous stems, cambiumis present between phloem and xylem. Suchvascular bundles because of the presence ofcambium possess the ability to form secondaryxylem and phloem tissues, and hence are calledopen vascular bundles. In the monocotyledons,the vascular bundles have no cambium presentin them. Hence, since they do not form secondarytissues they are referred to as closed. Whenxylem and phloem within a vascular bundle arearranged in an alternate manner along thedifferent radii, the arrangement is called radialsuch as in roots. In conjoint type of vascularbundles, the xylem and phloem are jointlysituated along the same radius of vascularbundles. Such vascular bundles are commonin stems and leaves. The conjoint vascularbundles usually have the phloem located onlyon the outer side of xylem.6.3 ANATOMY OF DICOTYLEDONOUS ANDMONOCOTYLEDONOUS PLANTSFor a better understanding of tissueorganisation of roots, stems and leaves, it isconvenient to study the transverse sections ofthe mature zones of these organs.6.3.1 Dicotyledonous RootLook at Figure 6.6 (a), it shows the transversesection of the sunflower root. The internal tissueorganisation is as follows:The outermost layer is epiblema. Many ofthe cells of epiblema protrude in the form ofunicellular root hairs. The cortex consists ofseveral layers of thin-walled parenchyma cellsFigure 6.5 Various types of vascular bundles :(a) radial (b) conjoint closed(c) conjoint open2022-23ANATOMY OF FLOWERING PLANTS 91with intercellular spaces. The innermostlayer of the cortex is called endodermis.It comprises a single layer of barrel-shapedcells without any intercellular spaces. Thetangential as well as radial walls of theendodermal cells have a deposition ofwater-impermeable, waxy material suberinin the form of casparian strips. Next toendodermis lies a few layers of thick-walledparenchyomatous cells referred to aspericycle. Initiation of lateral roots andvascular cambium during the secondarygrowth takes place in these cells. The pithis small or inconspicuous. Theparenchymatous cells which lie betweenthe xylem and the phloem are calledconjuctive tissue. There are usually twoto four xylem and phloem patches. Later,a cambium ring develops between thexylem and phloem. All tissues on theinnerside of the endodermis such aspericycle, vascular bundles and pithconstitute the stele.6.3.2 Monocotyledonous RootThe anatomy of the monocot root is similarto the dicot root in many respects (Figure6.6 b). It has epidermis, cortex, endodermis,pericycle, vascular bundles and pith. Ascompared to the dicot root which have fewerxylem bundles, there are usually more thansix (polyarch) xylem bundles in the monocotroot. Pith is large and well developed.Monocotyledonous roots do not undergoany secondary growth.6.3.3 Dicotyledonous StemThe transverse section of a typical youngdicotyledonous stem shows that the epidermisis the outermost protective layer of the stemRoot hairEpidermisCortexEndodermisProtoxylemMetaxylemPithPhloem(a)PericycleRoot hairCortexEndodermisPhloemProtoxylemPithMetaxylem(b)EpidermisPericycleFigure 6.6 T.S. : (a) Dicot root (Primary)(b) Monocot root2022-2392 BIOLOGY(Figure 6.7 a). Covered with a thin layer of cuticle, it may bear trichomes anda few stomata. The cells arranged in multiple layers between epidermis andpericycle constitute the cortex. It consists of three sub-zones. The outerhypodermis, consists of a few layers of collenchymatous cells just below theepidermis, which provide mechanical strength to the young stem. Corticallayers below hypodermis consist of rounded thin walled parenchymatouscells with conspicuous intercellular spaces. The innermost layer of the cortexis called the endodermis. The cells of the endodermis are rich in starchgrains and the layer is also referred to as the starch sheath. Pericycle isFigure 6.7 T.S. of stem : (a) Dicot (b) Monocot2022-23ANATOMY OF FLOWERING PLANTS 93present on the inner side of the endodermis and above the phloem in theform of semi-lunar patches of sclerenchyma. In between the vascular bundlesthere are a few layers of radially placed parenchymatous cells, which constitutemedullary rays. A large number of vascular bundles are arranged in a ring ;the âringâ arrangement of vascular bundles is a characteristic of dicot stem.Each vascular bundle is conjoint, open, and with endarch protoxylem. Alarge number of rounded, parenchymatous cells with large intercellularspaces which occupy the central portion of the stem constitute the pith.6.3.4 Monocotyledonous StemThe monocot stem has a sclerenchymatous hypodermis, a large numberof scattered vascular bundles, each surrounded by a sclerenchymatousbundle sheath, and a large, conspicuous parenchymatous ground tissue(Figure 6.7b). Vascular bundles are conjoint and closed. Peripheralvascular bundles are generally smaller than the centrally located ones.The phloem parenchyma is absent, and water-containing cavities arepresent within the vascular bundles.6.3.5 Dorsiventral (Dicotyledonous) LeafThe vertical section of a dorsiventral leaf through the lamina shows threemain parts, namely, epidermis, mesophyll and vascular system. Theepidermis which covers both the upper surface (adaxial epidermis) andlower surface (abaxial epidermis) of the leaf has a conspicuous cuticle.The abaxial epidermis generally bears more stomata than the adaxialepidermis. The latter may even lack stomata. The tissue between the upperand the lower epidermis is called the mesophyll. Mesophyll, whichpossesses chloroplasts and carry out photosynthesis, is made up ofparenchyma. It has two types of cells â the palisade parenchyma andthe spongy parenchyma. The adaxially placed palisade parenchyma ismade up of elongated cells, which are arranged vertically and parallel toeach other. The oval or round and loosely arranged spongy parenchymais situated below the palisade cells and extends to the lower epidermis.There are numerous large spaces and air cavities between these cells.Vascular system includes vascular bundles, which can be seen in theveins and the midrib. The size of the vascular bundles are dependent onthe size of the veins. The veins vary in thickness in the reticulate venationof the dicot leaves. The vascular bundles are surrounded by a layer ofthick walled bundle sheath cells. Look at Figure 6.8 (a) and find theposition of xylem in the vascular bundle.6.3.6 Isobilateral (Monocotyledonous) LeafThe anatomy of isobilateral leaf is similar to that of the dorsiventral leaf inmany ways. It shows the following characteristic differences. In an2022-2394 BIOLOGYisobilateral leaf, the stomata are presenton both the surfaces of the epidermis; andthe mesophyll is not differentiated intopalisade and spongy parenchyma (Figure6.8 b).In grasses, certain adaxial epidermalcells along the veins modify themselvesinto large, empty, colourless cells. Theseare called bulliform cells. When thebulliform cells in the leaves have absorbedwater and are turgid, the leaf surface isexposed. When they are flaccid due towater stress, they make the leaves curlinwards to minimise water loss.The parallel venation in monocotleaves is reflected in the near similar sizesof vascular bundles (except in main veins)as seen in vertical sections of the leaves.6.4 SECONDARY GROWTHThe growth of the roots and stems inlength with the help of apical meristem iscalled the primary growth. Apart fromprimary growth most dicotyledonousplants exhibit an increase in girth. Thisincrease is called the secondary growth.The tissues involved in secondary growthare the two lateral meristems: vascularcambium and cork cambium.6.4.1 Vascular CambiumThe meristematic layer that is responsiblefor cutting off vascular tissues â xylem andpholem â is called vascular cambium. Inthe young stem it is present in patches asa single layer between the xylem andphloem. Later it forms a complete ring.6.4.1.1 Formation of cambial ringIn dicot stems, the cells of cambium presentbetween primary xylem and primaryphloem is the intrafascicular cambium.Figure 6.8 T.S. of leaf : (a) Dicot (b) Monocot2022-23ANATOMY OF FLOWERING PLANTS 95The cells of medullary rays, adjoining these intrafascicular cambium becomemeristematic and form the interfascicular cambium. Thus, a continuousring of cambium is formed.6.4.1.2 Activity of the cambial ringThe cambial ring becomes active and begins to cut off new cells, bothtowards the inner and the outer sides. The cells cut off towards pith,mature into secondary xylem and the cells cut off towards peripherymature into secondary phloem. The cambium is generally more activeon the inner side than on the outer. As a result, the amount of secondaryxylem produced is more than secondary phloem and soon forms acompact mass. The primary and secondary phloems get graduallycrushed due to the continued formation and accumulation of secondaryxylem. The primary xylem however remains more or less intact, in oraround the centre. At some places, the cambium forms a narrow band ofparenchyma, which passes through the secondary xylem and thesecondary phloem in the radial directions. These are the secondarymedullary rays (Figure 6.9).Figure 6.9 Secondary growth in a dicot stem (diagrammatic) â stages in transverse views2022-2396 BIOLOGY6.4.1.3 Spring wood and autumn woodThe activity of cambium is under the control of many physiological andenvironmental factors. In temperate regions, the climatic conditions arenot uniform through the year. In the spring season, cambium is veryactive and produces a large number of xylary elements having vesselswith wider cavities. The wood formed during this season is called springwood or early wood. In winter, the cambium is less active and formsfewer xylary elements that have narrow vessels, and this wood is calledautumn wood or late wood.The spring wood is lighter in colour and has a lower density whereasthe autumn wood is darker and has a higher density. The two kinds ofwoods that appear as alternate concentric rings, constitute an annual ring.Annual rings seen in a cut stem give an estimate of the age of the tree.6.4.1.4 Heartwood and sapwoodIn old trees, the greater part of secondary xylem is dark brown due todeposition of organic compounds like tannins, resins, oils, gums, aromaticsubstances and essential oils in the central or innermost layers of the stem.These substances make it hard, durable and resistant to the attacks of microorganismsand insects. This region comprises dead elements with highlylignified walls and is called heartwood. The heartwood does not conductwater but it gives mechanical support to the stem. The peripheral region ofthe secondary xylem, is lighter in colour and is known as the sapwood. It isinvolved in the conduction of water and minerals from root to leaf.6.4.2 Cork CambiumAs the stem continues to increase in girth due to the activity of vascularcambium, the outer cortical and epidermis layers get broken and need tobe replaced to provide new protective cell layers. Hence, sooner or later,another meristematic tissue called cork cambium or phellogen develops,usually in the cortex region. Phellogen is a couple of layers thick. It ismade of narrow, thin-walled and nearly rectangular cells. Phellogen cutsoff cells on both sides. The outer cells differentiate into cork or phellemwhile the inner cells differentiate into secondary cortex or phelloderm.The cork is impervious to water due to suberin deposition in the cell wall.The cells of secondary cortex are parenchymatous. Phellogen, phellem,and phelloderm are collectively known as periderm. Due to activity ofthe cork cambium, pressure builds up on the remaining layers peripheral2022-23ANATOMY OF FLOWERING PLANTS 97Figure 6.10 (a) Lenticel and (b) Bark(b)(a)EpidermisComplimentarycellsCork cambiumSecondarycortex(a)to phellogen and ultimately theselayers die and slough off. Bark is anon-technical term that refers to alltissues exterior to the vascularcambium, therefore includingsecondary phloem. Bark refers to anumber of tissue types, viz.,periderm and secondary phloem.Bark that is formed early in theseason is called early or soft bark.Towards the end of the season, lateor hard bark is formed. Name thevarious kinds of cell layers whichconstitute the bark.At certain regions, the phellogencuts off closely arrangedparenchymatous cells on the outerside instead of cork cells. Theseparenchymatous cells soon rupturethe epidermis, forming a lensshapedopenings called lenticels.Lenticels permit the exchange ofgases between the outer atmosphereand the internal tissue of the stem.These occur in most woody trees(Figure 6.10).6.4.3 Secondary Growth inRootsIn the dicot root, the vascularcambium is completely secondary inorigin. It originates from the tissuelocated just below the phloembundles, a portion of pericycle tissue,above the protoxylem forming acomplete and continuous wavy ring,which later becomes circular (Figure6.11). Further events are similar tothose already described above for adicotyledon stem.2022-2398 BIOLOGYSecondary growth also occurs in stems and roots of gymnosperms.However, secondary growth does not occur in monocotyledons.EpidermisCortexPrimary phloemCambial ringEndodermisPericycleProtoxylemEpidermisVascular cambiumSecondary phloemPrimary xylemSecondary xylemCortexEpidermis/peridermCortexPrimary phloemAnnual ringSecondary xylemSecondaryphloem raysFigure 6.11 Different stages of the secondary growth in a typical dicot rootCortexSUMMARYAnatomically, a plant is made of different kinds of tissues. The plant tissues arebroadly classified into meristematic (apical, lateral and intercalary) and permanent(simple and complex). Assimilation of food and its storage, transportation of water,minerals and photosynthates, and mechanical support are the main functions oftissues. There are three types of tissue systems â epidermal, ground and vascular.The epidermal tissue systems are made of epidermal cells, stomata and theepidermal appendages. The ground tissue system forms the main bulk of theplant. It is divided into three zones â cortex, pericycle and pith. The vasculartissue system is formed by the xylem and phloem. On the basis of presence ofcambium, location of xylem and phloem, the vascular bundles are of differenttypes. The vascular bundles form the conducting tissue and translocate water,minerals and food material.2022-23ANATOMY OF FLOWERING PLANTS 99Monocotyledonous and dicotyledonous plants show marked variation in theirinternal structures. They differ in type, number and location of vascular bundles.The secondary growth occurs in most of the dicotyledonous roots and stems andit increases the girth (diameter) of the organs by the activity of the vascular cambiumand the cork cambium. The wood is actually a secondary xylem. There are differenttypes of wood on the basis of their composition and time of production.EXERCISES1. State the location and function of different types of meristems.2. Cork cambium forms tissues that form the cork. Do you agree with thisstatement? Explain.3. Explain the process of secondary growth in the stems of woody angiospermswith the help of schematic diagrams. What is its significance?4. Draw illustrations to bring out the anatomical difference between(a) Monocot root and Dicot root(b) Monocot stem and Dicot stem5. Cut a transverse section of young stem of a plant from your school garden andobserve it under the microscope. How would you ascertain whether it is amonocot stem or a dicot stem? Give reasons.6. The transverse section of a plant material shows the following anatomicalfeatures - (a) the vascular bundles are conjoint, scattered and surrounded by asclerenchymatous bundle sheaths. (b) phloem parenchyma is absent. Whatwill you identify it as?7. Why are xylem and phloem called complex tissues?8. What is stomatal apparatus? Explain the structure of stomata with a labelleddiagram.9. Name the three basic tissue systems in the flowering plants. Give the tissuenames under each system.10. How is the study of plant anatomy useful to us?11, What is periderm? How does periderm formation take place in the dicot stems?12. Describe the internal structure of a dorsiventral leaf with the help of labelleddiagrams.2022-23100 BIOLOGYIn the preceding chapters you came across a large variety of organisms,both unicellular and multicellular, of the animal kingdom. In unicellularorganisms, all functions like digestion, respiration and reproductionare performed by a single cell. In the complex body of multicellularanimals the same basic functions are carried out by different groups ofcells in a well organised manner. The body of a simple organism likeHydra is made of different types of cells and the number of cells in eachtype can be in thousands. The human body is composed of billions ofcells to perform various functions. How do these cells in the body worktogether? In multicellular animals, a group of similar cells alongwithintercellular substances perform a specific function. Such an organisationis called tissue.You may be surprised to know that all complex animals consist ofonly four basic types of tissues. These tissues are organised in specificproportion and pattern to form an organ like stomach, lung, heart andkidney. When two or more organs perform a common function by theirphysical and/or chemical interaction, they together form organ system,e.g., digestive system, respiratory system, etc. Cells, tissues, organs andorgan systems split up the work in a way that exhibits division of labourand contribute to the survival of the body as a whole.7.1 ANIMAL TISSUESThe structure of the cells vary according to their function. Therefore, thetissues are different and are broadly classified into four types : (i) Epithelial,(ii) Connective, (iii) Muscular and (iv) Neural.STRUCTURAL ORGANISATION INANIMALSCHAPTER 77.1 Animal Tissues7.2 Organ and OrganSystem7.3 Earthworm7.4 Cockroach7.5 Frogs2022-23STRUCTURAL ORGANISATION IN ANIMALS 1017.1.1 Epithelial TissueWe commonly refer to an epithelial tissue as epithelium (pl.: epithelia).This tissue has a free surface, which faces either a body fluid or the outsideenvironment and thus provides a covering or a lining for some part of thebody. The cells are compactly packed with little intercellular matrix. Thereare two types of epithelial tissues namely simple epithelium andcompound epithelium. Simple epithelium is composed of a single layerof cells and functions as a lining for body cavities, ducts, and tubes. Thecompound epithelium consists of two or more cell layers and has protectivefunction as it does in our skin.On the basis of structural modification of the cells, simple epitheliumis further divided into three types. These are (i) Squamous, (ii) Cuboidal,(iii) Columnar (Figure 7.1).The squamous epithelium is made of a single thin layer of flattenedcells with irregular boundaries. They are found in the walls of blood vesselsand air sacs of lungs and are involved in functions like forming a diffusionboundary. The cuboidal epithelium is composed of a single layer ofcube-like cells. This is commonly found in ducts of glands and tubularparts of nephrons in kidneys and its main functions are secretion andabsorption. The epithelium of proximal convoluted tubule (PCT) ofnephron in the kidney has microvilli. The columnar epithelium iscomposed of a single layer of tall and slender cells. Their nuclei are locatedat the base. Free surface may have microvilli. They are found in the liningof stomach and intestine and help in secretion and absorption. If thecolumnar or cuboidal cells bear cilia on their free surface they are calledciliated epithelium (Figure 7.1d). Their function is to move particles ormucus in a specific direction over the epithelium. They are mainly presentin the inner surface of hollow organs like bronchioles and fallopian tubes.Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar(d) Columnar cells bearing cilia(a)Flattened cellCube-like cellTall cell(b)(d)(c)2022-23102 BIOLOGYSome of the columnar or cuboidal cellsget specialised for secretion and are calledglandular epithelium (Figure 7.2). Theyare mainly of two types: unicellular,consisting of isolated glandular cells (gobletcells of the alimentary canal), andmulticellular, consisting of cluster of cells(salivary gland). On the basis of the mode ofpouring of their secretions, glands aredivided into two categories namelyexocrine and endocrine glands. Exocrineglands secrete mucus, saliva, earwax, oil,milk, digestive enzymes and other cellproducts. These products are releasedthrough ducts or tubes. In contrast,endocrine glands do not have ducts. Theirproducts called hormones are secreteddirectly into the fluid bathing the gland.Compound epithelium is made of morethan one layer (multi-layered) of cells and thushas a limited role in secretion and absorption(Figure 7.3). Their main function is to provideprotection against chemical and mechanicalstresses. They cover the dry surface of the skin,the moist surface of buccal cavity, pharynx,inner lining of ducts of salivary glands and ofpancreatic ducts.All cells in epithelium are held together with little intercellular material.In nearly all animal tissues, specialised junctions provide both structuraland functional links between its individual cells. Three types of cell junctionsare found in the epithelium and other tissues. These are called as tight,adhering and gap junctions. Tight junctions help to stop substancesfrom leaking across a tissue. Adhering junctions perform cementing tokeep neighbouring cells together. Gap junctions facilitate the cells tocommunicate with each other by connecting the cytoplasm of adjoiningcells, for rapid transfer of ions, small molecules and sometimes big molecules.7.1.2 Connective TissueConnective tissues are most abundant and widely distributed in the bodyof complex animals. They are named connective tissues because of theirspecial function of linking and supporting other tissues/organs of thebody. They range from soft connective tissues to specialised types, whichFigure 7.2 Glandular epithelium : (a) Unicellular(b) MulticellularunicellularglandMulticelluargland(a) (b)Figure 7.3 Compound epitheliumMultilayeredcells2022-23STRUCTURAL ORGANISATION IN ANIMALS 103Fat storageareaNucleusMacrophageMastcellFibroblastCollagenfibersPlasmaMembraneinclude cartilage, bone, adipose, and blood. In allconnective tissues except blood, the cells secrete fibres ofstructural proteins called collagen or elastin. The fibresprovide strength, elasticity and flexibility to the tissue.These cells also secrete modified polysaccharides, whichaccumulate between cells and fibres and act asmatrix (ground substance). Connective tissues areclassified into three types: (i) Loose connective tissue,(ii) Dense connective tissue and (iii) Specialisedconnective tissue.Loose connective tissue has cells and fibres looselyarranged in a semi-fluid ground substance, for example,areolar tissue present beneath the skin (Figure 7.4). Oftenit serves as a support framework for epithelium. Itcontains fibroblasts (cells that produce and secrete fibres),macrophages and mast cells. Adipose tissue is anothertype of loose connective tissue located mainly beneath theskin. The cells of this tissue are specialised to store fats.The excess of nutrients which are not used immediatelyare converted into fats and are stored in this tissue.Fibres and fibroblasts are compactly packed in thedense connective tissues. Orientation of fibres show aregular or irregular pattern and are called dense regularand dense irregular tissues. In the dense regularconnective tissues, the collagen fibres are present in rowsbetween many parallel bundles of fibres. Tendons, whichattach skeletal muscles to bones and ligaments whichattach one bone to another are examples of this tissue.Dense irregular connective tissue has fibroblasts andmany fibres (mostly collagen) that are oriented differently(Figure 7.5). This tissue is present in the skin. Cartilage,Figure 7.4 Loose connective tissue : (a) Areolar tissue (b) Adipose tissue(a)(b)fibresCollagen fibre(a)(b)Figure 7.5 Dense connective tissue:(a) Dense regular(b) Dense irregular2022-23104 BIOLOGYbones and blood are various types of specialisedconnective tissues.The intercellular material of cartilage is solid and pliableand resists compression. Cells of this tissue (chondrocytes)are enclosed in small cavities within the matrix secreted bythem (Figure 7.6a). Most of the cartilages in vertebrateembryos are replaced by bones in adults. Cartilage ispresent in the tip of nose, outer ear joints, between adjacentbones of the vertebral column, limbs and hands in adults.Bones have a hard and non-pliable ground substancerich in calcium salts and collagen fibres which give boneits strength (Figure 7.6b). It is the main tissue that providesstructural frame to the body. Bones support and protectsofter tissues and organs. The bone cells (osteocytes) arepresent in the spaces called lacunae. Limb bones, such asthe long bones of the legs, serve weight-bearing functions.They also interact with skeletal muscles attached to themto bring about movements. The bone marrow in some bonesis the site of production of blood cells.Blood is a fluid connective tissue containing plasma,red blood cells (RBC), white blood cells (WBC) and platelets(Figure 7.6c). It is the main circulating fluid that helps inthe transport of various substances. You will learn moreabout blood in Chapters 17 and 18.7.1.3 Muscle TissueEach muscle is made of many long, cylindrical fibresarranged in parallel arrays. These fibres are composed ofnumerous fine fibrils, called myofibrils. Muscle fibrescontract (shorten) in response to stimulation, then relax(lengthen) and return to their uncontracted state in acoordinated fashion. Their action moves the body to adjustto the changes in the environment and to maintain thepositions of the various parts of the body. In general,muscles play an active role in all the movements of the body.Muscles are of three types, skeletal, smooth, and cardiac.Skeletal muscle tissue is closely attached to skeletalbones. In a typical muscle such as the biceps, striated(striped) skeletal muscle fibres are bundled together in aparallel fashion (Figure 7.7a). A sheath of tough connectivetissue encloses several bundles of muscle fibres (You willlearn more about this in Chapter 20).PlateletsWBCRBC(a)(b)(c)Figure 7.6 Specialised connectivetissues : (a) Cartilage(b) Bone (c) Blood2022-23STRUCTURAL ORGANISATION IN ANIMALS 105The smooth muscle fibres taper at both ends (fusiform) and do notshow striations (Figure 7.7b). Cell junctions hold them together and theyare bundled together in a connective tissue sheath. The wall of internalorgans such as the blood vessels, stomach and intestine contains this typeof muscle tissue. Smooth muscles are âinvoluntaryâ as their functioningcannot be directly controlled. We usually are not able to make it contractmerely by thinking about it as we can do with skeletal muscles.Cardiac muscle tissue is a contractile tissue present only in the heart.Cell junctions fuse the plasma membranes of cardiac muscle cells andmake them stick together (Figure 7.7c). Communication junctions(intercalated discs) at some fusion points allow the cells to contract as aunit, i.e., when one cell receives a signal to contract, its neighbours arealso stimulated to contract.7.1.4 Neural TissueNeural tissue exerts the greatest control overthe bodyâs responsiveness to changingconditions. Neurons, the unit of neuralsystem are excitable cells (Figure 7.8). Theneuroglial cell which constitute the rest ofthe neural system protect and supportneurons. Neuroglia make up more than onehalfthe volume of neural tissue in our body.When a neuron is suitably stimulated,an electrical disturbance is generatedwhich swiftly travels along its plasmaNucleusStriationsJunctionbetweenadjacentcellsNucleusSmooth StriationsmusclefibersFigure 7.7 Muscle tissue : (a) Skeletal (striated) muscle tissue (b) Smooth muscle tissue(c) Cardiac muscle tissue(a) (b) (c)Figure 7.8 Neural tissue (Neuron withneuroglea)DendriteCellbodywithnucleusAxonNeuroglea2022-23106 BIOLOGYmembrane. Arrival of the disturbance at the neuronâs endings, or outputzone, triggers events that may cause stimulation or inhibition of adjacentneurons and other cells (You will study the details in Chapter 21).7.2 ORGAN AND ORGAN SYSTEMThe basic tissues mentioned above organise to form organs which in turnassociate to form organ systems in the multicellular organisms. Such anorganisation is essential for more efficient and better coordinated activitiesof millions of cells constituting an organism. Each organ in our body ismade of one or more type of tissues. For example, our heart consists of allthe four types of tissues, i.e., epithelial, connective, muscular and neural.We also notice, after some careful study that the complexity in organ andorgan systems displays certain discernable trend. This discernable trendis called evolutionary trend (You will study the details in class XII). Youare being introduced to morphology and anatomy of three organisms atdifferent evolutionary levels to show their organisation and functioning.Morphology refers to study of form or externally visible features. In thecase of plants or microbes, the term morphology precisely means onlythis. In case of animals this refers to the external appearance of the organsor parts of the body. The word anatomy conventionally is used for thestudy of morphology of internal organs in the animals. You will learn themorphology and anatomy of earthworm, cockroach and frog representinginvertebrates and vertebrates.7.3 EARTHWORMEarthworm is a reddish brown terrestrial invertebrate that inhabits theupper layer of the moist soil. During day time, they live in burrows madeby boring and swallowing the soil. In the gardens, they can be traced bytheir faecal deposits known as worm castings. The common Indianearthworms are Pheretima and Lumbricus.7.3.1 MorphologyEarthworms have long cylindrical body. The body is divided into morethan hundred short segments which are similar (metameres about100-120 in number). The dorsal surface of the body is marked by a darkmedian mid dorsal line (dorsal blood vessel) along the longitudinal axis ofthe body. The ventral surface is distinguished by the presence of genitalopenings (pores). Anterior end consists of the mouth and the prostomium,a lobe which serves as a covering for the mouth and as a wedge to forceopen cracks in the soil into which the earthworm may crawl. The prostomiumis sensory in function. The first body segment is called the peristomium(buccal segment) which contains the mouth. In a mature worm, segments2022-23STRUCTURAL ORGANISATION IN ANIMALS 10714-16 are covered by a prominent dark band of glandular tissue calledclitellum. Thus the body is divisible into three prominent regions âpreclitellar, clitellar and postclitellar segments (Figure 7.9).Four pairs of spermathecal apertures are situated on the ventro-lateralsides of the intersegmental grooves, i.e., 5th -9th segments. A single femalegenital pore is present in the mid-ventral line of 14th segment. A pair ofmale genital pores are present on the ventro-lateral sides of the 18thsegment. Numerous minute pores called nephridiopores open on thesurface of the body. In each body segment, except the first, last andclitellum, there are rows of S-shaped setae, embedded in the epidermalpits in the middle of each segment. Setae can be extended or retracted.Their principal role is in locomotion.7.3.2 AnatomyThe body wall of the earthworm is covered externally by a thin non-cellularcuticle below which is the epidermis, two muscle layers (circular andlongitudinal) and an innermost coelomic epithelium. The epidermis is madeFigure 7.9 Body of earthworm : (a) dorsal view (b) ventral view (c) lateral viewshowing mouth opening2022-23108 BIOLOGYup of a single layer of columnar epithelial cellswhich contain secretory gland cells.The alimentary canal is a straight tube andruns between first to last segment of the body.(Figure 7.10). A terminal mouth opens into thebuccal cavity (1-3 segments) which leads intomuscular pharynx. A small narrow tube,oesophagus (5-7 segments), continues into amuscular gizzard (8-9 segments). It helps ingrinding the soil particles and decaying leaves,etc. The stomach extends from 9-14 segments.The food of the earthworm is decaying leaves andorganic matter mixed with soil. Calciferousglands, present in the stomach, neutralise thehumic acid present in humus. Intestine startsfrom the 15th segment onwards and continuestill the last segment. A pair of short and conicalintestinal caecae project from the intestine on the26th segment. The characteristic feature of theintestine after 26th segment except the last23rd-25th segments is the presence of internalmedian fold of dorsal wall called typhlosole. Thisincreases the effective area of absorption in theintestine. The alimentary canal opens to theexterior by a small rounded aperture called anus.The ingested organic rich soil passes through thedigestive tract where digestive enzymesbreakdown complex food into smaller absorbableunits. These simpler molecules are absorbedthrough intestinal membranes and are utilised.Pheretima exhibits a closed type ofblood vascular system, consisting of bloodvessels, capillaries and heart. (Figure 7.11). Dueto closed circulatory system, blood is confinedto the heart and blood vessels. Contractionskeep blood circulating in one direction. Smallerblood vessels supply the gut, nerve cord, andthe body wall. Blood glands are present on the4th, 5th and 6th segments. They produce bloodcells and haemoglobin which is dissolved inblood plasma. Blood cells are phagocytic innature. Earthworms lack specialised breathingdevices. Respiratory exchange occurs throughmoist body surface into their blood stream.MouthPharynxOesophagusGizzardStomachPre-typhlosolarpart of intestineIntestinalcaecumLymph glandTyphlosolarpart of intestineIntestinal lumenTyphlosole12345678910111213141516171819202122232425262728293031323334Figure 7.10 Alimentary canal of earthworm2022-23STRUCTURAL ORGANISATION IN ANIMALS 109The excretory organs occur as segmentallyarranged coiled tubules called nephridia(sing.: nephridium). They are of three types:(i) septal nephridia, present on both the sides ofintersegmental septa of segment 15 to the lastthat open into intestine, (ii) integumentarynephridia, attached to lining of the body wall ofsegment 3 to the last that open on the bodysurface and (iii) pharyngeal nephridia, presentas three paired tufts in the 4th, 5th and 6thsegments (Figure 7.12). These different types ofnephridia are basically similar in structure.Nephridia regulate the volume and compositionof the body fluids. A nephridium starts out as afunnel that collects excess fluid from coelomicchamber. The funnel connects with a tubular partof the nephridium which delivers the wastesthrough a pore to the surface in the body wallinto the digestive tube.Nervous system is basically represented byganglia arranged segmentwise on the ventralpaired nerve cord. The nerve cord in the anteriorregion (3rd and 4th segments) bifurcates, laterallyencircling the pharynx and joins the cerebralganglia dorsally to form a nerve ring. The cerebralganglia alongwith other nerves in the ringintegrate sensory input as well as commandmuscular responses of the body.Figure 7.11 Closed circulatory systemFigure 7.12 Nephridial system in earthworm2022-23110 BIOLOGYSensory system does not have eyes but doespossess light and touch sensitive organs (receptorcells) to distinguish the light intensities and to feelthe vibrations in the ground. Worms havespecialised chemoreceptors (taste receptors) whichreact to chemical stimuli. These sense organs arelocated on the anterior part of the worm.Earthworm is hermaphrodite (bisexual), i.e.,testes and ovaries are present in the sameindividual (Figure 7.13). There are two pairs oftestes present in the 10th and 11th segments.Their vasa deferentia run up to the 18th segmentwhere they join the prostatic duct. Two pairs ofaccessory glands are present one pair each inthe 17th and 19th segments. The common prostateand spermatic duct (vasa deferentia) opens tothe exterior by a pair of male genital pores onthe ventro-lateral side of the 18th segment. Fourpairs of spermathecae are located in 6th-9thsegments (one pair in each segment). They receiveand store spermatozoa during copulation. Onepair of ovaries is attached at the inter-segmentalseptum of the 12th and 13th segments. Ovarianfunnels are present beneath the ovaries whichcontinue into oviduct, join together and open onthe ventral side as a single median female genitalpore on the 14th segment.A mutual exchange of sperm occurs betweentwo worms during mating. One worm has to findanother worm and they mate juxtaposingopposite gonadal openings exchanging packetsof sperms called spermatophores. Mature spermand egg cells and nutritive fluid are deposited incocoons produced by the gland cells of clitellum.Fertilisation and development occur within thecocoons which are deposited in soil. The ova(eggs) are fertilised by the sperm cells within thecocoon which then slips off the worm and isdeposited in or on the soil. The cocoon holds theworm embryos. After about 3 weeks, each cocoonproduces two to twenty baby worms with anaverage of four. Development of earthworms isdirect, i.e., there is no larva formed.Figure 7.13 Reproductive system of earthworm2022-23STRUCTURAL ORGANISATION IN ANIMALS 111Earthworms are known as âfriends of farmersâ because they makeburrows in the soil and make it porous which helps in respiration andpenetration of the developing plant roots. The process of increasing fertilityof soil by the earthworms is called vermicomposting. They are also usedas bait in game fishing.7.4 COCKROACHCockroaches are brown or black bodied animals that are included inclass Insecta of Phylum Arthropoda. Bright yellow, red and green colouredcockroaches have also been reported in tropical regions. Their size rangesfrom Âŧ inches to 3 inches (0.6-7.6 cm) and have long antenna, legs andflat extension of the upper body wall that conceals head. They arenocturnal omnivores that live in damp places throughout the world. Theyhave become residents of human homes and thus are serious pests andvectors of several diseases.7.4.1 MorphologyThe adults of the common species of cockroach, Periplaneta americanaare about 34-53 mm long with wings that extend beyond the tip of theabdomen in males. The body of the cockroach is segmented and divisibleinto three distinct regions â head, thorax and abdomen (Figure 7.14).The entire body is covered by a hard chitinous exoskeleton (brown incolour). In each segment, exoskeleton has hardened plates called sclerites(tergites dorsally and sternites ventrally) that are joined to each other bya thin and flexible articular membrane (arthrodial membrane).Figure 7.14 External features of cockroach2022-23112 BIOLOGYHead is triangular in shape and lies anteriorly at right angles to thelongitudinal body axis. It is formed by the fusion of six segments andshows great mobility in all directions due to flexible neck (Figure 7.15).The head capsule bears a pair of compound eyes. A pair of thread likeantennae arise from membranous sockets lying in front of eyes. Antennaehave sensory receptors that help in monitoring the environment. Anteriorend of the head bears appendages forming biting and chewing type ofmouth parts. The mouthparts consisting of a labrum (upper lip), a pairof mandibles, a pair of maxillae and a labium (lower lip). A median flexiblelobe, acting as tongue (hypopharynx), lies within the cavity enclosed bythe mouthparts (Figure 7.15b). Thorax consists of three parts â prothorax,mesothorax and metathorax. The head is connected with thorax by ashort extension of the prothorax known as the neck. Each thoracic segmentbears a pair of walking legs. The first pair of wings arises from mesothoraxand the second pair from metathorax. Forewings (mesothoracic) calledtegmina are opaque dark and leathery and cover the hind wings when atrest. The hind wings are transparent, membranous and are used in flight.The abdomen in both males and females consists of 10 segments. Infemales, the 7th sternum is boat shaped and together with the 8th and 9thsterna forms a brood or genital pouch whose anterior part contains femalegonopore, spermathecal pores and collateral glands. In males, genital pouchor chamber lies at the hind end of abdomen bounded dorsally by 9th and10th terga and ventrally by the 9th sternum. It contains dorsal anus, ventralmale genital pore and gonapophysis. Males bear a pair of short, threadlikeanal styles which are absent in females. In both sexes, the 10th segmentbears a pair of jointed filamentous structures called anal cerci.OcellusCompound eyeMandibleMaxillaLabrum(a) LabiumFigure 7.15 Head region of cockroach : (a) parts of head region (b) mouth partsGrindingregionIncisingMandible regionMaxillaLabrumHypopharynxLabiumMandibleMaxilla(b)2022-23STRUCTURAL ORGANISATION IN ANIMALS 1137.4.2 AnatomyThe alimentary canal present in the body cavityis divided into three regions: foregut, midgutand hindgut (Figure 7.16). The mouth opensinto a short tubular pharynx, leading to anarrow tubular passage called oesophagus.This in turn opens into a sac like structurecalled crop used for storing of food. The cropis followed by gizzard or proventriculus. It hasan outer layer of thick circular muscles andthick inner cuticle forming six highly chitinousplate called teeth. Gizzard helps in grinding thefood particles. The entire foregut is lined bycuticle. A ring of 6-8 blind tubules calledhepatic or gastric caeca is present at thejunction of foregut and midgut, which secretedigestive juice. At the junction of midgut andhindgut is present another ring of 100-150yellow coloured thin filamentous Malpighiantubules. They help in removal of excretoryproducts from haemolymph. The hindgut isbroader than midgut and is differentiated intoileum, colon and rectum. The rectum opensout through anus.Blood vascular system of cockroach is anopen type (Figure 7.17). Blood vessels arepoorly developed and open into space(haemocoel). Visceral organs located in thehaemocoel are bathed in blood (haemolymph).The haemolymph is composed of colourlessplasma and haemocytes. Heart of cockroachconsists of elongated muscular tube lyingalong mid dorsal line of thorax and abdomen.It is differentiated into funnel shaped chamberswith ostia on either side. Blood from sinusesenter heart through ostia and is pumpedanteriorly to sinuses again.The respiratory system consists of anetwork of trachea, that open through 10 pairsof small holes called spiracles present on thelateral side of the body. Thin branching tubes(tracheal tubes subdivided into tracheoles)carry oxygen from the air to all the parts. TheSalivary glandPharynxSalivaryreservoirOesophagusCropGizzardHepatic caecaMesenteronor midgutMalpighiantubulesIleumColonRectumFigure 7.16 Alimentary canal of cockroachAnterior aortaAlary musclesChambersof heartFigure 7.17 Open circulatory system of cockroach2022-23114 BIOLOGYopening of the spiracles is regulated by the sphincters. Exchange of gasestake place at the tracheoles by diffusion.Excretion is performed by Malpighian tubules. Each tubule is linedby glandular and ciliated cells. They absorb nitrogenous waste productsand convert them into uric acid which is excreted out through the hindgut.Therefore, this insect is called uricotelic. In addition, the fat body,nephrocytes and urecose glands also help in excretion.The nervous system of cockroach consists of a series of fused,segmentally arranged ganglia joined by paired longitudinal connectiveson the ventral side. Three ganglia lie in the thorax, and six in the abdomen.The nervous system of cockroach is spread throughout the body. Thehead holds a bit of a nervous system while the rest is situated along theventral (belly-side) part of its body. So, now you understand that if thehead of a cockroach is cut off, it will still live for as long as one week. Inthe head region, the brain is represented by supra-oesophageal ganglionwhich supplies nerves to antennae and compound eyes. In cockroach,the sense organs are antennae, eyes, maxillary palps, labial palps, analcerci, etc. The compound eyes are situated at the dorsal surface of thehead. Each eye consists of about 2000 hexagonal ommatidia(sing.: ommatidium). With the help of several ommatidia, a cockroach canreceive several images of an object. This kind of vision is known as mosaicvision with more sensitivity but less resolution, being common duringnight (hence called nocturnal vision).Cockroaches are dioecious and both sexes have well developedreproductive organs (Figure 7.18). Male reproductive system consists ofa pair of testes one lying on each lateral side in the 4th -6th abdominalsegments. From each testis arises a thin vas deferens, which opens intoejaculatory duct through seminal vesicle. The ejaculatory duct opens intomale gonopore situated ventral to anus. A characteristic mushroomshapedgland is present in the 6th-7th abdominal segments which functionsas an accessory reproductive gland. The external genitalia are representedby male gonapophysis or phallomere (chitinous asymmetrical structures,surrounding the male gonopore). The sperms are stored in the seminalvesicles and are glued together in the form of bundles calledspermatophores which are discharged during copulation. The femalereproductive sysytem consists of two large ovaries, lying laterally in the2nd â 6th abdominal segments. Each ovary is formed of a group of eightovarian tubules or ovarioles, containing a chain of developing ova.Oviducts of each ovary unite into a single median oviduct (also calledvagina) which opens into the genital chamber. A pair of spermatheca ispresent in the 6th segment which opens into the genital chamber.Sperms are transferred through spermatophores. Their fertilised eggsare encased in capsules called oothecae. Ootheca is a dark reddish toblackish brown capsule, about 3/8" (8 mm) long. They are dropped or2022-23STRUCTURAL ORGANISATION IN ANIMALS 115glued to a suitable surface, usually in a crack or crevice of high relativehumidity near a food source. On an average, females produce 9-10oothecae, each containing 14-16 eggs. The development of P. americanais paurometabolous, meaning there is development through nymphalstage. The nymphs look very much like adults. The nymph grows bymoulting about 13 times to reach the adult form. The next to last nymphalstage has wing pads but only adult cockroaches have wings.Many species of cockroaches are wild and are of no known economicimportance yet. A few species thrive in and around human habitat. They arepests because they spoil food and contaminate it with their smelly excreta.They can transmit a variety of bacterial diseases by contaminating food material.TestisPhallic glandSmall tubulesLong tubulesSeminal vesicleVas deferensEjaculatory ductRight phallomereVentral phallomereAnal cercusCaudal stylePseudopenisTitillatorLeft phallomereOvaryOviductCommon oviductor vaginaCollaterial glandsGenital chamberVestibulumGenitalpouchSpermathecagonapophyses ]Figure 7.18 Reproductive system of cockroach : (a) male (b) female(a)(b)2022-23116 BIOLOGY7.5 FROGSFrogs can live both on land and in freshwater and belong to classAmphibia of phylum Chordata. The most common species of frog foundin India is Rana tigrina.They do not have constant body temperature i.e., their bodytemperature varies with the temperature of the environment. Such animalsare called cold blooded or poikilotherms. You might have also noticedchanges in the colour of the frogs while they are in grasses and on dryland. They have the ability to change the colour to hide them from theirenemies (camouflage). This protective coloration is called mimicry. Youmay also know that frogs are not seen during peak summer and winter.During this period they take shelter in deep burrows to protect themfrom extreme heat and cold. This is known as summer sleep (aestivation)and winter sleep (hibernation) respectively.7.5.1 MorphologyHave you ever touched the skin of frog? The skin is smooth and slipperydue to the presence of mucus. The skin is always maintained in a moistcondition. The colour of dorsal side of body is generally olive green withdark irregular spots. On the ventral side the skin is uniformly pale yellow.The frog never drinks water but absorb it through the skin.Body of a frog is divisible into head and trunk (Figure 7.19). A neckand tail are absent. Above the mouth, a pair of nostrils is present. Eyesare bulged and covered by a nictitating membrane that protects themwhile in water. On either side of eyes a membranoustympanum (ear) receives sound signals. Theforelimbs and hind limbs help in swimming,walking, leaping and burrowing. The hind limbs endin five digits and they are larger and muscular thanfore limbs that end in four digits. Feet have webbeddigits that help in swimming. Frogs exhibit sexualdimorphism. Male frogs can be distinguished by thepresence of sound producing vocal sacs and also acopulatory pad on the first digit of the fore limbswhich are absent in female frogs.7.5.2 AnatomyThe body cavity of frogs accommodate different organ systems such asdigestive, circulatory, respiratory, nervous, excretory and reproductivesystems with well developed structures and functions (Figure 7.20).The digestive system consists of alimentary canal and digestive glands.The alimentary canal is short because frogs are carnivores and hence thelength of intestine is reduced. The mouth opens into the buccal cavity thatFigure 7.19 External features of frogEyeFore limbHind limbHeadTrunk2022-23STRUCTURAL ORGANISATION IN ANIMALS 117leads to the oesophagus through pharynx. Oesophagus is a short tubethat opens into the stomach which in turn continues as the intestine, rectumand finally opens outside by the cloaca. Liver secretes bile that is stored inthe gall bladder. Pancreas, a digestive gland produces pancreatic juicecontaining digestive enzymes. Food is captured by the bilobed tongue.Digestion of food takes place by the action of HCl and gastric juices secretedfrom the walls of the stomach. Partially digested food called chyme is passedfrom stomach to the first part of the small intestine, the duodenum. Theduodenum receives bile from gall bladder and pancreatic juices from thepancreas through a common bile duct. Bile emulsifies fat and pancreaticjuices digest carbohydrates and proteins. Final digestion takes place in theintestine. Digested food is absorbed by the numerous finger-like folds inthe inner wall of intestine called villi and microvilli. The undigested solidwaste moves into the rectum and passes out through cloaca.Frogs respire on land and in the water by two different methods. Inwater, skin acts as aquatic respiratory organ (cutaneous respiration).Dissolved oxygen in the water is exchanged through the skin by diffusion.Figure 7.20 Diagrammatic representation of internal organs of frog showingcomplete digestive systemUreter IntestineRectumCloacaUrinarybladderOesophagusLiverStomachKidneyGallbladderFat bodiesLungHeartCloacal Aperture2022-23118 BIOLOGYOn land, the buccal cavity, skin and lungs act as the respiratory organs.The respiration by lungs is called pulmonary respiration. The lungs area pair of elongated, pink coloured sac-like structures present in the upperpart of the trunk region (thorax). Air enters through the nostrils into thebuccal cavity and then to lungs. During aestivation and hibernationgaseous exchange takes place through skin.The vascular system of frog is well-developed closed type. Frogs havea lymphatic system also. The blood vascular system involves heart, bloodvessels and blood. The lymphatic system consists of lymph, lymphchannels and lymph nodes. Heart is a muscular structure situated in theupper part of the body cavity. It has three chambers, two atria and oneventricle and is covered by a membrane called pericardium. A triangularstructure called sinus venosus joins the right atrium. It receives bloodthrough the major veins called vena cava. The ventricle opens into a saclikeconus arteriosus on the ventral side of the heart. The blood from theheart is carried to all parts of the body by the arteries (arterial system).The veins collect blood from different parts of body to the heart and formthe venous system. Special venous connection between liver and intestineas well as the kidney and lower parts of the body are present in frogs. Theformer is called hepatic portal system and the latter is called renal portalsystem. The blood is composed of plasma and cells. The blood cells areRBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytesand platelets. RBCâs are nucleated and contain red coloured pigmentnamely haemoglobin. The lymph is different from blood. It lacks fewproteins and RBCs. The blood carries nutrients, gases and water to therespective sites during the circulation. The circulation of blood is achievedby the pumping action of the muscular heart.The elimination of nitrogenous wastes is carried out by a welldeveloped excretory system. The excretory system consists of a pair ofkidneys, ureters, cloaca and urinary bladder. These are compact, darkred and bean like structures situated a little posteriorly in the body cavityon both sides of vertebral column. Each kidney is composed of severalstructural and functional units called uriniferous tubules or nephrons.Two ureters emerge from the kidneys in the male frogs. The ureters act asurinogenital duct which opens into the cloaca. In females the ureters andoviduct open seperately in the cloaca. The thin-walled urinary bladder ispresent ventral to the rectum which also opens in the cloaca. The frogexcretes urea and thus is a ureotelic animal. Excretory wastes are carriedby blood into the kidney where it is separated and excreted.The system for control and coordination is highly evolved in the frog. Itincludes both neural system and endocrine glands. The chemicalcoordination of various organs of the body is achieved by hormones whichare secreted by the endocrine glands. The prominent endocrine glandsfound in frog are pituitary, thyroid, parathyroid, thymus, pineal body,pancreatic islets, adrenals and gonads. The nervous system is organised2022-23STRUCTURAL ORGANISATION IN ANIMALS 119into a central nervous system (brain and spinalcord), a peripheral nervous system (cranial andspinal nerves) and an autonomic nervous system(sympathetic and parasympathetic). There are tenpairs of cranial nerves arising from the brain. Brainis enclosed in a bony structure called brain box(cranium). The brain is divided into fore-brain,mid-brain and hind-brain. Forebrain includesolfactory lobes, paired cerebral hemispheres andunpaired diencephalon. The midbrain ischaracterised by a pair of optic lobes. Hind-brainconsists of cerebellum and medulla oblongata.The medulla oblongata passes out through theforamen magnum and continues into spinal cord,which is enclosed in the vertebral column.Frog has different types of sense organs, namelyorgans of touch (sensory papillae), taste (tastebuds), smell (nasal epithelium), vision (eyes) andhearing (tympanum with internal ears). Out ofthese, eyes and internal ears are well-organisedstructures and the rest are cellular aggregationsaround nerve endings. Eyes in a frog are a pair ofspherical structures situated in the orbit in skull.These are simple eyes (possessing only one unit).External ear is absent in frogs and only tympanumcan be seen externally. The ear is an organ ofhearing as well as balancing (equilibrium).Frogs have well organised male and femalereproductive systems. Male reproductive organsconsist of a pair of yellowish ovoid testes (Figure7.21), which are found adhered to the upper partof kidneys by a double fold of peritoneum calledmesorchium. Vasa efferentia are 10-12 innumber that arise from testes. They enter thekidneys on their side and open into Bidderâscanal. Finally it communicates with theurinogenital duct that comes out of the kidneysand opens into the cloaca. The cloaca is a small,median chamber that is used to pass faecalmatter, urine and sperms to the exterior.The female reproductive organs include a pairof ovaries (Figure 7.22). The ovaries are situatednear kidneys and there is no functionalconnection with kidneys. A pair of oviduct arising Figure 7.22 Female reproductive systemOviductOvaryOvaUreterCloacaCloacal apertureUrinarybladderFigure 7.21 Male reproductive systemFatbodiesKidneyUrinogenital ductCloacaCloacalapertureTestisAdrenalglandUrinarybladderRectumVasaefferentia2022-23120 BIOLOGYfrom the ovaries opens into the cloaca separately. A mature female canlay 2500 to 3000 ova at a time. Fertilisation is external and takes place inwater. Development involves a larval stage called tadpole. Tadpoleundergoes metamorphosis to form the adult.Frogs are beneficial for mankind because they eat insects and protectthe crop. Frogs maintain ecological balance because these serve as animportant link of food chain and food web in the ecosystem. In somecountries the muscular legs of frog are used as food by man.SUMMARYCells, tissues, organs and organ systems split up the work in a way that ensuresthe survival of the body as a whole and exhibit division of labour. A tissue isdefined as group of cells along with intercellular substances performing one ormore functions in the body. Epithelia are sheet like tissues lining the bodyâs surfaceand its cavities, ducts and tubes. Epithelia have one free surface facing a bodyfluid or the outside environment. Their cells are structurally and functionallyconnected at junctions.Diverse types of connective tissues bind together, support, strengthen, protect,and insulate other tissue in the body. Soft connective tissues consist of proteinfibres as well as a variety of cells arranged in a ground substance. Cartilage, bone,blood, and adipose tissue are specialised connective tissues. Cartilage and boneare both structural materials. Blood is a fluid tissue with transport functions.Adipose tissue is a reservoir of stored energy. Muscle tissue, which can contract(shorten) in response to stimulation, helps in movement of the body and specificbody parts. Skeletal muscle is the muscle tissue attached to bones. Smooth muscleis a component of internal organs. Cardiac muscle makes up the contractile wallsof the heart. Connective tissue covers all three types of tissues. Nervous tissueexerts greatest control over the response of body. Neurons are the basic units ofnervous tissue.Earthworm, Cockroach and Frog show characteristic features in bodyorganisation. In Pheretima posthuma (earthworm), the body is covered by cuticle.All segments of its body are alike except the 14th, 15th and 16th segment, which arethick and dark and glandular, forming clitellum. A ring of S-shaped chitinoussetae is found in each segment. These setae help in locomotion. On the ventralside spermathecal openings are present in between the grooves of 5 and 6, 6 and7, 7 and 8 and 8 and 9 segments. Female genital pores are present on 14th segmentand male genital pores on 18th segment. The alimentary canal is a narrow tubemade of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus.The blood vascular system is of closed type with heart and valves. Nervous systemis represented by ventral nerve cord. Earthworm is hermaphorodite. Two pairs of2022-23STRUCTURAL ORGANISATION IN ANIMALS 121testes occur in the 10th and 11th segment, respectively. A pair of ovaries are presenton 12 and 13th intersegmental septum. It is a protandrous animal with crossfertilisation.Fertilisation and development take place in cocoon secreted by theglands of clitellum.The body of Cockroach (Periplaneta americana) is covered by chitinousexoskeleton. It is divided into head, thorax and abdomen. Segments bear jointedappendages. There are three segments of thorax, each bearing a pair of walkinglegs. Two pairs of wings are present, one pair each on 2nd and 3rd segment. Thereare ten segments in abdomen. Alimentary canal is well developed with a mouthsurrounded by mouth parts, a pharynx, oesophagus, crop, gizzard, midgut,hindgut and anus. Hepatic caecae are present at the junction of foregut andmidgut. Malpighian tubules are present at the junction of midgut and hindgutand help in excretion. A pair of salivary gland is present near crop. The bloodvascular system is of open type. Respiration takes place by network of tracheae.Trachea opens outside with spiracles. Nervous system is represented bysegmentally arranged ganglia and ventral nerve cord. A pair of testes is present in4th-6th segments and ovaries in 2nd-6th segments. Fertilisation is internal. Femaleproduces 9-10 ootheca bearing developing embryos. After rupturing of singleootheca sixteen young ones, called nymphs come out.The Indian bullfrog, Rana tigrina, is the common frog found in India. Body iscovered by skin. Mucous glands are present in the skin which is highly vascularisedand helps in respiration in water and on land. Body is divisible into head and trunk.A muscular tongue is present, which is bilobed at the tip and is used in capturingthe prey. The alimentary canal consists of oesophagous, stomach, intestine andrectum, which open into the cloaca. The main digestive glands are liver and pancreas.It can respire in water through skin and through lungs on land. Circulatory systemis closed with single circulation. RBCs are nucleated. Nervous system is organisedinto central, peripheral and autonomic. The organs of urinogenital system are kidneysand urinogenital ducts, which open into the cloaca. The male reproductive organ isa pair of testes. The female reproductive organ is a pair of ovaries. A female lays2500-3000 ova at a time. The fertilisation and development are external. The eggshatch into tadpoles, which metamorphose into frogs.EXERCISES1. Answer in one word or one line.(i) Give the common name of Periplanata americana.(ii) How many spermathecae are found in earthworm?(iii) What is the position of ovaries in cockroach?(iv) How many segments are present in the abdomen of cockroach?(v) Where do you find Malpighian tubules?2022-23122 BIOLOGY2. Answer the following:(i) What is the function of nephridia?(ii) How many types of nephridia are found in earthworm based ontheir location?3. Draw a labelled diagram of the reproductive organs of an earthworm.4. Draw a labelled diagram of alimentary canal of a cockroach.5. Distinguish between the followings(a) Prostomium and peristomium(b) Septal nephridium and pharyngeal nephridium6. What are the cellular components of blood?7. What are the following and where do you find them in animal body.(a) Chondriocytes(b) Axons(c) Ciliated epithelium8. Describe various types of epithelial tissues with the help of labelled diagrams.9. Distinguish between(a) Simple epithelium and compound epithelium(b) Cardiac muscle and striated muscle(c) Dense regular and dense irregular connective tissues(d) Adipose and blood tissue(e) Simple gland and compound gland10. Mark the odd one in each series:(a) Areolar tissue; blood; neuron; tendon(b) RBC; WBC; platelets; cartilage(c) Exocrine; endocrine; salivary gland; ligament(d) Maxilla; mandible; labrum; antennae(e) Protonema; mesothorax; metathorax; coxa11. Match the terms in column I with those in column II:Column I Column II(a) Compound epithelium (i) Alimentary canal(b) Compound eye (ii) Cockroach(c) Septal nephridia (iii) Skin(d) Open circulatory system (iv) Mosaic vision(e) Typhlosole (v) Earthworm(f) Osteocytes (vi) Phallomere(g) Genitalia (vii) Bone12. Mention breifly about the circulatory system of earthworm13. Draw a neat diagram of digestive system of frog.14. Mention the function of the following(a) Ureters in frog(b) Malpighian tubules(c) Body wall in earthworm2022-23UNIT 3Biology is the study of living organisms. The detailed description oftheir form and appearance only brought out their diversity. It is thecell theory that emphasised the unity underlying this diversity of forms,i.e., the cellular organisation of all life forms. A description of cellstructure and cell growth by division is given in the chapters comprisingthis unit. Cell theory also created a sense of mystery around livingphenomena, i.e., physiological and behavioural processes. This mysterywas the requirement of integrity of cellular organisation for livingphenomena to be demonstrated or observed. In studying andunderstanding the physiological and behavioural processes, one cantake a physico-chemical approach and use cell-free systems toinvestigate. This approach enables us to describe the various processesin molecular terms. The approach is established by analysis of livingtissues for elements and compounds. It will tell us what types of organiccompounds are present in living organisms. In the next stage, one canask the question: What are these compounds doing inside a cell? And,in what way they carry out gross physiological processes like digestion,excretion, memory, defense, recognition, etc. In other words we answerthe question, what is the molecular basis of all physiological processes?It can also explain the abnormal processes that occur during anydiseased condition. This physico-chemical approach to study andunderstand living organisms is called âReductionist Biologyâ. Theconcepts and techniques of physics and chemistry are applied tounderstand biology. In Chapter 9 of this unit, a brief description ofbiomolecules is provided.CELL: STRUCTURE AND FUNCTIONSChapter 8Cell: The Unit of LifeChapter 9BiomoleculesChapter 10Cell Cycle andCell Division2022-23G.N. RAMACHANDRAN, an outstanding figure in the field of proteinstructure, was the founder of the âMadras schoolâ ofconformational analysis of biopolymers. His discovery of the triplehelical structure of collagen published in Nature in 1954 and hisanalysis of the allowed conformations of proteins through theuse of the âRamachandran plotâ rank among the most outstandingcontributions in structural biology. He was born on October 8,1922, in a small town, not far from Cochin on the southwesterncoast of India. His father was a professor of mathematics at alocal college and thus had considerable influence in shapingRamachandranâs interest in mathematics. After completing hisschool years, Ramachandran graduated in 1942 as the toprankingstudent in the B.Sc. (Honors) Physics course of theUniversity of Madras. He received a Ph.D. from CambridgeUniversity in 1949. While at Cambridge, Ramachandran metLinus Pauling and was deeply influenced by his publications onmodels of the a-helix and b-sheet structures that directed hisattention to solving the structure of collagen. He passed away atG.N. Ramachandran the age of 78, on April 7, 2001.(1922 â 2001)2022-23When you look around, you see both living and non-living things. Youmust have wondered and asked yourself â âwhat is it that makes anorganism living, or what is it that an inanimate thing does not have whicha living thing hasâ ? The answer to this is the presence of the basic unit oflife â the cell in all living organisms.All organisms are composed of cells. Some are composed of a singlecell and are called unicellular organisms while others, like us, composedof many cells, are called multicellular organisms.8.1 WHAT IS A CELL?Unicellular organisms are capable of (i) independent existence and(ii) performing the essential functions of life. Anything less than a completestructure of a cell does not ensure independent living. Hence, cell is thefundamental structural and functional unit of all living organisms.Anton Von Leeuwenhoek first saw and described a live cell. RobertBrown later discovered the nucleus. The invention of the microscope andits improvement leading to the electron microscope revealed all thestructural details of the cell.8.2 CELL THEORYIn 1838, Matthias Schleiden, a German botanist, examined a large numberof plants and observed that all plants are composed of different kinds ofcells which form the tissues of the plant. At about the same time, TheodoreCELL: THE UNIT OF LIFECHAPTER 88.1 What is a Cell?8.2 Cell Theory8.3 An Overview ofCell8.4 Prokaryotic Cells8.5 Eukaryotic Cells2022-23126 BIOLOGYSchwann (1839), a British Zoologist, studied different types of animal cellsand reported that cells had a thin outer layer which is today known as theâplasma membraneâ. He also concluded, based on his studies on planttissues, that the presence of cell wall is a unique character of the plantcells. On the basis of this, Schwann proposed the hypothesis that the bodiesof animals and plants are composed of cells and products of cells.Schleiden and Schwann together formulated the cell theory. This theoryhowever, did not explain as to how new cells were formed. Rudolf Virchow(1855) first explained that cells divided and new cells are formed frompre-existing cells (Omnis cellula-e cellula). He modified the hypothesis ofSchleiden and Schwann to give the cell theory a final shape. Cell theoryas understood today is:(i) all living organisms are composed of cells and products of cells.(ii) all cells arise from pre-existing cells.8.3 AN OVERVIEW OF CELLYou have earlier observed cells in an onion peel and/or human cheekcells under the microscope. Let us recollect their structure. The onion cellwhich is a typical plant cell, has a distinct cell wall as its outer boundaryand just within it is the cell membrane. The cells of the human cheekhave an outer membrane as the delimiting structure of the cell. Insideeach cell is a dense membrane bound structure called nucleus. Thisnucleus contains the chromosomes which in turn contain the geneticmaterial, DNA. Cells that have membrane bound nuclei are calledeukaryotic whereas cells that lack a membrane bound nucleus areprokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrixcalled cytoplasm occupies the volume of the cell. The cytoplasm is themain arena of cellular activities in both the plant and animal cells. Variouschemical reactions occur in it to keep the cell in the âliving stateâ.Besides the nucleus, the eukaryotic cells have other membrane bounddistinct structures called organelles like the endoplasmic reticulum (ER),the golgi complex, lysosomes, mitochondria, microbodies and vacuoles.The prokaryotic cells lack such membrane bound organelles.Ribosomes are non-membrane bound organelles found in all cells âboth eukaryotic as well as prokaryotic. Within the cell, ribosomes arefound not only in the cytoplasm but also within the two organelles âchloroplasts (in plants) and mitochondria and on rough ER.Animal cells contain another non-membrane bound organelle calledcentrosome which helps in cell division.Cells differ greatly in size, shape and activities (Figure 8.1). For example,Mycoplasmas, the smallest cells, are only 0.3 Îŧm in length while bacteria2022-23CELL: THE UNIT OF LIFE 127Red blood cells(round and biconcave)White blood cells(Branched and long)Columnar epithelial cells(amoeboid) (long and narrow)Nerve cellMesophyll cells(round and oval)A tracheid(elongated)Figure 8.1 Diagram showing different shapes of the cellscould be 3 to 5 Îŧm. The largest isolated single cell is the egg of an ostrich.Among multicellular organisms, human red blood cells are about 7.0Îŧm in diameter. Nerve cells are some of the longest cells. Cells also varygreatly in their shape. They may be disc-like, polygonal, columnar, cuboid,thread like, or even irregular. The shape of the cell may vary with thefunction they perform.8.4 PROKARYOTIC CELLSThe prokaryotic cells are represented by bacteria, blue-green algae,mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They aregenerally smaller and multiply more rapidly than the eukaryotic cells(Figure 8.2). They may vary greatly in shape and size. The four basicshapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (commashaped) and spirillum (spiral).The organisation of the prokaryotic cell is fundamentally similar eventhough prokaryotes exhibit a wide variety of shapes and functions. All2022-23128 BIOLOGYprokaryotes have a cell wall surrounding thecell membrane except in mycoplasma. The fluidmatrix filling the cell is the cytoplasm. There isno well-defined nucleus. The genetic material isbasically naked, not enveloped by a nuclearmembrane. In addition to the genomic DNA (thesingle chromosome/circular DNA), manybacteria have small circular DNA outside thegenomic DNA. These smaller DNA are calledplasmids. The plasmid DNA confers certainunique phenotypic characters to such bacteria.One such character is resistance to antibiotics.In higher classes you will learn that this plasmidDNA is used to monitor bacterial transformationwith foreign DNA. Nuclear membrane is foundin eukaryotes. No organelles, like the ones ineukaryotes, are found in prokaryotic cells exceptfor ribosomes. Prokaryotes have somethingunique in the form of inclusions. A specialiseddifferentiated form of cell membrane called mesosome is the characteristicof prokaryotes. They are essentially infoldings of cell membrane.8.4.1 Cell Envelope and its ModificationsMost prokaryotic cells, particularly the bacterial cells, have a chemicallycomplex cell envelope. The cell envelope consists of a tightly bound threelayered structure i.e., the outermost glycocalyx followed by the cell wall andthen the plasma membrane. Although each layer of the envelope performsdistinct function, they act together as a single protective unit. Bacteria canbe classified into two groups on the basis of the differences in the cell envelopesand the manner in which they respond to the staining procedure developedby Gram viz., those that take up the gram stain are Gram positive and theothers that do not are called Gram negative bacteria.Glycocalyx differs in composition and thickness among differentbacteria. It could be a loose sheath called the slime layer in some, whilein others it may be thick and tough, called the capsule. The cell walldetermines the shape of the cell and provides a strong structural supportto prevent the bacterium from bursting or collapsing.The plasma membrane is selectively permeable in nature and interactswith the outside world. This membrane is similar structurally to that ofthe eukaryotes.A special membranous structure is the mesosome which is formedby the extensions of plasma membrane into the cell. These extensions arein the form of vesicles, tubules and lamellae. They help in cell wallTypical bacteria(1-2 mm)PPLO(about 0.1 mm)Viruses(0.02-0.2 mm)A typical eukaryotic cell(10-20 mm)Figure 8.2 Diagram showing comparison ofeukaryotic cell with otherorganisms2022-23CELL: THE UNIT OF LIFE 129formation, DNA replication and distribution to daughter cells. They alsohelp in respiration, secretion processes, to increase the surface area ofthe plasma membrane and enzymatic content. In some prokaryotes likecyanobacteria, there are other membranous extensions into the cytoplasmcalled chromatophores which contain pigments.Bacterial cells may be motile or non-motile. If motile, they have thinfilamentous extensions from their cell wall called flagella. Bacteria show arange in the number and arrangement of flagella. Bacterial flagellum iscomposed of three parts â filament, hook and basal body. The filamentis the longest portion and extends from the cell surface to the outside.Besides flagella, Pili and Fimbriae are also surface structures of thebacteria but do not play a role in motility. The pili are elongated tubularstructures made of a special protein. The fimbriae are small bristle likefibres sprouting out of the cell. In some bacteria, they are known to helpattach the bacteria to rocks in streams and also to the host tissues.8.4.2 Ribosomes and Inclusion BodiesIn prokaryotes, ribosomes are associated with the plasma membrane ofthe cell. They are about 15 nm by 20 nm in size and are made of twosubunits - 50S and 30S units which when present together form 70Sprokaryotic ribosomes. Ribosomes are the site of protein synthesis. Severalribosomes may attach to a single mRNA and form a chain calledpolyribosomes or polysome. The ribosomes of a polysome translate themRNA into proteins.Inclusion bodies: Reserve material in prokaryotic cells are stored inthe cytoplasm in the form of inclusion bodies. These are not bound byany membrane system and lie free in the cytoplasm, e.g., phosphategranules, cyanophycean granules and glycogen granules. Gas vacuolesare found in blue green and purple and green photosynthetic bacteria.8.5 EUKARYOTIC CELLSThe eukaryotes include all the protists, plants, animals and fungi. Ineukaryotic cells there is an extensive compartmentalisation of cytoplasmthrough the presence of membrane bound organelles. Eukaryotic cellspossess an organised nucleus with a nuclear envelope. In addition,eukaryotic cells have a variety of complex locomotory and cytoskeletalstructures. Their genetic material is organised into chromosomes.All eukaryotic cells are not identical. Plant and animal cells are differentas the former possess cell walls, plastids and a large central vacuole whichare absent in animal cells. On the other hand, animal cells have centrioleswhich are absent in almost all plant cells (Figure 8.3).2022-23130 BIOLOGYRough endoplasmicreticulumLysosomeSmoothendoplasmicreticulumPlasmodesmataMicrotubuleNucleusNucleolusGolgiapparatusNuclearenvelopeVacuoleMiddle lamellaPlasmamembraneCell wallMitochondrionChloroplast RibosomesCytoplasmPeroxisomeFigure 8.3 Diagram showing : (a) Plant cell (b) Animal cellGolgiapparatusSmoothendoplasmicreticulumNuclearenvelopeNucleolusNucleusMicrovilliPlasmamembraneCentriolePeroxiomeLysosomeRibosomesMitochondrionRoughendoplasmicreticulumCytoplasm(a)(b)2022-23CELL: THE UNIT OF LIFE 131CholesterolSugar PeripheralProteinPhospholipidbilayerLet us now look at individual cell organelles to understand theirstructure and functions.8.5.1 Cell MembraneThe detailed structure of the membrane was studied only after the adventof the electron microscope in the 1950s. Meanwhile, chemical studies onthe cell membrane, especially in human red blood cells (RBCs), enabledthe scientists to deduce the possible structure of plasma membrane.These studies showed that the cell membrane is mainly composed oflipids and proteins. The major lipids are phospholipids that are arrangedin a bilayer. Also, the lipids are arranged within the membrane with thepolar head towards the outer sides and the hydrophobic tails towardsthe inner part.This ensures that the nonpolar tail of saturatedhydrocarbons is protected from the aqueous environment (Figure 8.4).In addition to phospholipids membrane also contains cholesterol.Later, biochemical investigation clearly revealed that the cell membranesalso possess protein and carbohydrate. The ratio of protein and lipid variesconsiderably in different cell types. In human beings, the membrane of theerythrocyte has approximately 52 per cent protein and 40 per cent lipids.Depending on the ease of extraction, membrane proteins can beclassified as integral and peripheral. Peripheral proteins lie on the surfaceof membrane while the integral proteins are partially or totally buried inthe membrane.Figure 8.4 Fluid mosaic model of plasma membrane2022-23132 BIOLOGYAn improved model of the structure of cell membrane was proposedby Singer and Nicolson (1972) widely accepted as fluid mosaic model(Figure 8.4). According to this, the quasi-fluid nature of lipid enableslateral movement of proteins within the overall bilayer. This ability to movewithin the membrane is measured as its fluidity.The fluid nature of the membrane is also important from the point ofview of functions like cell growth, formation of intercellular junctions,secretion, endocytosis, cell division etc.One of the most important functions of the plasma membrane is thetransport of the molecules across it. The membrane is selectively permeableto some molecules present on either side of it. Many molecules can movebriefly across the membrane without any requirement of energy and thisis called the passive transport. Neutral solutes may move across themembrane by the process of simple diffusion along the concentrationgradient, i.e., from higher concentration to the lower. Water may also moveacross this membrane from higher to lower concentration. Movement ofwater by diffusion is called osmosis. As the polar molecules cannot passthrough the nonpolar lipid bilayer, they require a carrier protein of themembrane to facilitate their transport across the membrane. A few ionsor molecules are transported across the membrane against theirconcentration gradient, i.e., from lower to the higher concentration. Sucha transport is an energy dependent process, in which ATP is utilised andis called active transport, e.g., Na+/K+ Pump.8.5.2 Cell WallAs you may recall, a non-living rigid structure called the cell wall formsan outer covering for the plasma membrane of fungi and plants. Cell wallnot only gives shape to the cell and protects the cell from mechanicaldamage and infection, it also helps in cell-to-cell interaction and providesbarrier to undesirable macromolecules. Algae have cell wall, made ofcellulose, galactans, mannans and minerals like calcium carbonate, whilein other plants it consists of cellulose, hemicellulose, pectins and proteins.The cell wall of a young plant cell, the primary wall is capable of growth,which gradually diminishes as the cell matures and the secondary wall isformed on the inner (towards membrane) side of the cell.The middle lamella is a layer mainly of calcium pectate which holdsor glues the different neighbouring cells together. The cell wall and middlelamellae may be traversed by plasmodesmata which connect the cytoplasmof neighbouring cells.8.5.3 Endomembrane SystemWhile each of the membranous organelles is distinct in terms of its2022-23CELL: THE UNIT OF LIFE 133NucleusNuclear pore RoughRibosomeendoplasmicEndoplasmicreticulumSmoothreticulumstructure and function, many of these areconsidered together as an endomembrane systembecause their functions are coordinated. Theendomembrane system include endoplasmicreticulum (ER), golgi complex, lysosomes andvacuoles. Since the functions of the mitochondria,chloroplast and peroxisomes are not coordinatedwith the above components, these are notconsidered as part of the endomembrane system.8.5.3.1 The Endoplasmic Reticulum (ER)Electron microscopic studies of eukaryotic cellsreveal the presence of a network or reticulum oftiny tubular structures scattered in the cytoplasmthat is called the endoplasmic reticulum (ER)(Figure 8.5). Hence, ER divides the intracellularspace into two distinct compartments, i.e., luminal(inside ER) and extra luminal (cytoplasm)compartments.The ER often shows ribosomes attached totheir outer surface. The endoplasmic reticulunbearing ribosomes on their surface is called roughendoplasmic reticulum (RER). In the absence ofribosomes they appear smooth and are calledsmooth endoplasmic reticulum (SER).RER is frequently observed in the cells activelyinvolved in protein synthesis and secretion. Theyare extensive and continuous with the outermembrane of the nucleus.The smooth endoplasmic reticulum is the majorsite for synthesis of lipid. In animal cells lipid-likesteroidal hormones are synthesised in SER.8.5.3.2 Golgi apparatusCamillo Golgi (1898) first observed densely stainedreticular structures near the nucleus. These werelater named Golgi bodies after him. They consistof many flat, disc-shaped sacs or cisternae of0.5Îŧm to 1.0Îŧm diameter (Figure 8.6). These arestacked parallel to each other. Varied number ofcisternae are present in a Golgi complex. The Golgicisternae are concentrically arranged near thenucleus with distinct convex cis or the formingFigure 8.5 Endoplasmic reticulumCisternaeFigure 8.6 Golgi apparatus2022-23134 BIOLOGYface and concave trans or the maturing face.The cis and the trans faces of the organelle are entirely different, butinterconnected.The golgi apparatus principally performs the function of packagingmaterials, to be delivered either to the intra-cellular targets or secretedoutside the cell. Materials to be packaged in the form of vesicles fromthe ER fuse with the cis face of the golgi apparatus and move towardsthe maturing face. This explains, why the golgi apparatus remains inclose association with the endoplasmic reticulum. A number of proteinssynthesised by ribosomes on the endoplasmic reticulum are modifiedin the cisternae of the golgi apparatus before they are released from itstrans face. Golgi apparatus is the important site of formation ofglycoproteins and glycolipids.8.5.3.3 LysosomesThese are membrane bound vesicular structures formed by the processof packaging in the golgi apparatus. The isolated lysosomal vesicleshave been found to be very rich in almost all types of hydrolyticenzymes (hydrolases â lipases, proteases, carbohydrases) optimallyactive at the acidic pH. These enzymes are capable of digestingcarbohydrates, proteins, lipids and nucleic acids.8.5.3.4 VacuolesThe vacuole is the membrane-bound space found in the cytoplasm. It containswater, sap, excretory product and other materials not useful for the cell. Thevacuole is bound by a single membrane called tonoplast. In plant cells thevacuoles can occupy up to 90 per cent of the volume of the cell.In plants, the tonoplast facilitates the transport of a number of ionsand other materials against concentration gradients into the vacuole, hencetheir concentration is significantly higher in the vacuole than in thecytoplasm.In Amoeba the contractile vacuole is important for osmoregulationand excretion. In many cells, as in protists, food vacuoles are formed byengulfing the food particles.8.5.4 MitochondriaMitochondria (sing.: mitochondrion), unless specifically stained, are noteasily visible under the microscope. The number of mitochondria per cellis variable depending on the physiological activity of the cells. In terms ofshape and size also, considerable degree of variability is observed. Typicallyit is sausage-shaped or cylindrical having a diameter of 0.2-1.0Îŧm (average0.5Îŧm) and length 1.0-4.1Îŧm. Each mitochondrion is a double2022-23CELL: THE UNIT OF LIFE 135membrane-bound structure with the outer membrane and the innermembrane dividing its lumen distinctly into two aqueous compartments,i.e., the outer compartment and the inner compartment. The innercompartment is filled with a dense homogeneous substance called thematrix. The outer membrane forms the continuous limiting boundary ofthe organelle. The inner membrane forms a number of infoldings calledthe cristae (sing.: crista) towards the matrix (Figure 8.7). The cristaeincrease the surface area. The two membranes have their own specificenzymes associated with the mitochondrial function. Mitochondria arethe sites of aerobic respiration. They produce cellular energy in the formof ATP, hence they are called âpower housesâ of the cell. The matrix alsopossesses single circular DNA molecule, a few RNA molecules, ribosomes(70S) and the components required for the synthesis of proteins. Themitochondria divide by fission.8.5.5 PlastidsPlastids are found in all plant cells and in euglenoides. These are easilyobserved under the microscope as they are large. They bear some specificpigments, thus imparting specific colours to the plants. Based on thetype of pigments plastids can be classified into chloroplasts,chromoplasts and leucoplasts.The chloroplasts contain chlorophyll and carotenoid pigments whichare responsible for trapping light energy essential for photosynthesis. Inthe chromoplasts fat soluble carotenoid pigments like carotene,xanthophylls and others are present. This gives the part of the plant ayellow, orange or red colour. The leucoplasts are the colourless plastidsof varied shapes and sizes with stored nutrients: Amyloplasts storecarbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereasOutermembraneInnermembraneMatrix CristaFigure 8.7 Structure of mitochondrion (Longitudinal section)Inter-membranespace2022-23136 BIOLOGYthe aleuroplasts store proteins.Majority of the chloroplasts of the greenplants are found in the mesophyll cells ofthe leaves. These are lens-shaped, oval,spherical, discoid or even ribbon-likeorganelles having variable length (5-10Îŧm)and width (2-4Îŧm). Their number variesfrom 1 per cell of the Chlamydomonas, agreen alga to 20-40 per cell in the mesophyll.Like mitochondria, the chloroplasts arealso double membrane bound. Of the two,the inner chloroplast membrane is relativelyless permeable. The space limited by theinner membrane of the chloroplast is called the stroma. A number of organisedflattened membranous sacs called the thylakoids, are present in the stroma(Figure 8.8). Thylakoids are arranged in stacks like the piles of coins calledgrana (singular: granum) or the intergranal thylakoids. In addition, there areflat membranous tubules called the stroma lamellae connecting the thylakoidsof the different grana. The membrane of the thylakoids enclose a space calleda lumen. The stroma of the chloroplast contains enzymes required for thesynthesis of carbohydrates and proteins. It also contains small, doublestrandedcircular DNA molecules and ribosomes. Chlorophyll pigments arepresent in the thylakoids. The ribosomes of the chloroplasts are smaller (70S)than the cytoplasmic ribosomes (80S).8.5.6 RibosomesRibosomes are the granular structures first observed under the electronmicroscope as dense particles by George Palade (1953). They arecomposed of ribonucleic acid (RNA) and proteins andare not surrounded by any membrane.The eukaryotic ribosomes are 80S while theprokaryotic ribosomes are 70S. Each ribosome has twosubunits, larger and smaller subunits (Fig 8.9). The twosubunits of 80S ribosomes are 60S and 40S while thatof 70S ribosomes are 50S and 30S. Here âSâ (SvedbergâsUnit) stands for the sedimentation coefficient; it isindirectly a measure of density and size. Both 70S and80S ribosomes are composed of two subunits.Figure 8.8 Sectional view of chloroplastFigure 8.9 Ribosome8.5.7 CytoskeletonAn elaborate network of filamentous proteinaceous structures consistingof microtubules, microfilaments and intermediate filaments present inthe cytoplasm is collectively referred to as the cytoskeleton. Thecytoskeleton in a cell are involved in many functions such as mechanicalsupport, motility, maintenance of the shape of the cell.2022-23CELL: THE UNIT OF LIFE 1378.5.8 Cilia and FlagellaCilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowthsof the cell membrane. Cilia are small structures which work like oars,causing the movement of either the cell or the surrounding fluid. Flagellaare comparatively longer and responsible for cell movement. Theprokaryotic bacteria also possess flagella but these are structurallydifferent from that of the eukaryotic flagella.The electron microscopic study of a cilium or the flagellum show thatthey are covered with plasma membrane. Their core called the axoneme,possesses a number of microtubules running parallel to the long axis.The axoneme usually has nine doublets of radially arranged peripheralmicrotubules, and a pair of centrally located microtubules. Such anarrangement of axonemal microtubules is referred to as the 9+2 array(Figure 8.10). The central tubules are connected by bridges and is alsoenclosed by a central sheath, which is connected to one of the tubules ofeach peripheral doublets by a radial spoke. Thus, there are nine radialspokes. The peripheral doublets are also interconnected by linkers. Boththe cilium and flagellum emerge from centriole-like structure called thebasal bodies.8.5.9 Centrosome and CentriolesCentrosome is an organelle usually containing two cylindrical structurescalled centrioles. They are surrounded by amorphous pericentriolarmaterials. Both the centrioles in a centrosome lie perpendicular to eachother in which each has an organisation like the cartwheel. They arePlasmamembranePeripheralmicrotubules(doublets)InterdoubletbridgeCentralRadial microtublespokeCentralsheathFigure 8.10 Section of cilia/flagella showing different parts : (a) Electron micrograph(b) Diagrammatic representation of internal structure(a) (b)2022-23138 BIOLOGYmade up of nine evenly spaced peripheral fibrils of tubulin protein. Eachof the peripheral fibril is a triplet.The adjacent triplets are also linked.The central part of the proximal region of the centriole is also proteinaceousand called the hub, which is connected with tubules of the peripheraltriplets by radial spokes made of protein. The centrioles form the basalbody of cilia or flagella, and spindle fibres that give rise to spindleapparatus during cell division in animal cells.8.5.10 NucleusNucleus as a cell organelle was first described by Robert Brown as earlyas 1831. Later the material of the nucleus stained by the basic dyes wasgiven the name chromatin by Flemming.The interphase nucleus (nucleus of acell when it is not dividing) has highlyextended and elaborate nucleoproteinfibres called chromatin, nuclear matrixand one or more spherical bodies callednucleoli (sing.: nucleolus) (Figure 8.11).Electron microscopy has revealed that thenuclear envelope, which consists of twoparallel membranes with a space between(10 to 50 nm) called the perinuclearspace, forms a barrier between thematerials present inside the nucleus andthat of the cytoplasm. The outermembrane usually remains continuouswith the endoplasmic reticulum and alsobears ribosomes on it. At a number ofplaces the nuclear envelope is interrupted by minute pores, which areformed by the fusion of its two membranes. These nuclear pores are thepassages through which movement of RNA and protein molecules takesplace in both directions between the nucleus and the cytoplasm. Normally,there is only one nucleus per cell, variations in the number of nuclei arealso frequently observed. Can you recollect names of organisms thathave more than one nucleus per cell? Some mature cells even lacknucleus, e.g., erythrocytes of many mammals and sieve tube cells ofvascular plants. Would you consider these cells as âlivingâ?The nuclear matrix or the nucleoplasm contains nucleolus andchromatin. The nucleoli are spherical structures present in thenucleoplasm. The content of nucleolus is continuous with the rest of thenucleoplasm as it is not a membrane bound structure. It is a site foractive ribosomal RNA synthesis. Larger and more numerous nucleoli arepresent in cells actively carrying out protein synthesis.NucleoplasmNucleolusNuclear poreNuclearmembraneFigure 8.11 Structure of nucleus2022-23CELL: THE UNIT OF LIFE 139KinetochoreFigure 8.12 Chromosome withkinetochoreFigure 8.13 Types of chromosomes based on the position of centromereYou may recall that the interphase nucleus has a looseand indistinct network of nucleoprotein fibres calledchromatin. But during different stages of cell division, cellsshow structured chromosomes in place of the nucleus.Chromatin contains DNA and some basic proteins calledhistones, some non-histone proteins and also RNA. Asingle human cell has approximately two metre longthread of DNA distributed among its forty six (twenty threepairs) chromosomes. You will study the details of DNApackaging in the form of a chromosome in class XII.Every chromosome (visible only in dividing cells)essentially has a primary constriction or the centromereon the sides of which disc shaped structures calledkinetochores are present (Figure 8.12). Centromere holdstwo chromatids of a chromosome. Based on the positionof the centromere, the chromosomes can be classified intofour types (Figure 8.13). The metacentric chromosomehas middle centromere forming two equal arms of thechromosome. The sub-metacentric chromosome hascentromere slightly away from the middle of thechromosome resulting into one shorter arm and onelonger arm. In case of acrocentric chromosome thecentromere is situated close to its end forming oneextremely short and one very long arm, whereas thetelocentric chromosome has a terminal centromere.2022-23140 BIOLOGYSometimes a few chromosomes have non-staining secondaryconstrictions at a constant location. This gives the appearance of a smallfragment called the satellite.8.5.11 MicrobodiesMany membrane bound minute vesicles called microbodies that containvarious enzymes, are present in both plant and animal cells.SUMMARYAll organisms are made of cells or aggregates of cells. Cells vary in their shape, sizeand activities/functions. Based on the presence or absence of a membrane boundnucleus and other organelles, cells and hence organisms can be named aseukaryotic or prokaryotic.A typical eukaryotic cell consists of a cell membrane, nucleus and cytoplasm.Plant cells have a cell wall outside the cell membrane. The plasma membrane isselectively permeable and facilitates transport of several molecules. Theendomembrane system includes ER, golgi complex, lysosomes and vacuoles. Allthe cell organelles perform different but specific functions. Centrosome and centrioleform the basal body of cilia and flagella that facilitate locomotion. In animal cells,centrioles also form spindle apparatus during cell division. Nucleus containsnucleoli and chromatin network. It not only controls the activities of organellesbut also plays a major role in heredity.Endoplasmic reticulum contains tubules or cisternae. They are of two types:rough and smooth. ER helps in the transport of substances, synthesis ofproteins, lipoproteins and glycogen. The golgi body is a membranous organellecomposed of flattened sacs. The secretions of cells are packed in them andtransported from the cell. Lysosomes are single membrane structurescontaining enzymes for digestion of all types of macromolecules. Ribosomesare involved in protein synthesis. These occur freely in the cytoplasm or areassociated with ER. Mitochondria help in oxidative phosphorylation andgeneration of adenosine triphosphate. They are bound by double membrane;the outer membrane is smooth and inner one folds into several cristae. Plastidsare pigment containing organelles found in plant cells only. In plant cells,chloroplasts are responsible for trapping light energy essential forphotosynthesis. The grana, in the plastid, is the site of light reactions and thestroma of dark reactions. The green coloured plastids are chloroplasts, whichcontain chlorophyll, whereas the other coloured plastids are chromoplasts,which may contain pigments like carotene and xanthophyll. The nucleus isenclosed by nuclear envelope, a double membrane structure with nuclear pores.The inner membrane encloses the nucleoplasm and the chromatin material.Thus, cell is the structural and functional unit of life.2022-23CELL: THE UNIT OF LIFE 141EXERCISES1. Which of the following is not correct?(a) Robert Brown discovered the cell.(b) Schleiden and Schwann formulated the cell theory.(c) Virchow explained that cells are formed from pre-existing cells.(d) A unicellular organism carries out its life activities within a single cell.2. New cells generate from(a) bacterial fermentation (b) regeneration of old cells(c) pre-existing cells (d) abiotic materials3. Match the followingColumn I Column II(a) Cristae (i) Flat membranous sacs in stroma(b) Cisternae (ii) Infoldings in mitochondria(c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus4. Which of the following is correct:(a) Cells of all living organisms have a nucleus.(b) Both animal and plant cells have a well defined cell wall.(c) In prokaryotes, there are no membrane bound organelles.(d) Cells are formed de novo from abiotic materials.5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.6. How do neutral solutes move across the plasma membrane? Can the polarmolecules also move across it in the same way? If not, then how are thesetransported across the membrane?7. Name two cell-organelles that are double membrane bound. What are thecharacteristics of these two organelles? State their functions and draw labelleddiagrams of both.8. What are the characteristics of prokaryotic cells?9. Multicellular organisms have division of labour. Explain.10. Cell is the basic unit of life. Discuss in brief.11. What are nuclear pores? State their function.12. Both lysosomes and vacuoles are endomembrane structures, yet they differ interms of their functions. Comment.13. Describe the structure of the following with the help of labelled diagrams.(i) Nucleus (ii) Centrosome14. What is a centromere? How does the position of centromere form the basis ofclassification of chromosomes. Support your answer with a diagram showingthe position of centromere on different types of chromosomes.2022-23142 BIOLOGYThere is a wide diversity in living organisms in our biosphere. Now aquestion that arises in our minds is: Are all living organisms made of thesame chemicals, i.e., elements and compounds? You have learnt inchemistry how elemental analysis is performed. If we perform such ananalysis on a plant tissue, animal tissue or a microbial paste, we obtain alist of elements like carbon, hydrogen, oxygen and several others andtheir respective content per unit mass of a living tissue. If the same analysisis performed on a piece of earthâs crust as an example of non-living matter,we obtain a similar list. What are the differences between the two lists? Inabsolute terms, no such differences could be made out. All the elementspresent in a sample of earthâs crust are also present in a sample of livingtissue. However, a closer examination reveals that the relative abundanceof carbon and hydrogen with respect to other elements is higher in anyliving organism than in earthâs crust (Table 9.1).9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?We can continue asking in the same way, what type of organic compoundsare found in living organisms? How does one go about finding the answer?To get an answer, one has to perform a chemical analysis. We can take anyliving tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroaceticacid (Cl3CCOOH) using a mortar and a pestle. We obtain a thick slurry. Ifwe were to strain this through a cheesecloth or cotton we would obtain twofractions. One is called the filtrate or more technically, the acid-solublepool, and the second, the retentate or the acid-insoluble fraction. Scientistshave found thousands of organic compounds in the acid-soluble pool.BIOMOLECULESCHAPTER 99.1 How to AnalyseChemicalComposition?9.2 Primary andSecondaryMetabolites9.3 Biomacromolecules9.4 Proteins9.5 Polysaccharides9.6 Nucleic Acids9.7 Structure ofProteins9.8 Nature of BondLinking Monomersin a Polymer9.9 Dynamic State ofBody Constituents- Concept ofMetabolism9.10 Metabolic Basis forLiving9.11 The Living State9.12 Enzymes2022-23BIOMOLECULES 143In higher classes you will learn about howto analyse a living tissue sample and identify aparticular organic compound. It will suffice tosay here that one extracts the compounds, thensubjects the extract to various separationtechniques till one has separated a compoundfrom all other compounds. In other words, oneisolates and purifies a compound. Analyticaltechniques, when applied to the compound giveus an idea of the molecular formula and theprobable structure of the compound. All thecarbon compounds that we get from livingtissues can be called âbiomoleculesâ. However,living organisms have also got inorganicelements and compounds in them. How do weknow this? A slightly different but destructiveexperiment has to be done. One weighs a smallamount of a living tissue (say a leaf or liver andthis is called wet weight) and dry it. All the water,evaporates. The remaining material gives dryweight. Now if the tissue is fully burnt, all thecarbon compounds are oxidised to gaseousform (CO2, water vapour) and are removed. Whatis remaining is called âashâ. This ash containsinorganic elements (like calcium, magnesiumetc). Inorganic compounds like sulphate,phosphate, etc., are also seen in the acid-solublefraction. Therefore elemental analysis giveselemental composition of living tissues in theform of hydrogen, oxygen, chlorine, carbon etc.while analysis for compounds gives an idea ofElement % Weight ofEarthâs crust Human bodyHydrogen (H) 0.14 0.5Carbon (C) 0.03 18.5Oxygen (O) 46.6 65.0Nitrogen (N) very little 3.3Sulphur (S) 0.03 0.3Sodium (Na) 2.8 0.2Calcium (Ca) 3.6 1.5Magnesium (Mg) 2.1 0.1Silicon (Si) 27.7 negligible* Adapted from CNR Rao, Understanding Chemistry,Universities Press, Hyderabad.TABLE 9.1 A Comparison of Elements Presentin Non-living and Living Matter*Component FormulaSodium Na+Potassium K+Calcium Ca++Magnesium Mg++Water H2OCompounds NaCl, CaCO3,PO SO 4342 â â ,TABLE 9.2 A List of Representative InorganicConstituents of Living Tissuesthe kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)present in living tissues. From a chemistry point of view, one can identifyfunctional groups like aldehydes, ketones, aromatic compounds, etc. Butfrom a biological point of view, we shall classify them into amino acids,nucleotide bases, fatty acids etc.Amino acids are organic compounds containing an amino group andan acidic group as substituents on the same carbon i.e., the a-carbon.Hence, they are called a-amino acids. They are substituted methanes. Thereare four substituent groups occupying the four valency positions. Theseare hydrogen, carboxyl group, amino group and a variable groupdesignated as R group. Based on the nature of R group there are manyamino acids. However, those which occur in proteins are only of twenty2022-23144 BIOLOGYtypes. The R group in these proteinaceous amino acids could be a hydrogen(the amino acid is called glycine), a methyl group (alanine), hydroxy methyl(serine), etc. Three of the twenty are shown in Figure 9.1.The chemical and physical properties of amino acids are essentiallyof the amino, carboxyl and the R functional groups. Based on number ofamino and carboxyl groups, there are acidic (e.g., glutamic acid), basic(lysine) and neutral (valine) amino acids. Similarly, there are aromaticamino acids (tyrosine, phenylalanine, tryptophan). A particular propertyof amino acids is the ionizable nature of âNH2 and âCOOH groups. Hencein solutions of different pH, the structure of amino acids changes.B is called zwitterionic form.Lipids are generally water insoluble. They could be simple fatty acids.A fatty acid has a carboxyl group attached to an R group. The R groupcould be a methyl (âCH3), or ethyl (âC2H5) or higher number of âCH2groups (1 carbon to 19 carbons). For example, palmitic acid has 16carbons including carboxyl carbon. Arachidonic acid has 20 carbonatoms including the carboxyl carbon. Fatty acids could be saturated(without double bond) or unsaturated (with one or more C=C doublebonds). Another simple lipid is glycerol which is trihydroxy propane. Manylipids have both glycerol and fatty acids. Here the fatty acids are foundesterified with glycerol. They can be then monoglycerides, diglyceridesand triglycerides. These are also called fats and oils based on meltingpoint. Oils have lower melting point (e.g., gingelly oil) and hence remainas oil in winters. Can you identify a fat from the market? Some lipidshave phosphorous and a phosphorylated organic compound in them.These are phospholipids. They are found in cell membrane. Lecithin isone example. Some tissues especially the neural tissues have lipids withmore complex structures.Living organisms have a number of carbon compounds in whichheterocyclic rings can be found. Some of these are nitrogen bases âadenine, guanine, cytosine, uracil, and thymine. When found attached toa sugar, they are called nucleosides. If a phosphate group is also foundesterified to the sugar they are called nucleotides. Adenosine, guanosine,thymidine, uridine and cytidine are nucleosides. Adenylic acid, thymidylicacid, guanylic acid, uridylic acid and cytidylic acid are nucleotides. Nucleicacids like DNA and RNA consist of nucleotides only. DNA and RNA functionas genetic material.2022-23BIOMOLECULES 145Phospholipid (Lecithin) CholesterolFats and oils (lipids)(CH ) 2 14 CH3 COOHFatty acid(Palmitic acid)Glycerol Triglyceride (R1, R2and R3 are fatty acids)Nitrogen basesOH OHO AdenineOCH2 HO POHOAdenylic acidNucleotideOH OHHOCH2O AdenineOH OHHOCH2O UracilAdenosineUridineNucleosidesOHOH OHHOCH2OOHOHHO OHCH OH 2OC6H12O6 (Glucose) C5H10O5 (Ribose)Sugars (Carbohydrates)Glycine SerineAmino acidsAlanineFigure 9.1 Diagrammatic representation of small molecular weight organiccompounds in living tissuesOOHNNHAdenine (Purine)Uracil (Pyrimidine)2022-23146 BIOLOGY9.2 PRIMARY AND SECONDARY METABOLITESThe most exciting aspect of chemistry deals with isolating thousands ofcompounds, small and big, from living organisms, determining theirstructure and if possible synthesising them.If one were to make a list of biomolecules, such a list would havethousands of organic compounds including amino acids, sugars, etc.For reasons that are given in section 9.10, we can call these biomoleculesas âmetabolitesâ. In animal tissues, one notices the presence of all suchcategories of compounds shown in Figure 9.1. These are called primarymetabolites. However, when one analyses plant, fungal and microbial cells,one would see thousands of compounds other than these called primarymetabolites, e.g. alkaloids, flavonoids, rubber, essential oils, antibiotics,coloured pigments, scents, gums, spices. Theseare called secondary metabolites (Table 9.3).While primary metabolites have identifiablefunctions and play known roles in normalphysiologial processes, we do not at the moment,understand the role or functions of all theâsecondary metabolitesâ in host organisms.However, many of them are useful to âhumanwelfareâ (e.g., rubber, drugs, spices, scents andpigments). Some secondary metabolites haveecological importance. In the later chapters andyears you will learn more about this.9.3 BIOMACROMOLECULESThere is one feature common to all those compounds found in the acidsoluble pool. They have molecular weights ranging from 18 to around800 daltons (Da) approximately.The acid insoluble fraction, has only four types of organic compoundsi.e., proteins, nucleic acids, polysaccharides and lipids. These classes ofcompounds with the exception of lipids, have molecular weights in therange of ten thousand daltons and above. For this very reason,biomolecules, i.e., chemical compounds found in living organisms are oftwo types. One, those which have molecular weights less than onethousand dalton and are usually referred to as micromolecules or simplybiomolecules while those which are found in the acid insoluble fractionare called macromolecules or biomacromolecules.The molecules in the insoluble fraction with the exception of lipidsare polymeric substances. Then why do lipids, whose molecular weightsdo not exceed 800 Da, come under acid insoluble fraction, i.e.,macromolecular fraction? Lipids are indeed small molecular weightPigments Carotenoids, Anthocyanins,etc.Alkaloids Morphine, Codeine, etc.Terpenoides Monoterpenes, Diterpenes etc.Essential oils Lemon grass oil, etc.Toxins Abrin, RicinLectins Concanavalin ADrugs Vinblastin, curcumin, etc.Polymeric Rubber, gums, cellulosesubstancesTABLE 9.3 Some Secondary Metabolites2022-23BIOMOLECULES 147Component % of the totalcellular massWater 70-90Proteins 10-15Carbohydrates 3Lipids 2Nucleic acids 5-7Ions 1TABLE 9.4 Average Composition of Cellscompounds and are present not only as such but alsoarranged into structures like cell membrane and othermembranes. When we grind a tissue, we are disruptingthe cell structure. Cell membrane and othermembranes are broken into pieces, and form vesicleswhich are not water soluble. Therefore, thesemembrane fragments in the form of vesicles getseparated along with the acid insoluble pool and hencein the macromolecular fraction. Lipids are not strictlymacromolecules.The acid soluble pool represents roughly thecytoplasmic composition. The macromolecules fromcytoplasm and organelles become the acid insolublefraction. Together they represent the entire chemicalcomposition of living tissues or organisms.In summary if we represent the chemicalcomposition of living tissue from abundance point ofview and arrange them class-wise, we observe thatwater is the most abundant chemical in livingorganisms (Table 9.4).9.4 PROTEINSProteins are polypeptides. They are linear chains ofamino acids linked by peptide bonds as shown inFigure 9.3.Each protein is a polymer of amino acids. As thereare 20 types of amino acids (e.g., alanine, cysteine,proline, tryptophan, lysine, etc.), a protein is aheteropolymer and not a homopolymer. Ahomopolymer has only one type of monomer repeatingânâ number of times. This information about the aminoacid content is important as later in your nutritionlessons, you will learn that certain amino acids areessential for our health and they have to be suppliedthrough our diet. Hence, dietary proteins are thesource of essential amino acids. Therefore, amino acidscan be essential or non-essential. The latter are thosewhich our body can make, while we get essential aminoacids through our diet/food. Proteins carry out manyfunctions in living organisms, some transportnutrients across cell membrane, some fight infectiousorganisms, some are hormones, some are enzymes,TABLE 9.5 Some Proteins and theirFunctionsProtein FunctionsCollagen Intercellular groundsubstanceTrypsin EnzymeInsulin HormoneAntibody Fights infectious agentsReceptor Sensory reception(smell, taste, hormone,etc.)GLUT-4 Enables glucosetransportinto cells2022-23148 BIOLOGYCH OH 2 CH OH 2CH2OHOHOH OHOHOO OOO OOO O O OOOFigure 9.2 Diagrammatic representation of a portion of glycogenetc. (Table 9.5). Collagen is the most abundant protein in animal worldand Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is themost abundant protein in the whole of the biosphere.9.5 POLYSACCHARIDESThe acid insoluble pellet also has polysaccharides (carbohydrates) asanother class of macromolecules. Polysaccharides are long chains ofsugars. They are threads (literally a cotton thread) containing differentmonosaccharides as building blocks. For example, cellulose is apolymeric polysaccharide consisting of only one type of monosaccharidei.e., glucose. Cellulose is a homopolymer. Starch is a variant of this butpresent as a store house of energy in plant tissues. Animals have anothervariant called glycogen. Inulin is a polymer of fructose. In apolysaccharide chain (say glycogen), the right end is called the reducingend and the left end is called the non-reducing end. It has branches asshown in the form of a cartoon (Figure 9.2). Starch forms helicalsecondary structures. In fact, starch can hold I2 molecules in the helicalportion. The starch-I2 is blue in colour. Cellulose does not containcomplex helices and hence cannot hold I2.2022-23BIOMOLECULES 149Plant cell walls are made of cellulose. Paper made from plant pulpand cotton fibre is cellulosic. There are more complex polysaccharidesin nature. They have as building blocks, amino-sugars and chemicallymodified sugars (e.g., glucosamine, N-acetyl galactosamine, etc.).Exoskeletons of arthropods, for example, have a complexpolysaccharide called chitin. These complex polysaccharides are mostlyhomopolymers.9.6 NUCLEIC ACIDSThe other type of macromolecule that one would find in the acidinsoluble fraction of any living tissue is the nucleic acid. These arepolynucleotides. Together with polysaccharides and polypeptides thesecomprise the true macromolecular fraction of any living tissue or cell.For nucleic acids, the building block is a nucleotide. A nucleotide hasthree chemically distinct components. One is a heterocyclic compound,the second is a monosaccharide and the third a phosphoric acid orphosphate.As you notice in Figure 9.1, the heterocyclic compounds in nucleicacids are the nitrogenous bases named adenine, guanine, uracil,cytosine, and thymine. Adenine and Guanine are substituted purineswhile the rest are substituted pyrimidines. The skeletal heterocyclic ringis called as purine and pyrimidine respectively. The sugar found inpolynucleotides is either ribose (a monosaccharide pentose) or 2âdeoxyribose. A nucleic acid containing deoxyribose is calleddeoxyribonucleic acid (DNA) while that which contains ribose is calledribonucleic acid (RNA).9.7 STRUCTURE OF PROTEINSProteins, as mentioned earlier, are heteropolymers containing stringsof amino acids. Structure of molecules means different things indifferent contexts. In inorganic chemistry, the structure invariablyrefers to the molecular formulae (e.g., NaCl, MgCl2, etc.). Organicchemists always write a two dimensional view of the molecules whilerepresenting the structure of the molecules (e.g., benzene,naphthalene, etc.). Physicists conjure up the three dimensional viewsof molecular structures while biologists describe the protein structureat four levels. The sequence of amino acids i.e., the positionalinformation in a protein â which is the first amino acid, which issecond, and so on â is called the primary structure (Figure 9.3a) of aprotein. A protein is imagined as a line, the left end represented bythe first amino acid and the right end represented by the last amino2022-23150 BIOLOGYacid. The first amino acid is alsocalled as N-terminal amino acid. Thelast amino acid is called the Cterminalamino acid. A proteinthread does not exist throughout asan extended rigid rod. The thread isfolded in the form of a helix (similarto a revolving staircase). Of course,only some portions of the proteinthread are arranged in the form of ahelix. In proteins, only right handedhelices are observed. Other regionsof the protein thread are folded intoother forms in what is called thesecondary structure (Fig. 9.4 b). Inaddition, the long protein chain isalso folded upon itself like a hollowwoolen ball, giving rise to thetertiary structure (Figure 9.4 c).This gives us a 3-dimensional viewof a protein. Tertiary structure isabsolutely necessary for the manybiological activities of proteins.Some proteins are an assemblyof more than one polypeptide orsubunits. The manner in whichthese individual folded polypeptidesor subunits are arranged withrespect to each other (e.g. linearstring of spheres, spheres arrangedone upon each other in the form ofa cube or plate etc.) is thearchitecture of a protein otherwisecalled the quaternary structure ofa protein (Fig. 9.4 d). Adult humanhaemoglobin consists of 4 subunits.Two of these are identical to eachother. Hence, two subunits of a typeand two subunits of b type togetherconstitute the human haemoglobin(Hb).(a) (b)NCFigure 9.4 Cartoon showing : (a) A secondary structureand (b) A tertiary structure of proteinsFigure 9.3 Various levels of Protein Structure(a) Primary(b) Secondary(d) QuaternaryHydrogenDisulphide bondBetaâplated sheetPolypeptideTertiaryAlphaâ Helix(c)2022-23BIOMOLECULES 1519.8 NATURE OF BOND LINKING MONOMERS IN A POLYMERIn a polypeptide or a protein, amino acids are linked by a peptidebond which is formed when the carboxyl (-COOH) group of one aminoacid reacts with the amino (-NH2) group of the next amino acid withthe elimination of a water moiety (the process is called dehydration).In a polysaccharide the individual monosaccharides are linked by aglycosidic bond. This bond is also formed by dehydration. This bondis formed between two carbon atoms of two adjacent monosaccharides.In a nucleic acid a phosphate moiety links the 3â-carbon of one sugarof one nucleotide to the 5â-carbon of the sugar of the succeedingnucleotide. The bond between the phosphate and hydroxyl group ofsugar is an ester bond. As there is one such ester bond on either side,it is called phosphodiester bond (Figure 9.5).Nucleic acids exhibit a wide variety of secondary structures. Forexample, one of the secondary structures exhibited by DNA is thefamous Watson-Crick model. This model says that DNA exists as adouble helix. The two strands of polynucleotides are antiparallel i.e.,run in the opposite direction. The backbone is formed by the sugarphosphate-sugar chain. The nitrogen bases are projected more or lessperpendicular to this backbone but face inside. A and G of one strandcompulsorily base pairs with T and C, respectively, on the other strand.Figure 9.5 Diagram indicating secondary structure of DNA5' 3'5' 3'2022-23152 BIOLOGYThere are two hydrogen bonds between A and T and three hydrogenbonds between G and C. Each strand appears like a helical staircase.Each step of ascent is represented by a pair of bases. At each step ofascent, the strand turns 36°. One full turn of the helical strand wouldinvolve ten steps or ten base pairs. Attempt drawing a line diagram.The pitch would be 34à . The rise per base pair would be 3.4à . Thisform of DNA with the above mentioned salient features is called BDNA.In higher classes, you will be told that there are more than adozen forms of DNA named after English alphabets with uniquestructural features.9.9 DYNAMIC STATE OF BODY CONSTITUENTS â CONCEPT OFMETABOLISMWhat we have learnt till now is that living organisms, be it a simple bacterialcell, a protozoan, a plant or an animal, contain thousands of organiccompounds. These compounds or biomolecules are present in certainconcentrations (expressed as mols/cell or mols/litre etc.). One of the greatestdiscoveries ever made was the observation that all these biomolecules havea turn over. This means that they are constantly being changed into someother biomolecules and also made from some other biomolecules. Thisbreaking and making is through chemical reactions constantly occuringin living organisms. Together all these chemical reactions are calledmetabolism. Each of the metabolic reactions results in the transformationof biomolecules. A few examples for such metabolic transformations are:removal of CO2 from amino acids making an amino acid into an amine,removal of amino group in a nucleotide base; hydrolysis of a glycosidicbond in a disaccharide, etc. We can list tens and thousands of suchexamples. Majority of these metabolic reactions do not occur in isolationbut are always linked to some other reactions. In other words, metabolitesare converted into each other in a series of linked reactions called metabolicpathways. These metabolic pathways are similar to the automobile trafficin a city. These pathways are either linear or circular. These pathways crisscrosseach other, i.e., there are traffic junctions. Flow of metabolites throughmetabolic pathway has a definite rate and direction like automobile traffic.This metabolite flow is called the dynamic state of body constituents. Whatis most important is that this interlinked metabolic traffic is very smoothand without a single reported mishap for healthy conditions. Another featureof these metabolic reactions is that every chemical reaction is a catalysedreaction. There is no uncatalysed metabolic conversion in living systems.Even CO2 dissolving in water, a physical process, is a catalysed reaction inliving systems. The catalysts which hasten the rate of a given metabolicconversation are also proteins. These proteins with catalytic power arenamed enzymes.2022-23BIOMOLECULES 1539.10 METABOLIC BASIS FOR LIVINGMetabolic pathways can lead to a more complex structure from a simplerstructure (for example, acetic acid becomes cholesterol) or lead to a simplerstructure from a complex structure (for example, glucose becomes lacticacid in our skeletal muscle). The former cases are called biosyntheticpathways or anabolic pathways. The latter constitute degradation andhence are called catabolic pathways. Anabolic pathways, as expected,consume energy. Assembly of a protein from amino acids requires energyinput. On the other hand, catabolic pathways lead to the release of energy.For example, when glucose is degraded to lactic acid in our skeletal muscle,energy is liberated. This metabolic pathway from glucose to lactic acid whichoccurs in 10 metabolic steps is called glycolysis. Living organisms havelearnt to trap this energy liberated during degradation and store it in theform of chemical bonds. As and when needed, this bond energy is utilisedfor biosynthetic, osmotic and mechanical work that we perform. The mostimportant form of energy currency in living systems is the bond energy ina chemical called adenosine triphosphate (ATP).How do living organisms derive their energy? What strategies have theyevolved? How do they store this energy and in what form? How do theyconvert this energy into work? You will study and understand all this undera sub-discipline called âBioenergeticsâ later in your higher classes.9.11 THE LIVING STATEAt this level, you must understand that the tens and thousands ofchemical compounds in a living organism, otherwise called metabolites,or biomolecules, are present at concentrations characteristic of each ofthem. For example, the blood concentration of glucose in a normal healthyindividual is 4.2 mmol/Lâ6.1 mmol/L, while that of hormones wouldbe nanograms/mL. The most important fact of biological systems is thatall living organisms exist in a steady-state characterised byconcentrations of each of these biomolecules. These biomolecules are ina metabolic flux. Any chemical or physical process moves spontaneouslyto equilibrium. The steady state is a non-equilibrium state. One shouldremember from physics that systems at equilibrium cannot performwork. As living organisms work continuously, they cannot afford to reachequilibrium. Hence the living state is a non-equilibrium steadystateto be able to perform work; living process is a constant effort toprevent falling into equilibrium. This is achieved by energy input.Metabolism provides a mechanism for the production of energy. Hencethe living state and metabolism are synonymous. Without metabolismthere cannot be a living state.2022-23154 BIOLOGY9.12 ENZYMESAlmost all enzymes are proteins. There are some nucleic acids that behavelike enzymes. These are called ribozymes. One can depict an enzyme by aline diagram. An enzyme like any protein has a primary structure, i.e.,amino acid sequence of the protein. An enzyme like any protein has thesecondary and the tertiary structure. When you look at a tertiary structure(Figure 9.4 b) you will notice that the backbone of the protein chain foldsupon itself, the chain criss-crosses itself and hence, many crevices orpockets are made. One such pocket is the âactive siteâ. An active site of anenzyme is a crevice or pocket into which the substrate fits. Thus enzymes,through their active site, catalyse reactions at a high rate. Enzyme catalystsdiffer from inorganic catalysts in many ways, but one major differenceneeds mention. Inorganic catalysts work efficiently at high temperaturesand high pressures, while enzymes get damaged at high temperatures(say above 40°C). However, enzymes isolated from organisms who normallylive under extremely high temperatures (e.g., hot vents and sulphursprings), are stable and retain their catalytic power even at hightemperatures (upto 80°-90°C). Thermal stability is thus an importantquality of such enzymes isolated from thermophilic organisms.9.12.1 Chemical ReactionsHow do we understand these enzymes? Let us first understand a chemicalreaction. Chemical compounds undergo two types of changes. A physicalchange simply refers to a change in shape without breaking of bonds.This is a physical process. Another physical process is a change in stateof matter: when ice melts into water, or when water becomes a vapour.These are also physical processes. However, when bonds are broken andnew bonds are formed during transformation, this will be called a chemicalreaction. For example:Ba(OH)2 + H2SO4 ÂŽ BaSO4 + 2H2Ois an inorganic chemical reaction. Similarly, hydrolysis of starch intoglucose is an organic chemical reaction. Rate of a physical or chemicalprocess refers to the amount of product formed per unit time. It can beexpressed as:rate =ddPtRate can also be called velocity if the direction is specified. Rates of physicaland chemical processes are influenced by temperature among otherfactors. A general rule of thumb is that rate doubles or decreases by half2022-23BIOMOLECULES 155for every 10°C change in either direction. Catalysed reactions proceed atrates vastly higher than that of uncatalysed ones. When enzyme catalysedreactions are observed, the rate would be vastly higher than the same butuncatalysed reaction. For exampleCO2 + H2O žžžžžžžžžžŽCarbonic anhydrase H2CO3carbon dioxide water carbonic acidIn the absence of any enzyme this reaction is very slow, with about200 molecules of H2CO3 being formed in an hour. However, by using theenzyme present within the cytoplasm called carbonic anhydrase, thereaction speeds dramatically with about 600,000 molecules being formedevery second. The enzyme has accelerated the reaction rate by about 10million times. The power of enzymes is incredible indeed!There are thousands of types of enzymes each catalysing a uniquechemical or metabolic reaction. A multistep chemical reaction, when eachof the steps is catalysed by the same enzyme complex or different enzymes,is called a metabolic pathway. For example,Glucose ÂŽ2 Pyruvic acidC6H12O6 + O2 ÂŽ 2C3H4 O3 + 2H2Ois actually a metabolic pathway in which glucose becomes pyruvic acidthrough ten different enzyme catalysed metabolic reactions. When youstudy respiration in Chapter 14 you will study these reactions. At thisstage you should know that this very metabolic pathway with one or twoadditional reactions gives rise to a variety of metabolic end products. Inour skeletal muscle, under anaerobic conditions, lactic acid is formed.Under normal aerobic conditions, pyruvic acid is formed. In yeast, duringfermentation, the same pathway leads to the production of ethanol(alcohol). Hence, in different conditions different products are possible.9.12.2 How do Enzymes bring about such High Rates ofChemical Conversions?To understand this we should study enzymes a little more. We have alreadyunderstood the idea of an âactive siteâ. The chemical or metabolic conversionrefers to a reaction. The chemical which is converted into a product iscalled a âsubstrateâ. Hence enzymes, i.e. proteins with three dimensionalstructures including an âactive siteâ, convert a substrate (S) into a product(P). Symbolically, this can be depicted as:S ÂŽPIt is now understood that the substrate âSâ has to bind the enzyme atits âactive siteâ within a given cleft or pocket. The substrate has to diffuseÂŦžžžž2022-23156 BIOLOGYtowards the âactive siteâ. There is thus, anobligatory formation of an âESâ complex. Estands for enzyme. This complex formation isa transient phenomenon. During the statewhere substrate is bound to the enzyme activesite, a new structure of the substrate calledtransition state structure is formed. Very soon,after the expected bond breaking/making iscompleted, the product is released from theactive site. In other words, the structure ofsubstrate gets transformed into the structureof product(s). The pathway of thistransformation must go through the so-calledtransition state structure. There could bemany more âaltered structural statesâ betweenthe stable substrate and the product. Implicitin this statement is the fact that all otherintermediate structural states are unstable. Stability is something relatedto energy status of the molecule or the structure. Hence, when we look atthis pictorially through a graph it looks like something as in Figure 9.6.The y-axis represents the potential energy content. The x-axisrepresents the progression of the structural transformation or statesthrough the âtransition stateâ. You would notice two things. The energylevel difference between S and P. If âPâ is at a lower level than âSâ, the reactionis an exothermic reaction. One need not supply energy (by heating) inorder to form the product. However, whether it is an exothermic orspontaneous reaction or an endothermic or energy requiring reaction,the âSâ has to go through a much higher energy state or transition state.The difference in average energy content of âSâ from that of this transitionstate is called âactivation energyâ.Enzymes eventually bring down this energy barrier making thetransition of âSâ to âPâ more easy.9.12.3 Nature of Enzyme ActionEach enzyme (E) has a substrate (S) binding site in its molecule so that ahighly reactive enzyme-substrate complex (ES) is produced. Thiscomplex is short-lived and dissociates into its product(s) P and theunchanged enzyme with an intermediate formation of the enzyme-productcomplex (EP).The formation of the ES complex is essential for catalysis.E + S ES žžŽ EP žžŽ E + PActivation energywithout enzymePotential EnergyActivationenergy with enzymeSubstrate (s)Product (P)Progress of reactionTransition stateFigure 9.6 Concept of activation energy2022-23BIOMOLECULES 157The catalytic cycle of an enzyme action can be described in the followingsteps:1. First, the substrate binds to the active site of the enzyme, fittinginto the active site.2. The binding of the substrate induces the enzyme to alter its shape,fitting more tightly around the substrate.3. The active site of the enzyme, now in close proximity of thesubstrate breaks the chemical bonds of the substrate and thenew enzyme- product complex is formed.4. The enzyme releases the products of the reaction and the freeenzyme is ready to bind to another molecule of the substrate andrun through the catalytic cycle once again.9.12.4 Factors Affecting Enzyme ActivityThe activity of an enzyme can be affected by a change in the conditionswhich can alter the tertiary structure of the protein. These includetemperature, pH, change in substrate concentration or binding of specificchemicals that regulate its activity.Temperature and pHEnzymes generally function in a narrow range of temperature and pH(Figure 9.7). Each enzyme shows its highest activity at a particulartemperature and pH called the optimum temperature and optimum pH.Activity declines both below and above the optimum value. Lowtemperature preserves the enzyme in a temporarily inactive state whereashigh temperature destroys enzymatic activity because proteins aredenatured by heat.Figure 9.7 Effect of change in : (a) pH (b) Temperature and (c) Concentration ofsubstrate on enzyme activityVmaxVelocity of reaction (V)[S]V2maxKm(a) (b) (c)pH TemperatureEnzyme activity2022-23158 BIOLOGYConcentration of SubstrateWith the increase in substrate concentration, the velocity of the enzymaticreaction rises at first. The reaction ultimately reaches a maximum velocity(Vmax) which is not exceeded by any further rise in concentration of thesubstrate. This is because the enzyme molecules are fewer than thesubstrate molecules and after saturation of these molecules, there are nofree enzyme molecules to bind with the additional substrate molecules(Figure 9.7).The activity of an enzyme is also sensitive to the presence of specificchemicals that bind to the enzyme. When the binding of the chemicalshuts off enzyme activity, the process is called inhibition and the chemicalis called an inhibitor.When the inhibitor closely resembles the substrate in its molecularstructure and inhibits the activity of the enzyme, it is known ascompetitive inhibitor. Due to its close structural similarity with thesubstrate, the inhibitor competes with the substrate for the substratebindingsite of the enzyme. Consequently, the substrate cannot bind andas a result, the enzyme action declines, e.g., inhibition of succinicdehydrogenase by malonate which closely resembles the substratesuccinate in structure. Such competitive inhibitors are often used in thecontrol of bacterial pathogens.9.12.5 Classification and Nomenclature of EnzymesThousands of enzymes have been discovered, isolated and studied. Mostof these enzymes have been classified into different groups based on thetype of reactions they catalyse. Enzymes are divided into 6 classes eachwith 4-13 subclasses and named accordingly by a four-digit number.Oxidoreductases/dehydrogenases: Enzymes which catalyseoxidoreduction between two substrates S and Sâ e.g.,S reduced + Sâ oxidised žžŽ S oxidised + Sâ reduced.Transferases: Enzymes catalysing a transfer of a group, G (other thanhydrogen) between a pair of substrate S and Sâ e.g.,S - G + Sâ žžŽ S + Sâ - GHydrolases: Enzymes catalysing hydrolysis of ester, ether, peptide,glycosidic, C-C, C-halide or P-N bonds.Lyases: Enzymes that catalyse removal of groups from substrates bymechanisms other than hydrolysis leaving double bonds.2022-23BIOMOLECULES 159Isomerases: Includes all enzymes catalysing inter-conversion of optical,geometric or positional isomers.Ligases: Enzymes catalysing the linking together of 2 compounds, e.g.,enzymes which catalyse joining of C-O, C-S, C-N, P-O etc. bonds.9.12.6 Co-factorsEnzymes are composed of one or several polypeptide chains. However,there are a number of cases in which non-protein constituents called cofactorsare bound to the the enzyme to make the enzyme catalyticallyactive. In these instances, the protein portion of the enzymes is called theapoenzyme. Three kinds of cofactors may be identified: prosthetic groups,co-enzymes and metal ions.Prosthetic groups are organic compounds and are distinguished fromother cofactors in that they are tightly bound to the apoenzyme. Forexample, in peroxidase and catalase, which catalyze the breakdown ofhydrogen peroxide to water and oxygen, haem is the prosthetic groupand it is a part of the active site of the enzyme.Co-enzymes are also organic compounds but their association withthe apoenzyme is only transient, usually occurring during the course ofcatalysis. Furthermore, co-enzymes serve as co-factors in a number ofdifferent enzyme catalyzed reactions. The essential chemical componentsof many coenzymes are vitamins, e.g., coenzyme nicotinamide adeninedinucleotide (NAD) and NADP contain the vitamin niacin.A number of enzymes require metal ions for their activity which formcoordination bonds with side chains at the active site and at the sametime form one or more cordination bonds with the substrate, e.g., zinc isa cofactor for the proteolytic enzyme carboxypeptidase.Catalytic activity is lost when the co-factor is removed from the enzymewhich testifies that they play a crucial role in the catalytic activity of theenzyme.SUMMARYAlthough there is a bewildering diversity of living organisms, their chemicalcomposition and metabolic reactions appear to be remarkably similar. Theelemental composition of living tissues and non-living matter appear also to besimilar when analysed qualitatively. However, a closer examination reveals thatthe relative abundance of carbon, hydrogen and oxygen is higher in living systemswhen compared to inanimate matter. The most abundant chemical in livingorganisms is water. There are thousands of small molecular weight (<1000 Da)2022-23160 BIOLOGYbiomolecules. Amino acids, monosaccharide and disaccharide sugars, fatty acids,glycerol, nucleotides, nucleosides and nitrogen bases are some of the organiccompounds seen in living organisms. There are 20 types of amino acids and 5types of nucleotides. Fats and oils are glycerides in which fatty acids are esterifiedto glycerol. Phospholipids contain, in addition, a phosphorylated nitrogenouscompound.Only three types of macromolecules, i.e., proteins, nucleic acids andpolysaccharides are found in living systems. Lipids, because of their associationwith membranes separate in the macromolecular fraction. Biomacromoleculesare polymers. They are made of building blocks which are different. Proteinsare heteropolymers made of amino acids. Nucleic acids (RNA and DNA) arecomposed of nucleotides. Biomacromolecules have a hierarchy of structures âprimary, secondary, tertiary and quaternary. Nucleic acids serve as geneticmaterial. Polysaccharides are components of cell wall in plants, fungi and alsoof the exoskeleton of arthropods. They also are storage forms of energy (e.g.,starch and glycogen). Proteins serve a variety of cellular functions. Many ofthem are enzymes, some are antibodies, some are receptors, some are hormonesand some others are structural proteins. Collagen is the most abundant proteinin animal world and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO)is the most abundant protein in the whole of the biosphere.Enzymes are proteins which catalyse biochemical reactions in the cells.Ribozymes are nucleic acids with catalytic power. Proteinaceous enzymesexhibit substrate specificity, require optimum temperature and pH for maximalactivity. They are denatured at high temperatures. Enzymes lower activationenergy of reactions and enhance greatly the rate of the reactions. Nucleic acidscarry hereditary information and are passed on from parental generation toprogeny.EXERCISES1. What are macromolecules? Give examples.2. Illustrate a glycosidic, peptide and a phospho-diester bond.3. What is meant by tertiary structure of proteins?4. Find and write down structures of 10 interesting small molecular weightbiomolecules. Find if there is any industry which manufactures the compoundsby isolation. Find out who are the buyers.5. Proteins have primary structure. If you are given a method to know which aminoacid is at either of the two termini (ends) of a protein, can you connect thisinformation to purity or homogeneity of a protein?6. Find out and make a list of proteins used as therapeutic agents. Find otherapplications of proteins (e.g., Cosmetics etc.)7. Explain the composition of triglyceride.2022-23BIOMOLECULES 1618. Can you describe what happens when milk is converted into curd or yoghurt,from your understanding of proteins.9. Can you attempt building models of biomolecules using commercially availableatomic models (Ball and Stick models).10. Attempt titrating an amino acid against a weak base and discover the numberof dissociating ( ionizable ) functional groups in the amino acid.11. Draw the structure of the amino acid, alanine.12. What are gums made of? Is Fevicol different?13. Find out a qualitative test for proteins, fats and oils, amino acids and test anyfruit juice, saliva, sweat and urine for them.14. Find out how much cellulose is made by all the plants in the biosphere andcompare it with how much of paper is manufactured by man and hence what isthe consumption of plant material by man annually. What a loss of vegetation!15. Describe the important properties of enzymes.2022-23162 BIOLOGYAre you aware that all organisms, even the largest, start their life from asingle cell? You may wonder how a single cell then goes on to form suchlarge organisms. Growth and reproduction are characteristics of cells,indeed of all living organisms. All cells reproduce by dividing into two,with each parental cell giving rise to two daughter cells each time theydivide. These newly formed daughter cells can themselves grow and divide,giving rise to a new cell population that is formed by the growth anddivision of a single parental cell and its progeny. In other words, suchcycles of growth and division allow a single cell to form a structureconsisting of millions of cells.10.1 CELL CYCLECell division is a very important process in all living organisms. Duringthe division of a cell, DNA replication and cell growth also take place. Allthese processes, i.e., cell division, DNA replication, and cell growth, hence,have to take place in a coordinated way to ensure correct division andformation of progeny cells containing intact genomes. The sequence ofevents by which a cell duplicates its genome, synthesises the otherconstituents of the cell and eventually divides into two daughter cells istermed cell cycle. Although cell growth (in terms of cytoplasmic increase)is a continuous process, DNA synthesis occurs only during one specificstage in the cell cycle. The replicated chromosomes (DNA) are thendistributed to daughter nuclei by a complex series of events during celldivision. These events are themselves under genetic control.CELL CYCLE AND CELL DIVISIONCHAPTER 1010.1 Cell Cycle10.2 M Phase10.3 Significance ofMitosis10.4 Meiosis10.5 Significance ofMeiosis2022-23CELL CYCLE AND CELL DIVISION 16310.1.1 Phases of Cell CycleA typical eukaryotic cell cycle is illustrated byhuman cells in culture. These cells divide oncein approximately every 24 hours (Figure 10.1).However, this duration of cell cycle can vary fromorganism to organism and also from cell typeto cell type. Yeast for example, can progressthrough the cell cycle in only about 90 minutes.The cell cycle is divided into two basicphases:l Interphasel M Phase (Mitosis phase)The M Phase represents the phase when theactual cell division or mitosis occurs and theinterphase represents the phase between twosuccessive M phases. It is significant to notethat in the 24 hour average duration of cellcycle of a human cell, cell division proper lastsfor only about an hour. The interphase lastsmore than 95% of the duration of cell cycle.The M Phase starts with the nuclear division, corresponding to theseparation of daughter chromosomes (karyokinesis) and usually endswith division of cytoplasm (cytokinesis). The interphase, though calledthe resting phase, is the time during which the cell is preparing for divisionby undergoing both cell growth and DNA replication in an orderly manner.The interphase is divided into three further phases:l G1 phase (Gap 1)l S phase (Synthesis)l G2 phase (Gap 2)G1 phase corresponds to the interval between mitosis and initiationof DNA replication. During G1 phase the cell is metabolically active andcontinuously grows but does not replicate its DNA. S or synthesis phasemarks the period during which DNA synthesis or replication takes place.During this time the amount of DNA per cell doubles. If the initial amountof DNA is denoted as 2C then it increases to 4C. However, there is noincrease in the chromosome number; if the cell had diploid or 2n numberof chromosomes at G1, even after S phase the number of chromosomesremains the same, i.e., 2n.In animal cells, during the S phase, DNA replication begins in thenucleus, and the centriole duplicates in the cytoplasm. During the G2phase, proteins are synthesised in preparation for mitosis while cell growthcontinues.How do plants andanimals continue togrow all their lives?Do all cells in a plantdivide all the time?Do you think all cellscontinue to divide inall plants andanimals? Can youtell the name and thelocation of tissueshaving cells thatdivide all their life inhigher plants? Doanimals have similarm e r i s t e m a t i ctissues?Figure 10.1 A diagrammatic view of cell cycleindicating formation of two cellsfrom one cellM Phase2022-23164 BIOLOGYSome cells in the adult animals do not appear to exhibit division (e.g.,heart cells) and many other cells divide only occasionally, as needed toreplace cells that have been lost because of injury or cell death. Thesecells that do not divide further exit G1 phase to enter an inactive stagecalled quiescent stage (G0) of the cell cycle. Cells in this stage remainmetabolically active but no longer proliferate unless called on to do sodepending on the requirement of the organism.In animals, mitotic cell division is only seen in the diploid somaticcells. However, there are few exceptions to this where haploid cells divideby mitosis, for example, male honey bees. Against this, the plants canshow mitotic divisions in both haploid and diploid cells. From yourrecollection of examples of alternation of generations in plants (Chapter 3)identify plant species and stages at which mitosis is seen in haploid cells.10.2 M PHASEThis is the most dramatic period of the cell cycle, involving a majorreorganisation of virtually all components of the cell. Since the number ofchromosomes in the parent and progeny cells is the same, it is also called asequational division. Though for convenience mitosis has been dividedinto four stages of nuclear division (karyokinesis), it is very essential tounderstand that cell division is a progressive process and very clear-cutlines cannot be drawn between various stages. Karyokinesis involvesfollowing four stages:l Prophasel Metaphasel Anaphasel Telophase10.2.1 ProphaseProphase which is the first stage of karyokinesis of mitosis follows theS and G2 phases of interphase. In the S and G2 phases the new DNAmolecules formed are not distinct but intertwined. Prophase is markedby the initiation of condensation of chromosomal material. Thechromosomal material becomes untangled during the process ofchromatin condensation (Figure 10.2 a). The centrosome, which hadundergone duplication during S phase of interphase, now begins to movetowards opposite poles of the cell. The completion of prophase can thusbe marked by the following characteristic events:l Chromosomal material condenses to form compact mitoticchromosomes. Chromosomes are seen to be composed of twochromatids attached together at the centromere.l Centrosome which had undergone duplication during interphase,begins to move towards opposite poles of the cell. Each centrosomeradiates out microtubules called asters. The two asters togetherwith spindle fibres forms mitotic apparatus.You have studiedmitosis in onion roottip cells. It has 16chromosomes ineach cell. Can youtell how manychromosomes willthe cell have at G1phase, after S phase,and after M phase?Also, what will be theDNA content of thecells at G1, after Sand at G2, if thecontent after Mphase is 2C?2022-23CELL CYCLE AND CELL DIVISION 165Cells at the end of prophase, when viewed under themicroscope, do not show golgi complexes, endoplasmicreticulum, nucleolus and the nuclear envelope.10.2.2 MetaphaseThe complete disintegration of the nuclear envelope marksthe start of the second phase of mitosis, hence thechromosomes are spread through the cytoplasm of the cell.By this stage, condensation of chromosomes is completedand they can be observed clearly under the microscope. Thisthen, is the stage at which morphology of chromosomes ismost easily studied. At this stage, metaphase chromosomeis made up of two sister chromatids, which are held togetherby the centromere (Figure 10.2 b). Small disc-shapedstructures at the surface of the centromeres are calledkinetochores. These structures serve as the sites of attachmentof spindle fibres (formed by the spindle fibres) to thechromosomes that are moved into position at the centre ofthe cell. Hence, the metaphase is characterised by all thechromosomes coming to lie at the equator with one chromatidof each chromosome connected by its kinetochore to spindlefibres from one pole and its sister chromatid connected byits kinetochore to spindle fibres from the opposite pole (Figure10.2 b). The plane of alignment of the chromosomes atmetaphase is referred to as the metaphase plate. The keyfeatures of metaphase are:l Spindle fibres attach to kinetochores ofchromosomes.l Chromosomes are moved to spindle equator and getaligned along metaphase plate through spindle fibresto both poles.10.2.3 AnaphaseAt the onset of anaphase, each chromosome arranged at themetaphase plate is split simultaneously and the two daughterchromatids, now referred to as daughter chromosomes ofthe future daughter nuclei, begin their migration towardsthe two opposite poles. As each chromosome moves awayfrom the equatorial plate, the centromere of each chromosomeremains directed towards the pole and hence at the leadingedge, with the arms of the chromosome trailing behind(Figure 10.2 c). Thus, anaphase stage is characterised byFigure 10.2 a and b : A diagrammaticview of stages in mitosis2022-23166 BIOLOGYthe following key events:l Centromeres split and chromatids separate.l Chromatids move to opposite poles.10.2.4 TelophaseAt the beginning of the final stage of karyokinesis, i.e.,telophase, the chromosomes that have reached theirrespective poles decondense and lose their individuality. Theindividual chromosomes can no longer be seen and each setof chromatin material tends to collect at each of the two poles(Figure 10.2 d). This is the stage which shows the followingkey events:l Chromosomes cluster at opposite spindle poles and theiridentity is lost as discrete elements.l Nuclear envelope develops around the chromosomeclusters at each pole forming two daughter nuclei.l Nucleolus, golgi complex and ER reform.10.2.5 CytokinesisMitosis accomplishes not only the segregation of duplicatedchromosomes into daughter nuclei (karyokinesis), but thecell itself is divided into two daughter cells by the separationof cytoplasm called cytokinesis at the end of which celldivision gets completed (Figure 10.2 e). In an animal cell,this is achieved by the appearance of a furrow in the plasmamembrane. The furrow gradually deepens and ultimatelyjoins in the centre dividing the cell cytoplasm into two. Plantcells however, are enclosed by a relatively inextensible cellwall, thererfore they undergo cytokinesis by a differentmechanism. In plant cells, wall formation starts in the centreof the cell and grows outward to meet the existing lateralwalls. The formation of the new cell wall begins with theformation of a simple precursor, called the cell-plate thatrepresents the middle lamella between the walls of twoadjacent cells. At the time of cytoplasmic division, organelleslike mitochondria and plastids get distributed between thetwo daughter cells. In some organisms karyokinesis is notfollowed by cytokinesis as a result of which multinucleatecondition arises leading to the formation of syncytium (e.g.,liquid endosperm in coconut).Figure 10.2 c to e : A diagrammaticview of stages in Mitosis2022-23CELL CYCLE AND CELL DIVISION 16710.3 Significance of MitosisMitosis or the equational division is usually restricted to the diploid cellsonly. However, in some lower plants and in some social insects haploidcells also divide by mitosis. It is very essential to understand thesignificance of this division in the life of an organism. Are you aware ofsome examples where you have studied about haploid and diploid insects?Mitosis usually results in the production of diploid daughter cellswith identical genetic complement. The growth of multicellular organismsis due to mitosis. Cell growth results in disturbing the ratio between thenucleus and the cytoplasm. It therefore becomes essential for the cell todivide to restore the nucleo-cytoplasmic ratio. A very significantcontribution of mitosis is cell repair. The cells of the upper layer of theepidermis, cells of the lining of the gut, and blood cells are being constantlyreplaced. Mitotic divisions in the meristematic tissues â the apical andthe lateral cambium, result in a continuous growth of plants throughouttheir life.10.4 MEIOSISThe production of offspring by sexual reproduction includes the fusionof two gametes, each with a complete haploid set of chromosomes. Gametesare formed from specialised diploid cells. This specialised kind of celldivision that reduces the chromosome number by half results in theproduction of haploid daughter cells. This kind of division is calledmeiosis. Meiosis ensures the production of haploid phase in the life cycleof sexually reproducing organisms whereas fertilisation restores the diploidphase. We come across meiosis during gametogenesis in plants andanimals. This leads to the formation of haploid gametes. The key featuresof meiosis are as follows:l Meiosis involves two sequential cycles of nuclear and cell division calledmeiosis I and meiosis II but only a single cycle of DNA replication.l Meiosis I is initiated after the parental chromosomes have replicatedto produce identical sister chromatids at the S phase.l Meiosis involves pairing of homologous chromosomes andrecombination between non-sister chromatids of homologouschromosomes.l Four haploid cells are formed at the end of meiosis II.Meiotic events can be grouped under the following phases:Meiosis I Meiosis IIProphase I Prophase IIMetaphase I Metaphase IIAnaphase I Anaphase IITelophase I Telophase II2022-23168 BIOLOGY10.4.1 Meiosis IProphase I: Prophase of the first meiotic division is typically longer andmore complex when compared to prophase of mitosis. It has been furthersubdivided into the following five phases based on chromosomalbehaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.During leptotene stage the chromosomes become gradually visibleunder the light microscope. The compaction of chromosomes continuesthroughout leptotene. This is followed by the second stage of prophaseI called zygotene. During this stage chromosomes start pairing togetherand this process of association is called synapsis. Such pairedchromosomes are called homologous chromosomes. Electronmicrographs of this stage indicate that chromosome synapsis isaccompanied by the formation of complex structure calledsynaptonemal complex. The complex formed by a pair of synapsedhomologous chromosomes is called a bivalent or a tetrad. However,these are more clearly visible at the next stage. The first two stages ofprophase I are relatively short-lived compared to the next stage that ispachytene. During this stage, the four chromatids of each bivalentchromosomes becomes distinct and clearly appears as tetrads. This stageis characterised by the appearance of recombination nodules, the sitesat which crossing over occurs between non-sister chromatids of thehomologous chromosomes. Crossing over is the exchange of geneticmaterial between two homologous chromosomes. Crossing over is alsoan enzyme-mediated process and the enzyme involved is calledrecombinase. Crossing over leads to recombination of genetic materialon the two chromosomes. Recombination between homologouschromosomes is completed by the end of pachytene, leaving thechromosomes linked at the sites of crossing over.The beginning of diplotene is recognised by the dissolution of thesynaptonemal complex and the tendency of the recombinedhomologous chromosomes of the bivalents to separate from each otherexcept at the sites of crossovers. These X-shaped structures, are calledchiasmata. In oocytes of some vertebrates, diplotene can last formonths or years.The final stage of meiotic prophase I is diakinesis. This is marked byterminalisation of chiasmata. During this phase the chromosomes arefully condensed and the meiotic spindle is assembled to prepare thehomologous chromosomes for separation. By the end of diakinesis, thenucleolus disappears and the nuclear envelope also breaks down.Diakinesis represents transition to metaphase.Metaphase I: The bivalent chromosomes align on the equatorial plate(Figure 10.3). The microtubules from the opposite poles of the spindleattach to the kinetochore of homologous chromosomes.2022-23CELL CYCLE AND CELL DIVISION 169Anaphase I: The homologous chromosomes separate, while sisterchromatids remain associated at their centromeres (Figure 10.3).Telophase I: The nuclear membrane and nucleolus reappear, cytokinesisfollows and this is called as dyad of cells (Figure 10.3). Although in manycases the chromosomes do undergo some dispersion, they do not reachthe extremely extended state of the interphase nucleus. The stage betweenthe two meiotic divisions is called interkinesis and is generally short lived.There is no replication of DNA during interkinesis. Interkinesis is followedby prophase II, a much simpler prophase than prophase I.10.4.2 Meiosis IIProphase II: Meiosis II is initiated immediately after cytokinesis, usuallybefore the chromosomes have fully elongated. In contrast to meiosis I,meiosis II resembles a normal mitosis. The nuclear membrane disappearsby the end of prophase II (Figure 10.4). The chromosomes again becomecompact.Metaphase II: At this stage the chromosomes align at the equator andthe microtubules from opposite poles of the spindle get attached to thekinetochores (Figure 10.4) of sister chromatids.Anaphase II: It begins with the simultaneous splitting of the centromereof each chromosome (which was holding the sister chromatids together),allowing them to move toward opposite poles of the cell (Figure 10.4) byshortening of microtubules attached to kinetochores.Figure 10.3 Stages of Meiosis I2022-23170 BIOLOGYTelophase II: Meiosis ends with telophase II, in which the twogroups of chromosomes once again get enclosed by a nuclearenvelope; cytokinesis follows resulting in the formation of tetradof cells i.e., four haploid daughter cells (Figure 10.4).10.5 SIGNIFICANCE OF MEIOSISMeiosis is the mechanism by which conservation of specificchromosome number of each species is achieved acrossgenerations in sexually reproducing organisms, even though theprocess, per se, paradoxically, results in reduction of chromosomenumber by half. It also increases the genetic variability in thepopulation of organisms from one generation to the next. Variationsare very important for the process of evolution.Figure 10.4 Stages of Meiosis IISUMMARYAccording to the cell theory, cells arise from preexisting cells. The process bywhich this occurs is called cell division. Any sexually reproducing organismstarts its life cycle from a single-celled zygote. Cell division does not stop withthe formation of the mature organism but continues throughout its life cycle.2022-23CELL CYCLE AND CELL DIVISION 171The stages through which a cell passes from one division to the next is calledthe cell cycle. Cell cycle is divided into two phases called (i) Interphase â aperiod of preparation for cell division, and (ii) Mitosis (M phase) â the actualperiod of cell division. Interphase is further subdivided into G1, S and G2. G1phase is the period when the cell grows and carries out normal metabolism.Most of the organelle duplication also occurs during this phase. S phase marksthe phase of DNA replication and chromosome duplication. G2 phase is theperiod of cytoplasmic growth. Mitosis is also divided into four stages namelyprophase, metaphase, anaphase and telophase. Chromosome condensationoccurs during prophase. Simultaneously, the centrioles move to the oppositepoles. The nuclear envelope and the nucleolus disappear and the spindlefibres start appearing. Metaphase is marked by the alignment of chromosomesat the equatorial plate. During anaphase the centromeres divide and thechromatids start moving towards the two opposite poles. Once the chromatidsreach the two poles, the chromosomal elongation starts, nucleolus and thenuclear membrane reappear. This stage is called the telophase. Nucleardivision is then followed by the cytoplasmic division and is called cytokinesis.Mitosis thus, is the equational division in which the chromosome number ofthe parent is conserved in the daughter cell.In contrast to mitosis, meiosis occurs in the diploid cells, which are destined toform gametes. It is called the reduction division since it reduces the chromosomenumber by half while making the gametes. In sexual reproduction when the twogametes fuse the chromosome number is restored to the value in the parent.Meiosis is divided into two phases â meiosis I and meiosis II. In the first meioticdivision the homologous chromosomes pair to form bivalents, and undergo crossingover. Meiosis I has a long prophase, which is divided further into five phases.These are leptotene, zygotene, pachytene, diplotene and diakinesis. Duringmetaphase I the bivalents arrange on the equatorial plate. This is followed byanaphase I in which homologous chromosomes move to the opposite poles withboth their chromatids. Each pole receives half the chromosome number of theparent cell. In telophase I, the nuclear membrane and nucleolus reappear. MeiosisII is similar to mitosis. During anaphase II the sister chromatids separate. Thus atthe end of meiosis four haploid cells are formed.EXERCISES1. What is the average cell cycle span for a mammalian cell?2. Distinguish cytokinesis from karyokinesis.3. Describe the events taking place during interphase.4. What is Go (quiescent phase) of cell cycle?2022-23172 BIOLOGY5. Why is mitosis called equational division?6. Name the stage of cell cycle at which one of the following events occur:(i) Chromosomes are moved to spindle equator.(ii) Centromere splits and chromatids separate.(iii) Pairing between homologous chromosomes takes place.(iv) Crossing over between homologous chromosomes takes place.7. Describe the following:(a) synapsis (b) bivalent (c) chiasmataDraw a diagram to illustrate your answer.8. How does cytokinesis in plant cells differ from that in animal cells?9. Find examples where the four daughter cells from meiosis are equal in size andwhere they are found unequal in size.10. Distinguish anaphase of mitosis from anaphase I of meiosis.11. List the main differences between mitosis and meiosis.12. What is the significance of meiosis?13. Discuss with your teacher about(i) haploid insects and lower plants where cell-division occurs, and(ii) some haploid cells in higher plants where cell-division does not occur.14. Can there be mitosis without DNA replication in âSâ phase?15. Can there be DNA replication without cell division?16. Analyse the events during every stage of cell cycle and notice how the followingtwo parameters change(i) number of chromosomes (N) per cell(ii) amount of DNA content (C) per cell2022-23UNIT 4The description of structure and variation of living organisms over aperiod of time, ended up as two, apparently irreconcilable perspectiveson biology. The two perspectives essentially rested on two levels oforganisation of life forms and phenomena. One described at organismicand above level of organisation while the second described at cellularand molecular level of organisation. The first resulted in ecology andrelated disciplines. The second resulted in physiology and biochemistry.Description of physiological processes, in flowering plants as anexample, is what is given in the chapters in this unit. The processes ofmineral nutrition of plants, photosynthesis, transport, respiration andultimately plant growth and development are described in molecularterms but in the context of cellular activities and even at organismlevel. Wherever appropriate, the relation of the physiological processesto environment is also discussed.PLANT PHYSIOLOGYChapter 11Transport in PlantsChapter 12Mineral NutritionChapter 13Photosynthesis in HigherPlantsChapter 14Respiration in PlantsChapter 15Plant Growth andDevelopment2022-23MELVIN CALVIN born in Minnesota in April, 1911, received hisPh.D. in Chemistry from the University of Minnesota. He servedas Professor of Chemistry at the University of California,Berkeley.Just after world war II, when the world was under shockafter the Hiroshima-Nagasaki bombings, and seeing the illeffectsof radio-activity, Calvin and co-workers put radioactivityto beneficial use. He along with J.A. Bassham studiedreactions in green plants forming sugar and other substancesfrom raw materials like carbon dioxide, water and mineralsby labelling the carbon dioxide with C14. Calvin proposed thatplants change light energy to chemical energy by transferringan electron in an organised array of pigment molecules andother substances. The mapping of the pathway of carbonassimilation in photosynthesis earned him Nobel Prize in 1961.The principles of photosynthesis as established by Calvinare, at present, being used in studies on renewable resourcefor energy and materials and basic studies in solar energyMelvin Calvin research.2022-23Have you ever wondered how water reaches the top of tall trees, or for thatmatter how and why substances move from one cell to the other, whetherall substances move in a similar way, in the same direction and whethermetabolic energy is required for moving substances. Plants need to movemolecules over very long distances, much more than animals do; they alsodo not have a circulatory system in place. Water taken up by the roots hasto reach all parts of the plant, up to the very tip of the growing stem. Thephotosynthates or food synthesised by the leaves have also to be moved toall parts including the root tips embedded deep inside the soil. Movementacross short distances, say within the cell, across the membranes and fromcell to cell within the tissue has also to take place. To understand some ofthe transport processes that take place in plants, one would have to recollectoneâs basic knowledge about the structure of the cell and the anatomy ofthe plant body. We also need to revisit our understanding of diffusion,besides gaining some knowledge about chemical potential and ions.When we talk of the movement of substances we need first to definewhat kind of movement we are talking about, and also what substanceswe are looking at. In a flowering plant the substances that would need tobe transported are water, mineral nutrients, organic nutrients and plantgrowth regulators. Over small distances substances move by diffusionand by cytoplasmic streaming supplemented by active transport.Transport over longer distances proceeds through the vascular system(the xylem and the phloem) and is called translocation.An important aspect that needs to be considered is the direction oftransport. In rooted plants, transport in xylem (of water and minerals) isessentially unidirectional, from roots to the stems. Organic and mineralnutrients however, undergo multidirectional transport. OrganicTRANSPORT IN PLANTSCHAPTER 1111.1 Means ofTransport11.2 Plant-WaterRelations11.3 Long DistanceTransport ofWater11.4 Transpiration11.5 Uptake andTransport ofMineralNutrients11.6 PhloemTransport: Flowfrom Source toSink2022-23176 BIOLOGYcompounds synthesised in the photosynthetic leaves are exported to allother parts of the plant including storage organs. From the storage organsthey are later re-exported. The mineral nutrients are taken up by theroots and transported upwards into the stem, leaves and the growingregions. When any plant part undergoes senescence, nutrients may bewithdrawn from such regions and moved to the growing parts. Hormonesor plant growth regulators and other chemical signals are also transported,though in very small amounts, sometimes in a strictly polarised orunidirectional manner from where they are synthesised to other parts.Hence, in a flowering plant there is a complex traffic of compounds (butprobably very orderly) moving in different directions, each organ receivingsome substances and giving out some others.11.1 MEANS OF TRANSPORT11.1.1 DiffusionMovement by diffusion is passive, and may be from one part of the cell tothe other, or from cell to cell, or over short distances, say, from the intercellularspaces of the leaf to the outside. No energy expenditure takes place.In diffusion, molecules move in a random fashion, the net result beingsubstances moving from regions of higher concentration to regions of lowerconcentration. Diffusion is a slow process and is not dependent on a âlivingsystemâ. Diffusion is obvious in gases and liquids, but diffusion in solids ismore likely rather than of solids. Diffusion is very important to plants sinceit is the only means for gaseous movement within the plant body.Diffusion rates are affected by the gradient of concentration, thepermeability of the membrane separating them, temperature and pressure.11.1.2 Facilitated DiffusionAs pointed out earlier, a gradient must already be present for diffusion tooccur. The diffusion rate depends on the size of the substances; obviouslysmaller substances diffuse faster. The diffusion of any substance across amembrane also depends on its solubility in lipids, the major constituent ofthe membrane. Substances soluble in lipids diffuse through the membranefaster. Substances that have a hydrophilic moiety, find it difficult to passthrough the membrane; their movement has to be facilitated. Membraneproteins provide sites at which such molecules cross the membrane. Theydo not set up a concentration gradient: a concentration gradient mustalready be present for molecules to diffuse even if facilitated by the proteins.This process is called facilitated diffusion.In facilitated diffusion special proteins help move substances acrossmembranes without expenditure of ATP energy. Facilitated diffusioncannot cause net transport of molecules from a low to a high concentrationâ this would require input of energy. Transport rate reaches a maximumwhen all of the protein transporters are being used (saturation). Facilitated2022-23TRANSPORT IN PLANTS 177diffusion is very specific: it allows cell toselect substances for uptake. It issensitive to inhibitors which react withprotein side chains.The proteins form channels in themembrane for molecules to pass through.Some channels are always open; otherscan be controlled. Some are large,allowing a variety of molecules to cross.The porins are proteins that form largepores in the outer membranes of theplastids, mitochondria and some bacteriaallowing molecules up to the size of smallproteins to pass through.Figure 11.1 shows an extracellularmolecule bound to the transport protein;the transport protein then rotates andreleases the molecule inside the cell, e.g.,water channels â made up of eightdifferent types of aquaporins.11.1.2.1 Passive symports andantiportsSome carrier or transport proteins allowdiffusion only if two types of moleculesmove together. In a symport, bothmolecules cross the membrane in the samedirection; in an antiport, they move inopposite directions (Figure 11.2). When aFigure 11.1 Facilitated diffusionUniportCarrier proteinMembraneAntiportSymportAAABBFigure 11.2 Facilitated diffusion2022-23178 BIOLOGYmolecule moves across a membrane independent of other molecules, theprocess is called uniport.11.1.3 Active TransportActive transport uses energy to transport and pump molecules against aconcentration gradient. Active transport is carried out by specificmembrane-proteins. Hence different proteins in the membrane play amajor role in both active as well as passive transport. Pumps are proteinsthat use energy to carry substances across the cell membrane. Thesepumps can transport substances from a low concentration to a highconcentration (âuphillâ transport). Transport rate reaches a maximumwhen all the protein transporters are being used or are saturated. Likeenzymes the carrier protein is very specific in what it carries across themembrane. These proteins are sensitive to inhibitors that react with proteinside chains.11.1.4 Comparison of Different Transport ProcessesTable 11.1 gives a comparison of the different transport mechanisms.Proteins in the membrane are responsible for facilitated diffusion andactive transport and hence show common characterstics of being highlyselective; they are liable to saturate, respond to inhibitors and are underhormonal regulation. But diffusion whether facilitated or not â take placeonly along a gradient and do not use energy.TABLE 11.1 Comparison of Different Transport MechanismsProperty Simple Facilitated ActiveDiffusion Transport TransportRequires special membrane proteins No Yes YesHighly selective No Yes YesTransport saturates No Yes YesUphill transport No No YesRequires ATP energy No No Yes11.2 PLANT-WATER RELATIONSWater is essential for all physiological activities of the plant and plays avery important role in all living organisms. It provides the medium inwhich most substances are dissolved. The protoplasm of the cells isnothing but water in which different molecules are dissolved and (severalparticles) suspended. A watermelon has over 92 per cent water; mostherbaceous plants have only about 10 to 15 per cent of its fresh weightas dry matter. Of course, distribution of water within a plant varies âwoody parts have relatively very little water, while soft parts mostly contain2022-23TRANSPORT IN PLANTS 179water. A seed may appear dry but it still has water â otherwise it wouldnot be alive and respiring!Terrestrial plants take up huge amount water daily but most of it islost to the air through evaporation from the leaves, i.e., transpiration. Amature corn plant absorbs almost three litres of water in a day, while amustard plant absorbs water equal to its own weight in about 5 hours.Because of this high demand for water, it is not surprising that water isoften the limiting factor for plant growth and productivity in bothagricultural and natural environments.11.2.1 Water PotentialTo comprehend plant-water relations, an understanding of certainstandard terms is necessary. Water potential (Yw) is a conceptfundamental to understanding water movement. Solute potential(Ys) and pressure potential (Yp) are the two main components thatdetermine water potential.Water molecules possess kinetic energy. In liquid and gaseous formthey are in random motion that is both rapid and constant. The greaterthe concentration of water in a system, the greater is its kinetic energy orâwater potentialâ. Hence, it is obvious that pure water will have the greatestwater potential. If two systems containing water are in contact, randommovement of water molecules will result in net movement of watermolecules from the system with higher energy to the one with lower energy.Thus water will move from the system containing water at higher waterpotential to the one having low water potential. This process of movementof substances down a gradient of free energy is called diffusion. Waterpotential is denoted by the Greek symbol Psi or Y and is expressed inpressure units such as pascals (Pa). By convention, the water potentialof pure water at standard temperatures, which is not under any pressure,is taken to be zero.If some solute is dissolved in pure water, the solution has fewer freewater molecules and the concentration (free energy) of water decreases,reducing its water potential. Hence, all solutions have a lower water potentialthan pure water; the magnitude of this lowering due to dissolution of asolute is called solute potential or Ys. Ys is always negative. The morethe solute molecules, the lower (more negative) is the Ys . For a solution atatmospheric pressure (water potential) Yw = (solute potential)Ys.If a pressure greater than atmospheric pressure is applied to purewater or a solution, its water potential increases. It is equivalent topumping water from one place to another. Can you think of any systemin our body where pressure is built up? Pressure can build up in a plantsystem when water enters a plant cell due to diffusion causing a pressurebuilt up against the cell wall, it makes the cell turgid (see section 11.2.2);2022-23180 BIOLOGYthis increases the pressure potential. Pressure potential is usuallypositive, though in plants negative potential or tension in the water columnin the xylem plays a major role in water transport up a stem. Pressurepotential is denoted as Yp.Water potential of a cell is affected by both solute and pressurepotential. The relationship between them is as follows:Yw = Ys + Yp11.2.2 OsmosisThe plant cell is surrounded by a cell membrane and a cell wall. The cellwall is freely permeable to water and substances in solution hence is nota barrier to movement. In plants the cells usually contain a large centralvacuole, whose contents, the vacuolar sap, contribute to the solutepotential of the cell. In plant cells, the cell membrane and the membraneof the vacuole, the tonoplast together are important determinants ofmovement of molecules in or out of the cell.Osmosis is the term used to refer specifically to the diffusion of water acrossa differentially- or selectively permeable membrane. Osmosis occursspontaneously in response to a driving force. The net direction and rate of osmosisdepends on both the pressure gradient and concentration gradient. Waterwill move from its region of higher chemical potential (or concentration) to itsregion of lower chemical potential until equilibrium is reached. At equilibriumthe two chambers should have nearly the same water potential.You may have made a potato osmometer in your earlier classes inschool. If the potato tuber is placed in water, the water enters the cavity inthe potato tuber containing a concentrated solution of sugar due to osmosis.Study Figure 11.3 in which the two chambers, A and B, containingsolutions are separated by a semi-permeable membrane.(a) Solution of which chamber has a lower water potential?(b) Solution of which chamber has a lower solute potential?(c) In which direction will osmosis occur?(d) Which solution has a higher solutepotential?(e) At equilibrium which chamber willhave lower water potential?(f) If one chamber has a Y of â 2000kPa, and the other â 1000 kPa, whichis the chamber that has the higherY?(g) What will be the direction of themovement of water when twosolutions with Yw = 0.2 MPa andYw = 0.1 MPa are separated by aselectively permeable membrane?Figure 11.3A BSolutemoleculeWaterSeSleecmtivi-eplye rpmeremaebaleblemembrane2022-23TRANSPORT IN PLANTS 181Let us discuss another experiment where asolution of sucrose in water taken in a funnel isseparated from pure water in a beaker by aselectively permeable membrane (Figure 11.4).You can get this kind of a membrane in an egg.Remove the yolk and albumin through a smallhole at one end of the egg, and place the shellin dilute solution of hydrochloric acid for a fewhours. The egg shell dissolves leaving themembrane intact. Water will move into the funnel,resulting in rise in the level of the solution in thefunnel. This will continue till the equilibrium isreached. In case sucrose does diffuse outthrough the membrane, will this equilibrium beever reached?External pressure can be applied from theupper part of the funnel such that no waterdiffuses into the funnel through the membrane.This pressure required to prevent water fromdiffusing is in fact, the osmotic pressure and thisis the function of the solute concentration; morethe solute concentration, greater will be thepressure required to prevent water from diffusingin. Numerically osmotic pressure is equivalentto the osmotic potential, but the sign isopposite.Osmotic pressure is the positivepressure applied, while osmotic potential isnegative.11.2.3 PlasmolysisThe behaviour of the plant cells (or tissues) withregard to water movement depends on thesurrounding solution. If the external solutionbalances the osmotic pressure of the cytoplasm,it is said to be isotonic. If the external solutionis more dilute than the cytoplasm, it ishypotonic and if the external solution is moreconcentrated, it is hypertonic. Cells swell inhypotonic solutions and shrink in hypertonicones.Plasmolysis occurs when water moves out ofthe cell and the cell membrane of a plant cellshrinks away from its cell wall. This occurs whenFigure 11.4 A demonstration of osmosis. Athistle funnel is filled withsucrose solution and keptinverted in a beaker containingwater. (a) Water will diffuseacross the membrane (asshown by arrows) to raise thelevel of the solution in thefunnel (b) Pressure can beapplied as shown to stop thewater movement into thefunnelSucrosesolutionMembranewater(a) (b)Pressure2022-23182 BIOLOGYthe cell (or tissue) is placed in a solution that is hypertonic (has more solutes)to the protoplasm. Water moves out; it is first lost from the cytoplasm andthen from the vacuole. The water when drawn out of the cell throughdiffusion into the extracellular (outside cell) fluid causes the protoplast toshrink away from the walls. The cell is said to be plasmolysed. The movementof water occurred across the membrane moving from an area of high waterpotential (i.e., the cell) to an area of lower water potential outside the cell(Figure 11.5).What occupies the space between the cell wall and the shrunkenprotoplast in the plasmolysed cell?When the cell (or tissue) is placed in an isotonic solution, there is nonet flow of water towards the inside or outside. If the external solutionbalances the osmotic pressure of the cytoplasm it is said to be isotonic.When water flows into the cell and out of the cell and are in equilibrium,the cells are said to be flaccid.The process of plasmolysis is usually reversible. When the cells areplaced in a hypotonic solution (higher water potential or dilute solutionas compared to the cytoplasm), water diffuses into the cell causing thecytoplasm to build up a pressure against the wall, that is called turgorpressure. The pressure exerted by the protoplasts due to entry of wateragainst the rigid walls is called pressure potential Yp.. Because of therigidity of the cell wall, the cell does not rupture. This turgor pressure isultimately responsible for enlargement and extension growth of cells.What would be the Yp of a flaccid cell? Which organisms other thanplants possess cell wall ?11.2.4 ImbibitionImbibition is a special type of diffusion when water is absorbed bysolids â colloids â causing them to increase in volume. The classicalFigure 11.5 Plant cell plasmolysis2022-23TRANSPORT IN PLANTS 183examples of imbibition are absorption of water by seeds and dry wood.The pressure that is produced by the swelling of wood had been used byprehistoric man to split rocks and boulders. If it were not for the pressuredue to imbibition, seedlings would not have been able to emerge out ofthe soil into the open; they probably would not have been able to establish!Imbibition is also diffusion since water movement is along aconcentration gradient; the seeds and other such materials have almost nowater hence they absorb water easily. Water potential gradient betweenthe absorbent and the liquid imbibed is essential for imbibition. In addition,for any substance to imbibe any liquid, affinity between the adsorbant andthe liquid is also a pre-requisite.11.3 LONG DISTANCE TRANSPORT OF WATERAt some earlier stage you might have carried out an experiment whereyou had placed a twig bearing white flowers in coloured water and hadwatched it turn colour. On examining the cut end of the twig after a fewhours you had noted the region through which the coloured water moved.That experiment very easily demonstrates that the path of water movementis through the vascular bundles, more specifically, the xylem. Now wehave to go further and try and understand the mechanism of movementof water and other substances up a plant.Long distance transport of substances within a plant cannot be bydiffusion alone. Diffusion is a slow process. It can account for only shortdistance movement of molecules. For example, the movement of a moleculeacross a typical plant cell (about 50 Îŧm) takes approximately 2.5 s. At thisrate, can you calculate how many years it would take for the movementof molecules over a distance of 1 m within a plant by diffusion alone?In large and complex organisms, often substances have to be movedto long distances. Sometimes the sites of production or absorption andsites of storage are too far from each other; diffusion or active transportwould not suffice. Special long distance transport systems becomenecessary so as to move substances across long distances and at a muchfaster rate. Water and minerals, and food are generally moved by a massor bulk flow system. Mass flow is the movement of substances in bulk oren masse from one point to another as a result of pressure differencesbetween the two points. It is a characteristic of mass flow that substances,whether in solution or in suspension, are swept along at the same pace,as in a flowing river. This is unlike diffusion where different substancesmove independently depending on their concentration gradients. Bulkflow can be achieved either through a positive hydrostatic pressuregradient (e.g., a garden hose) or a negative hydrostatic pressure gradient(e.g., suction through a straw).2022-23184 BIOLOGYThe bulk movement of substances through the conducting or vasculartissues of plants is called translocation.Do you remember studying cross sections of roots, stems and leavesof higher plants and studying the vascular system? The higher plantshave highly specialised vascular tissues â xylem and phloem. Xylem isassociated with translocation of mainly water, mineral salts, some organicnitrogen and hormones, from roots to the aerial parts of the plants. Thephloem translocates a variety of organic and inorganic solutes, mainlyfrom the leaves to other parts of the plants.11.3.1 How do Plants Absorb Water?We know that the roots absorb most of the water that goes into plants;obviously that is why we apply water to the soil and not on the leaves.The responsibility of absorption of water and minerals is more specificallythe function of the root hairs that are present in millions at the tips of theroots. Root hairs are thin-walled slender extensions of root epidermalcells that greatly increase the surface area for absorption. Water isabsorbed along with mineral solutes, by the root hairs, purely by diffusion.Once water is absorbed by the root hairs, it can move deeper into rootlayers by two distinct pathways:âĸ apoplast pathwayâĸ symplast pathwayThe apoplast is the system of adjacent cell walls that is continuousthroughout the plant, except at the casparian strips of the endodermisin the roots (Figure 11.6). The apoplastic movement of water occursexclusively through the intercellular spaces and the walls of the cells.Movement through the apoplast does not involve crossing the cellFigure 11.6 Pathway of water movement in the root2022-23TRANSPORT IN PLANTS 185membrane. This movement is dependent on the gradient. The apoplastdoes not provide any barrier to water movement and water movement isthrough mass flow. As water evaporates into the intercellular spaces orthe atmosphere, tension develop in the continuous stream of water in theapoplast, hence mass flow of water occurs due to the adhesive and cohesiveproperties of water.The symplastic system is the system of interconnected protoplasts.Neighbouring cells are connected through cytoplasmic strands thatextend through plasmodesmata. During symplastic movement, the watertravels through the cells â their cytoplasm; intercellular movement isthrough the plasmodesmata. Water has to enter the cells through thecell membrane, hence the movement is relatively slower. Movement is againdown a potential gradient. Symplastic movement may be aided bycytoplasmic streaming. You may have observed cytoplasmic streamingin cells of the Hydrilla leaf; the movement of chloroplast due to streamingis easily visible.Most of the water flow in the roots occurs via the apoplast since thecortical cells are loosely packed, and hence offer no resistance to watermovement. However, the inner boundary of the cortex, the endodermis,is impervious to water because of a band of suberised matrix called thecasparian strip. Water molecules are unable to penetrate the layer, sothey are directed to wall regions that are not suberised, into the cellsproper through the membranes. The water then moves through thesymplast and again crosses a membrane to reach the cells of the xylem.The movement of water through the root layers is ultimately symplasticin the endodermis. This is the onlyway water and other solutes canenter the vascular cylinder.Once inside the xylem, water isagain free to move between cells aswell as through them. In youngroots, water enters directly into thexylem vessels and/or tracheids.These are non-living conduits andso are parts of the apoplast. Thepath of water and mineral ions intothe root vascular system issummarised in Figure 11.7.Some plants have additionalstructures associated with themthat help in water (and mineral)absorption. A mycorrhiza is asymbiotic association of a funguswith a root system. The fungalPericycleCasparian PhloemApoplastic strippathSymplasticpathEndodermis XylemCortexFigure 11.7 Symplastic and apoplastic pathways ofwater and ion absorption and movement inroots2022-23186 BIOLOGYfilaments form a network around the young root or they penetrate theroot cells. The hyphae have a very large surface area that absorb mineralions and water from the soil from a much larger volume of soil that perhapsa root cannot do. The fungus provides minerals and water to the roots, inturn the roots provide sugars and N-containing compounds to themycorrhizae. Some plants have an obligate association with themycorrhizae. For example, Pinus seeds cannot germinate and establishwithout the presence of mycorrhizae.11.3.2 Water Movement up a PlantWe looked at how plants absorb water from the soil, and move it into thevascular tissues. We now have to try and understand how this water istransported to various parts of the plant. Is the water movement active, oris it still passive? Since the water has to be moved up a stem againstgravity, what provides the energy for this?11.3.2.1 Root PressureAs various ions from the soil are actively transported into the vasculartissues of the roots, water follows (its potential gradient) and increasesthe pressure inside the xylem. This positive pressure is called rootpressure, and can be responsible for pushing up water to small heightsin the stem. How can we see that root pressure exists? Choose a smallsoft-stemmed plant and on a day, when there is plenty of atmosphericmoisture, cut the stem horizontally near the base with a sharp blade,early in the morning. You will soon see drops of solution ooze out of thecut stem; this comes out due to the positive root pressure. If you fix arubber tube to the cut stem as a sleeve you can actually collect andmeasure the rate of exudation, and also determine the composition of theexudates. Effects of root pressure is also observable at night and earlymorning when evaporation is low, and excess water collects in the form ofdroplets around special openings of veins near the tip of grass blades,and leaves of many herbaceous parts. Such water loss in its liquid phaseis known as guttation.Root pressure can, at best, only provide a modest push in the overallprocess of water transport. They obviously do not play a major role inwater movement up tall trees. The greatest contribution of root pressuremay be to re-establish the continuous chains of water molecules in thexylem which often break under the enormous tensions created bytranspiration. Root pressure does not account for the majority of watertransport; most plants meet their need by transpiratory pull.11.3.2.2 Transpiration pullDespite the absence of a heart or a circulatory system in plants, theupward flow of water through the xylem in plants can achieve fairly high2022-23TRANSPORT IN PLANTS 187rates, up to 15 metres per hour. How is this movement accomplished? Along standing question is, whether water is âpushedâ or âpulledâ throughthe plant. Most researchers agree that water is mainly âpulledâ throughthe plant, and that the driving force for this process is transpiration fromthe leaves. This is referred to as the cohesion-tension-transpirationpull model of water transport. But, what generates this transpirational pull?Water is transient in plants. Less than 1 per cent of the water reachingthe leaves is used in photosynthesis and plant growth. Most of it is lostthrough the stomata in the leaves. This water loss is known astranspiration.You have studied transpiration in an earlier class by enclosing a healthyplant in polythene bag and observing the droplets of water formed insidethe bag. You could also study water loss from a leaf using cobalt chloridepaper, which turns colour on absorbing water.11.4 TRANSPIRATIONTranspiration is the evaporative loss of water by plants. It occurs mainlythrough stomata (sing. : stoma). Besides the loss of water vapour intranspiration, exchange of oxygen and carbon dioxide in the leaf also occursthrough these stomata. Normally stomata are open in the day time andclose during the night. The immediate cause of the opening or closing ofstomata is a change in the turgidity of the guard cells. The inner wall ofeach guard cell, towards the pore or stomatal aperture, is thick and elastic.When turgidity increases within the two guard cells flanking each stomatalaperture or pore, the thin outer walls bulge out and force the inner wallsinto a crescent shape. The opening of the stoma is also aided due to theorientation of the microfibrils in the cell walls of the guard cells. Cellulosemicrofibrils are oriented radially rather than longitudinally making it easierfor the stoma to open. When the guard cells lose turgor, due to water loss(or water stress) the elastic inner walls regain their original shape, the guardcells become flaccid and the stoma closes.Usually the lower surface of a dorsiventral (often dicotyledonous) leafhas a greater number of stomata while inan isobilateral (often monocotyledonous)leaf they are about equal on both surfaces.Transpiration is affected by severalexternal factors: temperature, light,humidity, wind speed. Plant factors thataffect transpiration include number anddistribution of stomata, per cent of openstomata, water status of the plant, canopystructure etc. Figure11.8 A stomatal aperture with guard cells2022-23188 BIOLOGYThe transpiration driven ascent of xylem sap depends mainly on thefollowing physical properties of water:âĸ Cohesion â mutual attraction between water molecules.âĸ Adhesion â attraction of water molecules to polar surfaces (suchas the surface of tracheary elements).âĸ Surface Tension â water molecules are attracted to each other inthe liquid phase more than to water in the gas phase.These properties give water high tensile strength, i.e., an ability toresist a pulling force, and high capillarity, i.e., the ability to rise in thintubes. In plants capillarity is aided by the small diameter of the trachearyelements â the tracheids and vessel elements.The process of photosynthesis requires water. The system of xylemvessels from the root to the leaf vein can supply the needed water. Butwhat force does a plant use to move water molecules into the leafparenchyma cells where they are needed? As water evaporates throughthe stomata, since the thin film of water over the cells is continuous, itresults in pulling of water, molecule by molecule, into the leaf from thexylem. Also, because of lower concentration of water vapour in theatmosphere as compared to the substomatal cavity and intercellularspaces, water diffuses into the surrounding air. This creates a âpullâ(Figure 11.9).Measurements reveal that the forces generated by transpiration cancreate pressures sufficient to lift a xylem sized column of water over 130metres high.XylemPhloemDiffusion intosurrounding airStomaGuard CellPalisadeFigure11.9 Water movement in the leaf. Evaporation from the leaf sets upa pressure gradient between the outside air and the air spaces of theleaf. The gradient is transmitted into the photosynthetic cells and onthe water-filled xylem in the leaf vein.Stomatalpore2022-23TRANSPORT IN PLANTS 18911.4.1 Transpiration and Photosynthesis â a CompromiseTranspiration has more than one purpose; itâĸ creates transpiration pull for absorption and transport of plantsâĸ supplies water for photosynthesisâĸ transports minerals from the soil to all parts of the plantâĸ cools leaf surfaces, sometimes 10 to 15 degrees, by evaporativecoolingâĸ maintains the shape and structure of the plants by keeping cellsturgidAn actively photosynthesising plant has an insatiable need for water.Photosynthesis is limited by available water which can be swiftly depletedby transpiration. The humidity of rainforests is largely due to this vastcycling of water from root to leaf to atmosphere and back to the soil.The evolution of the C4 photosynthetic system is probably one of thestrategies for maximising the availability of CO2 while minimising waterloss. C4 plants are twice as efficient as C3 plants in terms of fixing carbondioxide (making sugar). However, a C4 plant loses only half as much wateras a C3 plant for the same amount of CO2 fixed.11.5 UPTAKE AND TRANSPORT OF MINERAL NUTRIENTSPlants obtain their carbon and most of their oxygen from CO2 in theatmosphere. However, their remaining nutritional requirements areobtained from water and minerals in the soil.11.5.1 Uptake of Mineral IonsUnlike water, all minerals cannot be passively absorbed by the roots.Two factors account for this: (i) minerals are present in the soil as chargedparticles (ions) which cannot move across cell membranes and (ii) theconcentration of minerals in the soil is usually lower than the concentrationof minerals in the root. Therefore, most minerals must enter the root byactive absorption into the cytoplasm of epidermal cells. This needs energyin the form of ATP. The active uptake of ions is partly responsible for thewater potential gradient in roots, and therefore for the uptake of water byosmosis. Some ions also move into the epidermal cells passively.Ions are absorbed from the soil by both passive and active transport.Specific proteins in the membranes of root hair cells actively pump ionsfrom the soil into the cytoplasms of the epidermal cells. Like all cells, theendodermal cells have many transport proteins embedded in their plasmamembrane; they let some solutes cross the membrane, but not others.Transport proteins of endodermal cells are control points, where a plantadjusts the quantity and types of solutes that reach the xylem. Notethat the root endodermis because of the layer of suberin has the ability toactively transport ions in one direction only.2022-23190 BIOLOGY11.5.2 Translocation of Mineral IonsAfter the ions have reached xylem through active or passive uptake, or acombination of the two, their further transport up the stem to all parts ofthe plant is through the transpiration stream.The chief sinks for the mineral elements are the growing regions of theplant, such as the apical and lateral meristems, young leaves, developingflowers, fruits and seeds, and the storage organs. Unloading of mineralions occurs at the fine vein endings through diffusion and active uptakeby these cells.Mineral ions are frequently remobilised, particularly from older,senescing parts. Older dying leaves export much of their mineral contentto younger leaves. Similarly, before leaf fall in decidous plants, mineralsare removed to other parts. Elements most readily mobilised arephosphorus, sulphur, nitrogen and potassium. Some elements that arestructural components like calcium are not remobilised.An analysis of the xylem exudates shows that though some of thenitrogen travels as inorganic ions, much of it is carried in the organicform as amino acids and related compounds. Similarly, small amountsof P and S are carried as organic compounds. In addition, small amountof exchange of materials does take place between xylem and phloem.Hence, it is not that we can clearly make a distinction and say categoricallythat xylem transports only inorganic nutrients while phloem transportsonly organic materials, as was traditionally believed.11.6 PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINKFood, primarily sucrose, is transported by the vascular tissue phloemfrom a source to a sink. Usually the source is understood to be thatpart of the plant which synthesises the food, i.e., the leaf, and sink, thepart that needs or stores the food. But, the source and sink may bereversed depending on the season, or the plantâs needs. Sugar storedin roots may be mobilised to become a source of food in the early springwhen the buds of trees, act as sink; they need energy for growth anddevelopment of the photosynthetic apparatus. Since the source-sinkrelationship is variable, the direction of movement in the phloem canbe upwards or downwards, i.e., bi-directional. This contrasts withthat of the xylem where the movement is always unidirectional, i.e.,upwards. Hence, unlike one-way flow of water in transpiration, foodin phloem sap can be transported in any required direction so longas there is a source of sugar and a sink able to use, store or removethe sugar.Phloem sap is mainly water and sucrose, but other sugars, hormonesand amino acids are also transported or translocated through phloem.2022-23TRANSPORT IN PLANTS 19111.6.1 The Pressure Flow or Mass Flow HypothesisThe accepted mechanism used for the translocation of sugars from sourceto sink is called the pressure flow hypothesis. (see Figure 11.10). Asglucose is prepared at the source (by photosynthesis) it is converted tosucrose (a dissacharide). The sugar is then moved in the form of sucroseinto the companion cells and then into the living phloem sieve tube cellsby active transport. This process of loading at the source produces ahypertonic condition in the phloem. Water in the adjacent xylem movesinto the phloem by osmosis. As osmotic pressure builds up the phloemsap will move to areas of lower pressure. At the sink osmotic pressuremust be reduced. Again active transport is necessary to move the sucroseout of the phloem sap and into the cells which will use the sugar âconverting it into energy, starch, or cellulose. As sugars are removed, theosmotic pressure decreases and water moves out of the phloem.To summarise, the movement of sugars in the phloem begins at thesource, where sugars are loaded (actively transported) into a sieve tube.Loading of the phloem sets up a water potential gradient that facilitatesthe mass movement in the phloem.Phloem tissue is composed of sieve tube cells, which form long columnswith holes in their end walls called sieve plates. Cytoplasmic strands passthrough the holes in the sieve plates, so forming continuous filaments. Ashydrostatic pressure in the sieve tube of phloem increases, pressure flowbegins, and the sap moves through the phloem. Meanwhile, at the sink,incoming sugars are actively transported out of the phloem and removedSugars leave sieve tubefor metabolism andstorage; water followsby osmosis=HighPhloemturgorpressureRootSugars enter sieve tubes;water follows by osmosisSugar solution flowsto regions of lowturgor pressureTip of stemSugars leave sieve tubes;water follows by osmosisFigure11.10 Diagrammatic presentation of mechanism of translocation2022-23192 BIOLOGYas complex carbohydrates. The loss of solute produces a high waterpotential in the phloem, and water passes out, returning eventually to xylem.A simple experiment, called girdling, was used to identify the tissuesthrough which food is transported. On the trunk of a tree a ring of barkup to a depth of the phloem layer, can be carefully removed. In the absenceof downward movement of food the portion of the bark above the ring onthe stem becomes swollen after a few weeks. This simple experimentshows that phloem is the tissue responsible for translocation of food; andthat transport takes place in one direction, i.e., towards the roots. Thisexperiment can be performed by you easily.SUMMARYPlants obtain a variety of inorganic elements (ions) and salts from theirsurroundings especially from water and soil. The movement of these nutrientsfrom environment into the plant as well as from one plant cell to another plant cellessentially involves movement across a cell membrane. Transport across cellmembrane can be through diffusion, facilitated transport or active transport. Waterand minerals absorbed by roots are transported by xylem and the organic materialsynthesised in the leaves is transported to other parts of plant through phloem.Passive transport (diffusion, osmosis) and active transport are the two modesof nutrient transport across cell membranes in living organisms. In passivetransport, nutrients move across the membrane by diffusion, without any use ofenergy as it is always down the concentration gradient and hence entropy driven.This diffusion of substances depends on their size, solubility in water or organicsolvents. Osmosis is the special type of diffusion of water across a selectivelypermeable membrane which depends on pressure gradient and concentrationgradient. In active transport, energy in the form of ATP is utilised to pumpmolecules against a concentration gradient across membranes. Water potential isthe potential energy of water molecules which helps in the movement of water. It isdetermined by solute potential and pressure potential. The osmotic behaviour ofcells depends on the surrounding solution. If the surrounding solution of the cellis hypertonic, it gets plasmolysed. The absorption of water by seeds and drywoodtakes place by a special type of diffusion called imbibition.In higher plants, there is a vascular system comprising of xylem and phloem,responsible for translocation. Water minerals and food cannot be moved withinthe body of a plant by diffusion alone. They are therefore, transported by a massflow system â movement of substance in bulk from one point to another as aresult of pressure differences between the two points.Water absorbed by root hairs moves into the root tissue by two distinctpathways, i.e., apoplast and symplast. Various ions, and water from soil can betransported upto a small height in stems by root pressure. Transpiration pullmodel is the most acceptable to explain the transport of water. Transpiration is2022-23TRANSPORT IN PLANTS 193the loss of water in the form of vapours from the plant parts through stomata.Temperature, light, humidity, wind speed and number of stomata affect the rateof transpiration. Excess water is also removed through tips of leaves of plants byguttation.Phloem is responsible for transport of food (primarily) sucrose from the sourceto the sink. The translocation in phloem is bi-directional; the source-sinkrelationship is variable. The translocation in phloem is explained by the pressureflowhypothesis.EXERCISES1. What are the factors affecting the rate of diffusion?2. What are porins? What role do they play in diffusion?3. Describe the role played by protein pumps during active transport in plants.4. Explain why pure water has the maximum water potential.5. Differentiate between the following:(a) Diffusion and Osmosis(b) Transpiration and Evaporation(c) Osmotic Pressure and Osmotic Potential(d) Imbibition and Diffusion(e) Apoplast and Symplast pathways of movement of water in plants.(f) Guttation and Transpiration.6. Briefly describe water potential. What are the factors affecting it?7. What happens when a pressure greater than the atmospheric pressure is appliedto pure water or a solution?8. (a) With the help of well-labelled diagrams, describe the process of plasmolysisin plants, giving appropriate examples.(b) Explain what will happen to a plant cell if it is kept in a solution havinghigher water potential.9. How is the mycorrhizal association helpful in absorption of water and mineralsin plants?10. What role does root pressure play in water movement in plants?11. Describe transpiration pull model of water transport in plants. What are thefactors influencing transpiration? How is it useful to plants?12. Discuss the factors responsible for ascent of xylem sap in plants.13. What essential role does the root endodermis play during mineral absorption inplants?14. Explain why xylem transport is unidirectional and phloem transportbi-directional.15. Explain pressure flow hypothesis of translocation of sugars in plants.16. What causes the opening and closing of guard cells of stomata duringtranspiration?2022-23194 BIOLOGYThe basic needs of all living organisms are essentially the same. Theyrequire macromolecules, such as carbohydrates, proteins and fats, andwater and minerals for their growth and development.This chapter focusses mainly on inorganic plant nutrition, whereinyou will study the methods to identify elements essential to growth anddevelopment of plants and the criteria for establishing the essentiality.You will also study the role of the essential elements, their major deficiencysymptoms and the mechanism of absorption of these essential elements.The chapter also introduces you briefly to the significance and themechanism of biological nitrogen fixation.12.1 METHODS TO STUDY THE MINERAL REQUIREMENTS OF PLANTSIn 1860, Julius von Sachs, a prominent German botanist, demonstrated,for the first time, that plants could be grown to maturity in a definednutrient solution in complete absence of soil. This technique of growingplants in a nutrient solution is known as hydroponics. Since then, anumber of improvised methods have been employed to try and determinethe mineral nutrients essential for plants. The essence of all these methodsinvolves the culture of plants in a soil-free, defined mineral solution. Thesemethods require purified water and mineral nutrient salts. Can youexplain why is this so essential?After a series of experiments in which the roots of the plants wereimmersed in nutrient solutions and wherein an element was added /substituted / removed or given in varied concentration, a mineral solutionMINERAL NUTRITIONCHAPTER 1212.1 Methods toStudy theMineralRequirements ofPlants12.2 EssentialMineralElements12.3 Mechanism ofAbsorption ofElements12.4 Translocation ofSolutes12.5 Soil as Reservoirof EssentialElements12.6 Metabolism ofNitrogen2022-23MINERAL NUTRITION 195suitable for the plant growth was obtained. By thismethod, essential elements were identified andtheir deficiency symptoms discovered. Hydroponicshas been successfully employed as a technique forthe commercial production of vegetables such astomato, seedless cucumber and lettuce. It must beemphasised that the nutrient solutions must beadequately aerated to obtain the optimum growth.What would happen if solutions were poorlyaerated? Diagrammatic views of the hydroponictechnique is given in Figures 12.1 and 12.2.12.2 ESSENTIAL MINERAL ELEMENTSMost of the minerals present in soil can enter plantsthrough roots. In fact, more than sixty elements ofthe 105 discovered so far are found in differentplants. Some plant species accumulate selenium,some others gold, while some plants growing nearnuclear test sites take up radioactive strontium.There are techniques that are able to detect theminerals even at a very low concentration (10-8 g/mL). The question is, whether all the diverse mineralelements present in a plant, for example, gold andselenium as mentioned above, are really necessaryfor plants? How do we decide what is essential forplants and what is not?12.2.1 Criteria for EssentialityThe criteria for essentiality of an element are givenbelow:(a) The element must be absolutely necessary forsupporting normal growth and reproduction.In the absence of the element the plants do notcomplete their life cycle or set the seeds.(b) The requirement of the element must be specificand not replaceable by another element. Inother words, deficiency of any one elementcannot be met by supplying some otherelement.(c) The element must be directly involved in themetabolism of the plant.Figure 12.1 Diagram of a typical set-up fornutrient solution cultureFigure 12.2 Hydroponic plant production.Plants are grown in a tube ortrough placed on a slightincline. A pump circulates anutrient solution from areservoir to the elevated end ofthe tube. The solution flowsdown the tube and returns tothe reservoir due to gravity.Inset shows a plant whoseroots are continuously bathedin aerated nutrient solution.The arrows indicates thedirection of the flow.Nutrientsolution Pump2022-23196 BIOLOGYBased upon the above criteria only a few elements have been found tobe absolutely essential for plant growth and metabolism. These elementsare further divided into two broad categories based on their quantitativerequirements.(i) Macronutrients, and(ii) MicronutrientsMacronutrients are generally present in plant tissues in large amounts(in excess of 10 mmole Kg â1 of dry matter). The macronutrients includecarbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium,calcium and magnesium. Of these, carbon, hydrogen and oxygen aremainly obtained from CO2 and H2O, while the others are absorbed fromthe soil as mineral nutrition.Micronutrients or trace elements, are needed in very small amounts(less than 10 mmole Kg â1 of dry matter). These include iron, manganese,copper, molybdenum, zinc, boron, chlorine and nickel.In addition to the 17 essential elements named above, there are somebeneficial elements such as sodium, silicon, cobalt and selenium. Theyare required by higher plants.Essential elements can also be grouped into four broad categories onthe basis of their diverse functions. These categories are:(i) Essential elements as components of biomolecules and hencestructural elements of cells (e.g., carbon, hydrogen, oxygen andnitrogen).(ii) Essential elements that are components of energy-related chemicalcompounds in plants (e.g., magnesium in chlorophyll andphosphorous in ATP).(iii) Essential elements that activate or inhibit enzymes, for exampleMg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenaseand phosphoenol pyruvate carboxylase, both of whichare critical enzymes in photosynthetic carbon fixation; Zn2+ is anactivator of alcohol dehydrogenase and Mo of nitrogenase duringnitrogen metabolism. Can you name a few more elements thatfall in this category? For this, you will need to recollect some ofthe biochemical pathways you have studied earlier.(iv) Some essential elements can alter the osmotic potential of a cell.Potassium plays an important role in the opening and closing ofstomata. You may recall the role of minerals as solutes indetermining the water potential of a cell.12.2.2 Role of Macro- and Micro-nutrientsEssential elements perform several functions. They participate in variousmetabolic processes in the plant cells such as permeability of cell2022-23MINERAL NUTRITION 197membrane, maintenance of osmotic concentration of cell sap, electrontransportsystems, buffering action, enzymatic activity and act as majorconstituents of macromolecules and co-enzymes.Various forms and functions of essential nutrient elements are givenbelow.Nitrogen: This is the essential nutrient element required by plants in thegreatest amount. It is absorbed mainly as NO3â though some are also takenup as NO2â or NH4+. Nitrogen is required by all parts of a plant, particularlythe meristematic tissues and the metabolically active cells. Nitrogen is one ofthe major constituents of proteins, nucleic acids, vitamins and hormones.Phosphorus: Phosphorus is absorbed by the plants from soil in the formof phosphate ions (either as H PO 2 4â or HPO42â ). Phosphorus is aconstituent of cell membranes, certain proteins, all nucleic acids andnucleotides, and is required for all phosphorylation reactions.Potassium: It is absorbed as potassium ion (K+). In plants, this is requiredin more abundant quantities in the meristematic tissues, buds, leavesand root tips. Potassium helps to maintain an anion-cation balance incells and is involved in protein synthesis, opening and closing of stomata,activation of enzymes and in the maintenance of the turgidity of cells.Calcium: Plant absorbs calcium from the soil in the form of calcium ions(Ca2+). Calcium is required by meristematic and differentiating tissues.During cell division it is used in the synthesis of cell wall, particularly ascalcium pectate in the middle lamella. It is also needed during theformation of mitotic spindle. It accumulates in older leaves. It is involvedin the normal functioning of the cell membranes. It activates certainenzymes and plays an important role in regulating metabolic activities.Magnesium: It is absorbed by plants in the form of divalent Mg2+. Itactivates the enzymes of respiration, photosynthesis and are involved inthe synthesis of DNA and RNA. Magnesium is a constituent of the ringstructure of chlorophyll and helps to maintain the ribosome structure.Sulphur: Plants obtain sulphur in the form of sulphate (SO ) 42â . Sulphur ispresent in two amino acids â cysteine and methionine and is the mainconstituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A)and ferredoxin.Iron: Plants obtain iron in the form of ferric ions (Fe3+). It is required inlarger amounts in comparison to other micronutrients. It is an importantconstituent of proteins involved in the transfer of electrons like ferredoxinand cytochromes. It is reversibly oxidised from Fe2+ to Fe3+ during electrontransfer. It activates catalase enzyme, and is essential for the formation ofchlorophyll.2022-23198 BIOLOGYManganese: It is absorbed in the form of manganous ions (Mn2+). Itactivates many enzymes involved in photosynthesis, respiration andnitrogen metabolism. The best defined function of manganese is in thesplitting of water to liberate oxygen during photosynthesis.Zinc: Plants obtain zinc as Zn2+ ions. It activates various enzymes,especially carboxylases. It is also needed in the synthesis of auxin.Copper: It is absorbed as cupric ions (Cu2+). It is essential for the overallmetabolism in plants. Like iron, it is associated with certain enzymesinvolved in redox reactions and is reversibly oxidised from Cu+ to Cu2+.Boron : It is absorbed as BO33â or B O 4 72â . Boron is required for uptakeand utilisation of Ca2+, membrane functioning, pollen germination, cellelongation, cell differentiation and carbohydrate translocation.Molybdenum: Plants obtain it in the form of molybdate ions (MoO ) 22+ . Itis a component of several enzymes, including nitrogenase and nitratereductase both of which participate in nitrogen metabolism.Chlorine: It is absorbed in the form of chloride anion (Clâ). Along withNa+ and K+, it helps in determining the solute concentration and the anioncationbalance in cells. It is essential for the water-splitting reaction inphotosynthesis, a reaction that leads to oxygen evolution.12.2.3 Deficiency Symptoms of Essential ElementsWhenever the supply of an essential element becomes limited, plant growthis retarded. The concentration of the essential element below which plantgrowth is retarded is termed as critical concentration. The element issaid to be deficient when present below the critical concentration.Since each element has one or more specific structural or functionalrole in plants, in the absence of any particular element, plants show certainmorphological changes. These morphological changes are indicative ofcertain element deficiencies and are called deficiency symptoms. Thedeficiency symptoms vary from element to element and they disappearwhen the deficient mineral nutrient is provided to the plant. However, ifdeprivation continues, it may eventually lead to the death of the plant. Theparts of the plants that show the deficiency symptoms also depend on themobility of the element in the plant. For elements that are actively mobilisedwithin the plants and exported to young developing tissues, the deficiencysymptoms tend to appear first in the older tissues. For example, thedeficiency symptoms of nitrogen, potassium and magnesium are visiblefirst in the senescent leaves. In the older leaves, biomolecules containingthese elements are broken down, making these elements available formobilising to younger leaves.The deficiency symptoms tend to appear first in the young tissueswhenever the elements are relatively immobile and are not transportedout of the mature organs, for example, element like sulphur and2022-23MINERAL NUTRITION 199calcium are a part of the structural component of the cell and hence arenot easily released. This aspect of mineral nutrition of plants is of a greatsignificance and importance to agriculture and horticulture.The kind of deficiency symptoms shown in plants include chlorosis,necrosis, stunted plant growth, premature fall of leaves and buds, andinhibition of cell division. Chlorosis is the loss of chlorophyll leading toyellowing in leaves. This symptom is caused by the deficiency of elementsN, K, Mg, S, Fe, Mn, Zn and Mo. Likewise, necrosis, or death of tissue,particularly leaf tissue, is due to the deficiency of Ca, Mg, Cu, K. Lack orlow level of N, K, S, Mo causes an inhibition of cell division. Some elementslike N, S, Mo delay flowering if their concentration in plants is low.You can see from the above that the deficiency of any element cancause multiple symptoms and that the same symptoms may be causedby the deficiency of one of several different elements. Hence, to identifythe deficient element, one has to study all the symptoms developed in allthe various parts of the plant and compare them with the availablestandard tables. We must also be aware that different plants also responddifferently to the deficiency of the same element.12.2.4 Toxicity of MicronutrientsThe requirement of micronutrients is always in low amounts while theirmoderate decrease causes the deficiency symptoms and a moderate increasecauses toxicity. In other words, there is a narrow range of concentration atwhich the elements are optimum. Any mineral ion concentration in tissuesthat reduces the dry weight of tissues by about 10 per cent is consideredtoxic. Such critical concentrations vary widely among differentmicronutrients. The toxicity symptoms are difficult to identify. Toxicity levelsfor any element also vary for different plants. Many a times, excess of anelement may inhibit the uptake of another element. For example, theprominent symptom of manganese toxicity is the appearance of brownspots surrounded by chlorotic veins. It is important to know thatmanganese competes with iron and magnesium for uptake and withmagnesium for binding with enzymes. Manganese also inhibit calciumtranslocation in shoot apex. Therefore, excess of manganese may, in fact,induce deficiencies of iron, magnesium and calcium. Thus, what appearsas symptoms of manganese toxicity may actually be the deficiencysymptoms of iron, magnesium and calcium. Can this knowledge be of someimportance to a farmer? a gardener? or even for you in your kitchen-garden?12.3 MECHANISM OF ABSORPTION OF ELEMENTSMuch of the studies on mechanism of absorption of elements by plantshas been carried out in isolated cells, tissues or organs. These studies2022-23200 BIOLOGYrevealed that the process of absorption can be demarcated into two mainphases. In the first phase, an initial rapid uptake of ions into the âfreespaceâ or âouter spaceâ of cells â the apoplast, is passive. In the secondphase of uptake, the ions are taken in slowly into the âinner spaceâ â thesymplast of the cells. The passive movement of ions into the apoplastusually occurs through ion-channels, the trans-membrane proteins thatfunction as selective pores. On the other hand, the entry or exit of ions toand from the symplast requires the expenditure of metabolic energy, whichis an active process. The movement of ions is usually called flux; theinward movement into the cells is influx and the outward movement, efflux.You have read the aspects of mineral nutrient uptake and translocationin plants in Chapter 11.12.4 TRANSLOCATION OF SOLUTESMineral salts are translocated through xylem along with the ascendingstream of water, which is pulled up through the plant by transpirationalpull. Analysis of xylem sap shows the presence of mineral salts in it. Useof radioisotopes of mineral elements also substantiate the view that theyare transported through the xylem. You have already discussed themovement of water in xylem in Chapter 11.12.5 SOIL AS RESERVOIR OF ESSENTIAL ELEMENTSMajority of the nutrients that are essential for the growth anddevelopment of plants become available to the roots due to weatheringand breakdown of rocks. These processes enrich the soil with dissolvedions and inorganic salts. Since they are derived from the rock minerals,their role in plant nutrition is referred to as mineral nutrition. Soilconsists of a wide variety of substances. Soil not only supplies mineralsbut also harbours nitrogen-fixing bacteria, other microbes, holds water,supplies air to the roots and acts as a matrix that stabilises the plant.Since deficiency of essential minerals affect the crop-yield, there is oftena need for supplying them through fertilisers. Both macro-nutrients(N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.) formcomponents of fertilisers and are applied as per need.12.6 METABOLISM OF NITROGEN12.6.1 Nitrogen CycleApart from carbon, hydrogen and oxygen, nitrogen is the mostprevalent element in living organisms. Nitrogen is a constituent ofamino acids, proteins, hormones, chlorophylls and many of thevitamins. Plants compete with microbes for the limited nitrogen that2022-23MINERAL NUTRITION 201is available in soil. Thus, nitrogen isa limiting nutrient for both naturaland agricultural eco-systems.Nitrogen exists as two nitrogen atomsjoined by a very strong triple covalentbond (N Âē N). The process ofconversion of nitrogen (N2) toammonia is termed as nitrogenfixation.In nature, lightning andultraviolet radiation provide enoughenergy to convert nitrogen to nitrogenoxides (NO, NO2, N2O). Industrialcombustions, forest fires, automobileexhausts and power-generatingstations are also sources ofatmospheric nitrogen oxides.Decomposition of organic nitrogen ofdead plants and animals intoammonia is called ammonification.Some of this ammonia volatilises andre-enters the atmosphere but most ofit is converted into nitrate by soilbacteria in the following steps:Figure 12.3 The nitrogen cycle showingrelationship between the threemain nitrogen pools â atmosphericsoil, and biomass2 3 2 2 2 3 2 2 2 NH + O žžŽ NO + H + H Oâ + .... (i)2 2 2 2 3 NO O NO â â+ žžŽ ...... (ii)Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/orNitrococcus. The nitrite is further oxidised to nitrate with the help of thebacterium Nitrobacter. These steps are called nitrification (Figure 12.3).These nitrifying bacteria are chemoautotrophs.The nitrate thus formed is absorbed by plants and is transported tothe leaves. In leaves, it is reduced to form ammonia that finally forms theamine group of amino acids. Nitrate present in the soil is also reduced tonitrogen by the process of denitrification. Denitrification is carried bybacteria Pseudomonas and Thiobacillus.12.6.2 Biological Nitrogen FixationVery few living organisms can utilise the nitrogen in the form N2, availableabundantly in the air. Only certain prokaryotic species are capable offixing nitrogen. Reduction of nitrogen to ammonia by living organisms is2022-23202 BIOLOGYcalled biological nitrogen fixation. The enzyme, nitrogenase which iscapable of nitrogen reduction is present exclusively in prokaryotes. Suchmicrobes are called N2- fixers.N N NHNitrogenaseÂē žžžžžžžŽ3The nitrogen-fixing microbes could be free-living or symbiotic. Examplesof free-living nitrogen-fixing aerobic microbes are Azotobacter andBeijerinckia while Rhodospirillum is anaerobic and free-living. In addition,a number of cyanobacteria such as Anabaena and Nostoc are also freelivingnitrogen-fixers.Symbiotic biological nitrogen fixationSeveral types of symbiotic biological nitrogen fixing associations are known.The most prominent among them is the legume-bacteria relationship.Species of rod-shaped Rhizobium has such relationship with the roots ofseveral legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea,broad bean, clover beans, etc. The most common association on roots isas nodules. These nodules are small outgrowths on the roots. The microbe,Frankia, also produces nitrogen-fixing nodules on the roots of nonleguminousplants (e.g., Alnus). Both Rhizobium and Frankia are freelivingin soil, but as symbionts, can fix atmospheric nitrogen.Uproot any one plant of a common pulse, just before flowering. Youwill see near-spherical outgrowths on the roots. These are nodules. Ifyou cut through them you will notice that the central portion is red orpink. What makes the nodules pink? This is due to the presence ofleguminous haemoglobin or leg-haemoglobin.Nodule FormationNodule formation involves a sequence of multiple interactions betweenRhizobium and roots of the host plant. Principal stages in the noduleformation are summarised as follows:Rhizobia multiply and colonise the surroundings of roots and get attachedto epidermal and root hair cells. The root-hairs curl and the bacteria invadethe root-hair. An infection thread is produced carrying the bacteria intothe cortex of the root, where they initiate the nodule formation in the cortexof the root. Then the bacteria are released from the thread into the cellswhich leads to the differentiation of specialised nitrogen fixing cells. Thenodule thus formed, establishes a direct vascular connection with the hostfor exchange of nutrients. These events are depicted in Figure 12.4.The nodule contains all the necessary biochemical components, suchas the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenaseis a Mo-Fe protein and catalyses the conversion of atmospheric nitrogento ammonia, (Figure 12.5) the first stable product of nitrogen fixation.2022-23MINERAL NUTRITION 203The reaction is as follows:â2 3 2 i N 8e 8H 16ATP 2NH H 16ADP 16P ++ + + žžŽ + + +The enzyme nitrogenase is highly sensitive to the molecular oxygen; itrequires anaerobic conditions. The nodules have adaptations that ensurethat the enzyme is protected from oxygen. To protect these enzymes, thenodule contains an oxygen scavenger called leg-haemoglobin. It is interestingto note that these microbes live as aerobes under free-living conditions (wherenitrogenase is not operational), but during nitrogen-fixing events, they becomeanaerobic (thus protecting the nitrogenase enzyme). You must have noticedin the above reaction that the ammonia synthesis by nitrogenease requires aSoilparticlesRoot hairBacteriaInner cortex andpericycle cellsunder divisionInfectionthreadcontainingbacteriaMature noduleHookBacteriaFigure 12.4 Development of root nodules in soyabean : (a) Rhizobium bacteriacontact a susceptible root hair, divide near it, (b) Successful infectionof the root hair causes it to curl, (c) Infected thread carries the bacteriato the inner cortex. The bacteria get modified into rod-shapedbacteroids and cause inner cortical and pericycle cells to divide.Division and growth of cortical and pericycle cells lead to noduleformation, (d) A mature nodule is complete with vascular tissuescontinuous with those of the root(a)N+2 HHHH HHHHHH HHHHHHNN NN NNEnzymeSubstrate[nitrogen gas (N )] 2Reduction ReductionReductionBinding(nitrogenase) of substrateProduct[ammonia (NH )] 3Releaseof productsFree nitrogenasecan bind anothermolecule of N2+2 H +2 HNNHN HHNNFigure 12.5 Steps of conversion of atmospheric nitrogen to ammonia by nitrogenaseenzyme complex found in nitrogen-fixing bacteria2022-23204 BIOLOGYvery high input of energy (8 ATP for each NH3 produced). The energy required,thus, is obtained from the respiration of the host cells.Fate of ammonia: At physiological pH, the ammonia is protonated to formNH4+ (ammonium) ion. While most of the plants can assimilate nitrate as wellas ammonium ions, the latter is quite toxic to plants and hence cannotaccumulate in them. Let us now see how the NH4+ is used to synthesiseamino acids in plants. There are two main ways in which this can take place:(i) Reductive amination : In these processes, ammonia reacts witha-ketoglutaric acid and forms glutamic acid as indicated in theequation given below :(ii) Transamination : It involves the transfer of amino group from oneamino acid to the keto group of a keto acid. Glutamic acid is the mainamino acid from which the transfer of NH2, the amino group takesplace and other amino acids are formed through transamination. Theenzyme transaminase catalyses all such reactions. For example,a â + + žžžžžž ÂŽ+ ketoglutaric acid NH NADPH Glutamate4 Dehydrogenasežž glutamate + H O + NADP 2The two most important amides â asparagine and glutamine â found inplants are a structural part of proteins. They are formed from two aminoacids, namely aspartic acid and glutamic acid, respectively, by additionof another amino group to each. The hydroxyl part of the acid is replacedby another NH2â radicle. Since amides contain more nitrogen than theamino acids, they are transported to other parts of the plant via xylemvessels. In addition, along with the transpiration stream the nodules ofsome plants (e.g., soyabean) export the fixed nitrogen as ureides. Thesecompounds also have a particularly high nitrogen to carbon ratio.SUMMARYPlants obtain their inorganic nutrients from air, water and soil. Plants absorb awide variety of mineral elements. Not all the mineral elements that they absorb arerequired by plants. Out of the more than 105 elements discovered so far, less than21 are essential and beneficial for normal plant growth and development. Theelements required in large quantities are called macronutrients while those requiredin less quantities or in trace are termed as micronutrients. These elements areeither essential constituents of proteins, carbohydrates, fats, nucleic acid etc.,2022-23MINERAL NUTRITION 205and/or take part in various metabolic processes. Deficiency of each of theseessential elements may lead to symptoms called deficiency symptoms. Chlorosis,necrosis, stunted growth, impaired cell division, etc., are some prominent deficiencysymptoms. Plants absorb minerals through roots by either passive or activeprocesses. They are carried to all parts of the organism through xylem along withwater transport.Nitrogen is very essential for the sustenance of life. Plants cannot useatmospheric nitrogen directly. But some of the plants in association with N2-fixingbacteria, especially roots of legumes, can fix this atmospheric nitrogen intobiologically usable forms. Nitrogen fixation requires a strong reducing agent andenergy in the form of ATP. N2 -fixation is accomplished with the help of nitrogenfixingmicrobes, mainly Rhizobium. The enzyme nitrogenase which plays animportant role in biological N2 fixation is very sensitive to oxygen. Most of theprocesses take place in anaerobic environment. The energy, ATP, required isprovided by the respiration of the host cells. Ammonia produced following N2 fixationis incorporated into amino acids as the amino group.EXERCISES1. âAll elements that are present in a plant need not be essential to its survivalâ.Comment.2. Why is purification of water and nutrient salts so important in studies involvingmineral nutrition using hydroponics?3. Explain with examples: macronutrients, micronutrients, beneficial nutrients,toxic elements and essential elements.4. Name at least five different deficiency symptoms in plants. Describe them andcorrelate them with the concerned mineral deficiency.5. If a plant shows a symptom which could develop due to deficiency of more thanone nutrient, how would you find out experimentally, the real deficient mineralelement?6. Why is that in certain plants deficiency symptoms appear first in younger partsof the plant while in others they do so in mature organs?7. How are the minerals absorbed by the plants?8. What are the conditions necessary for fixation of atmospheric nitrogen byRhizobium. What is their role in N2 -fixation?9. What are the steps involved in formation of a root nodule?10. Which of the following statements are true? If false, correct them:(a) Boron deficiency leads to stout axis.(b) Every mineral element that is present in a cell is needed by the cell.(c) Nitrogen as a nutrient element, is highly immobile in the plants.(d) It is very easy to establish the essentiality of micronutrients because theyare required only in trace quantities.2022-23206 BIOLOGYAll animals including human beings depend on plants for their food. Haveyou ever wondered from where plants get their food? Green plants, in fact,have to make or rather synthesise the food they need and all other organismsdepend on them for their needs. The green plants make or rather synthesisethe food they need through photosynthesis and are therefore called autotrophs.You have already learnt that the autotrophic nutrition is found only in plantsand all other organisms that depend on the green plants for food areheterotrophs. Green plants carry out âphotosynthesisâ, a physico-chemicalprocess by which they use light energy to drive the synthesis of organiccompounds. Ultimately, all living forms on earth depend on sunlight forenergy. The use of energy from sunlight by plants doing photosynthesis isthe basis of life on earth. Photosynthesis is important due to two reasons: itis the primary source of all food on earth. It is also responsible for the releaseof oxygen into the atmosphere by green plants. Have you ever thought whatwould happen if there were no oxygen to breath? This chapter focusses onthe structure of the photosynthetic machinery and the various reactionsthat transform light energy into chemical energy.13.1 WHAT DO WE KNOW?Let us try to find out what we already know about photosynthesis. Somesimple experiments you may have done in the earlier classes have shownthat chlorophyll (green pigment of the leaf), light and CO2 are required forphotosynthesis to occur.You may have carried out the experiment to look for starch formationin two leaves â a variegated leaf or a leaf that was partially covered withblack paper, and exposed to light. On testing these leaves for the presenceof starch it was clear that photosynthesis occurred only in the green partsof the leaves in the presence of light.PHOTOSYNTHESIS IN HIGHER PLANTSCHAPTER 1313.1 What do weKnow?13.2 EarlyExperiments13.3 Where doesPhotosynthesistake place?13.4 How manyPigments areinvolved inPhotosynthesis?13.5 What is LightReaction?13.6 The ElectronTransport13.7 Where are theATP and NADPHUsed?13.8 The C4 Pathway13.9 Photorespiration13.10 FactorsaffectingPhotosynthesis2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 207Another experiment you may have carried outwhere a part of a leaf is enclosed in a test tubecontaining some KOH soaked cotton (whichabsorbs CO2), while the other half is exposed to air.The setup is then placed in light for some time. Ontesting for the presence of starch later in the twoparts of the leaf, you must have found that theexposed part of the leaf tested positive for starchwhile the portion that was in the tube, testednegative. This showed that CO2 was required forphotosynthesis. Can you explain how thisconclusion could be drawn?13.2 EARLY EXPERIMENTSIt is interesting to learn about those simpleexperiments that led to a gradual development inour understanding of photosynthesis.Joseph Priestley (1733-1804) in 1770performed a series of experiments that revealed theessential role of air in the growth of green plants.Priestley, you may recall, discovered oxygen in1774. Priestley observed that a candle burning ina closed space â a bell jar, soon gets extinguished(Figure 13.1 a, b, c, d). Similarly, a mouse wouldsoon suffocate in a closed space. He concluded thata burning candle or an animal that breathe the air,both somehow, damage the air. But when he placed a mint plant in thesame bell jar, he found that the mouse stayed alive and the candlecontinued to burn. Priestley hypothesised as follows: Plants restore tothe air whatever breathing animals and burning candles remove.Can you imagine how Priestley would have conducted the experimentusing a candle and a plant? Remember, he would need to rekindle thecandle to test whether it burns after a few days. How many differentways can you think of to light the candle without disturbing the set-up?Using a similar setup as the one used by Priestley, but by placing itonce in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)showed that sunlight is essential to the plant process that somehowpurifies the air fouled by burning candles or breathing animals.Ingenhousz in an elegant experiment with an aquatic plant showed thatin bright sunlight, small bubbles were formed around the green partswhile in the dark they did not. Later he identified these bubbles to be ofoxygen. Hence he showed that it is only the green part of the plants thatcould release oxygen.(a)(c)(b)(d)Figure 13.1 Priestleyâs experiment2022-23208 BIOLOGYIt was not until about 1854 that Julius von Sachs provided evidencefor production of glucose when plants grow. Glucose is usually stored asstarch. His later studies showed that the green substance in plants(chlorophyll as we know it now) is located in special bodies (later calledchloroplasts) within plant cells. He found that the green parts in plants iswhere glucose is made, and that the glucose is usually stored as starch.Now consider the interesting experiments done by T.W Engelmann(1843 â 1909). Using a prism he split light into its spectral componentsand then illuminated a green alga, Cladophora, placed in a suspensionof aerobic bacteria. The bacteria were used to detect the sites of O2evolution. He observed that the bacteria accumulated mainly in the regionof blue and red light of the split spectrum. A first action spectrum ofphotosynthesis was thus described. It resembles roughly the absorptionspectra of chlorophyll a and b (discussed in section 13.4).By the middle of the nineteenth century the key features of plantphotosynthesis were known, namely, that plants could use light energyto make carbohydrates from CO2 and water. The empirical equationrepresenting the total process of photosynthesis for oxygen evolvingorganisms was then understood as:CO H O CH O OLight2 2 2 2+ žžžžžŽ[ ]+where [CH2O] represented a carbohydrate (e.g., glucose, a six-carbonsugar).A milestone contribution to the understanding of photosynthesis wasthat made by a microbiologist, Cornelius van Niel (1897-1985), who,based on his studies of purple and green bacteria, demonstrated thatphotosynthesis is essentially a light-dependent reaction in whichhydrogen from a suitable oxidisable compound reduces carbon dioxideto carbohydrates. This can be expressed by:2 2 2 2 2 2 H A CO A CH O H OLight+ žžžžžŽ + +In green plants H2O is the hydrogen donor and is oxidised to O2. Someorganisms do not release O2 during photosynthesis. When H2S, insteadis the hydrogen donor for purple and green sulphur bacteria, theâoxidationâ product is sulphur or sulphate depending on the organismand not O2. Hence, he inferred that the O2 evolved by the green plantcomes from H2O, not from carbon dioxide. This was later proved by usingradioisotopic techniques. The correct equation, that would represent theoverall process of photosynthesis is therefore:6 12 6 6 2 2 6 12 6 2 2 CO H O C H O H O OLight+ žžžžžŽ + +where C6 H12 O6 represents glucose. The O2 released is from water; thiswas proved using radio isotope techniques. Note that this is not a single2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 209reaction but description of a multistep process called photosynthesis.Can you explain why twelve molecules of water as substrate are usedin the equation given above?13.3 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?You would of course answer: in âthe green leafâ or âin the chloroplastsâ,based on what you earlier read in Chapter 8. You are definitely right.Photosynthesis does take place in the green leaves of plants but it does soalso in other green parts of the plants. Can you name some other partswhere you think photosynthesis may occur?You would recollect from previous unit that the mesophyll cells in theleaves, have a large number of chloroplasts. Usually the chloroplasts alignthemselves along the walls of the mesophyll cells, such that they get theoptimum quantity of the incident light. When do you think thechloroplasts will be aligned with their flat surfaces parallel to the walls?When would they be perpendicular to the incident light?You have studied the structure of chloroplast in Chapter 8. Withinthe chloroplast there is membranous system consisting of grana, thestroma lamellae, and the matrix stroma (Figure 13.2). There is a cleardivision of labour within the chloroplast. The membrane system isresponsible for trapping the light energy and also for the synthesis of ATPand NADPH. In stroma, enzymatic reactions synthesise sugar, which inturn forms starch. The former set of reactions, since they are directly lightdriven are called light reactions (photochemical reactions). The latterare not directly light driven but are dependent on the products of lightreactions (ATP and NADPH). Hence, to distinguish the latter they are called,by convention, as dark reactions (carbon reactions). However, this shouldnot be construed to mean that they occur in darkness or that they are notlight-dependent.Figure 13.2 Diagrammatic representation of an electron micrograph of a section ofchloroplastOuter membraneInner membraneStromal lamellaGranaStromaRibosomesStarch granuleLipid droplet2022-23210 BIOLOGY13.4 HOW MANY TYPES OF PIGMENTS AREINVOLVED IN PHOTOSYNTHESIS?Looking at plants have you ever wondered whyand how there are so many shades of green intheir leaves â even in the same plant? We canlook for an answer to this question by trying toseparate the leaf pigments of any green plantthrough paper chromatography. Achromatographic separation of the leaf pigmentsshows that the colour that we see in leaves isnot due to a single pigment but due to fourpigments: Chlorophyll a (bright or blue greenin the chromatogram), chlorophyll b (yellowgreen), xanthophylls (yellow) and carotenoids(yellow to yellow-orange). Let us now see whatroles various pigments play in photosynthesis.Pigments are substances that have an abilityto absorb light, at specific wavelengths. Can youguess which is the most abundant plantpigment in the world? Let us study the graphshowing the ability of chlorophyll a pigment toabsorb lights of different wavelengths (Figure13.3 a). Of course, you are familiar with thewavelength of the visible spectrum of light aswell as the VIBGYOR.From Figure 13.3a can you determine thewavelength (colour of light) at which chlorophylla shows the maximum absorption? Does itshow another absorption peak at any otherwavelengths too? If yes, which one?Now look at Figure 13.3b showing thewavelengths at which maximum photosynthesisoccurs in a plant. Can you see that thewavelengths at which there is maximumabsorption by chlorophyll a, i.e., in the blue andthe red regions, also shows higher rate ofphotosynthesis. Hence, we can conclude thatchlorophyll a is the chief pigment associatedwith photosynthesis. But by looking at Figure13.3c can you say that there is a completeone-to-one overlap between the absorptionspectrum of chlorophyll a and the actionspectrum of photosynthesis?Figure 13.3a Graph showing the absorptionspectrum of chlorophyll a, b andthe carotenoidsFigure 13.3b Graph showing actionspectrum of photosynthesisFigure 13.3c Graph showing actionspectrum of photosynthesissuperimposed on absorptionspectrum of chlorophyll a2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 211These graphs, together, show that most of the photosynthesis takesplace in the blue and red regions of the spectrum; some photosynthesisdoes take place at the other wavelengths of the visible spectrum. Let ussee how this happens. Though chlorophyll is the major pigmentresponsible for trapping light, other thylakoid pigments like chlorophyllb, xanthophylls and carotenoids, which are called accessory pigments,also absorb light and transfer the energy to chlorophyll a. Indeed, theynot only enable a wider range of wavelength of incoming light to be utilisedfor photosyntesis but also protect chlorophyll a from photo-oxidation.13.5 WHAT IS LIGHT REACTION?Light reactions or the âPhotochemicalâ phaseinclude light absorption, water splitting, oxygenrelease, and the formation of high-energychemical intermediates, ATP and NADPH.Several protein complexes are involved in theprocess. The pigments are organised into twodiscrete photochemical light harvestingcomplexes (LHC) within the Photosystem I (PSI) and Photosystem II (PS II). These are namedin the sequence of their discovery, and not inthe sequence in which they function during thelight reaction. The LHC are made up ofhundreds of pigment molecules bound toproteins. Each photosystem has all the pigments(except one molecule of chlorophyll a) forminga light harvesting system also called antennae(Figure 13.4). These pigments help to makephotosynthesis more efficient by absorbingdifferent wavelengths of light. The single chlorophyll a molecule formsthe reaction centre. The reaction centre is different in both thephotosystems. In PS I the reaction centre chlorophyll a has an absorptionpeak at 700 nm, hence is called P700, while in PS II it has absorptionmaxima at 680 nm, and is called P680.13.6 THE ELECTRON TRANSPORTIn photosystem II the reaction centre chlorophyll a absorbs 680 nmwavelength of red light causing electrons to become excited and jumpinto an orbit farther from the atomic nucleus. These electrons are pickedup by an electron acceptor which passes them to an electrons transportPhoton ReactioncentrePigmentmoleculesPrimary acceptorFigure 13.4 The light harvesting complex2022-23212 BIOLOGYsystem consisting of cytochromes (Figure13.5). This movement of electrons is downhill,in terms of an oxidation-reduction or redoxpotential scale. The electrons are not used upas they pass through the electron transportchain, but are passed on to the pigments ofphotosystem PS I. Simultaneously, electronsin the reaction centre of PS I are also excitedwhen they receive red light of wavelength 700nm and are transferred to another acceptermolecule that has a greater redox potential.These electrons then are moved downhillagain, this time to a molecule of energy-richNADP+. The addition of these electrons reducesNADP+ to NADPH + H+. This whole scheme oftransfer of electrons, starting from the PS II,uphill to the acceptor, down the electrontransport chain to PS I, excitation of electrons,transfer to another acceptor, and finally down hill to NADP+ reducing it toNADPH + H+ is called the Z scheme, due to its characterstic shape (Figure13.5). This shape is formed when all the carriers are placed in a sequenceon a redox potential scale.13.6.1 Splitting of WaterYou would then ask, How does PS II supply electrons continuously? Theelectrons that were moved from photosystem II must be replaced. This isachieved by electrons available due to splitting of water. The splitting ofwater is associated with the PS II; water is split into 2H+, [O] and electrons.This creates oxygen, one of the net products of photosynthesis. Theelectrons needed to replace those removed from photosystem I are providedby photosystem II.2 4 4 2 2 H O žžŽ H +O + e+ âWe need to emphasise here that the water splitting complex is associatedwith the PS II, which itself is physically located on the inner side of themembrane of the thylakoid. Then, where are the protons and O2 formedlikely to be released â in the lumen? or on the outer side of the membrane?13.6.2 Cyclic and Non-cyclic Photo-phosphorylationLiving organisms have the capability of extracting energy from oxidisablesubstances and store this in the form of bond energy. Special substances likeATP, carry this energy in their chemical bonds. The process through whichElectrontransportsystem--e acceptore acceptorLightPhotosystem II Photosystem INADPHNADP+LHCLHCH2O 2e + 2H + [O] - +ADP+iP ATPFigure 13.5 Z scheme of light reaction2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 213ATP is synthesised by cells (in mitochondria andchloroplasts) is named phosphorylation. Photophosphorylationis the synthesis of ATP fromADP and inorganic phosphate in the presence oflight. When the two photosystems work in aseries, first PS II and then the PS I, a process callednon-cyclic photo-phosphorylation occurs. Thetwo photosystems are connected through anelectron transport chain, as seen earlier â in theZ scheme. Both ATP and NADPH + H+ aresynthesised by this kind of electron flow (Figure13.5).When only PS I is functional, the electron iscirculated within the photosystem and thephosphorylation occurs due to cyclic flow ofelectrons (Figure 13.6). A possible locationwhere this could be happening is in the stromalamellae. While the membrane or lamellae of the grana have both PS Iand PS II the stroma lamellae membranes lack PS II as well as NADPreductase enzyme. The excited electron does not pass on to NADP+ but iscycled back to the PS I complex through the electron transport chain(Figure 13.6). The cyclic flow hence, results only in the synthesis of ATP,but not of NADPH + H+. Cyclic photophosphorylation also occurs whenonly light of wavelengths beyond 680 nm are available for excitation.13.6.3 Chemiosmotic HypothesisLet us now try and understand how actually ATP is synthesised in thechloroplast. The chemiosmotic hypothesis has been put forward to explainthe mechanism. Like in respiration, in photosynthesis too, ATP synthesis islinked to development of a proton gradient across a membrane. This timethese are the membranes of thylakoid. There is one difference though, herethe proton accumulation is towards the inside of the membrane, i.e., in thelumen. In respiration, protons accumulate in the intermembrane space ofthe mitochondria when electrons move through the ETS (Chapter 14).Let us understand what causes the proton gradient across themembrane. We need to consider again the processes that take place duringthe activation of electrons and their transport to determine the steps thatcause a proton gradient to develop (Figure 13.7).(a) Since splitting of the water molecule takes place on the inner side ofthe membrane, the protons or hydrogen ions that are produced bythe splitting of water accumulate within the lumen of the thylakoids.Figure 13.6 Cyclic photophosphorylationPhotosystem ILighte- acceptorElectrontransportsystemChlorophyllP 700ADP+iP ATP2022-23214 BIOLOGY(b) As electrons move through the photosystems, protons are transportedacross the membrane. This happens because the primary accepter ofelectron which is located towards the outer side of the membranetransfers its electron not to an electron carrier but to an H carrier.Hence, this molecule removes a proton from the stroma whiletransporting an electron. When this molecule passes on its electronto the electron carrier on the inner side of the membrane, the protonis released into the inner side or the lumen side of the membrane.(c) The NADP reductase enzyme is located on the stroma side of themembrane. Along with electrons that come from the acceptor ofelectrons of PS I, protons are necessary for the reduction of NADP+ toNADPH+ H+. These protons are also removed from the stroma.Hence, within the chloroplast, protons in the stroma decrease innumber, while in the lumen there is accumulation of protons. This createsa proton gradient across the thylakoid membrane as well as a measurabledecrease in pH in the lumen.Why are we so interested in the proton gradient? This gradient isimportant because it is the breakdown of this gradient that leads to thesynthesis of ATP. The gradient is broken down due to the movement ofprotons across the membrane to the stroma through the transmembraneFigure 13.7 ATP synthesis through chemiosmosis2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 215channel of the CF0 of the ATP synthase. The ATP synthase enzyme consistsof two parts: one called the CF0 is embedded in the thylakoid membraneand forms a transmembrane channel that carries out facilitated diffusionof protons across the membrane. The other portion is called CF1 andprotrudes on the outer surface of the thylakoid membrane on the sidethat faces the stroma. The break down of the gradient provides enoughenergy to cause a conformational change in the CF1 particle of the ATPsynthase, which makes the enzyme synthesise several molecules of energypackedATP.Chemiosmosis requires a membrane, a proton pump, a protongradient and ATP synthase. Energy is used to pump protons across amembrane, to create a gradient or a high concentration of protons withinthe thylakoid lumen. ATP synthase has a channel that allows diffusion ofprotons back across the membrane; this releases enough energy to activateATP synthase enzyme that catalyses the formation of ATP.Along with the NADPH produced by the movement of electrons, theATP will be used immediately in the biosynthetic reaction taking place inthe stroma, responsible for fixing CO2, and synthesis of sugars.13.7 WHERE ARE THE ATP AND NADPH USED?We learnt that the products of light reaction are ATP, NADPH and O2. Ofthese O2 diffuses out of the chloroplast while ATP and NADPH are used todrive the processes leading to the synthesis of food, more accurately, sugars.This is the biosynthetic phase of photosynthesis. This process does notdirectly depend on the presence of light but is dependent on the productsof the light reaction, i.e., ATP and NADPH, besides CO2 and H2O. You maywonder how this could be verified; it is simple: immediately after lightbecomes unavailable, the biosynthetic process continues for some time,and then stops. If then, light is made available, the synthesis starts again.Can we, hence, say that calling the biosynthetic phase as the darkreaction is a misnomer? Discuss this amongst yourselves.Let us now see how the ATP and NADPH are used in the biosyntheticphase. We saw earlier that CO2 is combined with H2O to produce (CH2O)nor sugars. It was of interest to scientists to find out how this reactionproceeded, or rather what was the first product formed when CO2 is takeninto a reaction or fixed. Just after world war II, among the several effortsto put radioisotopes to beneficial use, the work of Melvin Calvin isexemplary. The use of radioactive 14C by him in algal photosynthesisstudies led to the discovery that the first CO2 fixation product was a3-carbon organic acid. He also contributed to working out the completebiosynthetic pathway; hence it was called Calvin cycle after him. Thefirst product identified was 3-phosphoglyceric acid or in short PGA.How many carbon atoms does it have?2022-23216 BIOLOGYScientists also tried to know whether all plants have PGA as the firstproduct of CO2 fixation, or whether any other product was formed inother plants. Experiments conducted over a wide range of plants led tothe discovery of another group of plants, where the first stable product ofCO2 fixation was again an organic acid, but one which had 4 carbonatoms in it. This acid was identified to be oxaloacetic acid or OAA. Sincethen CO2 assimilation during photosynthesis was said to be of two maintypes: those plants in which the first product of CO2 fixation is a C3 acid(PGA), i.e., the C3 pathway, and those in which the first product was a C4acid (OAA), i.e., the C4 pathway. These two groups of plants showedother associated characteristics that we will discuss later.13.7.1 The Primary Acceptor of CO2Let us now ask ourselves a question that was asked by the scientists whowere struggling to understand the âdark reactionâ. How many carbon atomswould a molecule have which after accepting (fixing) CO2, would have 3carbons (of PGA)?The studies very unexpectedly showed that the acceptor moleculewas a 5-carbon ketose sugar â ribulose bisphosphate (RuBP). Did anyof you think of this possibility? Do not worry; the scientists also tooka long time and conducted many experiments to reach this conclusion.They also believed that since the first product was a C3 acid, the primaryacceptor would be a 2-carbon compound; they spent many years tryingto identify a 2-carbon compound before they discovered the 5-carbonRuBP.13.7.2 The Calvin CycleCalvin and his co-workers then worked out the whole pathway and showedthat the pathway operated in a cyclic manner; the RuBP was regenerated.Let us now see how the Calvin pathway operates and where the sugar issynthesised. Let us at the outset understand very clearly that the Calvinpathway occurs in all photosynthetic plants; it does not matter whetherthey have C3 or C4 (or any other) pathways (Figure 13.8).For ease of understanding, the Calvin cycle can be described underthree stages: carboxylation, reduction and regeneration.1. Carboxylation â Carboxylation is the fixation of CO2 into a stable organicintermediate. Carboxylation is the most crucial step of the Calvin cyclewhere CO2 is utilised for the carboxylation of RuBP. This reaction iscatalysed by the enzyme RuBP carboxylase which results in the formationof two molecules of 3-PGA. Since this enzyme also has an oxygenationactivity it would be more correct to call it RuBP carboxylase-oxygenaseor RuBisCO.2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 2172. Reduction â These are a series of reactions that lead to the formationof glucose. The steps involve utilisation of 2 molecules of ATP forphosphorylation and two of NADPH for reduction per CO2 moleculefixed. The fixation of six molecules of CO2 and 6 turns of the cycle arerequired for the formation of one molecule of glucose from the pathway.3. Regeneration â Regeneration of the CO2 acceptor molecule RuBP iscrucial if the cycle is to continue uninterrupted. The regenerationsteps require one ATP for phosphorylation to form RuBP.Ribulose-1,5-bisphosphateAtmosphereC02 + H2OCarboxylationADPRegeneration3-phosphoglycerateTriosephosphateReductionATP+NADPHADP+Pi +NADP+Sucrose, starchATPFigure 13.8 The Calvin cycle proceeds in three stages : (1) carboxylation, during whichCO2 combines with ribulose-1,5-bisphosphate; (2) reduction, during whichcarbohydrate is formed at the expense of the photochemically made ATPand NADPH; and (3) regeneration during which the CO2 acceptor ribulose-1,5-bisphosphate is formed again so that the cycle continues2022-23218 BIOLOGYHence for every CO2 molecule entering the Calvin cycle, 3 moleculesof ATP and 2 of NADPH are required. It is probably to meet this differencein number of ATP and NADPH used in the dark reaction that the cyclicphosphorylation takes place.To make one molecule of glucose 6 turns of the cycle are required.Work out how many ATP and NADPH molecules will be required to makeone molecule of glucose through the Calvin pathway.It might help you to understand all of this if we look at what goes inand what comes out of the Calvin cycle.In OutSix CO2 One glucose18 ATP 18 ADP12 NADPH 12 NADP13.8 THE C4 PATHWAYPlants that are adapted to dry tropical regions have the C4 pathwaymentioned earlier. Though these plants have the C4 oxaloacetic acid asthe first CO2 fixation product they use the C3 pathway or the Calvin cycleas the main biosynthetic pathway. Then, in what way are they differentfrom C3 plants? This is a question that you may reasonably ask.C4 plants are special: They have a special type of leaf anatomy, theytolerate higher temperatures, they show a response to high light intensities,they lack a process called photorespiration and have greater productivityof biomass. Let us understand these one by one.Study vertical sections of leaves, one of a C3 plant and the other of a C4plant. Do you notice the differences? Do both have the same types ofmesophylls? Do they have similar cells around the vascular bundle sheath?The particularly large cells around the vascular bundles of the C4plants are called bundle sheath cells, and the leaves which have suchanatomy are said to have âKranzâ anatomy. âKranzâ means âwreathâ andis a reflection of the arrangement of cells. The bundle sheath cells mayform several layers around the vascular bundles; they are characterisedby having a large number of chloroplasts, thick walls impervious togaseous exchange and no intercellular spaces. You may like to cut asection of the leaves of C4 plants â maize or sorghum â to observe theKranz anatomy and the distribution of mesophyll cells.It would be interesting for you to collect leaves of diverse species ofplants around you and cut vertical sections of the leaves. Observe underthe microscope â look for the bundle sheath around the vascularbundles. The presence of the bundle sheath would help you identifythe C4 plants.2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 219Now study the pathway shown in Figure 13.9. This pathway that hasbeen named the Hatch and Slack Pathway, is again a cyclic process. Letus study the pathway by listing the steps.The primary CO2 acceptor is a 3-carbon molecule phosphoenolpyruvate (PEP) and is present in the mesophyll cells. The enzymeresponsible for this fixation is PEP carboxylase or PEPcase. It is importantto register that the mesophyll cells lack RuBisCO enzyme. The C4 acidOAA is formed in the mesophyll cells.It then forms other 4-carbon compounds like malic acid or asparticacid in the mesophyll cells itself, which are transported to the bundlesheath cells. In the bundle sheath cells these C4 acids are broken downto release CO2 and a 3-carbon molecule.The 3-carbon molecule is transported back to the mesophyll where itis converted to PEP again, thus, completing the cycle.The CO2 released in the bundle sheath cells enters the C3 or the Calvinpathway, a pathway common to all plants. The bundle sheath cells areFigure 13.9 Diagrammatic representation of the Hatch and Slack Pathway2022-23220 BIOLOGYrich in an enzyme Ribulose bisphosphate carboxylase-oxygenase(RuBisCO), but lack PEPcase. Thus, the basic pathway that results inthe formation of the sugars, the Calvin pathway, is common to the C3 andC4 plants.Did you note that the Calvin pathway occurs in all the mesophyllcells of the C3 plants? In the C4 plants it does not take place in themesophyll cells but does so only in the bundle sheath cells.13.9 PHOTORESPIRATIONLet us try and understand one more process that creates an importantdifference between C3 and C4 plants â Photorespiration. To understandphotorespiration we have to know a little bit more about the first step ofthe Calvin pathway â the first CO2 fixation step. This is the reactionwhere RuBP combines with CO2 to form 2 molecules of 3PGA, that iscatalysed by RuBisCO.RuBP CO RuBisCo PGA+ 2 žžžžžžŽ à 2 3RuBisCO that is the most abundant enzyme in the world (Do youwonder why?) is characterised by the fact that its active site can bind toboth CO2 and O2 â hence the name. Can you think how this could bepossible? RuBisCO has a much greater affinity for CO2 when the CO2: O2is nearly equal. Imagine what would happen if this were not so! Thisbinding is competitive. It is the relative concentration of O2 and CO2 thatdetermines which of the two will bind to the enzyme.In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation isdecreased. Here the RuBP instead of being converted to 2 molecules ofPGA binds with O2 to form one molecule of phosphoglycerate andphosphoglycolate (2 Carbon) in a pathway called photorespiration. Inthe photorespiratory pathway, there is neither synthesis of sugars, nor ofATP. Rather it results in the release of CO2 with the utilisation of ATP. Inthe photorespiratory pathway there is no synthesis of ATP or NADPH.The biological function of photorespiration is not known yet.In C4 plants photorespiration does not occur. This is because theyhave a mechanism that increases the concentration of CO2 at the enzymesite. This takes place when the C4 acid from the mesophyll is brokendown in the bundle sheath cells to release CO2 â this results in increasingthe intracellular concentration of CO2. In turn, this ensures that theRuBisCO functions as a carboxylase minimising the oxygenase activity.Now that you know that the C4 plants lack photorespiration, youprobably can understand why productivity and yields are better in theseplants. In addition these plants show tolerance to higher temperatures.Based on the above discussion can you compare plants showingthe C3 and the C4 pathway? Use the table format given and fill in theinformation.2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 221TABLE 13.1 Fill in the Columns 2 and 3 in this table to highlight the differencesbetween C3 and C4 PlantsCharacteristics C3 Plants C4 Plants Choose fromCell type in which the Calvin Mesophyll/Bundle sheath/bothcycle takes placeCell type in which the initial Mesophyll/Bundle sheath /bothcarboxylation reaction occursHow many cell types does the Two: Bundle sheath andleaf have that fix CO2. mesophyllOne: MesophyllThree: Bundle sheath, palisade,spongy mesophyllWhich is the primary CO2 acceptor RuBP/PEP/PGANumber of carbons in the 5 / 4 / 3primary CO2 acceptorWhich is the primary CO2 PGA/OAA/RuBP/PEPfixation productNo. of carbons in the primary 3 / 4 / 5CO2 fixation productDoes the plant have RuBisCO? Yes/No/Not alwaysDoes the plant have PEP Case? Yes/No/Not alwaysWhich cells in the plant have Mesophyll/Bundle sheath/noneRubisco?CO2 fixation rate under high Low/ high/ mediumlight conditionsWhether photorespiration is High/negligible/sometimespresent at low light intensitiesWhether photorespiration is High/negligible/sometimespresent at high light intensitiesWhether photorespiration would be High/negligible/sometimespresent at low CO2 concentrationsWhether photorespiration would be High/negligible/sometimespresent at high CO2 concentrationsTemperature optimum 30-40 C/20-25C/above 40 CExamples Cut vertical sections of leaves ofdifferent plants and observe underthe microscope for Kranz anatomyand list them in the appropriatecolumns.2022-23222 BIOLOGY13.10 FACTORS AFFECTING PHOTOSYNTHESISAn understanding of the factors that affect photosynthesis is necessary.The rate of photosynthesis is very important in determining the yield ofplants including crop plants. Photosynthesis is under the influence ofseveral factors, both internal (plant) and external. The plant factors includethe number, size, age and orientation of leaves, mesophyll cells andchloroplasts, internal CO2 concentration and the amount of chlorophyll.The plant or internal factors are dependent on the genetic predispositionand the growth of the plant.The external factors would include the availability of sunlight,temperature, CO2 concentration and water. As a plant photosynthesises,all these factors will simultaneously affect its rate. Hence, though severalfactors interact and simultaneously affect photosynthesis or CO2 fixation,usually one factor is the major cause or is the one that limits the rate.Hence, at any point the rate will be determined by the factor available atsub-optimal levels.When several factors affect any [bio] chemical process, Blackmanâs(1905) Law of Limiting Factors comes into effect. This states the following:If a chemical process is affected by more than one factor, then itsrate will be determined by the factor which is nearest to its minimalvalue: it is the factor which directly affects the process if its quantity ischanged.For example, despite the presence of a greenleaf and optimal light and CO2 conditions, theplant may not photosynthesise if the temperatureis very low. This leaf, if given the optimaltemperature, will start photosynthesising.13.10.1 LightWe need to distinguish between light quality, lightintensity and the duration of exposure to light,while discussing light as a factor that affectsphotosynthesis. There is a linear relationshipbetween incident light and CO2 fixation rates atlow light intensities. At higher light intensities,gradually the rate does not show further increaseas other factors become limiting (Figure 13.10).What is interesting to note is that light saturationoccurs at 10 per cent of the full sunlight. Hence,except for plants in shade or in dense forests, lightis rarely a limiting factor in nature. Increase inFigure 13.10 Graph of light intensity on therate of photosynthesisRate of photosynthesisLight intensityAB CDE2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 223incident light beyond a point causes the breakdown of chlorophyll and adecrease in photosynthesis.13.10.2 Carbon dioxide ConcentrationCarbon dioxide is the major limiting factor for photosynthesis. Theconcentration of CO2 is very low in the atmosphere (between 0.03 and0.04 per cent). Increase in concentration upto 0.05 per cent can cause anincrease in CO2 fixation rates; beyond this the levels can become damagingover longer periods.The C3 and C4 plants respond differently to CO2 concentrations. Atlow light conditions neither group responds to high CO2 conditions. Athigh light intensities, both C3 and C4 plants show increase in the rates ofphotosynthesis. What is important to note is that the C4 plants showsaturation at about 360 ÎŧlL-1 while C3 responds to increased CO2concentration and saturation is seen only beyond 450 ÎŧlL-1. Thus, currentavailability of CO2 levels is limiting to the C3 plants.The fact that C3 plants respond to higher CO2 concentration byshowing increased rates of photosynthesis leading to higher productivityhas been used for some greenhouse crops such as tomatoes and bellpepper. They are allowed to grow in carbon dioxide enriched atmospherethat leads to higher yields.13.10.3 TemperatureThe dark reactions being enzymatic are temperature controlled. Thoughthe light reactions are also temperature sensitive they are affected to amuch lesser extent. The C4 plants respond to higher temperatures andshow higher rate of photosynthesis while C3 plants have a much lowertemperature optimum.The temperature optimum for photosynthesis of different plants alsodepends on the habitat that they are adapted to. Tropical plants have ahigher temperature optimum than the plants adapted to temperateclimates.13.10.4 WaterEven though water is one of the reactants in the light reaction, the effect ofwater as a factor is more through its effect on the plant, rather than directlyon photosynthesis. Water stress causes the stomata to close hence reducingthe CO2 availability. Besides, water stress also makes leaves wilt, thus,reducing the surface area of the leaves and their metabolic activity as well.2022-23224 BIOLOGYSUMMARYGreen plants make their own food by photosynthesis. During this process carbondioxide from the atmosphere is taken in by leaves through stomata and used formaking carbohydrates, principally glucose and starch. Photosynthesis takes placeonly in the green parts of the plants, mainly the leaves. Within the leaves, themesophyll cells have a large number of chloroplasts that are responsible for CO2fixation. Within the chloroplasts, the membranes are sites for the light reaction,while the chemosynthetic pathway occurs in the stroma. Photosynthesis has twostages: the light reaction and the carbon fixing reactions. In the light reaction thelight energy is absorbed by the pigments present in the antenna, and funnelled tospecial chlorophyll a molecules called reaction centre chlorophylls. There are twophotosystems, PS I and PS II. PS I has a 700 nm absorbing chlorophyll a P700molecule at its reaction centre, while PS II has a P680 reaction centre that absorbsred light at 680 nm. After absorbing light, electrons are excited and transferredthrough PS II and PS I and finally to NAD forming NADH. During this process aproton gradient is created across the membrane of the thylakoid. The breakdownof the protons gradient due to movement through the F0 part of the ATPase enzymereleases enough energy for synthesis of ATP. Splitting of water molecules isassociated with PS II resulting in the release of O2, protons and transfer of electronsto PS II.In the carbon fixation cycle, CO2 is added by the enzyme, RuBisCO, to a 5-carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. Thisis then converted to sugar by the Calvin cycle, and the RuBP is regenerated. Duringthis process ATP and NADPH synthesised in the light reaction are utilised. RuBisCOalso catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.Some tropical plants show a special type of photosynthesis called C4 pathway.In these plants the first product of CO2 fixation that takes place in the mesophyll,is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carriedout for the synthesis of carbohydrates.EXERCISES1. By looking at a plant externally can you tell whether a plant is C3 or C4? Why andhow?2. By looking at which internal structure of a plant can you tell whether a plant isC3 or C4? Explain.3. Even though a very few cells in a C4 plant carry out the biosynthetic â Calvinpathway, yet they are highly productive. Can you discuss why?2022-23PHOTOSYNTHESIS IN HIGHER PLANTS 2254. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why doyou think RuBisCO carries out more carboxylation in C4 plants?5. Suppose there were plants that had a high concentration of Chlorophyll b, butlacked chlorophyll a, would it carry out photosynthesis? Then why do plantshave chlorophyll b and other accessory pigments?6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green?Which pigment do you think is more stable?7. Look at leaves of the same plant on the shady side and compare it with theleaves on the sunny side. Or, compare the potted plants kept in the sunlight withthose in the shade. Which of them has leaves that are darker green ? Why?8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on thegraph, answer the following questions:(a) At which point/s (A, B or C) in the curve is light a limiting factor?(b) What could be the limiting factor/s in region A?(c) What do C and D represent on the curve?9. Give comparison between the following:(a) C3 and C4 pathways(b) Cyclic and non-cyclic photophosphorylation(c) Anatomy of leaf in C3 and C4 plants2022-23226 BIOLOGYAll of us breathe to live, but why is breathing so essential to life? Whathappens when we breathe? Also, do all living organisms, including plantsand microbes, breathe? If so, how?All living organisms need energy for carrying out daily life activities,be it absorption, transport, movement, reproduction or even breathing.Where does all this energy come from? We know we eat food for energy âbut how is this energy taken from food? How is this energy utilised? Doall foods give the same amount of energy? Do plants âeatâ? Where do plantsget their energy from? And micro-organisms â for their energyrequirements, do they eat âfoodâ?You may wonder at the several questions raised above â they mayseem to be very disconnected. But in reality, the process of breathing isvery much connected to the process of release of energy from food. Let ustry and understand how this happens.All the energy required for âlifeâ processes is obtained by oxidation ofsome macromolecules that we call âfoodâ. Only green plants andcyanobacteria can prepare their own food; by the process of photosynthesisthey trap light energy and convert it into chemical energy that is stored inthe bonds of carbohydrates like glucose, sucrose and starch. We mustremember that in green plants too, not all cells, tissues and organsphotosynthesise; only cells containing chloroplasts, that are most oftenlocated in the superficial layers, carry out photosynthesis. Hence, evenin green plants all other organs, tissues and cells that are non-green,need food for oxidation. Hence, food has to be translocated to all nongreenparts. Animals are heterotrophic, i.e., they obtain food from plantsRESPIRATION IN PLANTSCHAPTER 1414.1 Do PlantsBreathe?14.2 Glycolysis14.3 Fermentation14.4 AerobicRespiration14.5 The RespiratoryBalance Sheet14.6 AmphibolicPathway14.7 RespiratoryQuotient2022-23RESPIRATION IN PLANTS 227directly (herbivores) or indirectly (carnivores). Saprophytes like fungi aredependent on dead and decaying matter. What is important to recogniseis that ultimately all the food that is respired for life processes comes fromphotosynthesis. This chapter deals with cellular respiration or themechanism of breakdown of food materials within the cell to releaseenergy, and the trapping of this energy for synthesis of ATP.Photosynthesis, of course, takes place within the chloroplasts (in theeukaryotes), whereas the breakdown of complex molecules to yield energytakes place in the cytoplasm and in the mitochondria (also only ineukaryotes). The breaking of the C-C bonds of complex compoundsthrough oxidation within the cells, leading to release of considerableamount of energy is called respiration. The compounds that are oxidisedduring this process are known as respiratory substrates. Usuallycarbohydrates are oxidised to release energy, but proteins, fats and evenorganic acids can be used as respiratory substances in some plants, undercertain conditions. During oxidation within a cell, all the energy containedin respiratory substrates is not released free into the cell, or in a singlestep. It is released in a series of slow step-wise reactions controlled byenzymes, and it is trapped as chemical energy in the form of ATP. Hence,it is important to understand that the energy released by oxidation inrespiration is not (or rather cannot be) used directly but is used tosynthesise ATP, which is broken down whenever (and wherever) energyneeds to be utilised. Hence, ATP acts as the energy currency of the cell.This energy trapped in ATP is utilised in various energy-requiringprocesses of the organisms, and the carbon skeleton produced duringrespiration is used as precursors for biosynthesis of other molecules inthe cell.14.1 DO PLANTS BREATHE?Well, the answer to this question is not quite so direct. Yes, plants requireO2 for respiration to occur and they also give out CO2. Hence, plants havesystems in place that ensure the availability of O2. Plants, unlike animals,have no specialised organs for gaseous exchange but they have stomataand lenticels for this purpose. There are several reasons why plants canget along without respiratory organs. First, each plant part takes care ofits own gas-exchange needs. There is very little transport of gases fromone plant part to another. Second, plants do not present great demandsfor gas exchange. Roots, stems and leaves respire at rates far lower thananimals do. Only during photosynthesis are large volumes of gasesexchanged and, each leaf is well adapted to take care of its own needsduring these periods. When cells photosynthesise, availability of O2 is nota problem in these cells since O2 is released within the cell. Third, the2022-23228 BIOLOGYdistance that gases must diffuse even in large, bulky plants is not great.Each living cell in a plant is located quite close to the surface of the plant.âThis is true for leavesâ, you may ask, âbut what about thick, woody stemsand roots?â In stems, the âlivingâ cells are organised in thin layers insideand beneath the bark. They also have openings called lenticels. The cellsin the interior are dead and provide only mechanical support. Thus, mostcells of a plant have at least a part of their surface in contact with air. Thisis also facilitated by the loose packing of parenchyma cells in leaves, stemsand roots, which provide an interconnected network of air spaces.The complete combustion of glucose, which produces CO2 and H2Oas end products, yields energy most of which is given out as heat.C H O O CO H O Energy 6 12 6 2 2 2 + 6 žžŽ 6 + 6 +If this energy is to be useful to the cell, it should be able to utilise it tosynthesise other molecules that the cell requires. The strategy that theplant cell uses is to catabolise the glucose molecule in such a way thatnot all the liberated energy goes out as heat. The key is to oxidise glucosenot in one step but in several small steps enabling some steps to be justlarge enough such that the energy released can be coupled to ATPsynthesis. How this is done is, essentially, the story of respiration.During the process of respiration, oxygen is utilised, and carbondioxide, water and energy are released as products. The combustionreaction requires oxygen. But some cells live where oxygen may or maynot be available. Can you think of such situations (and organisms) whereO2 is not available? There are sufficient reasons to believe that the firstcells on this planet lived in an atmosphere that lacked oxygen. Evenamong present-day living organisms, we know of several that are adaptedto anaerobic conditions. Some of these organisms are facultativeanaerobes, while in others the requirement for anaerobic condition isobligate. In any case, all living organisms retain the enzymatic machineryto partially oxidise glucose without the help of oxygen. This breakdownof glucose to pyruvic acid is called glycolysis.14.2 GLYCOLYSISThe term glycolysis has originated from the Greek words, glycos for sugar,and lysis for splitting. The scheme of glycolysis was given by GustavEmbden, Otto Meyerhof, and J. Parnas, and is often referred to as theEMP pathway. In anaerobic organisms, it is the only process in respiration.Glycolysis occurs in the cytoplasm of the cell and is present in all livingorganisms. In this process, glucose undergoes partial oxidation to formtwo molecules of pyruvic acid. In plants, this glucose is derived fromsucrose, which is the end product of photosynthesis, or from storage2022-23RESPIRATION IN PLANTS 229carbohydrates. Sucrose is converted into glucoseand fructose by the enzyme, invertase, and thesetwo monosaccharides readily enter the glycolyticpathway. Glucose and fructose arephosphorylated to give rise to glucose-6-phosphate by the activity of the enzymehexokinase. This phosphorylated form of glucosethen isomerises to produce fructose-6-phosphate. Subsequent steps of metabolism ofglucose and fructose are same. The various stepsof glycolysis are depicted in Figure 14.1. Inglycolysis, a chain of ten reactions, under thecontrol of different enzymes, takes place toproduce pyruvate from glucose. While studyingthe steps of glycolysis, please note the steps atwhich utilisation or synthesis of ATP or (in thiscase) NADH + H+ take place.ATP is utilised at two steps: first in theconversion of glucose into glucose 6-phosphateand second in the conversion of fructose6-phosphate to fructose 1, 6-bisphosphate.The fructose 1, 6-bisphosphate is splitinto dihydroxyacetone phosphate and3-phosphoglyceraldehyde (PGAL). We findthat there is one step where NADH + H+ isformed from NAD+; this is when3-phosphoglyceraldehyde (PGAL) is convertedto 1, 3-bisphosphoglycerate (BPGA). Tworedox-equivalents are removed (in the form oftwo hydrogen atoms) from PGAL and transferredto a molecule of NAD+. PGAL is oxidised andwith inorganic phosphate to get converted intoBPGA. The conversion of BPGA to3-phosphoglyceric acid (PGA), is also an energyyielding process; this energy is trapped by theformation of ATP. Another ATP is synthesisedduring the conversion of PEP to pyruvic acid.Can you then calculate how many ATPmolecules are directly synthesised in thispathway from one glucose molecule?Pyruvic acid is then the key product ofglycolysis. What is the metabolic fate ofpyruvate? This depends on the cellular need.Glucose(6C)Glucose-6-phosphate(6C)Fructose-6-phosphate(6C)Fructose1, 6-bisphosphate(6C)Triose phosphate(glyceraldehyde-3-phosphate)(3C)Triose phosphate(Dihydroxy acetonephosphate)(3C)2 à Triose bisphosphate(1,3 bisphosphoglyceric acid)(3C)2 à Triose phosphate(3-phosphoglyceric acid)(3C)2 à 2-phosphoglycerate2 à phosphoenolpyruvate2 à Pyruvic acid(3C)ADPATPADPATPADPATPADPNADH+H+NAD+H2OATPFigure 14.1 Steps of glycolysis2022-23230 BIOLOGYThere are three major ways in which different cells handle pyruvic acidproduced by glycolysis. These are lactic acid fermentation, alcoholicfermentation and aerobic respiration. Fermentation takes place underanaerobic conditions in many prokaryotes and unicellular eukaryotes.For the complete oxidation of glucose to CO2 and H2O, however, organismsadopt Krebsâ cycle which is also called as aerobic respiration. This requiresO2 supply.14.3 FERMENTATIONIn fermentation, say by yeast, the incomplete oxidation of glucose isachieved under anaerobic conditions by sets of reactions where pyruvicacid is converted to CO2 and ethanol. The enzymes, pyruvic aciddecarboxylase and alcohol dehydrogenase catalyse these reactions. Otherorganisms like some bacteria produce lactic acid from pyruvic acid. Thesteps involved are shown in Figure 14.2. In animal cells also, like musclesduring exercise, when oxygen is inadequate for cellular respiration pyruvicacid is reduced to lactic acid by lactate dehydrogenase. The reducingagent is NADH+H+ which is reoxidised to NAD+ in both the processes.In both lactic acid and alcoholfermentation not much energy is released; lessthan seven per cent of the energy in glucoseis released and not all of it is trapped as highenergy bonds of ATP. Also, the processes arehazardous â either acid or alcohol isproduced. What is the net ATPs that issynthesised (calculate how many ATP aresynthesised and deduct the number of ATPutilised during glycolysis) when one moleculeof glucose is fermented to alcohol or lacticacid? Yeasts poison themselves to death whenthe concentration of alcohol reaches about 13per cent. What then would be themaximum concentration of alcohol inbeverages that are naturally fermented?How do you think alcoholic beverages ofalcohol content greater than this concentrationare obtained?What then is the process by whichorganisms can carry out complete oxidationof glucose and extract the energy stored toFigure 14.2 Major pathways of anaerobicrespiration2022-23RESPIRATION IN PLANTS 231synthesise a larger number of ATP molecules needed for cellularmetabolism? In eukaryotes these steps take place within the mitochondriaand this requires O2. Aerobic respiration is the process that leads to acomplete oxidation of organic substances in the presence of oxygen, andreleases CO2, water and a large amount of energy present in the substrate.This type of respiration is most common in higher organisms. We willlook at these processes in the next section.14.4 AEROBIC RESPIRATIONFor aerobic respiration to take place within the mitochondria, the finalproduct of glycolysis, pyruvate is transported from the cytoplasm intothe mitochondria. The crucial events in aerobic respiration are:âĸ The complete oxidation of pyruvate by the stepwise removal of allthe hydrogen atoms, leaving three molecules of CO2.âĸ The passing on of the electrons removed as part of the hydrogenatoms to molecular O2 with simultaneous synthesis of ATP.What is interesting to note is that the first process takes place in thematrix of the mitochondria while the second process is located on theinner membrane of the mitochondria.Pyruvate, which is formed by the glycolytic catabolism of carbohydratesin the cytosol, after it enters mitochondrial matrix undergoes oxidativedecarboxylation by a complex set of reactions catalysed by pyruvicdehydrogenase. The reactions catalysed by pyruvic dehydrogenase requirethe participation of several coenzymes, including NAD+ and Coenzyme A.Pyruvic acid CoA NADMgPyruvate dehydrogenase+ + žžžžžžžžžžžžŽ+2+ž + + ++ Acetyl CoA CO NADH H 2During this process, two molecules of NADH are produced from themetabolism of two molecules of pyruvic acid (produced from one glucosemolecule during glycolysis).The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle,more commonly called as Krebsâ cycle after the scientist Hans Krebs whofirst elucidated it.14.4.1 Tricarboxylic Acid CycleThe TCA cycle starts with the condensation of acetyl group with oxaloaceticacid (OAA) and water to yield citric acid (Figure 14.3). The reaction iscatalysed by the enzyme citrate synthase and a molecule of CoA is released.Citrate is then isomerised to isocitrate. It is followed by two successivesteps of decarboxylation, leading to the formation of a-ketoglutaric acid2022-23232 BIOLOGYFigure 14.3 The Citric acid cyclePyruvate(3C)Acetyl coenzyme A(2C)Citric acid(6C)Oxaloacetic acid(4C)CO2NAD+NADH+H+NADH+H+NAD+NAD+CO2CITRIC ACID CYCLEa-ketoglutaric acid(5C)NADH+H+GDPGTPSuccinic acid(4C)Malic acid(4C)FADH2FAD+Pyruvic acid NAD FAD H O ADP Pi Mitochondrial + + + + + žžžž Matrix ÂŽ+ + 4 2 2žžžžžžžž + ++ 3 4 4 2 CO NADH Hand then succinyl-CoA. In the remaining stepsof citric acid cycle, succinyl-CoA is oxidisedto OAA allowing the cycle to continue. Duringthe conversion of succinyl-CoA to succinicacid a molecule of GTP is synthesised. This isa substrate level phosphorylation. In acoupled reaction GTP is converted to GDP withthe simultaneous synthesis of ATP from ADP.Also there are three points in the cycle whereNAD+ is reduced to NADH + H+ and one pointwhere FAD+ is reduced to FADH2. Thecontinued oxidation of acetyl CoA via the TCAcycle requires the continued replenishment ofoxaloacetic acid, the first member of the cycle.In addition it also requires regeneration ofNAD+ and FAD+ from NADH and FADH2respectively. The summary equation for thisphase of respiration may be written as follows:+ +2 FADH ATPCoA NAD+NADH+H+CO2We have till now seen that glucose has been broken down to releaseCO2 and eight molecules of NADH + H+; two of FADH2 have beensynthesised besides just two molecules of ATP in TCA cycle. You may bewondering why we have been discussing respiration at all â neither O2has come into the picture nor the promised large number of ATP has yetbeen synthesised. Also what is the role of the NADH + H+ and FADH2 thatis synthesised? Let us now understand the role of O2 in respiration andhow ATP is synthesised.14.4.2 Electron Transport System (ETS) and OxidativePhosphorylationThe following steps in the respiratory process are to release and utilisethe energy stored in NADH+H+ and FADH2. This is accomplished whenthey are oxidised through the electron transport system and the electronsare passed on to O2 resulting in the formation of H2O. The metabolicpathway through which the electron passes from one carrier to another,is called the electron transport system (ETS) (Figure 14.4) and it ispresent in the inner mitochondrial membrane. Electrons from NADH2022-23RESPIRATION IN PLANTS 233produced in the mitochondrial matrixduring citric acid cycle are oxidised by anNADH dehydrogenase (complex I), andelectrons are then transferred toubiquinone locatedwithin the inner membrane. Ubiquinonealso receives reducing equivalents viaFADH2 (complex II) that is generatedduring oxidation of succinate in the citricacid cycle. The reduced ubiquinone(ubiquinol) is then oxidised with thetransfer of electrons to cytochrome c viacytochrome bc1 complex (complex III).Cytochrome c is a small protein attachedto the outer surface of the innermembrane and acts as a mobile carrierfor transfer of electrons between complexIII and IV. Complex IV refers tocytochrome c oxidase complex containingcytochromes a and a3, and two coppercentres.When the electrons pass from onecarrier to another via complex I to IV inthe electron transport chain, they arecoupled to ATP synthase (complex V) forthe production of ATP from ADP andinorganic phosphate. The number of ATPmolecules synthesised depends on thenature of the electron donor. Oxidation ofone molecule of NADH gives rise to 3molecules of ATP, while that of onemolecule of FADH2 produces 2 moleculesof ATP. Although the aerobic process ofrespiration takes place only in thepresence of oxygen, the role of oxygen islimited to the terminal stage of theprocess. Yet, the presence of oxygen is vital, since it drives the wholeprocess by removing hydrogen from the system. Oxygen acts as the finalhydrogen acceptor. Unlike photophosphorylation where it is the lightenergy that is utilised for the production of proton gradient required forphosphorylation, in respiration it is the energy of oxidation-reductionutilised for the same process. It is for this reason that the process is calledoxidative phosphorylation.You have already studied about the mechanism of membrane-linkedATP synthesis as explained by chemiosmotic hypothesis in the earlierchapter. As mentioned earlier, the energy released during the electronFigure 14.4 Electron Transport System (ETS)2022-23234 BIOLOGYtransport system is utilised in synthesising ATPwith the help of ATP synthase (complex V). Thiscomplex consists of two major components, F1and F0 (Figure 14.5). The F1 headpiece is aperipheral membrane protein complex andcontains the site for synthesis of ATP from ADPand inorganic phosphate. F0 is an integralmembrane protein complex that forms thechannel through which protons cross the innermembrane. The passage of protons through thechannel is coupled to the catalytic site of the F1component for the production of ATP. For eachATP produced, 4H+ passes through F0 from theintermembrane space to the matrix down theelectrochemical proton gradient.14.5 THE RESPIRATORY BALANCE SHEETIt is possible to make calculations of the net gain of ATP for every glucosemolecule oxidised; but in reality this can remain only a theoretical exercise.These calculations can be made only on certain assumptions that:âĸ There is a sequential, orderly pathway functioning, with onesubstrate forming the next and with glycolysis, TCA cycle and ETSpathway following one after another.âĸ The NADH synthesised in glycolysis is transferred into themitochondria and undergoes oxidative phosphorylation.âĸ None of the intermediates in the pathway are utilised to synthesiseany other compound.âĸ Only glucose is being respired â no other alternative substrates areentering in the pathway at any of the intermediary stages.But this kind of assumptions are not really valid in a living system; allpathways work simultaneously and do not take place one after another;substrates enter the pathways and are withdrawn from it as and whennecessary; ATP is utilised as and when needed; enzymatic rates arecontrolled by multiple means. Yet, it is useful to do this exercise toappreciate the beauty and efficiency of the living system in extractionand storing energy. Hence, there can be a net gain of 38 ATP moleculesduring aerobic respiration of one molecule of glucose.Figure 14.5 Diagramatic presentation of ATPsynthesis in mitochondria2022-23RESPIRATION IN PLANTS 235Now let us compare fermentation and aerobic respiration:âĸ Fermentation accounts for only a partial breakdown of glucosewhereas in aerobic respiration it is completely degraded to CO2 andH2O.âĸ In fermentation there is a net gain of only two molecules of ATP foreach molecule of glucose degraded to pyruvic acid whereas manymore molecules of ATP are generated under aerobic conditions.âĸ NADH is oxidised to NAD+ rather slowly in fermentation, howeverthe reaction is very vigorous in case of aerobic respiration.14.6 AMPHIBOLIC PATHWAYGlucose is the favoured substrate for respiration. All carbohydrates areusually first converted into glucose before they are used for respiration.Other substrates can also be respired, as has been mentioned earlier, butthen they do not enter the respiratory pathway at the first step. See Figure14.6 to see the points of entry of different substrates in the respiratorypathway. Fats would need to be broken down into glycerol and fatty acidsfirst. If fatty acids were to be respired they would first be degraded toacetyl CoA and enter the pathway. Glycerol would enter the pathwayafter being converted to PGAL. The proteins would be degraded byproteases and the individual amino acids (after deamination) dependingon their structure would enter the pathway at some stage within the Krebsâcycle or even as pyruvate or acetyl CoA.Since respiration involves breakdown of substrates, the respiratoryprocess has traditionally been considered a catabolic process and therespiratory pathway as a catabolic pathway. But is this understandingcorrect? We have discussed above, at which points in the respiratorypathway different substrates would enter if they were to be respired andused to derive energy. What is important to recognise is that it is these verycompounds that would be withdrawn from the respiratory pathway for thesynthesis of the said substrates. Hence, fatty acids would be broken downto acetyl CoA before entering the respiratory pathway when it is used as asubstrate. But when the organism needs to synthesise fatty acids, acetylCoA would be withdrawn from the respiratory pathway for it. Hence, therespiratory pathway comes into the picture both during breakdown andsynthesis of fatty acids. Similarly, during breakdown and synthesis ofprotein too, respiratory intermediates form the link. Breaking downprocesses within the living organism is catabolism, and synthesis isanabolism. Because the respiratory pathway is involved in both anabolismand catabolism, it would hence be better to consider the respiratory pathwayas an amphibolic pathway rather than as a catabolic one.2022-23236 BIOLOGY14.7 RESPIRATORY QUOTIENTLet us now look at another aspect of respiration. As you know, duringaerobic respiration, O2 is consumed and CO2 is released. The ratio of thevolume of CO2 evolved to the volume of O2 consumed in respiration iscalled the respiratory quotient (RQ) or respiratory ratio.RQvolumeof CO evolvedvolumeof O consumed= 22The respiratory quotient depends upon the type of respiratorysubstrate used during respiration.When carbohydrates are used as substrate and are completelyoxidised, the RQ will be 1, because equal amounts of CO2 and O2 areevolved and consumed, respectively, as shown in the equation below :Figure 14.6 Interrelationship among metabolic pathways showing respirationmediated breakdown of different organic molecules to CO2 and H202022-23RESPIRATION IN PLANTS 237C H O O CO H O Energy 6 12 6 2 2 2 + 6 žžŽ 6 +6 +RQCOO= =661 0 22.When fats are used in respiration, the RQ is less than 1. Calculationsfor a fatty acid, tripalmitin, if used as a substrate is shown:2 145 102 98 51 98 6 2 2 2 (C H O )+ O žžŽ CO + H O+ energyTripalmitinRQCOO= =1021450 7 22.When proteins are respiratory substrates the ratio would be about0.9.What is important to recognise is that in living organisms respiratorysubstrates are often more than one; pure proteins or fats are never usedas respiratory substrates.SUMMARYPlants unlike animals have no special systems for breathing or gaseous exchange.Stomata and lenticels allow gaseous exchange by diffusion. Almost all living cellsin a plant have their surfaces exposed to air.The breaking of C-C bonds of complex organic molecules by oxidation cellsleading to the release of a lot of energy is called cellular respiration. Glucose is thefavoured substrate for respiration. Fats and proteins can also be broken down toyield energy. The initial stage of cellular respiration takes place in the cytoplasm.Each glucose molecule is broken through a series of enzyme catalysed reactionsinto two molecules of pyruvic acid. This process is called glycolysis. The fate of thepyruvate depends on the availability of oxygen and the organism. Under anaerobicconditions either lactic acid fermentation or alcohol fermentation occurs.Fermentation takes place under anaerobic conditions in many prokaryotes,unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobicrespiration occurs in the presence of oxygen. Pyruvic acid is transported into themitochondria where it is converted into acetyl CoA with the release of CO2. AcetylCoA then enters the tricarboxylic acid pathway or Krebsâ cycle operating in thematrix of the mitochondria. NADH + H+ and FADH2 are generated in the Krebsâcycle. The energy in these molecules as well as that in the NADH + H+ synthesisedduring glycolysis are used to synthesise ATP. This is accomplished through a2022-23238 BIOLOGYsystem of electron carriers called electron transport system (ETS) located on theinner membrane of the mitochondria. The electrons, as they move through thesystem, release enough energy that are trapped to synthesise ATP. This is calledoxidative phosphorylation. In this process O2 is the ultimate acceptor of electronsand it gets reduced to water.The respiratory pathway is an amphibolic pathway as it involves both anabolismand catabolism. The respiratory quotient depends upon the type of respiratorysubstance used during respiration.EXERCISES1. Differentiate between(a) Respiration and Combustion(b) Glycolysis and Krebsâ cycle(c) Aerobic respiration and Fermentation2. What are respiratory substrates? Name the most common respiratory substrate.3. Give the schematic representation of glycolysis?4. What are the main steps in aerobic respiration? Where does it take place?5. Give the schematic representation of an overall view of Krebsâ cycle.6. Explain ETS.7. Distinguish between the following:(a) Aerobic respiration and Anaerobic respiration(b) Glycolysis and Fermentation(c) Glycolysis and Citric acid Cycle8. What are the assumptions made during the calculation of net gain of ATP?9. Discuss âThe respiratory pathway is an amphibolic pathway.â10. Define RQ. What is its value for fats?11. What is oxidative phosphorylation?12. What is the significance of step-wise release of energy in respiration?2022-23PLANT GROWTH AND DEVELOPMENT 239You have already studied the organisation of a flowering plant in Chapter5. Have you ever thought about where and how the structures like roots,stems, leaves, flowers, fruits and seeds arise and that too in an orderlysequence? You are, by now, aware of the terms seed, seedling, plantlet,mature plant. You have also seen that trees continue to increase in heightor girth over a period of time. However, the leaves, flowers and fruits of thesame tree not only have limited dimensions but also appear and fallperiodically and some time repeatedly. Why does vegetative phase precedeflowering in a plant? All plant organs are made up of a variety of tissues; isthere any relationship between the structure of a cell, a tissue, an organand the function they perform? Can the structure and the function of thesebe altered? All cells of a plant are descendents of the zygote. The questionis, then, why and how do they have different structural and functionalattributes? Development is the sum of two processes: growth anddifferentiation. To begin with, it is essential and sufficient to know that thedevelopment of a mature plant from a zygote (fertilised egg) follow a preciseand highly ordered succession of events. During this process a complexbody organisation is formed that produces roots, leaves, branches, flowers,fruits, and seeds, and eventually they die (Figure 15.1). The first step in theprocess of plant growth is seed germination. The seed germinates whenfavourable conditions for growth exist in the environment. In absence ofsuch favourable conditions the seeds do not germinate and goes into aperiod of suspended growth or rest. Once favourable conditions return,the seeds resume metabolic activities and growth takes place.In this chapter, you shall also study some of the factors whichgovern and control these developmental processes. These factors are bothintrinsic (internal) and extrinsic (external) to the plant.PLANT GROWTH AND DEVELOPMENTCHAPTER 1515.1 Growth15.2 Differentiation,DedifferentiationandRedifferentiation15.3 Development15.4 Plant GrowthRegulators15.5 Photoperiodism15.6 Vernalisation2022-23240 BIOLOGY15.1 GROWTHGrowth is regarded as one of the most fundamental and conspicuouscharacteristics of a living being. What is growth? Growth can be definedas an irreversible permanent increase in size of an organ or its parts oreven of an individual cell. Generally, growth is accompanied by metabolicprocesses (both anabolic and catabolic), that occur at the expense ofenergy. Therefore, for example, expansion of a leaf is growth. How wouldyou describe the swelling of piece of wood when placed in water?15.1.1 Plant Growth Generally is IndeterminatePlant growth is unique because plants retain the capacity for unlimitedgrowth throughout their life. This ability of the plants is due to the presenceof meristems at certain locations in their body. The cells of such meristemshave the capacity to divide and self-perpetuate. The product, however,soon loses the capacity to divide and such cells make up the plant body.This form of growth wherein new cells are always being added to theplant body by the activity of the meristem is called the open form of growth.What would happen if the meristem ceases to divide? Does this everhappen?In Chapter 6, you have studied about the root apical meristem andthe shoot apical meristem. You know that they are responsible for theSeed coatEpicotylhookCotyledonsSoil line CotyledonEpicotylHypocotylHypocotylFigure 15.1 Germination and seedling development in bean2022-23PLANT GROWTH AND DEVELOPMENT 241primary growth of the plants and principallycontribute to the elongation of the plants alongtheir axis. You also know that in dicotyledonousplants and gymnosperms, the lateral meristems,vascular cambium and cork-cambium appearlater in life. These are the meristems that causethe increase in the girth of the organs in whichthey are active. This is known as secondarygrowth of the plant (see Figure 15.2).15.1.2 Growth is MeasurableGrowth, at a cellular level, is principally aconsequence of increase in the amount ofprotoplasm. Since increase in protoplasm isdifficult to measure directly, one generallymeasures some quantity which is more or lessproportional to it. Growth is, therefore,measured by a variety of parameters some ofwhich are: increase in fresh weight, dry weight,length, area, volume and cell number. You mayfind it amazing to know that one single maizeroot apical mersitem can give rise to more than17,500 new cells per hour, whereas cells in awatermelon may increase in size by upto3,50,000 times. In the former, growth isexpressed as increase in cell number; the latterexpresses growth as increase in size of the cell.While the growth of a pollen tube is measuredin terms of its length, an increase in surface areadenotes the growth in a dorsiventral leaf.15.1.3 Phases of GrowthThe period of growth is generally divided intothree phases, namely, meristematic, elongationand maturation (Figure 15.3). Let usunderstand this by looking at the root tips. Theconstantly dividing cells, both at the root apexand the shoot apex, represent the meristematicphase of growth. The cells in this region are richin protoplasm, possess large conspicuousnuclei. Their cell walls are primary in nature,thin and cellulosic with abundantplasmodesmatal connections. The cellsproximal (just next, away from the tip) to theShoot apicalmeristemVascularcambiumVascularcambiumRoot apicalmeristemShootRootFigure 15.2 Diagrammatic representation oflocations of root apical meristem,shoot aplical meristem andvascular cambium. Arrows exhibitthe direction of growth of cells andorganGFEDCBAFigure 15.3 Detection of zones of elongation bythe parallel line technique. ZonesA, B, C, D immediately behind theapex have elongated most.2022-23242 BIOLOGYmeristematic zone represent the phase of elongation. Increasedvacuolation, cell enlargement and new cell wall deposition are thecharacteristics of the cells in this phase. Further away from the apex, i.e.,more proximal to the phase of elongation, lies the portion of axis which isundergoing the phase of maturation. The cells of this zone, attain theirmaximal size in terms of wall thickening and protoplasmic modifications.Most of the tissues and cell types you have studied in Chapter 6 representthis phase.15.1.4 Growth RatesThe increased growth per unit time is termed as growth rate. Thus, rateof growth can be expressed mathematically. An organism, or a part of theorganism can produce more cells in a variety of ways.Figure15.4 Diagrammatic representation of : (a) Arithmetic (b) Geometric growth and(c) Stages during embryo development showing geometric and arithematicphases2022-23PLANT GROWTH AND DEVELOPMENT 243The growth rate shows an increase that may bearithmetic or geometrical (Figure 15.4).In arithmetic growth, following mitotic celldivision, only one daughter cell continues to dividewhile the other differentiates and matures. Thesimplest expression of arithmetic growth isexemplified by a root elongating at a constant rate.Look at Figure 15.5. On plotting the length of theorgan against time, a linear curve is obtained.Mathematically, it is expressed asLt = L0 + rtLt = length at time âtâL0 = length at time âzeroâr = growth rate / elongation per unit time.Let us now see what happens in geometricalgrowth. In most systems, the initial growth is slow(lag phase), and it increases rapidly thereafter â atan exponential rate (log or exponential phase). Here,both the progeny cells following mitotic cell divisionretain the ability to divide and continue to do so.However, with limited nutrient supply, the growthslows down leading to a stationary phase. If we plotthe parameter of growth against time, we get a typicalsigmoid or S-curve (Figure 15.6). A sigmoid curveis a characteristic of living organism growing in anatural environment. It is typical for all cells, tissuesand organs of a plant. Can you think of more similarexamples? What kind of a curve can you expect ina tree showing seasonal activities?The exponential growth can be expressed asW1 = W0 ertW1 = final size (weight, height, number etc.)W0 = initial size at the beginning of the periodr = growth ratet = time of growthe = base of natural logarithmsHere, r is the relative growth rate and is also themeasure of the ability of the plant to produce newplant material, referred to as efficiency index. Hence,the final size of W1 depends on the initial size, W0.Figure 15.5 Constant linear growth, a plotof length L against time tFigure 15.6 An idealised sigmoid growthcurve typical of cells in culture,and many higher plants andplant organsSize/weight of the organExponential phaseLag phaseTimeStationary phase2022-23244 BIOLOGYQuantitative comparisons between the growth of living system canalso be made in two ways : (i) measurement and the comparison of totalgrowth per unit time is called the absolute growth rate. (ii) The growth ofthe given system per unit time expressed on a common basis, e.g., perunit initial parameter is called the relative growth rate. In Figure 15.7two leaves, A and B, are drawn that are of different sizes but showsabsolute increase in area in the given time to give leaves, A1 and B1. However,one of them shows much higher relative growth rate. Which one and why?15.1.5 Conditions for GrowthWhy do you not try to write down what you think are necessary conditionsfor growth? This list may have water, oxygen and nutrients as very essentialelements for growth. The plant cells grow in size by cell enlargement whichin turn requires water. Turgidity of cells helps in extension growth. Thus,plant growth and further development is intimately linked to the waterstatus of the plant. Water also provides the medium for enzymatic activitiesneeded for growth. Oxygen helps in releasing metabolic energy essentialfor growth activities. Nutrients (macro and micro essential elements) arerequired by plants for the synthesis of protoplasm and act as source ofenergy.In addition, every plant organism has an optimum temperature rangebest suited for its growth. Any deviation from this range could bedetrimental to its survival. Environmental signals such as light and gravityalso affect certain phases/stages of growth.Figure15.7 Diagrammatic comparison of absolute and relative growth rates. Bothleaves A and B have increased their area by 5 cm2 in a given time toproduce A1, B1 leaves.2022-23PLANT GROWTH AND DEVELOPMENT 24515.2 DIFFERENTIATION, DEDIFFERENTIATION ANDREDIFFERENTIATIONThe cells derived from root apical and shoot-apical meristems andcambium differentiate and mature to perform specific functions. This actleading to maturation is termed as differentiation. During differentiation,cells undergo few to major structural changes both in their cell walls andprotoplasm. For example, to form a tracheary element, the cells wouldlose their protoplasm. They also develop a very strong, elastic,lignocellulosic secondary cell walls, to carry water to long distances evenunder extreme tension. Try to correlate the various anatomical featuresyou encounter in plants to the functions they perform.Plants show another interesting phenomenon. The living differentiatedcells, that by now have lost the capacity to divide can regain the capacityof division under certain conditions. This phenomenon is termed asdedifferentiation. For example, formation of meristems â interfascicularcambium and cork cambium from fully differentiated parenchyma cells.While doing so, such meristems/tissues are able to divide and producecells that once again lose the capacity to divide but mature to performspecific functions, i.e., get redifferentiated. List some of the tissues in awoody dicotyledenous plant that are the products of redifferentiation.How would you describe a tumour? What would you call the parenchymacells that are made to divide under controlled laboratory conditions duringplant tissue culture?Recall, in Section 15.1.1, we have mentioned that the growth in plantsis open, i.e., it can be indeterminate or determinate. Now, we may say thateven differentiation in plants is open, because cells/tissues arising out ofthe same meristem have different structures at maturity. The finalstructure at maturity of a cell/tissue is also determined by the location ofthe cell within. For example, cells positioned away from root apicalmeristems differentiate as root-cap cells, while those pushed to theperiphery mature as epidermis. Can you add a few more examples ofopen differentiation correlating the position of a cell to its position in anorgan?15.3 DEVELOPMENTDevelopment is a term that includes all changes that an organism goesthrough during its life cycle from germination of the seed to senescence.Diagrammatic representation of the sequence of processes whichconstitute the development of a cell of a higher plant is given in Figure15.8. It is also applicable to tissues/organs.2022-23246 BIOLOGYPlants follow different pathways in response to environment or phasesof life to form different kinds of structures. This ability is called plasticity,e.g., heterophylly in cotton, coriander and larkspur. In such plants, theleaves of the juvenile plant are different in shape from those in matureplants. On the other hand, difference in shapes of leaves produced in airand those produced in water in buttercup also represent theheterophyllous development due to environment (Figure 15.9). Thisphenomenon of heterophylly is an example of plasticity.Figure 15.8 Sequence of the developmental process in a plant cellCell Division DeathPlasmatic growth DifferentiationExpansion(Elongation)MaturationMERISTEMATICCELLSENESCENCEMATURECELLFigure 15.9 Heterophylly in (a) larkspur and (b) buttercup2022-23PLANT GROWTH AND DEVELOPMENT 247Thus, growth, differentiation and development are very closely relatedevents in the life of a plant. Broadly, development is considered as thesum of growth and differentiation. Development in plants (i.e., both growthand differentiation) is under the control of intrinsic and extrinsic factors.The former includes both intracellular (genetic) or intercellular factors(chemicals such as plant growth regulators) while the latter includes light,temperature, water, oxygen, nutrition, etc.15.4 PLANT GROWTH REGULATORS15.4.1 CharacteristicsThe plant growth regulators (PGRs) are small, simple molecules of diversechemical composition. They could be indole compounds (indole-3-aceticacid, IAA); adenine derivatives (N6-furfurylamino purine, kinetin),derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid,GA3) or gases (ethylene, C2H4). Plant growth regulators are variouslydescribed as plant growth substances, plant hormones or phytohormonesin literature.The PGRs can be broadly divided into two groups based on theirfunctions in a living plant body. One group of PGRs are involved in growthpromoting activities, such as cell division, cell enlargement, patternformation, tropic growth, flowering, fruiting and seed formation. Theseare also called plant growth promoters, e.g., auxins, gibberellins andcytokinins. The PGRs of the other group play an important role in plantresponses to wounds and stresses of biotic and abiotic origin. They arealso involved in various growth inhibiting activities such as dormancyand abscission. The PGR abscisic acid belongs to this group. The gaseousPGR, ethylene, could fit either of the groups, but it is largely an inhibitorof growth activities.15.4.2 The Discovery of Plant Growth RegulatorsInterestingly, the discovery of each of the fivemajor groups of PGRs have been accidental.All this started with the observation of CharlesDarwin and his son Francis Darwin when theyobserved that the coleoptiles of canary grassresponded to unilateral illumination bygrowing towards the light source(phototropism). After a series of experiments,it was concluded that the tip of coleoptile wasthe site of transmittable influence that causedthe bending of the entire coleoptile (Figure15.10). Auxin was isolated by F.W. Went fromtips of coleoptiles of oat seedlings.Figure 15.10 Experiment used to demonstratethat tip of the coleoptile is thesource of auxin. Arrows indicatedirection of lighta b c d2022-23248 BIOLOGYThe âbakanaeâ (foolish seedling) disease of rice seedlings, was causedby a fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reportedthe appearance of symptoms of the disease in rice seedlings when theywere treated with sterile filtrates of the fungus. The active substanceswere later identified as gibberellic acid.F. Skoog and his co-workers observed that from the internodalsegments of tobacco stems the callus (a mass of undifferentiated cells)proliferated only if, in addition to auxins the nutrients medium wassupplemented with one of the following: extracts of vascular tissues, yeastextract, coconut milk or DNA. Miller et al. (1955), later identified andcrystallised the cytokinesis promoting active substance that theytermed kinetin.During mid-1960s, three independent researches reported thepurification and chemical characterisation of three different kinds ofinhibitors: inhibitor-B, abscission II and dormin. Later all the three wereproved to be chemically identical. It was named abscisic acid (ABA).H.H. Cousins (1910) confirmed the release of a volatile substance fromripened oranges that hastened the ripening of stored unripened bananas.Later this volatile substance was identified as ethylene, a gaseous PGR.Let us study some of the physiological effects of these five categoriesof PGRs in the next section.15.4.3 Physiological Effects of Plant Growth Regulators15.4.3.1 AuxinsAuxins (from Greek âauxeinâ : to grow) was first isolated from human urine.The term âauxinâ is applied to the indole-3-acetic acid (IAA), and to othernatural and synthetic compounds having certain growth regulatingproperties. They are generally produced by the growing apices of the stemsand roots, from where they migrate to the regions of their action. Auxinslike IAA and indole butyric acid (IBA) have been isolated from plants.NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic)are synthetic auxins. All these auxins have been used extensively inagricultural and horticultural practices.They help to initiate rooting in stem cuttings, an application widelyused for plant propagation. Auxins promote flowering e.g. in pineapples.They help to prevent fruit and leaf drop at early stages but promote theabscission of older mature leaves and fruits.In most higher plants, the growing apical bud inhibits the growth ofthe lateral (axillary) buds, a phenomenon called apical dominance.Removal of shoot tips (decapitation) usually results in the growth of lateralbuds (Figure 15.11). It is widely applied in tea plantations, hedge-making.Can you explain why?2022-23PLANT GROWTH AND DEVELOPMENT 249Auxins also induce parthenocarpy, e.g., intomatoes. They are widely used as herbicides.2, 4-D, widely used to kill dicotyledonousweeds, does not affect maturemonocotyledonous plants. It is used to prepareweed-free lawns by gardeners. Auxin alsocontrols xylem differentiation and helps in celldivision.15.4.3.2 GibberellinsGibberellins are another kind of promotoryPGR. There are more than 100 gibberellinsreported from widely different organisms suchas fungi and higher plants. They are denotedas GA1, GA2, GA3 and so on. However,Gibberellic acid (GA3) was one of the firstgibberellins to be discovered and remains themost intensively studied form. All GAs areacidic. They produce a wide range ofphysiological responses in the plants. Their ability to cause an increasein length of axis is used to increase the length of grapes stalks. Gibberellins,cause fruits like apple to elongate and improve its shape. They also delaysenescence. Thus, the fruits can be left on the tree longer so as to extendthe market period. GA3 is used to speed up the malting process in brewingindustry.Sugarcane stores carbohydrate as sugar in their stems. Sprayingsugarcane crop with gibberellins increases the length of the stem, thusincreasing the yield by as much as 20 tonnes per acre.Spraying juvenile conifers with GAs hastens the maturity period, thusleading to early seed production. Gibberellins also promotes bolting(internode elongation just prior to flowering) in beet, cabbages and manyplants with rosette habit.15.4.3.3 CytokininsCytokinins have specific effects on cytokinesis, and were discovered askinetin (a modified form of adenine, a purine) from the autoclaved herringsperm DNA. Kinetin does not occur naturally in plants. Search for naturalsubstances with cytokinin-like activities led to the isolation of zeatin fromcorn-kernels and coconut milk. Since the discovery of zeatin, severalnaturally occurring cytokinins, and some synthetic compounds with celldivision promoting activity, have been identified. Natural cytokinins aresynthesised in regions where rapid cell division occurs, for example, rootapices, developing shoot buds, young fruits etc. It helps to produce newFigure 15.11 Apical dominance in plants :(a) A plant with apical bud intact(b) A plant with apical bud removedNote the growth of lateral buds intobranches after decapitation.(a) (b)2022-23250 BIOLOGYleaves, chloroplasts in leaves, lateral shoot growth and adventitious shootformation. Cytokinins help overcome the apical dominance. They promotenutrient mobilisation which helps in the delay of leaf senescence.15.4.3.4 EthyleneEthylene is a simple gaseous PGR. It is synthesised in large amountsby tissues undergoing senescence and ripening fruits. Influences ofethylene on plants include horizontal growth of seedlings, swelling ofthe axis and apical hook formation in dicot seedlings. Ethylene promotessenescence and abscission of plant organs especially of leaves andflowers. Ethylene is highly effective in fruit ripening. It enhances therespiration rate during ripening of the fruits. This rise in rate ofrespiration is called respiratory climactic.Ethylene breaks seed and bud dormancy, initiates germination inpeanut seeds, sprouting of potato tubers. Ethylene promotes rapidinternode/petiole elongation in deep water rice plants. It helps leaves/upper parts of the shoot to remain above water. Ethylene also promotesroot growth and root hair formation, thus helping the plants to increasetheir absorption surface.Ethylene is used to initiate flowering and for synchronising fruit-setin pineapples. It also induces flowering in mango. Since ethylene regulatesso many physiological processes, it is one of the most widely used PGR inagriculture. The most widely used compound as source of ethylene isethephon. Ethephon in an aqueous solution is readily absorbed andtransported within the plant and releases ethylene slowly. Ethephonhastens fruit ripening in tomatoes and apples and accelerates abscissionin flowers and fruits (thinning of cotton, cherry, walnut). It promotes femaleflowers in cucumbers thereby increasing the yield.15.4.3.5 Abscisic acidAs mentioned earlier, abscisic acid (ABA) was discovered for its role inregulating abscission and dormancy. But like other PGRs, it also hasother wide ranging effects on plant growth and development. It acts as ageneral plant growth inhibitor and an inhibitor of plant metabolism.ABA inhibits seed germination. ABA stimulates the closure of stomataand increases the tolerance of plants to various kinds of stresses.Therefore, it is also called the stress hormone. ABA plays an importantrole in seed development, maturation and dormancy. By inducingdormancy, ABA helps seeds to withstand desiccation and other factorsunfavourable for growth. In most situations, ABA acts as an antagonistto GAs.We may summarise that for any and every phase of growth,differentiation and development of plants, one or the other PGR has somerole to play. Such roles could be complimentary or antagonistic. Thesecould be individualistic or synergistic.2022-23PLANT GROWTH AND DEVELOPMENT 251Similarly, there are a number of events in the life of a plant wheremore than one PGR interact to affect that event, e.g., dormancy in seeds/buds, abscission, senescence, apical dominance, etc.Remember, the role of PGR is of only one kind of intrinsic control.Along with genomic control and extrinsic factors, they play an importantrole in plant growth and development. Many of the extrinsic factors suchas temperature and light, control plant growth and development via PGR.Some of such events could be: vernalisation, flowering, dormancy, seedgermination, plant movements, etc.We shall discuss briefly the role of light and temperature (both of them,the extrinsic factors) on initiation of flowering.15.5 PHOTOPERIODISMIt has been observed that some plants require a periodic exposure tolight to induce flowering. It is also seen that such plants are able tomeasure the duration of exposure to light. For example, some plantsrequire the exposure to light for a period exceeding a well defined criticalduration, while others must be exposed to light for a period less than thiscritical duration before the flowering is initiated in them. The former groupof plants are called long day plants while the latter ones are termedshort day plants. The critical duration is different for different plants.There are many plants, however, where there is no such correlationbetween exposure to light duration and induction of flowering response;such plants are called day-neutral plants (Figure 15.12). It is now alsoFigure 15.12 Photoperiodism : Long day, short day and day neutral plantsLong day plant Short day plant Day neutral plant2022-23252 BIOLOGYknown that not only the duration of light period but that the duration ofdark period is also of equal importance. Hence, it can be said that floweringin certain plants depends not only on a combination of light and darkexposures but also their relative durations. This response of plants toperiods of day/night is termed photoperiodism. It is also interesting tonote that while shoot apices modify themselves into flowering apices priorto flowering, they (i.e., shoot apices of plants) by themselves cannot percievephotoperiods. The site of perception of light/dark duration are the leaves.It has been hypothesised that there is a hormonal substance(s) that isresponsible for flowering. This hormonal substance migrates from leavesto shoot apices for inducing flowering only when the plants are exposedto the necessary inductive photoperiod.15.6 VERNALISATIONThere are plants for which flowering is either quantitatively or qualitativelydependent on exposure to low temperature. This phenomenon is termedvernalisation. It prevents precocious reproductive development late inthe growing season, and enables the plant to have sufficient time to reachmaturity. Vernalisation refers specially to the promotion of flowering by aperiod of low temperature. Some important food plants, wheat, barley,rye have two kinds of varieties: winter and spring varieties. The âspringâvariety are normally planted in the spring and come to flower and producegrain before the end of the growing season. Winter varieties, however, ifplanted in spring would normally fail to flower or produce mature grainwithin a span of a flowering season. Hence, they are planted in autumn.They germinate, and over winter come out as small seedlings, resumegrowth in the spring, and are harvested usually around mid-summer.Another example of vernalisation is seen in biennial plants. Biennialsare monocarpic plants that normally flower and die in the second season.Sugarbeet, cabbages, carrots are some of the common biennials.Subjecting the growing of a biennial plant to a cold treatment stimulatesa subsequent photoperiodic flowering response.15.7 SEED DORMANCYThere are certain seeds which fail to germinate even when externalconditions are favourable. Such seeds are understood to be undergoinga period of dormancy which is controlled not by external environmentbut are under endogenous control or conditions within the seed itself.Impermeable and hard seed coat; presence of chemical inhibitors suchas abscissic acids, phenolic acids, para-ascorbic acid; and immature2022-23PLANT GROWTH AND DEVELOPMENT 253embryos are some of the reasons which causes seed dormancy. This dormancyhowever can be overcome through natural means and various other man-mademeasures. For example, the seed coat barrier in some seeds can be broken bymechanical abrasions using knives, sandpaper, etc. or vigorous shaking. In nature,these abrasions are caused by microbial action, and passage through digestivetract of animals. Effect of inhibitory substances can be removed by subjecting theseeds to chilling conditions or by application of certain chemicals like gibberellicacid and nitrates. Changing the environmental conditions, such as light andtemperature are other methods to overcome seed dormancy.SUMMARYGrowth is one of the most conspicuous events in any living organism. It is anirreversible increase expressed in parameters such as size, area, length, height,volume, cell number etc. It conspicuously involves increased protoplasmic material.In plants, meristems are the sites of growth. Root and shoot apical meristemssometimes alongwith intercalary meristem, contribute to the elongation growth ofplant axes. Growth is indeterminate in higher plants. Following cell division in rootand shoot apical meristem cells, the growth could be arithmetic or geometrical.Growth may not be and generally is not sustained at a high rate throughout the lifeof cell/tissue/organ/organism. One can define three principle phases of growth âthe lag, the log and the senescent phase. When a cell loses the capacity to divide, itleads to differentiation. Differentiation results in development of structures that iscommensurate with the function the cells finally has to perform. General principlesfor differentiation for cell, tissues and organs are similar. A differentiated cell maydedifferentiate and then redifferentiate. Since differentiation in plants is open, thedevelopment could also be flexible, i.e., the development is the sum of growth anddifferentiation. Plant exhibit plasticity in development.Plant growth and development are under the control of both intrinsic andextrinsic factors. Intercellular intrinsic factors are the chemical substances, calledplant growth regulators (PGR). There are diverse groups of PGRs in plants, principallybelonging to five groups: auxins, gibberellins, cytokinins, abscisic acid and ethylene.These PGRs are synthesised in various parts of the plant; they control differentdifferentiation and developmental events. Any PGR has diverse physiological effectson plants. Diverse PGRs also manifest similar effects. PGRs may act synergisticallyor antagonistically. Plant growth and development is also affected by light,temperature, nutrition, oxygen status, gravity and such external factors.Flowering in some plants is induced only when exposed to certain duration ofphotoperiod. Depending on the nature of photoperiod requirements, the plants arecalled short day plants, long day plants and day-neutral plants. Certain plantsalso need to be exposed to low temperature so as to hasten flowering later in life.This treatement is known as vernalisation.2022-23254 BIOLOGYEXERCISES1. Define growth, differentiation, development, dedifferentiation, redifferentiation,determinate growth, meristem and growth rate.2. Why is not any one parameter good enough to demonstrate growth throughoutthe life of a flowering plant?3. Describe briefly:(a) Arithmetic growth(b) Geometric growth(c) Sigmoid growth curve(d) Absolute and relative growth rates4. List five main groups of natural plant growth regulators. Write a note ondiscovery, physiological functions and agricultural/horticultural applicationsof any one of them.5. What do you understand by photoperiodism and vernalisation? Describe theirsignificance.6. Why is abscisic acid also known as stress hormone?7. âBoth growth and differentiation in higher plants are openâ. Comment.8. âBoth a short day plant and a long day plant can produce can flowersimultaneously in a given placeâ. Explain.9. Which one of the plant growth regulators would you use if you are asked to:(a) induce rooting in a twig(b) quickly ripen a fruit(c) delay leaf senescence(d) induce growth in axillary buds(e) âboltâ a rosette plant(f) induce immediate stomatal closure in leaves.10. Would a defoliated plant respond to photoperiodic cycle? Why?11. What would be expected to happen if:(a) GA3 is applied to rice seedlings(b) dividing cells stop differentiating(c) a rotten fruit gets mixed with unripe fruits(d) you forget to add cytokinin to the culture medium.2022-23UNIT 5The reductionist approach to study of life forms resulted in increasinguse of physico-chemical concepts and techniques. Majority of thesestudies employed either surviving tissue model or straightaway cellfreesystems. An explosion of knowledge resulted in molecular biology.Molecular physiology became almost synonymous with biochemistryand biophysics. However, it is now being increasingly realised thatneither a purely organismic approach nor a purely reductionisticmolecular approach would reveal the truth about biological processesor living phenomena. Systems biology makes us believe that all livingphenomena are emergent properties due to interaction amongcomponents of the system under study. Regulatory network of molecules,supra molecular assemblies, cells, tissues, organisms and indeed,populations and communities, each create emergent properties. In thechapters under this unit, major human physiological processes likedigestion, exchange of gases, blood circulation, locomotion andmovement are described in cellular and molecular terms. The last twochapters point to the coordination and regulation of body events at theorganismic level.HUMAN PHYSIOLOGYChapter 16Digestion and AbsorptionChapter 17Breathing and Exchangeof GasesChapter 18Body Fluids andCirculationChapter 19Excretory Products andtheir EliminationChapter 20Locomotion and MovementChapter 21Neural Control andCoordinationChapter 22Chemical Coordinationand Integration2022-23ALFONSO CORTI, Italian anatomist, was born in 1822. Corti beganhis scientific career studying the cardiovascular systems ofreptiles. Later, he turned his attention to the mammalianauditory system. In 1851, he published a paper describing astructure located on the basilar membrane of the cochleacontaining hair cells that convert sound vibrations into nerveimpulses, the organ of Corti. He died in the year 1888.Alfonso Corti(1822 â 1888)2022-23Food is one of the basic requirements of all living organisms. The majorcomponents of our food are carbohydrates, proteins and fats. Vitaminsand minerals are also required in small quantities. Food provides energyand organic materials for growth and repair of tissues. The water we takein, plays an important role in metabolic processes and also preventsdehydration of the body. Biomacromolecules in food cannot be utilisedby our body in their original form. They have to be broken down andconverted into simple substances in the digestive system. This process ofconversion of complex food substances to simple absorbable forms iscalled digestion and is carried out by our digestive system by mechanicaland biochemical methods. General organisation of the human digestivesystem is shown in Figure 16.1.16.1 DIGESTIVE SYSTEMThe human digestive system consists of the alimentary canal and theassociated glands.16.1.1 Alimentary CanalThe alimentary canal begins with an anterior opening â the mouth, and itopens out posteriorly through the anus. The mouth leads to the buccalcavity or oral cavity. The oral cavity has a number of teeth and a musculartongue. Each tooth is embedded in a socket of jaw bone (Figure16.2).This type of attachment is called thecodont. Majority of mammalsincluding human being forms two sets of teeth during their life, a set oftemporary milk or deciduous teeth replaced by a set of permanent oradult teeth. This type of dentition is called diphyodont. An adult humanDIGESTION AND ABSORPTIONCHAPTER 1616.1 DigestiveSystem16.2 Digestion ofFood16.3 Absorption ofDigestedProducts16.4 Disorders ofDigestiveSystem2022-23258 BIOLOGYSigmoid ColonFigure 16.1 The human digestive systemhas 32 permanent teeth which are of four different types (Heterodontdentition), namely, incisors (I), canine (C), premolars (PM) and molars(M). Arrangement of teeth in each half of the upper and lower jaw in theorder I, C, PM, M is represented by a dental formula which in humanis21232123. The hard chewing surface of the teeth, made up of enamel, helpsin the mastication of food. The tongue is a freely movable muscular organattached to the floor of the oral cavity by the frenulum. The upper surfaceof the tongue has small projections called papillae, some of which beartaste buds.The oral cavity leads into a short pharynx which serves as a commonpassage for food and air. The oesophagus and the trachea (wind pipe)open into the pharynx. A cartilaginous flap called epiglottis prevents theentry of food into the glottis â opening of the wind pipe â during swallowing.The oesophagus is a thin, long tube which extends posteriorly passingthrough the neck, thorax and diaphragm and leads to a âJâ shaped bag2022-23DIGESTION AND ABSORPTION 259like structure called stomach. A muscularsphincter (gastro-oesophageal) regulates theopening of oesophagus into the stomach.The stomach, located in the upper leftportion of the abdominal cavity, has fourmajor parts â a cardiac portion into whichthe oesophagus opens, a fundic region, body(main central region) and a pyloric portionwhich opens into the first part of smallintestine (Figure 16.3). Small intestine isdistinguishable into three regions, a âCâshaped duodenum, a long coiled middleportion jejunum and a highly coiled ileum.The opening of the stomach into theduodenum is guarded by the pyloricsphincter. Ileum opens into the largeintestine. It consists of caecum, colon andrectum. Caecum is a small blind sac whichhosts some symbiotic micro-organisms. Anarrow finger-like tubular projection, thevermiform appendix which is a vestigialorgan, arises from the caecum. The caecumopens into the colon. The colon is dividedinto four parts â an ascending, a transverse,descending part and a sigmoid colon. Thedescending part opens into the rectumwhich opens out through the anus.The wall of alimentary canal fromoesophagus to rectum possesses four layers(Figure 16.4) namely serosa, muscularis,sub-mucosa and mucosa. Serosa is theoutermost layer and is made up of a thinmesothelium (epithelium of visceral organs)with some connective tissues. Muscularis isformed by smooth muscles usuallyarranged into an inner circular and an outerlongitudinal layer. An oblique muscle layermay be present in some regions. The submucosallayer is formed of loose connectivetissues containing nerves, blood and lymphvessels. In duodenum, glands are alsopresent in sub-mucosa. The innermostlayer lining the lumen of the alimentarycanal is the mucosa. This layer formsirregular folds (rugae) in the stomach andsmall finger-like foldings called villi in thesmall intestine (Figure 16.5). The cells liningthe villi produce numerous microscopicFigure 16.2 Arrangement of different types ofteeth in the jaws on one side andthe sockets on the other sideFigure 16.3 Anatomical regions of humanstomach2022-23260 BIOLOGYFigure 16.4 Diagrammatic representation of transverse section of gutprojections called microvilli giving a brush borderappearance. These modifications increase thesurface area enormously. Villi are supplied witha network of capillaries and a large lymph vesselcalled the lacteal. Mucosal epithelium has gobletcells which secrete mucus that help in lubrication.Mucosa also forms glands in the stomach (gastricglands) and crypts in between the bases of villi inthe intestine (crypts of Lieberkuhn). All the fourlayers show modifications in different parts of thealimentary canal.16.1.2 Digestive GlandsThe digestive glands associated with thealimentary canal include the salivary glands, theliver and the pancreas.Saliva is mainly produced by three pairs ofsalivary glands, the parotids (cheek), the submaxillary/sub-mandibular (lower jaw) and thesub- linguals (below the tongue). These glandssituated just outside the buccal cavity secretesalivary juice into the buccal cavity.Liver is the largest gland of the body weighing about 1.2 to 1.5 kg inan adult human. It is situated in the abdominal cavity, just below thediaphragm and has two lobes. The hepatic lobules are the structural andfunctional units of liver containing hepatic cells arranged in the form ofcords. Each lobule is covered by a thin connective tissue sheath calledthe Glissonâs capsule. The bile secreted by the hepatic cells passes throughthe hepatic ducts and is stored and concentrated in a thin muscular saccalled the gall bladder. The duct of gall bladder (cystic duct) along withFigure 16.5 A section of small intestinalmucosa showing villiVilliLactealCapillariesCryptsArteryVein2022-23DIGESTION AND ABSORPTION 261the hepatic duct from the liver forms the common bile duct (Figure 16.6).The bile duct and the pancreatic duct open together into the duodenumas the common hepato-pancreatic duct which is guarded by a sphinctercalled the sphincter of Oddi.The pancreas is a compound (both exocrine and endocrine) elongatedorgan situated between the limbs of the âCâ shaped duodenum. Theexocrine portion secretes an alkaline pancreatic juice containing enzymesand the endocrine portion secretes hormones, insulin and glucagon.Figure 16.6 The duct systems of liver, gall bladder and pancreas16.2 DIGESTION OF FOODThe process of digestion is accomplished by mechanical and chemicalprocesses.The buccal cavity performs two major functions, mastication of foodand facilitation of swallowing. The teeth and the tongue with the help ofsaliva masticate and mix up the food thoroughly. Mucus in saliva helpsin lubricating and adhering the masticated food particles into a bolus.The bolus is then conveyed into the pharynx and then into the oesophagusby swallowing or deglutition. The bolus further passes down throughthe oesophagus by successive waves of muscular contractions calledperistalsis. The gastro-oesophageal sphincter controls the passage of foodinto the stomach.The saliva secreted into the oral cavity containselectrolytes and enzymes, salivary amylase andlysozyme. The chemical process of digestion is initiated in the oral cavityby the hydrolytic action of the carbohydrate splitting enzyme, the salivary2022-23262 BIOLOGYamylase. About 30 per cent of starch is hydrolysed here by this enzyme(optimum pH 6.8) into a disaccharide â maltose. Lysozyme present insaliva acts as an antibacterial agent that prevents infections.StarchSalivary.AmylasepHMaltose6 8žžžžžžžžžŽThe mucosa of stomach has gastric glands. Gastric glands have threemajor types of cells namely -(i) mucus neck cells which secrete mucus;(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor(factor essential for absorption of vitamin B12).The stomach stores the food for 4-5 hours. The food mixes thoroughlywith the acidic gastric juice of the stomach by the churning movementsof its muscular wall and is called the chyme. The proenzyme pepsinogen,on exposure to hydrochloric acid gets converted into the active enzymepepsin, the proteolytic enzyme of the stomach. Pepsin converts proteinsinto proteoses and peptones (peptides). The mucus and bicarbonatespresent in the gastric juice play an important role in lubrication andprotection of the mucosal epithelium from excoriation by the highlyconcentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8)optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juiceof infants which helps in the digestion of milk proteins. Small amounts oflipases are also secreted by gastric glands.Various types of movements are generated by the muscularis layer ofthe small intestine. These movements help in a thorough mixing up ofthe food with various secretions in the intestine and thereby facilitatedigestion. The bile, pancreatic juice and the intestinal juice are thesecretions released into the small intestine. Pancreatic juice and bile arereleased through the hepato-pancreatic duct. The pancreatic juicecontains inactive enzymes â trypsinogen, chymotrypsinogen,procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen isactivated by an enzyme, enterokinase, secreted by the intestinal mucosainto active trypsin, which in turn activates the other enzymes in thepancreatic juice. The bile released into the duodenum contains bilepigments (bilirubin and bili-verdin), bile salts, cholesterol andphospholipids but no enzymes. Bile helps in emulsification of fats, i.e.,breaking down of the fats into very small micelles. Bile also activates lipases.The intestinal mucosal epithelium has goblet cells which secretemucus. The secretions of the brush border cells of the mucosa alongwiththe secretions of the goblet cells constitute the intestinal juice orsuccus entericus. This juice contains a variety of enzymes likedisaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.The mucus alongwith the bicarbonates from the pancreas protects theintestinal mucosa from acid as well as provide an alkaline medium (pH7.8) for enzymatic activities. Sub-mucosal glands (Brunnerâs glands) alsohelp in this.2022-23DIGESTION AND ABSORPTION 263Proteins, proteoses and peptones (partially hydrolysed proteins) inthe chyme reaching the intestine are acted upon by the proteolyticenzymes of pancreatic juice as given below:ProteinsPeptonesProteosesTrypsin/ChymotrypsinCarboxypÃŧÃŊ ïÞ ïeptidasežžžžžžžžžžžžŽ DipeptidesCarbohydrates in the chyme are hydrolysed by pancreatic amylaseinto disaccharides.Polysaccharides starch DisaccharidesAmylase( )žžžžžžŽFats are broken down by lipases with the help of bile into di-andmonoglycerides.Fats Diglycerides MonoglyceridesžžLipžažsesžŽ žžŽNucleases in the pancreatic juice acts on nucleic acids to formnucleotides and nucleosidesNucleic acids žžNužclžeažsežs ÂŽNucleotides žžŽNucleosidesThe enzymes in the succus entericus act on the end products of theabove reactions to form the respective simple absorbable forms. Thesefinal steps in digestion occur very close to the mucosal epithelial cells ofthe intestine.DipeptidesžžDižpežptiždažsežs ÂŽ Amino acidsMaltose žžMažltažsežŽGlucose + GlucoseLactose žžLažctažsežŽGlucose +GalactoseSucrose žžSužcržasže ÂŽGlucose+ FructoseNucleotidesžžNužclžeotžidažsežsžŽNucleosidesžžNužclžeosžidžasžesžŽSugars + BasesDi andMonoglycerides Fatty acids GlycerolžžLipžažsežs ÂŽ +The breakdown of biomacromolecules mentioned above occurs in theduodenum region of the small intestine. The simple substances thusformed are absorbed in the jejunum and ileum regions of the smallintestine. The undigested and unabsorbed substances are passed on tothe large intestine.2022-23264 BIOLOGYNo significant digestive activity occurs in the large intestine. Thefunctions of large intestine are:(i) absorption of some water, minerals and certain drugs;(ii) secretion of mucus which helps in adhering the waste (undigested)particles together and lubricating it for an easy passage.The undigested, unabsorbed substances called faeces enters into the caecumof the large intestine through ileo-caecal valve, which prevents the back flow ofthe faecal matter. It is temporarily stored in the rectum till defaecation.The activities of the gastro-intestinal tract are under neural andhormonal control for proper coordination of different parts. The sight, smelland/or the presence of food in the oral cavity can stimulate the secretion ofsaliva. Gastric and intestinal secretions are also, similarly, stimulated byneural signals. The muscular activities of different parts of the alimentarycanal can also be moderated by neural mechanisms, both local and throughCNS. Hormonal control of the secretion of digestive juices is carried out bylocal hormones produced by the gastric and intestinal mucosa.CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE AND FAT(Boxed item â Not for evaluation)The energy requirements of animals, and the energy content of food, areexpressed in terms of measure of heat energy because heat is the ultimate formof all energies. This is often measured to as calorie (cal) or joule (J), which is theamount of heat energy required to raise the temperature of 1 g of water by 1 °C.Since this value is tiny amount of energy, physiologists commonly use kilocalorie(kcal) or kilo joule (kJ). One kilo calorie is the amount of energy required to raisethe temperature of 1 kg of water by 1 °C. Nutritionists, traditionally refer to kcalas the Calorie or Joule (always capitalised). The amount of heat liberated fromcomplete combustion of 1 g food in a bomb calorimeter (a closed metal chamberfilled with O2) is its gross calorific or gross energy value. The actual amount ofenergy combustion of 1 g of food is the physiologic value of food. Gross calorificvalues of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/gand 9.0 kcal/g, respectively.16.3 ABSORPTION OF DIGESTED PRODUCTSAbsorption is the process by which the end products of digestion passthrough the intestinal mucosa into the blood or lymph. It is carried out bypassive, active or facilitated transport mechanisms. Small amounts ofmonosaccharides like glucose, amino acids and some electrolytes likechloride ions are generally absorbed by simple diffusion. The passage ofthese substances into the blood depends upon the concentration gradients.However, some substances like glucose and amino acids are absorbed withthe help of carrier proteins. This mechanism is called the facilitated transport.Transport of water depends upon the osmotic gradient. Activetransport occurs against the concentration gradient and hence requiresenergy. Various nutrients like amino acids, monosaccharides like glucose,electrolytes like Na+ are absorbed into the blood by this mechanism.Fatty acids and glycerol being insoluble, cannot be absorbed into the2022-23DIGESTION AND ABSORPTION 265blood. They are first incorporated into small droplets called micelles whichmove into the intestinal mucosa. They are re-formed into very small proteincoated fat globules called the chylomicrons which are transported intothe lymph vessels (lacteals) in the villi. These lymph vessels ultimatelyrelease the absorbed substances into the blood stream.Absorption of substances takes place in different parts of the alimentarycanal, like mouth, stomach, small intestine and large intestine. However,maximum absorption occurs in the small intestine. A summary of absorption(sites of absorption and substances absorbed) is given in Table 16.1.The absorbed substances finally reach the tissues which utilise themfor their activities. This process is called assimilation.The digestive wastes, solidified into coherent faeces in the rectuminitiate a neural reflex causing an urge or desire for its removal. Theegestion of faeces to the outside through the anal opening (defaecation) isa voluntary process and is carried out by a mass peristaltic movement.16.4 DISORDERS OF DIGESTIVE SYSTEMThe inflammation of the intestinal tract is the most common ailment dueto bacterial or viral infections. The infections are also caused by theparasites of the intestine like tapeworm, roundworm, threadworm,hookworm, pin worm, etc.Jaundice: The liver is affected, skin and eyes turn yellow due to thedeposit of bile pigments.Vomiting: It is the ejection of stomach contents through the mouth. Thisreflex action is controlled by the vomit centre in the medulla. A feeling ofnausea precedes vomiting.Diarrhoea: The abnormal frequency of bowel movement and increasedliquidity of the faecal discharge is known as diarrhoea. It reduces theabsorption of food.Constipation: In constipation, the faeces are retained within the colonas the bowel movements occur irregularly.Indigestion: In this condition, the food is not properly digested leading toa feeling of fullness. The causes of indigestion are inadequate enzymesecretion, anxiety, food poisoning, over eating, and spicy food.StomachAbsorption ofwater, simplesugars, andalcohol etc.takes place.Small IntestinePrincipal organ for absorptionof nutrients. The digestion iscompleted here and the finalproducts of digestion such asglucose, fructose, fatty acids,glycerol and amino acids areabsorbed through the mucosainto the blood stream andlymph.MouthCertain drugscoming in contactwith the mucosaof mouth andlower side of thetongue areabsorbed into theblood capillarieslining them.Large IntestineAbsorption ofwater, someminerals anddrugs takesplace.TABLE 16.1 The Summary of Absorption in Different Parts of Digestive System2022-23266 BIOLOGYPEMDietary deficiencies of proteins and total food calories are widespread inmany underdeveloped countries of South and South-east Asia, SouthAmerica, and West and Central Africa. Protein-energy malnutrition(PEM) may affect large sections of the population during drought, famineand political turmoil. This happened in Bangladesh during the liberationwar and in Ethiopia during the severe drought in mid-eighties. PEM affectsinfants and children to produce Marasmus and Kwashiorkar.Marasmus is produced by a simultaneous deficiency of proteins andcalories. It is found in infants less than a year in age, if motherâs milk isreplaced too early by other foods which are poor in both proteins andcaloric value. This often happens if the mother has second pregnancy orchildbirth when the older infant is still too young. In Marasmus, proteindeficiency impairs growth and replacement of tissue proteins; extremeemaciation of the body and thinning of limbs results, the skin becomesdry, thin and wrinkled. Growth rate and body weight decline considerably.Even growth and development of brain and mental faculties are impaired.Kwashiorkar is produced by protein deficiency unaccompanied by caloriedeficiency. It results from the replacement of motherâs milk by a high calorielowprotein diet in a child more than one year in age. Like marasmus,kwashiorkor shows wasting of muscles, thinning of limbs, failure of growthand brain development. But unlike marasmus, some fat is still left underthe skin; moreover, extensive oedema and swelling of body parts are seen.SUMMARYThe digestive system of humans consists of an alimentary canal andassociated digestive glands. The alimentary canal consists of the mouth,buccal cavity, pharynx, oesophagus, stomach, small intestine, largeintestine, rectum and the anus. The accessory digestive glands include thesalivary glands, the liver (with gall bladder) and the pancreas. Inside themouth the teeth masticates the food, the tongue tastes the food andmanipulates it for proper mastication by mixing with the saliva. Salivacontains a starch digestive enzyme, salivary amylase that digests the starchand converts it into maltose (disaccharide). The food then passes into thepharynx and enters the oesophagus in the form of bolus, which is furthercarried down through the oesophagus by peristalsis into the stomach. Instomach mainly protein digestion takes place. Absorption of simple sugars,alcohol and medicines also takes place in the stomach.The chyme (food) enters into the duodenum portion of the smallintestine and is acted on by the pancreatic juice, bile and finally by theenzymes in the succus entericus, so that the digestion of carbohydrates,proteins and fats is completed. The food then enters into the jejunum andileum portions of the small intestine. Carbohydrates are digested andconverted into monosaccharides like glucose. Proteins are finally brokendown into amino acids. The fats are converted to fatty acids and glycerol.2022-23DIGESTION AND ABSORPTION 267The digested end products are absorbed into the body through the epithelial liningof the intestinal villi. The undigested food (faeces) enters into the caecum of the largeintestine through ileo-caecal valve, which prevents the back flow of the faecal matter.Most of the water is absorbed in the large intestine. The undigested food becomessemi-solid in nature and then enters into the rectum, anal canal and is finally egestedout through the anus.EXERCISES1. Choose the correct answer among the following :(a) Gastric juice contains(i) pepsin, lipase and rennin(ii) trypsin, lipase and rennin(iii) trypsin, pepsin and lipase(iv) trypsin, pepsin and renin(b) Succus entericus is the name given to(i) a junction between ileum and large intestine(ii) intestinal juice(iii) swelling in the gut(iv) appendix2. Match column I with column IIColumn I Column II(a) Bilirubin and biliverdin (i) Parotid(b) Hydrolysis of starch (ii) Bile(c) Digestion of fat (iii) Lipases(d) Salivary gland (iv) Amylases3. Answer briefly:(a) Why are villi present in the intestine and not in the stomach?(b) How does pepsinogen change into its active form?(c) What are the basic layers of the wall of alimentary canal?(d) How does bile help in the digestion of fats?4. State the role of pancreatic juice in digestion of proteins.5. Describe the process of digestion of protein in stomach.6. Give the dental formula of human beings.7. Bile juice contains no digestive enzymes, yet it is important for digestion. Why?8. Describe the digestive role of chymotrypsin. Which two other digestive enzymesof the same category are secreted by its source gland?9. How are polysaccharides and disaccharides digested?10. What would happen if HCl were not secreted in the stomach?11. How does butter in your food get digested and absorbed in the body?12. Discuss the main steps in the digestion of proteins as the food passes throughdifferent parts of the alimentary canal.13. Explain the term thecodont and diphyodont.14. Name different types of teeth and their number in an adult human.15. What are the functions of liver?2022-23268 BIOLOGYAs you have read earlier, oxygen (O2) is utilised by the organisms toindirectly break down simple molecules like glucose, amino acids, fattyacids, etc., to derive energy to perform various activities. Carbon dioxide(CO2) which is harmful is also released during the above catabolicreactions. It is, therefore, evident that O2 has to be continuously providedto the cells and CO2 produced by the cells have to be released out. Thisprocess of exchange of O2 from the atmosphere with CO2 produced by thecells is called breathing, commonly known as respiration. Place yourhands on your chest; you can feel the chest moving up and down. Youknow that it is due to breathing. How do we breathe? The respiratoryorgans and the mechanism of breathing are described in the followingsections of this chapter.17.1 RESPIRATORY ORGANSMechanisms of breathing vary among different groups of animalsdepending mainly on their habitats and levels of organisation. Lowerinvertebrates like sponges, coelenterates, flatworms, etc., exchange O2with CO2 by simple diffusion over their entire body surface. Earthwormsuse their moist cuticle and insects have a network of tubes (trachealtubes) to transport atmospheric air within the body. Special vascularisedstructures called gills (branchial respiration) are used by most of theaquatic arthropods and molluscs whereas vascularised bags called lungs(pulmonary respiration) are used by the terrestrial forms for the exchangeof gases. Among vertebrates, fishes use gills whereas amphibians, reptiles,birds and mammals respire through lungs. Amphibians like frogs canrespire through their moist skin (cutaneous respiration) also.BREATHING AND EXCHANGE OF GASESCHAPTER 1717.1 RespiratoryOrgans17.2 Mechanism ofBreathing17.3 Exchange ofGases17.4 Transport ofGases17.5 Regulation ofRespiration17.6 Disorders ofRespiratorySystem2022-23BREATHING AND EXCHANGE OF GASES 26917.1.1 Human Respiratory SystemWe have a pair of external nostrils opening out above the upper lips.It leads to a nasal chamber through the nasal passage. The nasalchamber opens into the pharynx, a portion of which is the commonpassage for food and air. The pharynx opens through the larynx regioninto the trachea. Larynx is a cartilaginous box which helps in soundproduction and hence called the sound box. During swallowing glottiscan be covered by a thin elastic cartilaginous flap called epiglottis toprevent the entry of food into the larynx. Trachea is a straight tubeextending up to the mid-thoracic cavity, which divides at the level of5th thoracic vertebra into a right and left primary bronchi. Each bronchiundergoes repeated divisions to form the secondary and tertiary bronchiand bronchioles ending up in very thin terminal bronchioles. Thetracheae, primary, secondary and tertiary bronchi, and initialbronchioles are supported by incomplete cartilaginous rings. Eachterminal bronchiole gives rise to a number of very thin, irregular-walledand vascularised bag-like structures called alveoli. The branchingnetwork of bronchi, bronchioles and alveoli comprise the lungs (Figure17.1). We have two lungs which are covered by a double layered pleura,with pleural fluid between them. It reduces friction on the lung-surface.The outer pleural membrane is in close contact with the thoracicBronchusLungheartDiaphragmEpiglottisLarynxTracheaCut end of rib Pleural membranesAlveoliPleural fluidBronchioleFigure 17.1 Diagrammatic view of human respiratory system (sectional view ofthe left lung is also shown)2022-23270 BIOLOGYlining whereas the inner pleural membrane is in contact with the lungsurface. The part starting with the external nostrils up to the terminalbronchioles constitute the conducting part whereas the alveoli and theirducts form the respiratory or exchange part of the respiratory system.The conducting part transports the atmospheric air to the alveoli, clearsit from foreign particles, humidifies and also brings the air to bodytemperature. Exchange part is the site of actual diffusion of O2 and CO2between blood and atmospheric air.The lungs are situated in the thoracic chamber which is anatomicallyan air-tight chamber. The thoracic chamber is formed dorsally by thevertebral column, ventrally by the sternum, laterally by the ribs and onthe lower side by the dome-shaped diaphragm. The anatomical setup oflungs in thorax is such that any change in the volume of the thoraciccavity will be reflected in the lung (pulmonary) cavity. Such anarrangement is essential for breathing, as we cannot directly alter thepulmonary volume.Respiration involves the following steps:(i) Breathing or pulmonary ventilation by which atmospheric airis drawn in and CO2 rich alveolar air is released out.(ii) Diffusion of gases (O2 and CO2) across alveolar membrane.(iii) Transport of gases by the blood.(iv) Diffusion of O2 and CO2 between blood and tissues.(v) Utilisation of O2 by the cells for catabolic reactions and resultantrelease of CO2 (cellular respiration as dealt in the Chapter 14).17.2 MECHANISM OF BREATHINGBreathing involves two stages : inspiration during which atmosphericair is drawn in and expiration by which the alveolar air is released out.The movement of air into and out of the lungs is carried out by creating apressure gradient between the lungs and the atmosphere. Inspirationcan occur if the pressure within the lungs (intra-pulmonary pressure) isless than the atmospheric pressure, i.e., there is a negative pressure inthe lungs with respect to atmospheric pressure. Similarly, expiration takesplace when the intra-pulmonary pressure is higher than the atmosphericpressure. The diaphragm and a specialised set of muscles â external andinternal intercostals between the ribs, help in generation of such gradients.Inspiration is initiated by the contraction of diaphragm which increasesthe volume of thoracic chamber in the antero-posterior axis. Thecontraction of external inter-costal muscles lifts up the ribs and the2022-23BREATHING AND EXCHANGE OF GASES 271sternum causing an increase in the volume ofthe thoracic chamber in the dorso-ventral axis.The overall increase in the thoracic volumecauses a similar increase in pulmonaryvolume. An increase in pulmonary volumedecreases the intra-pulmonary pressure to lessthan the atmospheric pressure which forcesthe air from outside to move into the lungs,i.e., inspiration (Figure 17.2a). Relaxation ofthe diaphragm and the inter-costal musclesreturns the diaphragm and sternum to theirnormal positions and reduce the thoracicvolume and thereby the pulmonary volume.This leads to an increase in intra-pulmonarypressure to slightly above the atmosphericpressure causing the expulsion of air from thelungs, i.e., expiration (Figure 17.2b). We havethe ability to increase the strength ofinspiration and expiration with the help ofadditional muscles in the abdomen. On anaverage, a healthy human breathes 12-16times/minute. The volume of air involved inbreathing movements can be estimated byusing a spirometer which helps in clinicalassessment of pulmonary functions.17.2.1 Respiratory Volumes andCapacitiesTidal Volume (TV): Volume of air inspired orexpired during a normal respiration. It isapprox. 500 mL., i.e., a healthy man caninspire or expire approximately 6000 to 8000mL of air per minute.Inspiratory Reserve Volume (IRV):Additional volume of air, a person can inspireby a forcible inspiration. This averages 2500mL to 3000 mL.Expiratory Reserve Volume (ERV):Additional volume of air, a person can expireby a forcible expiration. This averages 1000mL to 1100 mL.Figure 17.2 Mechanism of breathing showing :(a) inspiration (b) expiration2022-23272 BIOLOGYResidual Volume (RV): Volume of air remaining in the lungs even after aforcible expiration. This averages 1100 mL to 1200 mL.By adding up a few respiratory volumes described above, one canderive various pulmonary capacities, which can be used in clinicaldiagnosis.Inspiratory Capacity (IC): Total volume of air a person can inspireafter a normal expiration. This includes tidal volume and inspiratoryreserve volume ( TV+IRV).Expiratory Capacity (EC): Total volume of air a person can expire aftera normal inspiration. This includes tidal volume and expiratory reservevolume (TV+ERV).Functional Residual Capacity (FRC): Volume of air that will remain inthe lungs after a normal expiration. This includes ERV+RV.Vital Capacity (VC): The maximum volume of air a person can breathe inafter a forced expiration. This includes ERV, TV and IRV or the maximumvolume of air a person can breathe out after a forced inspiration.Total Lung Capacity (TLC): Total volume of air accommodated in thelungs at the end of a forced inspiration. This includes RV, ERV, TV andIRV or vital capacity + residual volume.17.3 EXCHANGE OF GASESAlveoli are the primary sites of exchange of gases. Exchange of gases alsooccur between blood and tissues. O2 and CO2 are exchanged in thesesites by simple diffusion mainly based on pressure/concentrationgradient. Solubility of the gases as well as the thickness of the membranesinvolved in diffusion are also some important factors that can affect therate of diffusion.Pressure contributed by an individual gas in a mixture of gases iscalled partial pressure and is represented as pO2 for oxygen and pCO2 forcarbon dioxide. Partial pressures of these two gases in the atmosphericair and the two sites of diffusion are given in Table 17.1 and inFigure 17.3. The data given in the table clearly indicates a concentrationgradient for oxygen from alveoli to blood and blood to tissues. Similarly,TABLE 17.1 Partial Pressures (in mm Hg) of Oxygen and Carbon dioxide at DifferentParts Involved in Diffusion in Comparison to those in AtmosphereRespiratory Atmospheric Alveoli Blood Blood TissuesGas Air (Deoxygenated) (Oxygenated)O2 159 104 40 95 40CO2 0.3 40 45 40 452022-23BREATHING AND EXCHANGE OF GASES 273a gradient is present for CO2 in the opposite direction, i.e., from tissues toblood and blood to alveoli. As the solubility of CO2 is 20-25 times higherthan that of O2, the amount of CO2 that can diffuse through the diffusionmembrane per unit difference in partial pressure is much higher comparedto that of O2. The diffusion membraneis made up of three major layers(Figure 17.4) namely, the thin squamousepithelium of alveoli, the endothelium ofalveolar capillaries and the basementsubstance (composed of a thin basementmembrane supporting the squamousepithelium and the basement membranesurrounding the single layer endothelialcells of capillaries) in between them.However, its total thickness is much lessthan a millimetre. Therefore, all the factorsin our body are favourable for diffusion ofO2 from alveoli to tissues and that of CO2from tissues to alveoli.Figure 17.4 A Diagram of a section of analveolus with a pulmonarycapillary.Figure 17.3 Diagrammatic representation of exchange of gases at the alveolus andthe body tissues with blood and transport of oxygen and carbon dioxide2022-23274 BIOLOGY17.4 TRANSPORT OF GASESBlood is the medium of transport for O2 and CO2. About 97 per cent of O2 istransported by RBCs in the blood. The remaining 3 per cent of O2 is carriedin a dissolved state through the plasma. Nearly 20-25 per cent of CO2 istransported by RBCs whereas 70 per cent of it is carried as bicarbonate.About 7 per cent of CO2 is carried in a dissolved state through plasma.17.4.1 Transport of OxygenHaemoglobin is a red coloured iron containing pigment present in theRBCs. O2 can bind with haemoglobin in a reversible manner to formoxyhaemoglobin. Each haemoglobin molecule can carry a maximum offour molecules of O2. Binding of oxygen with haemoglobin is primarilyrelated to partial pressure of O2. Partial pressure of CO2, hydrogen ionconcentration and temperature are the other factors which can interferewith this binding. A sigmoid curve is obtained when percentage saturationof haemoglobin with O2 is plotted against thepO2. This curve is called the Oxygendissociation curve (Figure 17.5) and is highlyuseful in studying the effect of factors likepCO2, H+ concentration, etc., on binding of O2with haemoglobin. In the alveoli, where thereis high pO2, low pCO2, lesser H+ concentrationand lower temperature, the factors areall favourable for the formation ofoxyhaemoglobin, whereas in the tissues,where low pO2, high pCO2, high H+concentration and higher temperature exist,the conditions are favourable for dissociationof oxygen from the oxyhaemoglobin. Thisclearly indicates that O2 gets bound tohaemoglobin in the lung surface and getsdissociated at the tissues. Every 100 ml ofoxygenated blood can deliver around 5 ml ofO2 to the tissues under normal physiologicalconditions.17.4.2 Transport of Carbon dioxideCO2 is carried by haemoglobin as carbamino-haemoglobin (about20-25 per cent). This binding is related to the partial pressure of CO2.pO2 is a major factor which could affect this binding. When pCO2 is highand pO2 is low as in the tissues, more binding of carbon dioxide occurswhereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation200 20404060608080100100Partial pressure of oxygen (mm Hg)Percentage saturation of haemoglobin with oxygenFigure 17.5 Oxygen dissociation curve2022-23BREATHING AND EXCHANGE OF GASES 275of CO2 from carbamino-haemoglobin takes place, i.e., CO2 which is boundto haemoglobin from the tissues is delivered at the alveoli. RBCs containa very high concentration of the enzyme, carbonic anhydrase and minutequantities of the same is present in the plasma too. This enzyme facilitatesthe following reaction in both directions.CO H O H COCarbonicanhydraseCarbonicanhydra2 2 2 3+ žžžžžžŽÂŦžžžžžžsežžžžžžŽHCO HÂŦžžžžžž +â +3At the tissue site where partial pressure of CO2 is high due tocatabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3âand H+,. At the alveolar site where pCO2 is low, the reaction proceeds inthe opposite direction leading to the formation of CO2 and H2O. Thus,CO2 trapped as bicarbonate at the tissue level and transported to thealveoli is released out as CO2 (Figure 17.4). Every 100 ml of deoxygenatedblood delivers approximately 4 ml of CO2 to the alveoli.17.5 REGULATION OF RESPIRATIONHuman beings have a significant ability to maintain and moderate therespiratory rhythm to suit the demands of the body tissues. This is doneby the neural system. A specialised centre present in the medulla regionof the brain called respiratory rhythm centre is primarily responsible forthis regulation. Another centre present in the pons region of the braincalled pneumotaxic centre can moderate the functions of the respiratoryrhythm centre. Neural signal from this centre can reduce the duration ofinspiration and thereby alter the respiratory rate. A chemosensitive areais situated adjacent to the rhythm centre which is highly sensitive to CO2and hydrogen ions. Increase in these substances can activate this centre,which in turn can signal the rhythm centre to make necessary adjustmentsin the respiratory process by which these substances can be eliminated.Receptors associated with aortic arch and carotid artery also can recognisechanges in CO2 and H+ concentration and send necessary signals to therhythm centre for remedial actions. The role of oxygen in the regulation ofrespiratory rhythm is quite insignificant.17.6 DISORDERS OF RESPIRATORY SYSTEMAsthma is a difficulty in breathing causing wheezing due to inflammationof bronchi and bronchioles.Emphysema is a chronic disorder in which alveolar walls are damageddue to which respiratory surface is decreased. One of the major causes ofthis is cigarette smoking.2022-23276 BIOLOGYSUMMARYCells utilise oxygen for metabolism and produce energy along with substanceslike carbon dioxide which is harmful. Animals have evolved different mechanismsfor the transport of oxygen to the cells and for the removal of carbon dioxide fromthere. We have a well developed respiratory system comprising two lungs andassociated air passages to perform this function.The first step in respiration is breathing by which atmospheric air is taken in(inspiration) and the alveolar air is released out (expiration). Exchange of O2 andCO2 between deoxygenated blood and alveoli, transport of these gases throughoutthe body by blood, exchange of O2 and CO2 between the oxygenated blood andtissues and utilisation of O2 by the cells (cellular respiration) are the other stepsinvolved.Inspiration and expiration are carried out by creating pressure gradientsbetween the atmosphere and the alveoli with the help of specialised muscles âintercostals and diaphragm. Volumes of air involved in these activities can beestimated with the help of spirometer and are of clinical significance.Exchange of O2 and CO2 at the alveoli and tissues occur by diffusion. Rate ofdiffusion is dependent on the partial pressure gradients of O2 (pO2) and CO2 (pCO2),their solubility as well as the thickness of the diffusion surface. These factors inour body facilitate diffusion of O2 from the alveoli to the deoxygenated blood aswell as from the oxygenated blood to the tissues. The factors are favourable for thediffusion of CO2 in the opposite direction, i.e., from tissues to alveoli.Oxygen is transported mainly as oxyhaemoglobin. In the alveoli where pO2 ishigher, O2 gets bound to haemoglobin which is easily dissociated at the tissueswhere pO2 is low and pCO2 and H+ concentration are high. Nearly 70 per cent ofcarbon dioxide is transported as bicarbonate (HCO3â) with the help of the enzymecarbonic anhydrase. 20-25 per cent of carbon dioxide is carried by haemoglobinas carbamino-haemoglobin. In the tissues where pCO2 is high, it gets bound toblood whereas in the alveoli where pCO2 is low and pO2 is high, it gets removedfrom the blood.Respiratory rhythm is maintained by the respiratory centre in the medullaregion of brain. A pneumotaxic centre in the pons region of the brain and achemosensitive area in the medulla can alter respiratory mechanism.Occupational Respiratory Disorders: In certain industries, especiallythose involving grinding or stone-breaking, so much dust is producedthat the defense mechanism of the body cannot fully cope with thesituation. Long exposure can give rise to inflammation leading to fibrosis(proliferation of fibrous tissues) and thus causing serious lung damage.Workers in such industries should wear protective masks.2022-23BREATHING AND EXCHANGE OF GASES 277EXERCISES1. Define vital capacity. What is its significance?2. State the volume of air remaining in the lungs after a normal breathing.3. Diffusion of gases occurs in the alveolar region only and not in the other parts ofrespiratory system. Why?4. What are the major transport mechanisms for CO2? Explain.5. What will be the pO2 and pCO2 in the atmospheric air compared to those in thealveolar air ?(i) pO2 lesser, pCO2 higher(ii) pO2 higher, pCO2 lesser(iii) pO2 higher, pCO2 higher(iv) pO2 lesser, pCO2 lesser6. Explain the process of inspiration under normal conditions.7. How is respiration regulated?8. What is the effect of pCO2 on oxygen transport?9. What happens to the respiratory process in a man going up a hill?10. What is the site of gaseous exchange in an insect?11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidalpattern?12. Have you heard about hypoxia? Try to gather information about it, and discusswith your friends.13. Distinguish between(a) IRV and ERV(b) Inspiratory capacity and Expiratory capacity.(c) Vital capacity and Total lung capacity.14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthyhuman in an hour.2022-23278 BIOLOGYYou have learnt that all living cells have to be provided with nutrients, O2and other essential substances. Also, the waste or harmful substancesproduced, have to be removed continuously for healthy functioning oftissues. It is therefore, essential to have efficient mechanisms for themovement of these substances to the cells and from the cells. Differentgroups of animals have evolved different methods for this transport. Simpleorganisms like sponges and coelenterates circulate water from theirsurroundings through their body cavities to facilitate the cells to exchangethese substances. More complex organisms use special fluids within theirbodies to transport such materials. Blood is the most commonly used bodyfluid by most of the higher organisms including humans for this purpose.Another body fluid, lymph, also helps in the transport of certain substances.In this chapter, you will learn about the composition and properties ofblood and lymph (tissue fluid) and the mechanism of circulation of bloodis also explained herein.18.1 BLOODBlood is a special connective tissue consisting of a fluid matrix, plasma,and formed elements.18.1.1 PlasmaPlasma is a straw coloured, viscous fluid constituting nearly 55 per cent ofthe blood. 90-92 per cent of plasma is water and proteins contribute 6-8per cent of it. Fibrinogen, globulins and albumins are the major proteins.BODY FLUIDS AND CIRCULATIONCHAPTER 1818.1 Blood18.2 Lymph (TissueFluid)18.3 CirculatoryPathways18.4 DoubleCirculation18.5 Regulation ofCardiac Activity18.6 Disorders ofCirculatorySystem2022-23BODY FLUIDS AND CIRCULATION 279Fibrinogens are needed for clotting or coagulation of blood. Globulinsprimarly are involved in defense mechanisms of the body and the albuminshelp in osmotic balance. Plasma also contains small amounts of mineralslike Na+, Ca++, Mg++, HCO3â, Clâ, etc. Glucose, amino acids, lipids, etc., arealso present in the plasma as they are always in transit in the body. Factorsfor coagulation or clotting of blood are also present in the plasma in aninactive form. Plasma without the clotting factors is called serum.18.1.2 Formed ElementsErythrocytes, leucocytes and platelets are collectively called formedelements (Figure 18.1) and they constitute nearly 45 per cent of the blood.Erythrocytes or red blood cells (RBC) are the most abundant of allthe cells in blood. A healthy adult man has, on an average, 5 millions to5.5 millions of RBCs mmâ3 of blood. RBCs are formed in the red bonemarrow in the adults. RBCs are devoid of nucleus in most of the mammalsand are biconcave in shape. They have a red coloured, iron containingcomplex protein called haemoglobin, hence the colour and name of thesecells. A healthy individual has 12-16 gms of haemoglobin in every100 ml of blood. These molecules play a significant role in transport ofrespiratory gases. RBCs have an average life span of 120 days after whichthey are destroyed in the spleen (graveyard of RBCs).Leucocytes are also known as white blood cells (WBC) as they arecolourless due to the lack of haemoglobin. They are nucleated and arerelatively lesser in number which averages 6000-8000 mmâ3 of blood.Leucocytes are generally short lived. We have two main categories of WBCsâ granulocytes and agranulocytes. Neutrophils, eosinophils and basophilsare different types of granulocytes, while lymphocytes and monocytesare the agranulocytes. Neutrophils are the most abundant cells (60-65per cent) of the total WBCs and basophils are the least (0.5-1 per cent)among them. Neutrophils and monocytes (6-8 per cent) are phagocyticcells which destroy foreign organisms entering the body. Basophils secretehistamine, serotonin, heparin, etc., and are involved in inflammatoryreactions. Eosinophils (2-3 per cent) resist infections and are alsoR B CPlateletsEosinophilBasophilNeutrophilMonocyteT lymphocyteB lymphocyteFigure 18.1 Diagrammatic representation of formed elements in blood2022-23280 BIOLOGYassociated with allergic reactions. Lymphocytes (20-25 per cent) are oftwo major types â âBâ and âTâ forms. Both B and T lymphocytes areresponsible for immune responses of the body.Platelets also called thrombocytes, are cell fragments produced frommegakaryocytes (special cells in the bone marrow). Blood normallycontains 1,500,00-3,500,00 platelets mmâ3. Platelets can release a varietyof substances most of which are involved in the coagulation or clotting ofblood. A reduction in their number can lead to clotting disorders whichwill lead to excessive loss of blood from the body.18.1.3 Blood GroupsAs you know, blood of human beings differ in certain aspects though itappears to be similar. Various types of grouping of blood has been done.Two such groupings â the ABO and Rh â are widely used all over theworld.18.1.3.1 ABO groupingABO grouping is based on the presence or absence of two surface antigens(chemicals that can induce immune response) on the RBCs namely Aand B. Similarly, the plasma of different individuals contain two naturalantibodies (proteins produced in response to antigens). The distributionof antigens and antibodies in the four groups of blood, A, B, AB and Oare given in Table 18.1. You probably know that during blood transfusion,any blood cannot be used; the blood of a donor has to be carefully matchedwith the blood of a recipient before any blood transfusion to avoid severeproblems of clumping (destruction of RBC). The donorâs compatibility isalso shown in the Table 18.1.Blood Group Antigens on Antibodies Donorâs GroupRBCs in PlasmaA A anti-B A, OB B anti-A B, OAB A, B nil AB, A, B, OO nil anti-A, B OTABLE 18.1 Blood Groups and Donor CompatibilityFrom the above mentioned table it is evident that group âOâ blood canbe donated to persons with any other blood group and hence âOâ groupindividuals are called âuniversal donorsâ. Persons with âABâ group canaccept blood from persons with AB as well as the other groups of blood.Therefore, such persons are called âuniversal recipientsâ.2022-23BODY FLUIDS AND CIRCULATION 28118.1.3.2 Rh groupingAnother antigen, the Rh antigen similar to one present in Rhesus monkeys(hence Rh), is also observed on the surface of RBCs of majority (nearly 80per cent) of humans. Such individuals are called Rh positive (Rh+ve)and those in whom this antigen is absent are called Rh negative (Rh-ve).An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodiesagainst the Rh antigens. Therefore, Rh group should also be matchedbefore transfusions. A special case of Rh incompatibility (mismatching)has been observed between the Rh-ve blood of a pregnant mother withRh+ve blood of the foetus. Rh antigens of the foetus do not get exposed tothe Rh-ve blood of the mother in the first pregnancy as the two bloods arewell separated by the placenta. However, during the delivery of the firstchild, there is a possibility of exposure of the maternal blood to smallamounts of the Rh+ve blood from the foetus. In such cases, the motherstarts preparing antibodies against Rh antigen in her blood. In case ofher subsequent pregnancies, the Rh antibodies from the mother (Rh-ve)can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs.This could be fatal to the foetus or could cause severe anaemia andjaundice to the baby. This condition is called erythroblastosis foetalis.This can be avoided by administering anti-Rh antibodies to the motherimmediately after the delivery of the first child.18.1.4 Coagulation of BloodYou know that when you cut your finger or hurt yourself, your wounddoes not continue to bleed for a long time; usually the blood stops flowingafter sometime. Do you know why? Blood exhibits coagulation or clottingin response to an injury or trauma. This is a mechanism to preventexcessive loss of blood from the body. You would have observed a darkreddish brown scum formed at the site of a cut or an injury over a periodof time. It is a clot or coagulam formed mainly of a network of threadscalled fibrins in which dead and damaged formed elements of blood aretrapped. Fibrins are formed by the conversion of inactive fibrinogens inthe plasma by the enzyme thrombin. Thrombins, in turn are formed fromanother inactive substance present in the plasma called prothrombin. Anenzyme complex, thrombokinase, is required for the above reaction. Thiscomplex is formed by a series of linked enzymic reactions (cascadeprocess) involving a number of factors present in the plasma in an inactivestate. An injury or a trauma stimulates the platelets in the blood to releasecertain factors which activate the mechanism of coagulation. Certainfactors released by the tissues at the site of injury also can initiatecoagulation. Calcium ions play a very important role in clotting.2022-23282 BIOLOGY18.2 LYMPH (TISSUE FLUID)As the blood passes through the capillaries in tissues, some water alongwith many small water soluble substances move out into the spacesbetween the cells of tissues leaving the larger proteins and most of theformed elements in the blood vessels. This fluid released out is called theinterstitial fluid or tissue fluid. It has the same mineral distribution asthat in plasma. Exchange of nutrients, gases, etc., between the blood andthe cells always occur through this fluid. An elaborate network of vesselscalled the lymphatic system collects this fluid and drains it back to themajor veins. The fluid present in the lymphatic system is called the lymph.Lymph is a colourless fluid containing specialised lymphocytes whichare responsible for the immune responses of the body. Lymph is also animportant carrier for nutrients, hormones, etc. Fats are absorbed throughlymph in the lacteals present in the intestinal villi.18.3 CIRCULATORY PATHWAYSThe circulatory patterns are of two types â open or closed. Opencirculatory system is present in arthropods and molluscs in which bloodpumped by the heart passes through large vessels into open spaces orbody cavities called sinuses. Annelids and chordates have a closedcirculatory system in which the blood pumped by the heart is alwayscirculated through a closed network of blood vessels. This pattern isconsidered to be more advantageous as the flow of fluid can be moreprecisely regulated.All vertebrates possess a muscular chambered heart. Fishes have a2-chambered heart with an atrium and a ventricle. Amphibians and thereptiles (except crocodiles) have a 3-chambered heart with two atria and asingle ventricle, whereas crocodiles, birds and mammals possess a4-chambered heart with two atria and two ventricles. In fishes the heartpumps out deoxygenated blood which is oxygenated by the gills andsupplied to the body parts from where deoxygenated blood is returned tothe heart (single circulation). In amphibians and reptiles, the left atriumreceives oxygenated blood from the gills/lungs/skin and the right atriumgets the deoxygenated blood from other body parts. However, they get mixedup in the single ventricle which pumps out mixed blood (incomplete doublecirculation). In birds and mammals, oxygenated and deoxygenated bloodreceived by the left and right atria respectively passes on to the ventricles ofthe same sides. The ventricles pump it out without any mixing up, i.e., twoseparate circulatory pathways are present in these organisms, hence, theseanimals have double circulation. Let us study the human circulatorysystem.2022-23BODY FLUIDS AND CIRCULATION 28318.3.1 Human Circulatory SystemHuman circulatory system, also called the blood vascular system consistsof a muscular chambered heart, a network of closed branching bloodvessels and blood, the fluid which is circulated.Heart, the mesodermally derived organ, is situated in the thoraciccavity, in between the two lungs, slightly tilted to the left. It has the size ofa clenched fist. It is protected by a double walled membranous bag,pericardium, enclosing the pericardial fluid. Our heart has fourchambers, two relatively small upper chambers called atria and two largerlower chambers called ventricles. A thin, muscular wall called the interatrialseptum separates the right and the left atria, whereas a thick-walled,the inter-ventricular septum, separates the left and the right ventricles(Figure 18.2). The atrium and the ventricle of the same side are alsoseparated by a thick fibrous tissue called the atrio-ventricular septum.However, each of these septa are provided with an opening through whichthe two chambers of the same side are connected. The opening betweenthe right atrium and the right ventricle is guarded by a valve formed ofthree muscular flaps or cusps, the tricuspid valve, whereas a bicuspidor mitral valve guards the opening between the left atrium and the leftventricle. The openings of the right and the left ventricles into theFigure 18.2 Section of a human heart2022-23284 BIOLOGYpulmonary artery and the aorta respectively are provided with thesemilunar valves. The valves in the heart allows the flow of blood only inone direction, i.e., from the atria to the ventricles and from the ventriclesto the pulmonary artery or aorta. These valves prevent any backwardflow.The entire heart is made of cardiac muscles. The walls of ventriclesare much thicker than that of the atria. A specialised cardiac musculaturecalled the nodal tissue is also distributed in the heart (Figure 18.2). Apatch of this tissue is present in the right upper corner of the right atriumcalled the sino-atrial node (SAN). Another mass of this tissue is seen inthe lower left corner of the right atrium close to the atrio-ventricular septumcalled the atrio-ventricular node (AVN). A bundle of nodal fibres, atrioventricularbundle (AV bundle) continues from the AVN which passesthrough the atrio-ventricular septa to emerge on the top of the interventricularseptum and immediately divides into a right and left bundle.These branches give rise to minute fibres throughout the ventricularmusculature of the respective sides and are called purkinje fibres. Thenodal musculature has the ability to generate action potentials withoutany external stimuli, i.e., it is autoexcitable. However, the number of actionpotentials that could be generated in a minute vary at different parts ofthe nodal system. The SAN can generate the maximum number of actionpotentials, i.e., 70-75 minâ1, and is responsible for initiating andmaintaining the rhythmic contractile activity of the heart. Therefore, it iscalled the pacemaker. Our heart normally beats 70-75 times in a minute(average 72 beats minâ1).18.3.2 Cardiac CycleHow does the heart function? Let us take a look. To begin with, all thefour chambers of heart are in a relaxed state, i.e., they are in jointdiastole. As the tricuspid and bicuspid valves are open, blood from thepulmonary veins and vena cava flows into the left and the right ventriclerespectively through the left and right atria. The semilunar valves areclosed at this stage. The SAN now generates an action potential whichstimulates both the atria to undergo a simultaneous contraction â theatrial systole. This increases the flow of blood into the ventricles by about30 per cent. The action potential is conducted to the ventricular side bythe AVN and AV bundle from where the bundle of His transmits it throughthe entire ventricular musculature. This causes the ventricular musclesto contract, (ventricular systole), the atria undergoes relaxation(diastole), coinciding with the ventricular systole. Ventricular systoleincreases the ventricular pressure causing the closure of tricuspid and2022-23BODY FLUIDS AND CIRCULATION 285bicuspid valves due to attempted backflow of blood into the atria. Asthe ventricular pressure increases further, the semilunar valves guardingthe pulmonary artery (right side) and the aorta (left side) are forced open,allowing the blood in the ventricles to flow through these vessels intothe circulatory pathways. The ventricles now relax (ventricular diastole)and the ventricular pressure falls causing the closure of semilunar valveswhich prevents the backflow of blood into the ventricles. As theventricular pressure declines further, the tricuspid and bicuspid valvesare pushed open by the pressure in the atria exerted by the blood whichwas being emptied into them by the veins. The blood now once againmoves freely to the ventricles. The ventricles and atria are now again ina relaxed (joint diastole) state, as earlier. Soon the SAN generates a newaction potential and the events described above are repeated in thatsequence and the process continues.This sequential event in the heart which is cyclically repeated is calledthe cardiac cycle and it consists of systole and diastole of both the atriaand ventricles. As mentioned earlier, the heart beats 72 times per minute,i.e., that many cardiac cycles are performed per minute. From this it couldbe deduced that the duration of a cardiac cycle is 0.8 seconds. During acardiac cycle, each ventricle pumps out approximately 70 mL of bloodwhich is called the stroke volume. The stroke volume multiplied by theheart rate (no. of beats per min.) gives the cardiac output. Therefore, thecardiac output can be defined as the volume of blood pumped out by eachventricle per minute and averages 5000 mL or 5 litres in a healthy individual.The body has the ability to alter the stroke volume as well as the heart rateand thereby the cardiac output. For example, the cardiac output of anathlete will be much higher than that of an ordinary man.During each cardiac cycle two prominent sounds are produced whichcan be easily heard through a stethoscope. The first heart sound (lub) isassociated with the closure of the tricuspid and bicuspid valves whereasthe second heart sound (dub) is associated with the closure of thesemilunar valves. These sounds are of clinical diagnostic significance.18.3.3 Electrocardiograph (ECG)You are probably familiar with this scene from a typical hospital televisionshow: A patient is hooked up to a monitoring machine that shows voltagetraces on a screen and makes the sound â... pip... pip... pip.....peeeeeeeeeeeeeeeeeeeeeeâ as the patient goes into cardiac arrest. This typeof machine (electro-cardiograph) is used to obtain an electrocardiogram(ECG). ECG is a graphical representation of the electrical activity of theheart during a cardiac cycle. To obtain a standard ECG (as shown in the2022-23286 BIOLOGYFigure 18.3), a patient is connected to themachine with three electrical leads (one to eachwrist and to the left ankle) that continuouslymonitor the heart activity. For a detailedevaluation of the heartâs function, multipleleads are attached to the chest region. Here,we will talk only about a standard ECG.Each peak in the ECG is identified with aletter from P to T that corresponds to a specificelectrical activity of the heart.The P-wave represents the electricalexcitation (or depolarisation) of the atria,which leads to the contraction of both the atria.The QRS complex represents the depolarisation of the ventricles,which initiates the ventricular contraction. The contraction starts shortlyafter Q and marks the beginning of the systole.The T-wave represents the return of the ventricles from excited to normalstate (repolarisation). The end of the T-wave marks the end of systole.Obviously, by counting the number of QRS complexes that occur in agiven time period, one can determine the heart beat rate of an individual.Since the ECGs obtained from different individuals have roughly the sameshape for a given lead configuration, any deviation from this shape indicatesa possible abnormality or disease. Hence, it is of a great clinical significance.18.4 DOUBLE CIRCULATIONThe blood flows strictly by a fixed route through Blood Vesselsâthearteries and veins. Basically, each artery and vein consists of three layers:an inner lining of squamous endothelium, the tunica intima, a middlelayer of smooth muscle and elastic fibres, the tunica media, and anexternal layer of fibrous connective tissue with collagen fibres, the tunicaexterna. The tunica media is comparatively thin in the veins (Figure18.4).As mentioned earlier, the blood pumped by the right ventricle entersthe pulmonary artery, whereas the left ventricle pumps blood into theaorta. The deoxygenated blood pumped into the pulmonary artery ispassed on to the lungs from where the oxygenated blood is carried bythe pulmonary veins into the left atrium. This pathway constitutes thepulmonary circulation. The oxygenated blood entering the aorta iscarried by a network of arteries, arterioles and capillaries to the tissuesfrom where the deoxygenated blood is collected by a system of venules,veins and vena cava and emptied into the right atrium. This is thesystemic circulation (Figure 18.4). The systemic circulation providesnutrients, O2 and other essential substances to the tissues and takesCO2 and other harmful substances away for elimination. A uniquevascular connection exists between the digestive tract and liver calledFigure 18.3 Diagrammatic presentation of astandard ECG2022-23BODY FLUIDS AND CIRCULATION 287hepatic portal system. The hepatic portal vein carries blood from intestineto the liver before it is delivered to the systemic circulation. A specialcoronary system of blood vessels is present in our body exclusively forthe circulation of blood to and from the cardiac musculature.18.5 REGULATION OF CARDIAC ACTIVITYNormal activities of the heart are regulated intrinsically, i.e., auto regulatedby specialised muscles (nodal tissue), hence the heart is called myogenic.A special neural centre in the medulla oblangata can moderate the cardiacfunction through autonomic nervous system (ANS). Neural signals throughthe sympathetic nerves (part of ANS) can increase the rate of heart beat,the strength of ventricular contraction and thereby the cardiac output.On the other hand, parasympathetic neural signals (another componentof ANS) decrease the rate of heart beat, speed of conduction of actionpotential and thereby the cardiac output. Adrenal medullary hormonescan also increase the cardiac output.18.6 DISORDERS OF CIRCULATORY SYSTEMHigh Blood Pressure (Hypertension): Hypertension is the term for bloodpressure that is higher than normal (120/80). In this measurement 120mm Hg (millimetres of mercury pressure) is the systolic, or pumping,pressure and 80 mm Hg is the diastolic, or resting, pressure. If repeatedchecks of blood pressure of an individual is 140/90 (140 over 90) orFigure 18.4 Schematic plan of blood circulation in human2022-23288 BIOLOGYhigher, it shows hypertension. High blood pressure leads to heart diseasesand also affects vital organs like brain and kidney.Coronary Artery Disease (CAD): Coronary Artery Disease, often referredto as atherosclerosis, affects the vessels that supply blood to the heartmuscle. It is caused by deposits of calcium, fat, cholesterol and fibroustissues, which makes the lumen of arteries narrower.Angina: It is also called âangina pectorisâ. A symptom of acute chest painappears when no enough oxygen is reaching the heart muscle. Anginacan occur in men and women of any age but it is more common amongthe middle-aged and elderly. It occurs due to conditions that affect theblood flow.Heart Failure: Heart failure means the state of heart when it is not pumpingblood effectively enough to meet the needs of the body. It is sometimescalled congestive heart failure because congestion of the lungs is one ofthe main symptoms of this disease. Heart failure is not the same as cardiacarrest (when the heart stops beating) or a heart attack (when the heartmuscle is suddenly damaged by an inadequate blood supply).SUMMARYVertebrates circulate blood, a fluid connective tissue, in their body, to transport essentialsubstances to the cells and to carry waste substances from there. Another fluid, lymph(tissue fluid) is also used for the transport of certain substances.Blood comprises of a fluid matrix, plasma and formed elements. Red blood cells (RBCs,erythrocytes), white blood cells (WBCs, leucocytes) and platelets (thrombocytes) constitutethe formed elements. Blood of humans are grouped into A, B, AB and O systems basedon the presence or absence of two surface antigens, A, B on the RBCs. Another bloodgrouping is also done based on the presence or absence of another antigen called Rhesusfactor (Rh) on the surface of RBCs. The spaces between cells in the tissues contain a fluidderived from blood called tissue fluid. This fluid called lymph is almost similar to bloodexcept for the protein content and the formed elements.All vertebrates and a few invertebrates have a closed circulatory system. Our circulatorysystem consists of a muscular pumping organ, heart, a network of vessels and a fluid, blood.Heart has two atria and two ventricles. Cardiac musculature is auto-excitable. Sino-atrial node(SAN) generates the maximum number of action protentials per minute (70-75/min) andtherefore, it sets the pace of the activities of the heart. Hence it is called the Pacemaker. Theaction potential causes the atria and then the ventricles to undergo contraction (systole) followedby their relaxation (diastole). The systole forces the blood to move from the atria to the ventriclesand to the pulmonary artery and the aorta. The cardiac cycle is formed by sequential events inthe heart which is cyclically repeated and is called the cardiac cycle. A healthy person shows 72such cycles per minute. About 70 mL of blood is pumped out by each ventricle during acardiac cycle and it is called the stroke or beat volume. Volume of blood pumped out by eachventricle of heart per minute is called the cardiac output and it is equal to the product of strokevolume and heart rate (approx 5 litres). The electrical activity of the heart can be recorded from2022-23BODY FLUIDS AND CIRCULATION 289the body surface by using electrocardiograph and the recording is calledelectrocardiogram (ECG) which is of clinical importance.We have a complete double circulation, i.e., two circulatory pathways, namely,pulmonary and systemic are present. The pulmonary circulation starts by thepumping of deoxygenated blood by the right ventricle which is carried to the lungswhere it is oxygenated and returned to the left atrium. The systemic circulationstarts with the pumping of oxygenated blood by the left ventricle to the aortawhich is carried to all the body tissues and the deoxygenated blood from there iscollected by the veins and returned to the right atrium. Though the heart isautoexcitable, its functions can be moderated by neural and hormonal mechanisms.EXERCISES1. Name the components of the formed elements in the blood and mention onemajor function of each of them.2. What is the importance of plasma proteins?3. Match Column I with Column II :Column I Column II(a) Eosinophils (i) Coagulation(b) RBC (ii) Universal Recipient(c) AB Group (iii) Resist Infections(d) Platelets (iv) Contraction of Heart(e) Systole (v) Gas transport4. Why do we consider blood as a connective tissue?5. What is the difference between lymph and blood?6. What is meant by double circulation? What is its significance?7. Write the differences between :(a) Blood and Lymph(b) Open and Closed system of circulation(c) Systole and Diastole(d) P-wave and T-wave8. Describe the evolutionary change in the pattern of heart among the vertebrates.9. Why do we call our heart myogenic?10. Sino-atrial node is called the pacemaker of our heart. Why?11. What is the significance of atrio-ventricular node and atrio-ventricular bundlein the functioning of heart?12. Define a cardiac cycle and the cardiac output.13. Explain heart sounds.14. Draw a standard ECG and explain the different segments in it.2022-23290 BIOLOGYAnimals accumulate ammonia, urea, uric acid, carbon dioxide, waterand ions like Na+, K+, Clâ, phosphate, sulphate, etc., either by metabolicactivities or by other means like excess ingestion. These substances haveto be removed totally or partially. In this chapter, you will learn themechanisms of elimination of these substances with special emphasis oncommon nitrogenous wastes. Ammonia, urea and uric acid are the majorforms of nitrogenous wastes excreted by the animals. Ammonia is themost toxic form and requires large amount of water for its elimination,whereas uric acid, being the least toxic, can be removed with a minimumloss of water.The process of excreting ammonia is Ammonotelism. Many bony fishes,aquatic amphibians and aquatic insects are ammonotelic in nature.Ammonia, as it is readily soluble, is generally excreted by diffusion acrossbody surfaces or through gill surfaces (in fish) as ammonium ions. Kidneysdo not play any significant role in its removal. Terrestrial adaptationnecessitated the production of lesser toxic nitrogenous wastes like ureaand uric acid for conservation of water. Mammals, many terrestrialamphibians and marine fishes mainly excrete urea and are called ureotelicanimals. Ammonia produced by metabolism is converted into urea in theliver of these animals and released into the blood which is filtered andexcreted out by the kidneys. Some amount of urea may be retained in thekidney matrix of some of these animals to maintain a desired osmolarity.Reptiles, birds, land snails and insects excrete nitrogenous wastes as uricacid in the form of pellet or paste with a minimum loss of water and arecalled uricotelic animals.EXCRETORY PRODUCTS ANDTHEIR ELIMINATIONCHAPTER 1919.1 HumanExcretorySystem19.2 Urine Formation19.3 Function of theTubules19.4 Mechanism ofConcentration ofthe Filtrate19.5 Regulation ofKidney Function19.6 Micturition19.7 Role of otherOrgans inExcretion19.8 Disorders of theExcretorySystem2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 291A survey of animal kingdom presents a variety of excretory structures.In most of the invertebrates, these structures are simple tubular formswhereas vertebrates have complex tubular organs called kidneys. Someof these structures are mentioned here. Protonephridia or flame cells arethe excretory structures in Platyhelminthes (Flatworms, e.g., Planaria),rotifers, some annelids and the cephalochordate â Amphioxus.Protonephridia are primarily concerned with ionic and fluid volumeregulation, i.e., osmoregulation. Nephridia are the tubular excretorystructures of earthworms and other annelids. Nephridia help to removenitrogenous wastes and maintain a fluid and ionic balance. Malpighiantubules are the excretory structures of most of the insects includingcockroaches. Malpighian tubules help in the removal of nitrogenouswastes and osmoregulation. Antennal glands or green glands performthe excretory function in crustaceans like prawns.19.1 HUMAN EXCRETORY SYSTEMIn humans, the excretory system consistsof a pair of kidneys, one pair of ureters, aurinary bladder and a urethra (Figure19.1). Kidneys are reddish brown, beanshaped structures situated between thelevels of last thoracic and third lumbarvertebra close to the dorsal inner wall ofthe abdominal cavity. Each kidney of anadult human measures 10-12 cm inlength, 5-7 cm in width, 2-3 cm inthickness with an average weight of 120-170 g. Towards the centre of the innerconcave surface of the kidney is a notchcalled hilum through which ureter, bloodvessels and nerves enter. Inner to the hilumis a broad funnel shaped space called therenal pelvis with projections called calyces.The outer layer of kidney is a toughcapsule. Inside the kidney, there are twozones, an outer cortex and an innermedulla. The medulla is divided into a fewconical masses (medullary pyramids)projecting into the calyces (sing.: calyx).The cortex extends in between theFigure 19.1 Human Urinary system2022-23292 BIOLOGYFigure 19.3 A diagrammatic representation of a nephron showing blood vessels,duct and tubulemedullary pyramids as renal columns calledColumns of Bertini (Figure 19.2).Each kidney has nearly one millioncomplex tubular structures called nephrons(Figure 19.3), which are the functional units.Each nephron has two parts â theglomerulus and the renal tubule.Glomerulus is a tuft of capillaries formed bythe afferent arteriole â a fine branch of renalartery. Blood from the glomerulus is carriedaway by an efferent arteriole.The renal tubule begins with a doublewalled cup-like structure called Bowmanâscapsule, which encloses the glomerulus.Glomerulus alongwith Bowmanâs capsule, iscalled the malpighian body or renalcorpuscle (Figure 19.4). The tubulecontinues further to form a highly coilednetwork â proximal convoluted tubuleFigure 19.2 Longitudinal section (Diagrammatic)of Kidney2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 293(PCT). A hairpin shaped Henleâs loop is thenext part of the tubule which has adescending and an ascending limb. Theascending limb continues as another highlycoiled tubular region called distalconvoluted tubule (DCT). The DCTs of manynephrons open into a straight tube calledcollecting duct, many of which converge andopen into the renal pelvis through medullarypyramids in the calyces.The Malpighian corpuscle, PCT and DCTof the nephron are situated in the corticalregion of the kidney whereas the loop of Henledips into the medulla. In majority ofnephrons, the loop of Henle is too short andextends only very little into the medulla. Suchnephrons are called cortical nephrons. Insome of the nephrons, the loop of Henle isvery long and runs deep into the medulla.These nephrons are called juxta medullarynephrons.The efferent arteriole emerging from the glomerulus forms a finecapillary network around the renal tubule called the peritubularcapillaries. A minute vessel of this network runs parallel to the Henleâsloop forming a âUâ shaped vasa recta. Vasa recta is absent or highlyreduced in cortical nephrons.19.2 URINE FORMATIONUrine formation involves three main processes namely, glomerularfiltration, reabsorption and secretion, that takes place in different parts ofthe nephron.The first step in urine formation is the filtration of blood, which is carriedout by the glomerulus and is called glomerular filtration. On an average,1100-1200 ml of blood is filtered by the kidneys per minute which constituteroughly 1/5th of the blood pumped out by each ventricle of the heart in aminute. The glomerular capillary blood pressure causes filtration of bloodthrough 3 layers, i.e., the endothelium of glomerular blood vessels, theepithelium of Bowmanâs capsule and a basement membrane between thesetwo layers. The epithelial cells of Bowmanâs capsule called podocytes arearranged in an intricate manner so as to leave some minute spaces calledfiltration slits or slit pores. Blood is filtered so finely through thesemembranes, that almost all the constituents of the plasma except theproteins pass onto the lumen of the Bowmanâs capsule. Therefore, it isconsidered as a process of ultra filtration.Afferent arterioleEfferentarterioleBowmanâscapsuleProximalconvoluted tubuleFigure 19.4 Malpighian body (renal corpuscle)2022-23294 BIOLOGYThe amount of the filtrate formed by the kidneys per minute is calledglomerular filtration rate (GFR). GFR in a healthy individual isapproximately 125 ml/minute, i.e., 180 litres per day !The kidneys have built-in mechanisms for the regulation of glomerularfiltration rate. One such efficient mechanism is carried out by juxtaglomerular apparatus (JGA). JGA is a special sensitive region formed bycellular modifications in the distal convoluted tubule and the afferentarteriole at the location of their contact. A fall in GFR can activate the JGcells to release renin which can stimulate the glomerular blood flow andthereby the GFR back to normal.A comparison of the volume of the filtrate formed per day (180 litresper day) with that of the urine released (1.5 litres), suggest that nearly 99per cent of the filtrate has to be reabsorbed by the renal tubules. Thisprocess is called reabsorption. The tubular epithelial cells in differentsegments of nephron perform this either by active or passive mechanisms.For example, substances like glucose, amino acids, Na+, etc., in the filtrateare reabsorbed actively whereas the nitrogenous wastes are absorbed bypassive transport. Reabsorption of water also occurs passively in the initialsegments of the nephron (Figure 19.5).During urine formation, the tubular cells secrete substances like H+,K+ and ammonia into the filtrate. Tubular secretion is also an importantstep in urine formation as it helps in the maintenance of ionic and acidbase balance of body fluids.19.3 FUNCTION OF THE TUBULESProximal Convoluted Tubule (PCT): PCT is lined by simple cuboidalbrush border epithelium which increases the surface area for reabsorption.Nearly all of the essential nutrients, and 70-80 per cent of electrolytesand water are reabsorbed by this segment. PCT also helps to maintainthe pH and ionic balance of the body fluids by selective secretion ofhydrogen ions and ammonia into the filtrate and by absorption ofHCO3â from it.Henleâs Loop: Reabsorption is minimum in its ascending limb.However, this region plays a significant role in the maintenance of highosmolarity of medullary interstitial fluid. The descending limb of loop ofHenle is permeable to water but almost impermeable to electrolytes. Thisconcentrates the filtrate as it moves down. The ascending limb isimpermeable to water but allows transport of electrolytes actively orpassively. Therefore, as the concentrated filtrate pass upward, it getsdiluted due to the passage of electrolytes to the medullary fluid.Distal Convoluted Tubule (DCT): Conditional reabsorption of Na+and water takes place in this segment. DCT is also capable of reabsorptionof HCO3â and selective secretion of hydrogen and potassium ions andNH3 to maintain the pH and sodium-potassium balance in blood.2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 295Collecting Duct: This long duct extends from the cortex of the kidneyto the inner parts of the medulla. Large amounts of water could bereabsorbed from this region to produce a concentrated urine. This segmentallows passage of small amounts of urea into the medullary interstitiumto keep up the osmolarity. It also plays a role in the maintenance of pHand ionic balance of blood by the selective secretion of H+ and K+ ions(Figure 19.5).19.4 MECHANISM OF CONCENTRATION OF THE FILTRATEMammals have the ability to produce a concentrated urine. The Henleâsloop and vasa recta play a significant role in this. The flow of filtrate inthe two limbs of Henleâs loop is in opposite directions and thus forms acounter current. The flow of blood through the two limbs of vasa recta isFigure 19.5 Reabsorption and secretion of major substances at different parts ofthe nephron (Arrows indicate direction of movement of materials.)2022-23296 BIOLOGYalso in a counter current pattern. The proximity between the Henleâs loopand vasa recta, as well as the counter current in them help in maintainingan increasing osmolarity towards the inner medullary interstitium, i.e.,from 300 mOsmolLâ1 in the cortex to about 1200 mOsmolLâ1 in the innermedulla. This gradient is mainly caused by NaCl and urea. NaCl istransported by the ascending limb of Henleâs loop which is exchangedwith the descending limb of vasa recta. NaCl is returned to the interstitiumby the ascending portion of vasa recta. Similarly, small amounts of ureaenter the thin segment of the ascending limb of Henleâs loop which istransported back to the interstitium by the collecting tubule. The abovedescribed transport of substances facilitated by the special arrangementof Henleâs loop and vasa recta is called the counter current mechanism(Figure. 19.6). This mechanism helps to maintain a concentration gradientFigure 19.6 Diagrammatic representation of a nephron and vasa recta showingcounter current mechanisms2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 297in the medullary interstitium. Presence of such interstitial gradient helpsin an easy passage of water from the collecting tubule therebyconcentrating the filtrate (urine). Human kidneys can produce urine nearlyfour times concentrated than the initial filtrate formed.19.5 REGULATION OF KIDNEY FUNCTIONThe functioning of the kidneys is efficiently monitored and regulated byhormonal feedback mechanisms involving the hypothalamus, JGA andto a certain extent, the heart.Osmoreceptors in the body are activated by changes in blood volume,body fluid volume and ionic concentration. An excessive loss of fluid fromthe body can activate these receptors which stimulate the hypothalamusto release antidiuretic hormone (ADH) or vasopressin from theneurohypophysis. ADH facilitates water reabsorption from latter parts ofthe tubule, thereby preventing diuresis. An increase in body fluid volumecan switch off the osmoreceptors and suppress the ADH release to completethe feedback. ADH can also affect the kidney function by its constrictoryeffects on blood vessels. This causes an increase in blood pressure. Anincrease in blood pressure can increase the glomerular blood flow andthereby the GFR.The JGA plays a complex regulatory role. A fall in glomerular bloodflow/glomerular blood pressure/GFR can activate the JG cells to releaserenin which converts angiotensinogen in blood to angiotensin I andfurther to angiotensin II. Angiotensin II, being a powerfulvasoconstrictor, increases the glomerular blood pressure and therebyGFR. Angiotensin II also activates the adrenal cortex to releaseAldosterone. Aldosterone causes reabsorption of Na+ and water fromthe distal parts of the tubule. This also leads to an increase in bloodpressure and GFR. This complex mechanism is generally known asthe Renin-Angiotensin mechanism.An increase in blood flow to the atria of the heart can cause the releaseof Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation ofblood vessels) and thereby decrease the blood pressure. ANF mechanism,therefore, acts as a check on the renin-angiotensin mechanism.19.6 MICTURITIONUrine formed by the nephrons is ultimately carried to the urinary bladderwhere it is stored till a voluntary signal is given by the central nervoussystem (CNS). This signal is initiated by the stretching of the urinary bladderas it gets filled with urine. In response, the stretch receptors on the wallsof the bladder send signals to the CNS. The CNS passes on motor messages2022-23298 BIOLOGYto initiate the contraction of smooth muscles of the bladder andsimultaneous relaxation of the urethral sphincter causing the release ofurine. The process of release of urine is called micturition and the neuralmechanisms causing it is called the micturition reflex. An adult humanexcretes, on an average, 1 to 1.5 litres of urine per day. The urine formedis a light yellow coloured watery fluid which is slightly acidic (pH-6.0)and has a characterestic odour. On an average, 25-30 gm of urea isexcreted out per day. Various conditions can affect the characteristics ofurine. Analysis of urine helps in clinical diagnosis of many metabolicdiscorders as well as malfunctioning of the kidney. For example, presenceof glucose (Glycosuria) and ketone bodies (Ketonuria) in urine areindicative of diabetes mellitus.19.7 ROLE OF OTHER ORGANS IN EXCRETIONOther than the kidneys, lungs, liver and skin also help in the eliminationof excretory wastes.Our lungs remove large amounts of CO2 (approximately 200mL/minute) and also significant quantities of water every day. Liver, the largestgland in our body, secretes bile-containing substances like bilirubin,biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs.Most of these substances ultimately pass out alongwith digestive wastes.The sweat and sebaceous glands in the skin can eliminate certainsubstances through their secretions. Sweat produced by the sweatglands is a watery fluid containing NaCl, small amounts of urea, lacticacid, etc. Though the primary function of sweat is to facilitate a coolingeffect on the body surface, it also helps in the removal of some of thewastes mentioned above. Sebaceous glands eliminate certainsubstances like sterols, hydrocarbons and waxes through sebum. Thissecretion provides a protective oily covering for the skin. Do you knowthat small amounts of nitrogenous wastes could be eliminated throughsaliva too?19.8 DISORDERS OF THE EXCRETORY SYSTEMMalfunctioning of kidneys can lead to accumulation of urea in blood, acondition called uremia, which is highly harmful and may lead to kidneyfailure. In such patients, urea can be removed by a process calledhemodialysis. During the process of haemodialysis, the blood drainedfrom a convenient artery is pumped into a dialysing unit called artificialkidney. Blood drained from a convenient artery is pumped into a dialysingunit after adding an anticoagulant like heparin. The unit contains a coiledcellophane tube surrounded by a fluid (dialysing fluid) having the same2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 299SUMMARYMany nitrogen containing substances, ions, CO2, water, etc., that accumulate inthe body have to be eliminated. Nature of nitrogenous wastes formed and theirexcretion vary among animals, mainly depending on the habitat (availability ofwater). Ammonia, urea and uric acid are the major nitrogenous wastes excreted.Protonephridia, nephridia, malpighian tubules, green glands and the kidneys arethe common excretory organs in animals. They not only eliminate nitrogenous wastesbut also help in the maintenance of ionic and acid-base balance of body fluids.In humans, the excretory system consists of one pair of kidneys, a pair of ureters,a urinary bladder and a urethra. Each kidney has over a million tubular structurescalled nephrons. Nephron is the functional unit of kidney and has two portions âglomerulus and renal tubule. Glomerulus is a tuft of capillaries formed from afferentarterioles, fine branches of renal artery. The renal tubule starts with a double walledBowmanâs capsule and is further differentiated into a proximal convoluted tubule(PCT), Henleâs loop (HL) and distal convoluted tubule (DCT). The DCTs of manynephrons join to a common collecting duct many of which ultimately open into therenal pelvis through the medullary pyramids. The Bowmanâs capsule encloses theglomerulus to form Malpighian or renal corpuscle.Urine formation involves three main processes, i.e., filtration, reabsorption andsecretion. Filtration is a non-selective process performed by the glomerulus usingthe glomerular capillary blood pressure. About 1200 ml of blood is filtered by theglomerulus per minute to form 125 ml of filtrate in the Bowmanâs capsule percomposition as that of plasma except the nitrogenous wastes. The porouscellophane membrance of the tube allows the passage of molecules basedon concentration gradient. As nitrogenous wastes are absent in thedialysing fluid, these substances freely move out, thereby clearing theblood. The cleared blood is pumped back to the body through a veinafter adding anti-heparin to it. This method is a boon for thousands ofuremic patients all over the world.Kidney transplantation is the ultimate method in the correction ofacute renal failures (kidney failure). A functioning kidney is used intransplantation from a donor, preferably a close relative, to minimise itschances of rejection by the immune system of the host. Modern clinicalprocedures have increased the success rate of such a complicatedtechnique.Renal calculi: Stone or insoluble mass of crystallised salts (oxalates,etc.) formed within the kidney.Glomerulonephritis: Inflammation of glomeruli of kidney.2022-23300 BIOLOGYminute (GFR). JGA, a specialised portion of the nephrons, plays a significant rolein the regulation of GFR. Nearly 99 per cent reabsorption of the filtrate takes placethrough different parts of the nephrons. PCT is the major site of reabsorption andselective secretion. HL primarily helps to maintain osmolar gradient(300 mOsmolLâ1 -1200 mOsmolLâ1) within the kidney interstitium. DCT andcollecting duct allow extensive reabsorption of water and certain electrolytes, whichhelp in osmoregulation: H+, K+ and NH3 could be secreted into the filtrate by thetubules to maintain the ionic balance and pH of body fluids.A counter current mechanism operates between the two limbs of the loop ofHenle and those of vasa recta (capillary parallel to Henleâs loop). The filtrate getsconcentrated as it moves down the descending limb but is diluted by the ascendinglimb. Electrolytes and urea are retained in the interstitium by this arrangement.DCT and collecting duct concentrate the filtrate about four times, i.e., from 300mOsmolLâ1 to 1200 mOsmolLâ1, an excellent mechanism of conservation of water.Urine is stored in the urinary bladder till a voluntary signal from CNS carries outits release through urethra, i.e., micturition. Skin, lungs and liver also assist inexcretion.EXERCISES1. Define Glomerular Filtration Rate (GFR)2. Explain the autoregulatory mechanism of GFR.3. Indicate whether the following statements are true or false :(a) Micturition is carried out by a reflex.(b) ADH helps in water elimination, making the urine hypotonic.(c) Protein-free fluid is filtered from blood plasma into the Bowmanâs capsule.(d) Henleâs loop plays an important role in concentrating the urine.(e) Glucose is actively reabsorbed in the proximal convoluted tubule.4. Give a brief account of the counter current mechanism.5. Describe the role of liver, lungs and skin in excretion.6. Explain micturition.7. Match the items of column I with those of column II :Column I Column II(a) Ammonotelism (i) Birds(b) Bowmanâs capsule (ii) Water reabsorption(c) Micturition (iii) Bony fish(d) Uricotelism (iv) Urinary bladder(d) ADH (v) Renal tubule2022-23EXCRETORY PRODUCTS AND THEIR ELIMINATION 3018. What is meant by the term osmoregulation?9. Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic,why ?10. What is the significance of juxta glomerular apparatus (JGA) in kidney function?11. Name the following:(a) A chordate animal having flame cells as excretory structures(b) Cortical portions projecting between the medullary pyramids in the humankidney(c) A loop of capillary running parallel to the Henleâs loop.12. Fill in the gaps :(a) Ascending limb of Henleâs loop is _______ to water whereas the descendinglimb is _______ to it.(b) Reabsorption of water from distal parts of the tubules is facilitated by hormone_______.(c) Dialysis fluid contain all the constituents as in plasma except _______.(d) A healthy adult human excretes (on an average) _______ gm of urea/day.2022-23302 BIOLOGYMovement is one of the significant features of living beings. Animals andplants exhibit a wide range of movements. Streaming of protoplasm inthe unicellular organisms like Amoeba is a simple form of movement.Movement of cilia, flagella and tentacles are shown by many organisms.Human beings can move limbs, jaws, eyelids, tongue, etc. Some of themovements result in a change of place or location. Such voluntarymovements are called locomotion. Walking, running, climbing, flying,swimming are all some forms of locomotory movements. Locomotorystructures need not be different from those affecting other types ofmovements. For example, in Paramoecium, cilia helps in the movement offood through cytopharynx and in locomotion as well. Hydra can use itstentacles for capturing its prey and also use them for locomotion. We uselimbs for changes in body postures and locomotion as well. The aboveobservations suggest that movements and locomotion cannot be studiedseparately. The two may be linked by stating that all locomotions aremovements but all movements are not locomotions.Methods of locomotion performed by animals vary with their habitatsand the demand of the situation. However, locomotion is generally forsearch of food, shelter, mate, suitable breeding grounds, favourableclimatic conditions or to escape from enemies/predators.20.1 TYPES OF MOVEMENTCells of the human body exhibit three main types of movements, namely,amoeboid, ciliary and muscular.LOCOMOTION AND MOVEMENTCHAPTER 2020.1 Types ofMovement20.2 Muscle20.3 Skeletal System20.4 Joints20.5 Disorders ofMuscular andSkeletal System2022-23LOCOMOTION AND MOVEMENT 303Some specialised cells in our body like macrophages and leucocytesin blood exhibit amoeboid movement. It is effected by pseudopodia formedby the streaming of protoplasm (as in Amoeba). Cytoskeletal elementslike microfilaments are also involved in amoeboid movement.Ciliary movement occurs in most of our internal tubular organs whichare lined by ciliated epithelium. The coordinated movements of cilia inthe trachea help us in removing dust particles and some of the foreignsubstances inhaled alongwith the atmospheric air. Passage of ova throughthe female reproductive tract is also facilitated by the ciliary movement.Movement of our limbs, jaws, tongue, etc, require muscular movement.The contractile property of muscles are effectively used for locomotionand other movements by human beings and majority of multicellularorganisms. Locomotion requires a perfect coordinated activity of muscular,skeletal and neural systems. In this chapter, you will learn about thetypes of muscles, their structure, mechanism of their contraction andimportant aspects of the skeletal system.20.2 MUSCLEYou have studied in Chapter 8 that the cilia and flagella are the outgrowthsof the cell membrane. Flagellar movement helps in the swimming ofspermatozoa, maintenance of water current in the canal system of spongesand in locomotion of Protozoans like Euglena. Muscle is a specialisedtissue of mesodermal origin. About 40-50 per cent of the bodyweight of a human adult is contributed by muscles. They havespecial properties like excitability, contractility, extensibility andelasticity. Muscles have been classified using different criteria,namely location, appearance and nature of regulation of theiractivities. Based on their location, three types of muscles areidentified : (i) Skeletal (ii) Visceral and (iii) Cardiac.Skeletal muscles are closely associated with the skeletal componentsof the body. They have a striped appearance under the microscope andhence are called striated muscles. As their activities are under thevoluntary control of the nervous system, they are known as voluntarymuscles too. They are primarily involved in locomotory actions andchanges of body postures.Visceral muscles are located in the inner walls of hollow visceral organsof the body like the alimentary canal, reproductive tract, etc. They do notexhibit any striation and are smooth in appearance. Hence, they are calledsmooth muscles (nonstriated muscle). Their activities are not under thevoluntary control of the nervous system and are therefore known asinvoluntary muscles. They assist, for example, in the transportation of foodthrough the digestive tract and gametes through the genital tract.2022-23304 BIOLOGYmuscle fibre is lined by the plasma membrane called sarcolemmaenclosing the sarcoplasm. Muscle fibre is a syncitium as the sarcoplasmcontains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmicreticulum of the muscle fibres is the store house of calcium ions. Acharacteristic feature of the muscle fibre is the presence of a large numberof parallelly arranged filaments in the sarcoplasm called myofilaments ormyofibrils. Each myofibril has alternate dark and light bands on it. Adetailed study of the myofibril has established that the striated appearanceis due to the distribution pattern of two important proteins â Actin andMyosin. The light bands contain actin and is called I-band or Isotropicband, whereas the dark band called âAâ or Anisotropic band containsAs the name suggests, Cardiac muscles are the muscles of heart.Many cardiac muscle cells assemble in a branching pattern to form acardiac muscle. Based on appearance, cardiac muscles are striated. Theyare involuntary in nature as the nervous system does not control theiractivities directly.Let us examine a skeletal muscle in detail to understand the structureand mechanism of contraction. Each organised skeletal muscle in ourbody is made of a number of muscle bundles or fascicles held togetherby a common collagenous connective tissue layer called fascia. Eachmuscle bundle contains a number of muscle fibres (Figure 20.1). EachFascicle(muscle bundle)Muscle fibre(muscle cell)SarcolemmaBlood capillaryFigure 20.1 Diagrammatic cross sectional view of a muscle showing muscle bundlesand muscle fibres2022-23LOCOMOTION AND MOVEMENT 305myosin. Both the proteins are arranged as rod-like structures, parallel toeach other and also to the longitudinal axis of the myofibrils. Actinfilaments are thinner as compared to the myosin filaments, hence arecommonly called thin and thick filaments respectively. In the centre ofeach âIâ band is an elastic fibre called âZâ line which bisects it. The thinfilaments are firmly attached to the âZâ line. The thick filaments in theâAâ band are also held together in the middle of this band by a thin fibrousmembrane called âMâ line. The âAâ and âIâ bands are arranged alternatelythroughout the length of the myofibrils. The portion of the myofibrilbetween two successive âZâ lines is considered as the functional unit ofcontraction and is called a sarcomere (Figure 20.2). In a resting state, theedges of thin filaments on either side of the thick filaments partially overlapthe free ends of the thick filaments leaving the central part of the thickfilaments. This central part of thick filament, not overlapped by thinfilaments is called the âHâ zone.Figure 20.2 Diagrammatic representation of (a) anatomy of a muscle fibre showinga sarcomere (b) a sarcomere(a)(b)2022-23306 BIOLOGY20.2.1 Structure of Contractile ProteinsEach actin (thin) filament is made of two âFâ (filamentous) actinshelically wound to each other. Each âFâ actin is a polymer of monomericâGâ (Globular) actins. Two filaments of another protein, tropomyosinalso run close to the âFâ actins throughout its length. A complex proteinTroponin is distributed at regular intervals on the tropomyosin. In theresting state a subunit of troponin masks the active binding sites formyosin on the actin filaments (Figure 20.3a).Each myosin (thick) filament is also a polymerised protein. Manymonomeric proteins called Meromyosins (Figure 20.3b) constitute onethick filament. Each meromyosin has two important parts, a globularhead with a short arm and a tail, the former being called the heavymeromyosin (HMM) and the latter, the light meromyosin (LMM). The HMMcomponent, i.e.; the head and short arm projects outwards at regulardistance and angle from each other from the surface of a polymerised myosinfilament and is known as cross arm. The globular head is an active ATPaseenzyme and has binding sites for ATP and active sites for actin.Figure 20.3 (a) An actin (thin) filament (b) Myosin monomer (Meromyosin)Actin binding sitesATP binding sitesHeadCross arm(a)(b)20.2.2 Mechanism of Muscle ContractionMechanism of muscle contraction is best explained by the sliding filamenttheory which states that contraction of a muscle fibre takes place by thesliding of the thin filaments over the thick filaments.2022-23LOCOMOTION AND MOVEMENT 307Muscle contraction is initiated by a signal sent by the central nervoussystem (CNS) via a motor neuron. A motor neuron alongwith the musclefibres connected to it constitute a motor unit. The junction between amotor neuron and the sarcolemma of the muscle fibre is called theneuromuscular junction or motor-end plate. A neural signal reachingthis junction releases a neurotransmitter (Acetyl choline) which generatesan action potential in the sarcolemma. This spreads through the musclefibre and causes the release of calcium ions into the sarcoplasm. Increasein Ca++ level leads to the binding of calcium with a subunit of troponin onactin filaments and thereby remove the masking of active sites for myosin.Utilising the energy from ATP hydrolysis, the myosin head now binds tothe exposed active sites on actin to form a cross bridge (Figure 20.4). Thispulls the attached actin filaments towards the centre of âAâ band. TheâZâ line attached to these actins are also pulled inwards thereby causing ashortening of the sarcomere, i.e., contraction. It is clear from the abovesteps, that during shortening of the muscle, i.e., contraction, the âIâ bandsget reduced, whereas the âAâ bands retain the length (Figure 20.5). Themyosin, releasing the ADP and P1 goes back to its relaxed state. A newATP binds and the cross-bridge is broken (Figure 20.4). The ATP is againhydrolysed by the myosin head and the cycle of cross bridge formationFigure 20.4 Stages in cross bridge formation, rotation of head and breaking ofcross bridge2022-23308 BIOLOGYand breakage is repeated causing further sliding. The process continuestill the Ca++ ions are pumped back to the sarcoplasmic cisternae resultingin the masking of actin filaments. This causes the return of âZâ lines backto their original position, i.e., relaxation. The reaction time of the fibrescan vary in different muscles. Repeated activation of the muscles can leadto the accumulation of lactic acid due to anaerobic breakdown of glycogenin them, causing fatigue. Muscle contains a red coloured oxygen storingpigment called myoglobin. Myoglobin content is high in some of themuscles which gives a reddish appearance. Such muscles are called theRed fibres. These muscles also contain plenty of mitochondria which canutilise the large amount of oxygen stored in them for ATP production.These muscles, therefore, can also be called aerobic muscles. On theother hand, some of the muscles possess very less quantity of myoglobinand therefore, appear pale or whitish. These are the White fibres. Numberof mitochondria are also few in them, but the amount of sarcoplasmicreticulum is high. They depend on anaerobic process for energy.Figure 20.5 Sliding-filament theory of muscle contraction (movement of the thinfilaments and the relative size of the I band and H zones)2022-23LOCOMOTION AND MOVEMENT 30920.3 SKELETAL SYSTEMSkeletal system consists of a framework of bones and a few cartilages.This system has a significant role in movement shown by the body.Imagine chewing food without jaw bones and walking around withoutthe limb bones. Bone and cartilage are specialised connective tissues.The former has a very hard matrix due to calcium salts in it and the latterhas slightly pliable matrix due to chondroitin salts. In human beings,this system is made up of 206 bones and a few cartilages. It is groupedinto two principal divisions â the axial and the appendicular skeleton.Axial skeleton comprises 80 bones distributed along the main axisof the body. The skull, vertebral column, sternum and ribs constituteaxial skeleton. The skull (Figure 20.6) is composed of two sets of bones âcranial and facial, that totals to 22 bones. Cranial bones are 8 in number.They form the hard protective outer covering, cranium for the brain. Thefacial region is made up of 14 skeletal elements which form the front partof the skull. A single U-shaped bone called hyoid is present at the base ofthe buccal cavity and it is also included in the skull. Each middle earcontains three tiny bones â Malleus, Incus and Stapes, collectively calledEar Ossicles. The skull region articulates with the superior region of theParietalboneFrontal boneTemporalboneOccipitalboneOccipitalcondyleSphenoid boneEthmoid boneLacrimal boneNasal boneZygomatic boneMaxillaMandibleHyoid boneFigure 20.6 Diagrammatic view of human skull2022-23310 BIOLOGYvertebral column with the help of two occipitalcondyles (dicondylic skull).Our vertebral column (Figure 20.7) isformed by 26 serially arranged units calledvertebrae and is dorsally placed. It extends fromthe base of the skull and constitutes the mainframework of the trunk. Each vertebra has acentral hollow portion (neural canal) throughwhich the spinal cord passes. First vertebra isthe atlas and it articulates with the occipitalcondyles. The vertebral column is differentiatedinto cervical (7), thoracic (12), lumbar (5), sacral(1-fused) and coccygeal (1-fused) regionsstarting from the skull. The number of cervicalvertebrae are seven in almost all mammalsincluding human beings. The vertebral columnprotects the spinal cord, supports the head andserves as the point of attachment for the ribsand musculature of the back. Sternum is aflat bone on the ventral midline of thorax.There are 12 pairs of ribs. Each rib is athin flat bone connected dorsally to thevertebral column and ventrally to the sternum.It has two articulation surfaces on its dorsalend and is hence called bicephalic. First sevenpairs of ribs are called true ribs. Dorsally, theyare attached to the thoracic vertebrae andventrally connected to the sternum with thehelp of hyaline cartilage. The 8th, 9th and 10thpairs of ribs do not articulate directly with thesternum but join the seventh rib with the helpof hyaline cartilage. These are calledvertebrochondral (false) ribs. Last 2 pairs (11thand 12th) of ribs are not connected ventrallyand are therefore, called floating ribs. Thoracicvertebrae, ribs and sternum together form therib cage (Figure 20.8).The bones of the limbs alongwith theirgirdles constitute the appendicular skeleton.Each limb is made of 30 bones. The bones ofthe hand (fore limb) are humerus, radius andCervical vertebraIntervertebraldiscSacrumCoccyxThoracicvertebraLumbarvertebraFigure 20.7 Vertebral column (right lateral view)Figure 20.8 Ribs and rib cage2022-23LOCOMOTION AND MOVEMENT 311Figure 20.9 Right pectoral girdle and upperarm. (frontal view)ulna, carpals (wrist bones â 8 in number),metacarpals (palm bones â 5 in number) andphalanges (digits â 14 in number) (Figure20.9). Femur (thigh bone â the longest bone),tibia and fibula, tarsals (ankle bones â 7 innumber), metatarsals (5 in number) andphalanges (digits â 14 in number) are thebones of the legs (hind limb) (Figure 20.10). Acup shaped bone called patella cover the kneeventrally (knee cap).Pectoral and Pelvic girdle bones help inthe articulation of the upper and the lower limbsrespectively with the axial skeleton. Eachgirdle is formed of two halves. Each half ofpectoral girdle consists of a clavicle and ascapula (Figure 20.9). Scapula is a largetriangular flat bone situated in the dorsal partof the thorax between the second and theseventh ribs. The dorsal, flat, triangular bodyof scapula has a slightly elevated ridge calledthe spine which projects as a flat, expandedprocess called the acromion. The claviclearticulates with this. Below the acromion is adepression called the glenoid cavity whicharticulates with the head of the humerus toform the shoulder joint. Each clavicle is a longslender bone with two curvatures. This boneis commonly called the collar bone.Pelvic girdle consists of two coxal bones(Figure 20.10). Each coxal bone is formed bythe fusion of three bones â ilium, ischium andpubis. At the point of fusion of the above bonesis a cavity called acetabulum to which the thighbone articulates. The two halves of the pelvicgirdle meet ventrally to form the pubicsymphysis containing fibrous cartilage.20.4 JOINTSJoints are essential for all types of movementsinvolving the bony parts of the body.Locomotory movements are no exception toFigure 20.10 Right pelvic girdle and lower limbbones (frontal view)2022-23312 BIOLOGYthis. Joints are points of contact between bones, or between bones andcartilages. Force generated by the muscles is used to carry out movementthrough joints, where the joint acts as a fulcrum. The movability at thesejoints vary depending on different factors. Joints have been classified intothree major structural forms, namely, fibrous, cartilaginous and synovial.Fibrous joints do not allow any movement. This type of joint is shownby the flat skull bones which fuse end-to-end with the help of dense fibrousconnective tissues in the form of sutures, to form the cranium.In cartilaginous joints, the bones involved are joined together withthe help of cartilages. The joint between the adjacent vertebrae in thevertebral column is of this pattern and it permits limited movements.Synovial joints are characterised by the presence of a fluid filled synovialcavity between the articulating surfaces of the two bones. Such an arragementallows considerable movement. These joints help in locomotion and manyother movements. Ball and socket joint (between humerus and pectoralgirdle), hinge joint (knee joint), pivot joint (between atlas and axis), glidingjoint (between the carpals) and saddle joint (between carpal and metacarpalof thumb) are some examples.20.5 DISORDERS OF MUSCULAR AND SKELETAL SYSTEMMyasthenia gravis: Auto immune disorder affecting neuromuscularjunction leading to fatigue, weakening and paralysis of skeletal muscle.Muscular dystrophy: Progressive degeneration of skeletal muscle mostlydue to genetic disorder.Tetany: Rapid spasms (wild contractions) in muscle due to low Ca++ inbody fluid.Arthritis: Inflammation of joints.Osteoporosis: Age-related disorder characterised by decreased bone massand increased chances of fractures. Decreased levels of estrogen is acommon cause.Gout: Inflammation of joints due to accumulation of uric acid crystals.SUMMARYMovement is an essential feature of all living beings. Protoplasmic streaming, ciliarymovements, movements of fins, limbs, wings, etc., are some forms exhibited byanimals. A voluntary movement which causes the animal to change its place, is2022-23LOCOMOTION AND MOVEMENT 313called locomotion. Animals move generally in search of food, shelter, mate, breedingground, better climate or to protect themselves.The cells of the human body exhibit amoeboid, ciliary and muscularmovements. Locomotion and many other movements require coordinated muscularactivities. Three types of muscles are present in our body. Skeletal muscles areattached to skeletal elements. They appear striated and are voluntary in nature.Visceral muscles, present in the inner walls of visceral organs are nonstriated andinvoluntary. Cardiac muscles are the muscles of the heart. They are striated,branched and involuntary. Muscles possess excitability, contractility, extensibilityand elasticity.Muscle fibre is the anatomical unit of muscle. Each muscle fibre has manyparallelly arranged myofibrils. Each myofibril contains many serially arrangedunits called sarcomere which are the functional units. Each sarcomere has a centralâAâ band made of thick myosin filaments, and two half âIâ bands made of thin actinfilaments on either side of it marked by âZâ lines. Actin and myosin are polymerisedproteins with contractility. The active sites for myosin on resting actin filament aremasked by a protein-troponin. Myosin head contains ATPase and has ATP bindingsites and active sites for actin. A motor neuron carries signal to the muscle fibrewhich generates an action potential in it. This causes the release of Ca++ fromsarcoplasmic reticulum. Ca++ activates actin which binds to the myosin head toform a cross bridge. These cross bridges pull the actin filaments causing them toslide over the myosin filaments and thereby causing contraction. Ca++ are thenreturned to sarcoplasmic reticulum which inactivate the actin. Cross bridges arebroken and the muscles relax.Repeated stimulation of muscles leads to fatigue. Muscles are classified asRed and White fibres based primarily on the amount of red coloured myoglobinpigment in them.Bones and cartilages constitute our skeletal system. The skeletal system isdivisible into axial and appendicular. Skull, vertebral column, ribs and sternumconstitute the axial skeleton. Limb bones and girdles form the appendicularskeleton. Three types of joints are formed between bones or between bone andcartilage â fibrous, cartilaginous and synovial. Synovial joints allow considerablemovements and therefore, play a significant role in locomotion.EXERCISES1. Draw the diagram of a sarcomere of skeletal muscle showing different regions.2. Define sliding filament theory of muscle contraction.3. Describe the important steps in muscle contraction.2022-23314 BIOLOGY4. Write true or false. If false change the statement so that it is true.(a) Actin is present in thin filament(b) H-zone of striated muscle fibre represents both thick and thin filaments.(c) Human skeleton has 206 bones.(d) There are 11 pairs of ribs in man.(e) Sternum is present on the ventral side of the body.5. Write the difference between :(a) Actin and Myosin(b) Red and White muscles(c) Pectoral and Pelvic girdle6. Match Column I with Column II :Column I Column II(a) Smooth muscle (i) Myoglobin(b) Tropomyosin (ii) Thin filament(c) Red muscle (iii) Sutures(d) Skull (iv) Involuntary7. What are the different types of movements exhibited by the cells of humanbody?8. How do you distinguish between a skeletal muscle and a cardiac muscle?9. Name the type of joint between the following:-(a) atlas/axis(b) carpal/metacarpal of thumb(c) between phalanges(d) femur/acetabulum(e) between cranial bones(f) between pubic bones in the pelvic girdle10. Fill in the blank spaces:(a) All mammals (except a few) have __________ cervical vertebra.(b) The number of phalanges in each limb of human is __________(c) Thin filament of myofibril contains 2 âFâ actins and two other proteins namely__________ and __________.(d) In a muscle fibre Ca++ is stored in __________(e) __________ and __________ pairs of ribs are called floating ribs.(f) The human cranium is made of __________ bones.2022-23NEURAL CONTROL AND COORDINATION 315As you know, the functions of the organs/organ systems in our bodymust be coordinated to maintain homeostasis. Coordination is theprocess through which two or more organs interact and complement thefunctions of one another. For example, when we do physical exercises,the energy demand is increased for maintaining an increased muscularactivity. The supply of oxygen is also increased. The increased supply ofoxygen necessitates an increase in the rate of respiration, heart beat andincreased blood flow via blood vessels. When physical exercise is stopped,the activities of nerves, lungs, heart and kidney gradually return to theirnormal conditions. Thus, the functions of muscles, lungs, heart, bloodvessels, kidney and other organs are coordinated while performing physicalexercises. In our body the neural system and the endocrine system jointlycoordinate and integrate all the activities of the organs so that they functionin a synchronised fashion.The neural system provides an organised network of point-to-pointconnections for a quick coordination. The endocrine system provideschemical integration through hormones. In this chapter, you will learnabout the neural system of human, mechanisms of neural coordinationlike transmission of nerve impulse, impulse conduction across a synapseand the physiology of reflex action.NEURAL CONTROL ANDCOORDINATIONCHAPTER 2121.1 Neural System21.2 Human NeuralSystem21.3 Neuron asStructural andFunctional Unitof NeuralSystem21.4 Central NeuralSystem21.5 Reflex Actionand Reflex Arc21.6 SensoryReception andProcessing2022-23316 BIOLOGY21.1 NEURAL SYSTEMThe neural system of all animals is composed of highly specialised cells calledneurons which can detect, receive and transmit different kinds of stimuli.The neural organisation is very simple in lower invertebrates. Forexample, in Hydra it is composed of a network of neurons. The neuralsystem is better organised in insects, where a brain is present along witha number of ganglia and neural tissues. The vertebrates have a moredeveloped neural system.21.2 HUMAN NEURAL SYSTEMThe human neural system is divided into two parts :(i) the central neural system (CNS)(ii) the peripheral neural system (PNS)The CNS includes the brain and the spinal cord and is the site ofinformation processing and control. The PNS comprises of all the nervesof the body associated with the CNS (brain and spinal cord). The nervefibres of the PNS are of two types :(a) afferent fibres(b) efferent fibresThe afferent nerve fibres transmit impulses from tissues/organs tothe CNS and the efferent fibres transmit regulatory impulses from theCNS to the concerned peripheral tissues/organs.The PNS is divided into two divisions called somatic neural systemand autonomic neural system. The somatic neural system relaysimpulses from the CNS to skeletal muscles while the autonomic neuralsystem transmits impulses from the CNS to the involuntary organs andsmooth muscles of the body. The autonomic neural system is furtherclassified into sympathetic neural system and parasympathetic neuralsystem.Visceral nervous system is the part of the peripheral nervous systemthat comprises the whole complex of nerves, fibres, ganglia, and plexusesby which impulses travel from the central nervous system to the visceraand from the viscera to the central nervous system.21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OFNEURAL SYSTEMA neuron is a microscopic structure composed of three major parts, namely,cell body, dendrites and axon (Figure 21.1). The cell body contains cytoplasmwith typical cell organelles and certain granular bodies called Nisslâs granules.Short fibres which branch repeatedly and project out of the cell body also2022-23NEURAL CONTROL AND COORDINATION 317contain Nisslâs granules and are called dendrites. Thesefibres transmit impulses towards the cell body. Theaxon is a long fibre, the distal end of which is branched.Each branch terminates as a bulb-like structure calledsynaptic knob which possess synaptic vesiclescontaining chemicals called neurotransmitters. Theaxons transmit nerve impulses away from the cell bodyto a synapse or to a neuro-muscular junction. Basedon the number of axon and dendrites, the neurons aredivided into three types, i.e., multipolar (with one axonand two or more dendrites; found in the cerebral cortex),bipolar (with one axon and one dendrite, found in theretina of eye) and unipolar (cell body with one axononly; found usually in the embryonic stage). There aretwo types of axons, namely, myelinated and nonmyelinated.The myelinated nerve fibres are envelopedwith Schwann cells, which form a myelin sheatharound the axon. The gaps between two adjacentmyelin sheaths are called nodes of Ranvier.Myelinated nerve fibres are found in spinal and cranialnerves. Unmyelinated nerve fibre is enclosed by aSchwann cell that does not form a myelin sheatharound the axon, and is commonly found inautonomous and the somatic neural systems.21.3.1 Generation and Conduction ofNerve ImpulseNeurons are excitable cells because their membranes are in a polarisedstate. Do you know why the membrane of a neuron is polarised? Differenttypes of ion channels are present on the neural membrane. These ionchannels are selectively permeable to different ions. When a neuron is notconducting any impulse, i.e., resting, the axonal membrane iscomparatively more permeable to potassium ions (K+) and nearlyimpermeable to sodium ions (Na+). Similarly, the membrane isimpermeable to negatively charged proteins present in the axoplasm.Consequently, the axoplasm inside the axon contains high concentrationof K+ and negatively charged proteins and low concentration of Na+. Incontrast, the fluid outside the axon contains a low concentration of K+, ahigh concentration of Na+ and thus form a concentration gradient. Theseionic gradients across the resting membrane are maintained by the activetransport of ions by the sodium-potassium pump which transports 3Na+ outwards for 2 K+ into the cell. As a result, the outer surface of theaxonal membrane possesses a positive charge while its inner surfaceFigure 21.1 Structure of a neuron2022-23318 BIOLOGYbecomes negatively charged and therefore is polarised. The electricalpotential difference across the resting plasma membrane is called as theresting potential.You might be curious to know about the mechanisms of generationof nerve impulse and its conduction along an axon. When a stimulus isapplied at a site (Figure 21.2 e.g., point A) on the polarised membrane,the membrane at the site A becomes freely permeable to Na+. This leadsto a rapid influx of Na+ followed by the reversal of the polarity at that site,i.e., the outer surface of the membrane becomes negatively charged andthe inner side becomes positively charged. The polarity of the membraneat the site A is thus reversed and hence depolarised. The electrical potentialdifference across the plasma membrane at the site A is called theaction potential, which is in fact termed as a nerve impulse. At sitesimmediately ahead, the axon (e.g., site B) membrane has a positive chargeon the outer surface and a negative charge on its inner surface. As aresult, a current flows on the inner surface from site A to site B. On theouter surface current flows from site B to site A (Figure 21.2) to completethe circuit of current flow. Hence, the polarity at the site is reversed, andan action potential is generated at site B. Thus, the impulse (actionpotential) generated at site A arrives at site B. The sequence is repeatedalong the length of the axon and consequently the impulse is conducted.The rise in the stimulus-induced permeability to Na+ is extremely shortlived.It is quickly followed by a rise in permeability to K+. Within a fractionof a second, K+ diffuses outside the membrane and restores the restingpotential of the membrane at the site of excitation and the fibre becomesonce more responsive to further stimulation.- --- - - - - - - ---- - -- - - -++ ++ ++ + + + + +++ + + + + + + + ++ + +- - --- - - - - - - ---- - -- - - -++ ++ ++ + + + + +++ + + + + + + + ++ + +- ANaBNaFigure 21.2 Diagrammatic representation of impulse conduction through an axon(at points A and B)2022-23NEURAL CONTROL AND COORDINATION 31921.3.2 Transmission of ImpulsesA nerve impulse is transmitted from one neuron to another throughjunctions called synapses. A synapse is formed by the membranes of apre-synaptic neuron and a post-synaptic neuron, which may or may notbe separated by a gap called synaptic cleft. There are two types ofsynapses, namely, electrical synapses and chemical synapses. At electricalsynapses, the membranes of pre- and post-synaptic neurons are in veryclose proximity. Electrical current can flow directly from one neuron intothe other across these synapses. Transmission of an impulse acrosselectrical synapses is very similar to impulse conduction along a singleaxon. Impulse transmission across an electrical synapse is always fasterthan that across a chemical synapse. Electrical synapses are rare in oursystem.At a chemical synapse, the membranes of the pre- and post-synapticneurons are separated by a fluid-filled space called synaptic cleft(Figure 21.3). Do you know how the pre-synaptic neuron transmits animpulse (action potential) across the synaptic cleft to the post-synapticneuron? Chemicals called neurotransmitters are involved in thetransmission of impulses at these synapses. The axon terminals containvesicles filled with these neurotransmitters. When an impulse (actionpotential) arrives at the axon terminal, it stimulates the movement of thesynaptic vesicles towards the membrane where they fuse with the plasmaFigure 21.3 Diagram showing axon terminal and synapse2022-23320 BIOLOGYmembrane and release their neurotransmitters in the synaptic cleft. Thereleased neurotransmitters bind to their specific receptors, present onthe post-synaptic membrane. This binding opens ion channels allowingthe entry of ions which can generate a new potential in the post-synapticneuron. The new potential developed may be either excitatory orinhibitory.21.4 CENTRAL NEURAL SYSTEMThe brain is the central information processing organ of our body, andacts as the âcommand and control systemâ. It controls the voluntarymovements, balance of the body, functioning of vital involuntary organs(e.g., lungs, heart, kidneys, etc.), thermoregulation, hunger and thirst,circadian (24-hour) rhythms of our body, activities of several endocrineglands and human behaviour. It is also the site for processing of vision,hearing, speech, memory, intelligence, emotions and thoughts.The human brain is well protected by the skull. Inside the skull, thebrain is covered by cranial meninges consisting of an outer layer calleddura mater, a very thin middle layer called arachnoid and an inner layer(which is in contact with the brain tissue) called pia mater. The brain canbe divided into three major parts: (i) forebrain, (ii) midbrain, and(iii) hindbrain (Figure 21.4).Figure 21.4 Diagram showing sagital section of the human brainForebrain2022-23NEURAL CONTROL AND COORDINATION 32121.4.1 ForebrainThe forebrain consists of cerebrum, thalamus and hypothalamus(Figure 21.4). Cerebrum forms the major part of the human brain. A deepcleft divides the cerebrum longitudinally into two halves, which are termedas the left and right cerebral hemispheres. The hemispheres areconnected by a tract of nerve fibres called corpus callosum. The layer ofcells which covers the cerebral hemisphere is called cerebral cortex and isthrown into prominent folds. The cerebral cortex is referred to as the greymatter due to its greyish appearance. The neuron cell bodies areconcentrated here giving the colour. The cerebral cortex contains motorareas, sensory areas and large regions that are neither clearly sensorynor motor in function. These regions called as the association areas areresponsible for complex functions like intersensory associations, memoryand communication. Fibres of the tracts are covered with the myelin sheath,which constitute the inner part of cerebral hemisphere. They give anopaque white appearance to the layer and, hence, is called the white matter.The cerebrum wraps around a structure called thalamus, which is a majorcoordinating centre for sensory and motor signaling. Another veryimportant part of the brain called hypothalamus lies at the base of thethalamus. The hypothalamus contains a number of centres which controlbody temperature, urge for eating and drinking. It also contains severalgroups of neurosecretory cells, which secrete hormones calledhypothalamic hormones. The inner parts of cerebral hemispheres and agroup of associated deep structures like amygdala, hippocampus, etc.,form a complex structure called the limbic lobe or limbic system. Alongwith the hypothalamus, it is involved in the regulation of sexual behaviour,expression of emotional reactions (e.g., excitement, pleasure, rage andfear), and motivation.21.4.2 MidbrainThe midbrain is located between the thalamus/hypothalamus of theforebrain and pons of the hindbrain. A canal called the cerebral aqueductpassess through the midbrain. The dorsal portion of the midbrain consistsmainly of four round swellings (lobes) called corpora quadrigemina.21.4.3 HindbrainThe hindbrain comprises pons, cerebellum and medulla (also calledthe medulla oblongata). Pons consists of fibre tracts that interconnectdifferent regions of the brain. Cerebellum has very convoluted surface inorder to provide the additional space for many more neurons. The medullaof the brain is connected to the spinal cord. The medulla contains centreswhich control respiration, cardiovascular reflexes and gastric secretions.Three major regions make up the brain stem; mid brain, ponsand medulla oblongata. Brain stem forms the connections betweenthe brain and spinal cord.2022-23322 BIOLOGY21.5 REFLEX ACTION AND REFLEX ARCYou must have experienced a sudden withdrawal of a body part whichcomes in contact with objects that are extremely hot, cold pointed oranimals that are scary or poisonous. The entire process of response to aperipheral nervous stimulation, that occurs involuntarily, i.e., withoutconscious effort or thought and requires the involvment of a part of thecentral nervous system is called a reflex action. The reflex pathwaycomprises at least one afferent neuron (receptor) and one efferent (effectoror excitor) neuron appropriately arranged in a series (Figure 21.5). Theafferent neuron receives signal from a sensory organ and transmits theimpulse via a dorsal nerve root into the CNS (at the level of spinal cord).The efferent nueuron then carries signals from CNS to the effector. Thestimulus and response thus forms a reflex arc as shown below in theknee jerk reflex. You should carefully study Figure 21.5 to understandthe mechanism of a knee jerk reflex.21.6 SENSORY RECEPTION AND PROCESSINGHave you ever thought how do you feel the climatic changes in theenvironment? How do you see an object and its colour? How do youhear a sound? The sensory organs detect all types of changes in theenvironment and send appropriate signals to the CNS, where all the inputsare processed and analysed. Signals are then sent to different parts/centres of the brain. This is how you can sense changes in the environment.Figure 21.5 Diagrammatic presentation of reflex action (showing knee jerk reflex)2022-23NEURAL CONTROL AND COORDINATION 323Sense OrgansWe smell things by our nose, taste by tongue, hear by ear and see objectsby eyes.The nose contains mucus-coated receptors which are specialised forreceiving the sense of smell and called olfactory receptors. These aremade up of olfactory epithelium that consists of three kinds of cells. Theneurons of the olfactory epithelium extend from the outside environmentdirectly into a pair of broad bean-sized organs, called olfactory bulb,which are extensions of the brainâs limbic system.Both nose and tongue detect dissolved chemicals. The chemical sensesof gustation (taste) and olfactory (smell) are functionally similar andinterrelated. The tongue detects tastes through taste buds, containinggustatory receptors. With each taste of food or sip of drink, the brainintegrates the differential input from the taste buds and a complex flavouris perceived.In the following sections, you will be introduced to the structure andfunctioning of the eye (sensory organ for vision) and the ear (sensory organfor hearing).21.6.1 EyeOur paired eyes are located in sockets of the skull called orbits. A briefaccount of structure and functions of the human eye is given in thefollowing sections.21.6.1.1 Parts of an eyeThe adult human eye ball isnearly a spherical structure. Thewall of the eye ball is composedof three layers (Figure 21.6). Theexternal layer is composed of adense connective tissue and iscalled the sclera. The anteriorportion of this layer is called thecornea. The middle layer,choroid, contains many bloodvessels and looks bluish incolour. The choroid layer is thinover the posterior two-thirds ofthe eye ball, but it becomes thickin the anterior part to form theciliary body. The ciliary body Figure 21.6 Diagram showing parts of an eye2022-23324 BIOLOGYitself continues forward to form a pigmented and opaque structure calledthe iris which is the visible coloured portion of the eye. The eye ball containsa transparent crystalline lens which is held in place by ligaments attachedto the ciliary body. In front of the lens, the aperture surrounded by theiris is called the pupil. The diameter of the pupil is regulated by the musclefibres of iris.The inner layer is the retina and it contains three layers of neural cells âfrom inside to outside â ganglion cells, bipolar cells and photoreceptor cells.There are two types of photoreceptor cells, namely, rods and cones. Thesecells contain the light-sensitive proteins called the photopigments. Thedaylight (photopic) vision and colour vision are functions of cones andthe twilight (scotopic) vision is the function of the rods. The rods containa purplish-red protein called the rhodopsin or visual purple, whichcontains a derivative of Vitamin A. In the human eye, there are three typesof cones which possess their own characteristic photopigments thatrespond to red, green and blue lights. The sensations of different coloursare produced by various combinations of these cones and theirphotopigments. When these cones are stimulated equally, a sensation ofwhite light is produced.The optic nerves leave the eye and the retinal blood vessels enter it ata point medial to and slightly above the posterior pole of the eye ball.Photoreceptor cells are not present in that region and hence it is calledthe blind spot. At the posterior pole of the eye lateral to the blind spot,there is a yellowish pigmented spot called macula lutea with a central pitcalled the fovea. The fovea is a thinned-out portion of the retina whereonly the cones are densely packed. It is the point where the visual acuity(resolution) is the greatest.The space between the cornea and the lens is called the aqueouschamber and contains a thin watery fluid called aqueous humor. Thespace between the lens and the retina is called the vitreous chamberand is filled with a transparent gel called vitreous humor.21.6.1.2 Mechanism of VisionThe light rays in visible wavelength focussed on the retina through thecornea and lens generate potentials (impulses) in rods and cones. Asmentioned earlier, the photosensitive compounds (photopigments) in thehuman eyes is composed of opsin (a protein) and retinal (an aldehyde ofvitamin A). Light induces dissociation of the retinal from opsin resultingin changes in the structure of the opsin. This causes membranepermeability changes. As a result, potential differences are generated inthe photoreceptor cells. This produces a signal that generates actionpotentials in the ganglion cells through the bipolar cells. These actionpotentials (impulses) are transmitted by the optic nerves to the visual2022-23NEURAL CONTROL AND COORDINATION 325Figure 21.7 Diagrammatic view of earcortex area of the brain, where the neural impulses are analysed and theimage formed on the retina is recognised based on earlier memory andexperience.21.6.2 The EarThe ears perform two sensory functions, hearing and maintenance of bodybalance. Anatomically, the ear can be divided into three major sectionscalled the outer ear, the middle ear and the inner ear (Figure 21.7). Theouter ear consists of thepinna and externalauditory meatus (canal).The pinna collects thevibrations in the air whichproduce sound. Theexternal auditory meatusleads inwards and extendsup to the tympanicmembrane (the ear drum).There are very fine hairs andwax-secreting glands in theskin of the pinna and themeatus. The tympanicmembrane is composed ofconnective tissues coveredwith skin outside and withmucus membrane inside.The middle ear contains three ossicles called malleus, incus and stapeswhich are attached to one another in a chain-like fashion. The malleus isattached to the tympanic membrane and the stapes is attached to theoval window of the cochlea. The ear ossicles increase the efficiency oftransmission of sound waves to the inner ear. An Eustachian tubeconnects the middle ear cavity with the pharynx. The Eustachian tubehelps in equalising the pressures on either sides of the ear drum.The fluid-filled inner ear called labyrinth consists of two parts, thebony and the membranous labyrinths. The bony labyrinth is a series ofchannels. Inside these channels lies the membranous labyrinth, which issurrounded by a fluid called perilymph. The membranous labyrinth isfilled with a fluid called endolymph. The coiled portion of the labyrinth iscalled cochlea. The membranes constituting cochlea, the reissnerâs andbasilar, divide the surounding perilymph filled bony labyrinth into anupper scala vestibuli and a lower scala tympani (Figure 21.8). The space2022-23326 BIOLOGYThe organ of corti is a structure located on the basilar membranewhich contains hair cells that act as auditory receptors. The hair cellsare present in rows on the internal side of the organ of corti. The basalend of the hair cell is in close contact with the afferent nerve fibres. A largenumber of processes called stereo cilia are projected from the apical partof each hair cell. Above the rows of the hair cells is a thin elastic membranecalled tectorial membrane.The inner ear also contains a complex system called vestibularapparatus, located above the cochlea. The vestibular apparatus iscomposed of three semi-circular canals and the otolith (macula is thesensory part of saccule and utricle). Each semi-circular canal lies in adifferent plane at right angles to each other. The membranous canals aresuspended in the perilymph of the bony canals. The base of canals isFigure 21.8 Diagrammatic representation of the sectional view of cochleawithin cochlea called scala media is filled with endolymph. At the base ofthe cochlea, the scala vestibuli ends at the oval window, while the scalatympani terminates at the round window which opens to the middle ear.2022-23NEURAL CONTROL AND COORDINATION 327SUMMARYThe neural system coordinates and integrates functions as well as metabolicand homeostatic activities of all the organs. Neurons, the functional units ofneural system are excitable cells due to a differential concentration gradient ofions across the membrane. The electrical potential difference across the restingneural membrane is called the âresting potentialâ. The nerve impulse is conductedalong the axon membrane in the form of a wave of depolarisation andrepolarisation. A synapse is formed by the membranes of a pre-synaptic neuronand a post-synaptic neuron which may or may not be separated by a gap calledsynaptic cleft. Chemicals involved in the transmission of impulses at chemicalsynapses are called neurotransmitters.Human neural system consists of two parts : (i) central neural system (CNS)and (ii) the peripheral neural system. The CNS consists of the brain and spiralcord. The brain can be divided into three major parts : (i) forebrain, (ii) midbrainand (iii) hindbrain. The forebrain consists of cerebrum, thalamus andhypothalamus. The cerebrum is longitudinally divided into two halves that areconnected by the corpus callosum. A very important part of the forebrain calledhypothalamus controls the body temperature, eating and drinking. Inner partsswollen and is called ampulla, which contains a projecting ridge calledcrista ampullaris which has hair cells. The saccule and utricle contain aprojecting ridge called macula. The crista and macula are the specificreceptors of the vestibular apparatus responsible for maintenance ofbalance of the body and posture.20.6.2.1 Mechanism of HearingHow does ear convert sound waves into neural impulses, which aresensed and processed by the brain enabling us to recognise a sound ?The external ear receives sound waves and directs them to the ear drum.The ear drum vibrates in response to the sound waves and these vibrationsare transmitted through the ear ossicles (malleus, incus and stapes) tothe oval window. The vibrations are passed through the oval window onto the fluid of the cochlea, where they generate waves in the lymphs. Thewaves in the lymphs induce a ripple in the basilar membrane. Thesemovements of the basilar membrane bend the hair cells, pressing themagainst the tectorial membrane. As a result, nerve impulses are generatedin the associated afferent neurons. These impulses are transmitted bythe afferent fibres via auditory nerves to the auditory cortex of the brain,where the impulses are analysed and the sound is recognised.2022-23328 BIOLOGYof cerebral hemispheres and a group of associated deep structures form acomplex structure called limbic system which is concerned with olfaction,autonomic responses, regulation of sexual behaviour, expression of emotionalreactions, and motivation. The midbrain receives and integrates visual, tactileand auditory inputs. The hindbrain comprises pons, cerebellum and medulla.The cerebellum integrates information received from the semicircular canals ofthe ear and the auditory system. The medulla contains centres, which controlrespiration, cardiovascular reflexes, and gastric secretions. Pons consist of fibretracts that interconnect different regions of the brain. The entire process ofinvoluntary response to a peripheral nervous stimulation is called reflex action.Information regarding changes in the environment is received by the CNSthrough the sensory organs which are processed and analysed. Signals are thensent for necessary adjustments. The wall of the human eye ball is composed ofthree layers. The external layer is composed of cornea and sclera. Inside sclera isthe middle layer, which is called the choroid. Retina, the innermost layer, containstwo types of photoreceptor cells, namely rods and cones. The daylight (photopic)vision and colour vision are functions of cones and twilight (scotopic) vision is thefunction of the rods. The light enters through cornea, the lens and the images ofobjects are formed on the retina.The ear can be divided into the outer ear, the middle ear and the inner ear. Themiddle ear contains three ossicles called malleus, incus and stapes. The fluidfilled inner ear is called the labyrinth, and the coiled portion of the labyrinth iscalled cochlea. The organ of corti is a structure which contains hair cells that actas auditory receptors and is located on the basilar membrane. The vibrationsproduced in the ear drum are transmitted through the ear ossicles and oval windowto the fluid-filled inner ear. Nerve impulses are generated and transmitted by theafferent fibres to the auditory cortex of the brain. The inner ear also contains acomplex system located above the cochlea called vestibular apparatus. It isinfluenced by gravity and movements, and helps us in maintaining balance of thebody and posture.2022-23NEURAL CONTROL AND COORDINATION 329EXERCISES1. Briefly describe the structure of the following:(a) Brain (b) Eye (c) Ear2. Compare the following:(a) Central neural system (CNS) and Peripheral neural system (PNS)(b) Resting potential and action potential(c) Choroid and retina3. Explain the following processes:(a) Polarisation of the membrane of a nerve fibre(b) Depolarisation of the membrane of a nerve fibre(c) Conduction of a nerve impulse along a nerve fibre(d) Transmission of a nerve impulse across a chemical synapse4. Draw labelled diagrams of the following:(a) Neuron (b) Brain (c) Eye (d) Ear5. Write short notes on the following:(a) Neural coordination (b) Forebrain (c) Midbrain(d) Hindbrain (e) Retina (f) Ear ossicles(g) Cochlea (h) Organ of Corti (i) Synapse6. Give a brief account of:(a) Mechanism of synaptic transmission(b) Mechanism of vision(c) Mechanism of hearing7. Answer briefly:(a) How do you perceive the colour of an object?(b) Which part of our body helps us in maintaining the body balance?(c) How does the eye regulate the amount of light that falls on the retina.8. Explain the following:(a) Role of Na+ in the generation of action potential.(b) Mechanism of generation of light-induced impulse in the retina.(c) Mechanism through which a sound produces a nerve impulse in theinner ear.9. Differentiate between:(a) Myelinated and non-myelinated axons(b) Dendrites and axons(c) Rods and cones(d) Thalamus and Hypothalamus(e) Cerebrum and Cerebellum2022-23330 BIOLOGY10. Answer the following:(a) Which part of the ear determines the pitch of a sound?(b) Which part of the human brain is the most developed?(c) Which part of our central neural system acts as a master clock?11. The region of the vertebrate eye, where the optic nerve passes out of the retina, iscalled the(a) fovea(b) iris(c) blind spot(d) optic chaisma12. Distinguish between:(a) afferent neurons and efferent neurons(b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre(c) aqueous humor and vitreous humor(d) blind spot and yellow spot(f) cranial nerves and spinal nerves.2022-23CHEMICAL COORDINATION AND INTEGRATION 331You have already learnt that the neural system provides apoint-to-point rapid coordination among organs. The neuralcoordination is fast but short-lived. As the nerve fibres do not innervateall cells of the body and the cellular functions need to be continuouslyregulated; a special kind of coordination and integration has to beprovided. This function is carried out by hormones. The neural systemand the endocrine system jointly coordinate and regulate thephysiological functions in the body.22.1 ENDOCRINE GLANDS AND HORMONESEndocrine glands lack ducts and are hence, called ductless glands. Theirsecretions are called hormones. The classical definition of hormone as achemical produced by endocrine glands and released into the blood andtransported to a distantly located target organ has current scientificdefinition as follows: Hormones are non-nutrient chemicals whichact as intercellular messengers and are produced in trace amounts.The new definition covers a number of new molecules in addition to thehormones secreted by the organised endocrine glands. Invertebratespossess very simple endocrine systems with few hormones whereas a largenumber of chemicals act as hormones and provide coordination in thevertebrates. The human endocrine system is described here.CHEMICAL COORDINATIONAND INTEGRATIONCHAPTER 2222.1 EndocrineGlands andHormones22.2 HumanEndocrineSystem22.3 Hormones ofHeart, KidneyandGastrointestinalTract22.4 Mechanism ofHormone Action2022-23332 BIOLOGY22.2 HUMAN ENDOCRINE SYSTEMThe endocrine glands and hormoneproducing diffused tissues/cells locatedin different parts of our body constitutethe endocrine system. Pituitary, pineal,thyroid, adrenal, pancreas, parathyroid,thymus and gonads (testis in males andovary in females) are the organisedendocrine bodies in our body(Figure 22.1). In addition to these, someother organs, e.g., gastrointestinal tract,liver, kidney, heart also producehormones. A brief account of thestructure and functions of all majorendocrine glands and hypothalamusof the human body is given in thefollowing sections.22.2.1 The HypothalamusAs you know, the hypothalamus is thebasal part of diencephalon, forebrain(Figure 22.1) and it regulates a widespectrum of body functions. It containsseveral groups of neurosecretory cellscalled nuclei which produce hormones.These hormones regulate the synthesis and secretion of pituitaryhormones. However, the hormones produced by hypothalamus are oftwo types, the releasing hormones (which stimulate secretion of pituitaryhormones) and the inhibiting hormones (which inhibit secretions ofpituitary hormones). For example a hypothalamic hormone calledGonadotrophin releasing hormone (GnRH) stimulates the pituitarysynthesis and release of gonadotrophins. On the other hand, somatostatinfrom the hypothalamus inhibits the release of growth hormone from thepituitary. These hormones originating in the hypothalamic neurons, passthrough axons and are released from their nerve endings. These hormonesreach the pituitary gland through a portal circulatory system and regulatethe functions of the anterior pituitary. The posterior pituitary is underthe direct neural regulation of the hypothalamus (Figure 22.2).Figure 22.1 Location of endocrine glandsTestis(in male)Ovary(in female)AdrenalPancreasThyroid andParathyroidThymusPinealPituitaryHypothalamus2022-23CHEMICAL COORDINATION AND INTEGRATION 33322.2.2 The Pituitary GlandThe pituitary gland is located in a bony cavitycalled sella tursica and is attached tohypothalamus by a stalk (Figure 22.2). It isdivided anatomically into an adenohypophysisand a neurohypophysis. Adenohypophysisconsists of two portions, pars distalis and parsintermedia. The pars distalis region of pituitary,commonly called anterior pituitary, producesgrowth hormone (GH), prolactin (PRL), thyroidstimulating hormone (TSH),adrenocorticotrophic hormone (ACTH),luteinizing hormone (LH) and folliclestimulating hormone (FSH). Pars intermediasecretes only one hormone called melanocytestimulating hormone (MSH). However, inhumans, the pars intermedia is almost mergedwith pars distalis. Neurohypophysis (parsnervosa) also known as posterior pituitary, storesand releases two hormones called oxytocin andvasopressin, which are actually synthesised bythe hypothalamus and are transported axonally to neurohypophysis.Over-secretion of GH stimulates abnormal growth of the body leadingto gigantism and low secretion of GH results in stunted growth resultingin pituitary dwarfism. Excess secretion of growth hormone in adultsespecially in middle age can result in severe disfigurement (especially ofthe face) called Acromegaly, which may lead to serious complications,and premature death if unchecked. The disease is hard to diagnose inthe early stages and often goes undetected for many years, until changesin external features become noticeable. Prolactin regulates the growth ofthe mammary glands and formation of milk in them. TSH stimulates thesynthesis and secretion of thyroid hormones from the thyroid gland. ACTHstimulates the synthesis and secretion of steroid hormones calledglucocorticoids from the adrenal cortex. LH and FSH stimulate gonadalactivity and hence are called gonadotrophins. In males, LH stimulatesthe synthesis and secretion of hormones called androgens from testis. Inmales, FSH and androgens regulate spermatogenesis. In females, LHinduces ovulation of fully mature follicles (graafian follicles) and maintainsthe corpus luteum, formed from the remnants of the graafian folliclesPosteriorpituitaryAnteriorpituitaryHypothalamusHypothalamicneuronsPortal circulationFigure 22.2 Diagrammatic representation ofpituitary and its relationship withhypothalamus2022-23334 BIOLOGYafter ovulation. FSH stimulates growth anddevelopment of the ovarian follicles in females. MSHacts on the melanocytes (melanin containing cells) andregulates pigmentation of the skin. Oxytocin acts onthe smooth muscles of our body and stimulates theircontraction. In females, it stimulates a vigorouscontraction of uterus at the time of child birth, and milkejection from the mammary gland. Vasopressin actsmainly at the kidney and stimulates resorption of waterand electrolytes by the distal tubules and therebyreduces loss of water through urine (diuresis). Hence,it is also called as anti-diuretic hormone (ADH).An impairment affecting synthesis or release of ADHresults in a diminished ability of the kidney to conservewater leading to water loss and dehydration. Thiscondition is known as Diabetes Insipidus.22.2.3 The Pineal GlandThe pineal gland is located on the dorsal side offorebrain. Pineal secretes a hormone called melatonin.Melatonin plays a very important role in the regulationof a 24-hour (diurnal) rhythm of our body. Forexample, it helps in maintaining the normal rhythmsof sleep-wake cycle, body temperature. In addition,melatonin also influences metabolism, pigmentation,the menstrual cycle as well as our defense capability.22.2.4 Thyroid GlandThe thyroid gland is composed of two lobes which arelocated on either side of the trachea (Figure 22.3). Boththe lobes are interconnected with a thin flap of connectivetissue called isthmus. The thyroid gland is composed offollicles and stromal tissues. Each thyroid follicle iscomposed of follicular cells, enclosing a cavity. Thesefollicular cells synthesise two hormones,tetraiodothyronine or thyroxine (T4) andtriiodothyronine (T3). Iodine is essential for the normalrate of hormone synthesis in the thyroid. Deficiency ofiodine in our diet results in hypothyroidism andenlargement of the thyroid gland, commonly calledgoitre. Hypothyroidism during pregnancy causesdefective development and maturation of the growingFigure 22.3 Diagrammatic view of theposition of Thyroid andParathyroid(a) Ventral side(b) Dorsal side2022-23CHEMICAL COORDINATION AND INTEGRATION 335baby leading to stunted growth (cretinism), mental retardation, lowintelligence quotient, abnormal skin, deaf-mutism, etc. In adult women,hypothyroidism may cause menstrual cycle to become irregular. Due tocancer of the thyroid gland or due to development of nodules of the thyroidglands, the rate of synthesis and secretion of the thyroid hormones isincreased to abnormal high levels leading to a condition calledhyperthyroidism which adversely affects the body physiology.Exopthalmic goitre is a form of hyperthyroidism, characterised byenlargement of the thyroid gland, protrusion of the eyeballs, increasedbasal metabolic rate, and weight loss, also called Gravesâ disease.Thyroid hormones play an important role in the regulation of the basalmetabolic rate. These hormones also support the process of red blood cellformation. Thyroid hormones control the metabolism of carbohydrates, proteinsand fats. Maintenance of water and electrolyte balance is also influenced bythyroid hormones. Thyroid gland also secretes a protein hormone calledthyrocalcitonin (TCT) which regulates the blood calcium levels.22.2.5 Parathyroid GlandIn humans, four parathyroid glands are present on the back side of thethyroid gland, one pair each in the two lobes of the thyroid gland (Figure22.3b). The parathyroid glands secrete a peptide hormone calledparathyroid hormone (PTH). The secretion of PTH is regulated by thecirculating levels of calcium ions.Parathyroid hormone (PTH) increases the Ca2+ levels in the blood. PTHacts on bones and stimulates the process of bone resorption (dissolution/demineralisation). PTH also stimulates reabsorption of Ca2+ by the renaltubules and increases Ca2+ absorption from the digested food. It is, thus,clear that PTH is a hypercalcemic hormone, i.e., it increases the bloodCa2+ levels. Along with TCT, it plays a significant role in calcium balancein the body.22.2.6 ThymusThe thymus gland is a lobular structure located between lungs behindsternum on the ventral side of aorta. The thymus plays a major role inthe development of the immune system. This gland secretes the peptidehormones called thymosins. Thymosins play a major role in thedifferentiation of T-lymphocytes, which provide cell-mediatedimmunity. In addition, thymosins also promote production of antibodiesto provide humoral immunity. Thymus is degenerated in old individualsresulting in a decreased production of thymosins. As a result, the immuneresponses of old persons become weak.2022-23336 BIOLOGYThe adrenal medulla secretes two hormones called adrenaline orepinephrine and noradrenaline or norepinephrine. These arecommonly called as catecholamines. Adrenaline and noradrenaline arerapidly secreted in response to stress of any kind and during emergencysituations and are called emergency hormones or hormones of Fightor Flight. These hormones increase alertness, pupilary dilation,piloerection (raising of hairs), sweating etc. Both the hormones increasethe heart beat, the strength of heart contraction and the rate of respiration.Catecholamines also stimulate the breakdown of glycogen resulting in22.2.7 Adrenal GlandOur body has one pair of adrenal glands, one at the anterior part of eachkidney (Figure 22.4 a). The gland is composed of two types of tissues.The centrally located tissue is called the adrenal medulla, and outsidethis lies the adrenal cortex (Figure 22.4 b).Underproduction of hormones by the adrenal cortex alterscarbohydrate metabolism causing acute weakness and fatigue leadingto a disease called Addisonâs disease.Figure 22.4 Diagrammatic representation of : (a) Adrenal gland above kidney(b) Section showing two parts of adrenal glandAdrenal glandAdrenal cortexKidneyAdrenal medulla(a) (b)2022-23CHEMICAL COORDINATION AND INTEGRATION 337an increased concentration of glucose in blood. In addition, they alsostimulate the breakdown of lipids and proteins.The adrenal cortex can be divided into three layers, called zonareticularis (inner layer), zona fasciculata (middle layer) and zonaglomerulosa (outer layer). The adrenal cortex secretes many hormones,commonly called as corticoids. The corticoids, which are involved incarbohydrate metabolism are called glucocorticoids. In our body, cortisolis the main glucocorticoid. Corticoids, which regulate the balance of waterand electrolytes in our body are called mineralocorticoids. Aldosterone isthe main mineralocorticoid in our body.Glucocorticoids stimulate gluconeogenesis, lipolysis and proteolysis;and inhibit cellular uptake and utilisation of amino acids. Cortisol is alsoinvolved in maintaining the cardio-vascular system as well as the kidneyfunctions. Glucocorticoids, particularly cortisol, produces antiinflammatoryreactions and suppresses the immune response. Cortisolstimulates the RBC production. Aldosterone acts mainly at the renaltubules and stimulates the reabsorption of Na+ and water and excretionof K+ and phosphate ions. Thus, aldosterone helps in the maintenance ofelectrolytes, body fluid volume, osmotic pressure and blood pressure.Small amounts of androgenic steroids are also secreted by the adrenalcortex which play a role in the growth of axial hair, pubic hair and facialhair during puberty.22.2.8 PancreasPancreas is a composite gland (Figure 22.1) which acts as both exocrineand endocrine gland. The endocrine pancreas consists of âIslets ofLangerhansâ. There are about 1 to 2 million Islets of Langerhans in anormal human pancreas representing only 1 to 2 per cent of the pancreatictissue. The two main types of cells in the Islet of Langerhans are calleda-cells and b-cells. The a-cells secrete a hormone called glucagon, whilethe b-cells secrete insulin.Glucagon is a peptide hormone, and plays an important role inmaintaining the normal blood glucose levels. Glucagon acts mainly onthe liver cells (hepatocytes) and stimulates glycogenolysis resulting in anincreased blood sugar (hyperglycemia). In addition, this hormonestimulates the process of gluconeogenesis which also contributes tohyperglycemia. Glucagon reduces the cellular glucose uptake andutilisation. Thus, glucagon is a hyperglycemic hormone.Insulin is a peptide hormone, which plays a major role in theregulation of glucose homeostasis. Insulin acts mainly on hepatocytesand adipocytes (cells of adipose tissue), and enhances cellular glucose2022-23338 BIOLOGYuptake and utilisation. As a result, there is a rapid movement of glucosefrom blood to hepatocytes and adipocytes resulting in decreased bloodglucose levels (hypoglycemia). Insulin also stimulates conversion ofglucose to glycogen (glycogenesis) in the target cells. The glucosehomeostasis in blood is thus maintained jointly by the two â insulin andglucagons.Prolonged hyperglycemia leads to a complex disorder called diabetesmellitus which is associated with loss of glucose through urine andformation of harmful compounds known as ketone bodies. Diabeticpatients are successfully treated with insulin therapy.22.2.9 TestisA pair of testis is present in the scrotal sac (outside abdomen) of maleindividuals (Figure 22.1). Testis performs dual functions as a primarysex organ as well as an endocrine gland. Testis is composed ofseminiferous tubules and stromal or interstitial tissue. The Leydigcells or interstitial cells, which are present in the intertubularspaces produce a group of hormones called androgens mainlytestosterone.Androgens regulate the development, maturation and functions ofthe male accessory sex organs like epididymis, vas deferens, seminalvesicles, prostate gland, urethra etc. These hormones stimulate musculargrowth, growth of facial and axillary hair, aggressiveness, low pitch ofvoice etc. Androgens play a major stimulatory role in the process ofspermatogenesis (formation of spermatozoa). Androgens act on the centralneural system and influence the male sexual behaviour (libido). Thesehormones produce anabolic (synthetic) effects on protein and carbohydratemetabolism.22.2.10 OvaryFemales have a pair of ovaries located in the abdomen (Figure 22.1). Ovaryis the primary female sex organ which produces one ovum during eachmenstrual cycle. In addition, ovary also produces two groups of steroidhormones called estrogen and progesterone. Ovary is composed ofovarian follicles and stromal tissues. The estrogen is synthesised andsecreted mainly by the growing ovarian follicles. After ovulation, theruptured follicle is converted to a structure called corpus luteum, whichsecretes mainly progesterone.Estrogens produce wide ranging actions such as stimulation of growthand activities of female secondary sex organs, development of growing2022-23CHEMICAL COORDINATION AND INTEGRATION 339ovarian follicles, appearance of female secondary sex characters (e.g., highpitch of voice, etc.), mammary gland development. Estrogens also regulatefemale sexual behaviour.Progesterone supports pregnancy. Progesterone also acts on themammary glands and stimulates the formation of alveoli (sac-likestructures which store milk) and milk secretion.22.3 HORMONES OF HEART, KIDNEY AND GASTROINTESTINAL TRACTNow you know about the endocrine glands and their hormones. However,as mentioned earlier, hormones are also secreted by some tissues whichare not endocrine glands. For example, the atrial wall of our heart secretesa very important peptide hormone called atrial natriuretic factor (ANF),which decreases blood pressure. When blood pressure is increased, ANFis secreted which causes dilation of the blood vessels. This reduces theblood pressure.The juxtaglomerular cells of kidney produce a peptide hormone callederythropoietin which stimulates erythropoiesis (formation of RBC).Endocrine cells present in different parts of the gastro-intestinal tractsecrete four major peptide hormones, namely gastrin, secretin,cholecystokinin (CCK) and gastric inhibitory peptide (GIP). Gastrinacts on the gastric glands and stimulates the secretion of hydrochloricacid and pepsinogen. Secretin acts on the exocrine pancreas andstimulates secretion of water and bicarbonate ions. CCK acts on bothpancreas and gall bladder and stimulates the secretion of pancreaticenzymes and bile juice, respectively. GIP inhibits gastric secretion andmotility. Several other non-endocrine tissues secrete hormones calledgrowth factors. These factors are essential for the normal growth of tissuesand their repairing/regeneration.22.4 MECHANISM OF HORMONE ACTIONHormones produce their effects on target tissues by binding to specificproteins called hormone receptors located in the target tissues only.Hormone receptors present on the cell membrane of the target cells arecalled membrane-bound receptors and the receptors present inside thetarget cell are called intracellular receptors, mostly nuclear receptors(present in the nucleus). Binding of a hormone to its receptor leads to theformation of a hormone-receptor complex (Figure 22.5 a, b). Eachreceptor is specific to one hormone only and hence receptors are specific.Hormone-Receptor complex formation leads to certain biochemicalchanges in the target tissue. Target tissue metabolism and hence2022-23340 BIOLOGYphysiological functions are regulated by hormones. On the basis of theirchemical nature, hormones can be divided into groups :(i) peptide, polypeptide, protein hormones (e.g., insulin, glucagon,pituitary hormones, hypothalamic hormones, etc.)(ii) steroids (e.g., cortisol, testosterone, estradiol and progesterone)(iii) iodothyronines (thyroid hormones)(iv) amino-acid derivatives (e.g., epinephrine).Hormones which interact with membrane-bound receptors normallydo not enter the target cell, but generate second messengers (e.g., cyclicAMP, IP3, Ca++ etc) which in turn regulate cellular metabolism (Figure22.5a). Hormones which interact with intracellular receptors (e.g., steroidhormones, iodothyronines, etc.) mostly regulate gene expression orchromosome function by the interaction of hormone-receptor complexwith the genome. Cumulative biochemical actions result in physiologicaland developmental effects (Figure 22.5b).(a)2022-23CHEMICAL COORDINATION AND INTEGRATION 341SUMMARYThere are special chemicals which act as hormones and provide chemicalcoordination, integration and regulation in the human body. These hormonesregulate metabolism, growth and development of our organs, the endocrine glandsor certain cells. The endocrine system is composed of hypothalamus, pituitaryand pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testisand ovary). In addition to these, some other organs, e.g., gastrointestinal tract,kidney, heart etc., also produce hormones. The pituitary gland is divided intothree major parts, which are called as pars distalis, pars intermedia and parsnervosa. Pars distalis produces six trophic hormones. Pars intermedia secretesFigure 22.5 Diagramatic representation of the mechanism of hormone action :(a) Protein hormone (b) Steroid hormone(b)2022-23342 BIOLOGYonly one hormone, while pars nervosa (neurohypophysis) secretes two hormones.The pituitary hormones regulate the growth and development of somatic tissuesand activities of peripheral endocrine glands. Pineal gland secretes melatonin, whichplays a very important role in the regulation of 24-hour (diurnal) rhythms of ourbody (e.g., rhythms of sleep and state of being awake, body temperature, etc.). Thethyroid gland hormones play an important role in the regulation of the basalmetabolic rate, development and maturation of the central neural system,erythropoiesis, metabolism of carbohydrates, proteins and fats, menstrual cycle.Another thyroid hormone, i.e., thyrocalcitonin regulates calcium levels in our bloodby decreasing it. The parathyroid glands secrete parathyroid hormone (PTH) whichincreases the blood Ca2+ levels and plays a major role in calcium homeostasis. Thethymus gland secretes thymosins which play a major role in the differentiation ofT-lymphocytes, which provide cell-mediated immunity. In addition, thymosinsalso increase the production of antibodies to provide humoral immunity. Theadrenal gland is composed of the centrally located adrenal medulla and the outeradrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine.These hormones increase alertness, pupilary dilation, piloerection, sweating, heartbeat, strength of heart contraction, rate of respiration, glycogenolysis, lipolysis,proteolysis. The adrenal cortex secretes glucocorticoids and mineralocorticoids.Glucocorticoids stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis,cardio-vascular system, blood pressure, and glomerular filtration rate and inhibitinflammatory reactions by suppressing the immune response. Mineralocorticoidsregulate water and electrolyte contents of the body. The endocrine pancreas secretesglucagon and insulin. Glucagon stimulates glycogenolysis and gluconeogenesisresulting in hyperglycemia. Insulin stimulates cellular glucose uptake andutilisation, and glycogenesis resulting in hypoglycemia. Insulin deficiencyand/or insulin resistance result in a disease called diabetes mellitus.The testis secretes androgens, which stimulate the development, maturationand functions of the male accessory sex organs, appearance of the male secondarysex characters, spermatogenesis, male sexual behaviour, anabolic pathways anderythropoiesis. The ovary secretes estrogen and progesterone. Estrogen stimulatesgrowth and development of female accessory sex organs and secondary sexcharacters. Progesterone plays a major role in the maintenance of pregnancy aswell as in mammary gland development and lactation. The atrial wall of the heartproduces atrial natriuretic factor which decreases the blood pressure. Kidneyproduces erythropoietin which stimulates erythropoiesis. The gastrointestinal tractsecretes gastrin, secretin, cholecystokinin and gastric inhibitory peptide. Thesehormones regulate the secretion of digestive juices and help in digestion.2022-23CHEMICAL COORDINATION AND INTEGRATION 343EXERCISES1. Define the following:(a) Exocrine gland(b) Endocrine gland(c) Hormone2. Diagrammatically indicate the location of the various endocrine glands in ourbody.3. List the hormones secreted by the following:(a) Hypothalamus (b) Pituitary (c) Thyroid (d) Parathyroid(e) Adrenal (f) Pancreas (g) Testis (h) Ovary(i) Thymus (j) Atrium (k) Kidney (l) G-I Tract4. Fill in the blanks:Hormones Target gland(a) Hypothalamic hormones __________________(b) Thyrotrophin (TSH) __________________(c) Corticotrophin (ACTH) __________________(d) Gonadotrophins (LH, FSH) __________________(e) Melanotrophin (MSH) __________________5. Write short notes on the functions of the following hormones:(a) Parathyroid hormone (PTH) (b) Thyroid hormones(c) Thymosins (d) Androgens(e) Estrogens (f) Insulin and Glucagon6. Give example(s) of:(a) Hyperglycemic hormone and hypoglycemic hormone(b) Hypercalcemic hormone(c) Gonadotrophic hormones(d) Progestational hormone(e) Blood pressure lowering hormone(f) Androgens and estrogens7. Which hormonal deficiency is responsible for the following:(a) Diabetes mellitus (b) Goitre (c) Cretinism8. Briefly mention the mechanism of action of FSH.9. Match the following:Column I Column II(a) T4 (i) Hypothalamus(b) PTH (ii) Thyroid(c) GnRH (iii) Pituitary(d) LH (iv) Parathyroid2022-23NOTE2022-23CHEMICAL COORDINATION AND INTEGRATION 345NOTE2022-23NOTE2022-23BIOLOGYTEXTBOOK FOR CLASS XII2022-23First EditionDecember 2006 Pausa 1928ReprintedNovember 2007 Kartika 1929January 2009 Pausa 1930December 2009 Pausa 1931January 2011 Magha 1932January 2012 Magha 1933November 2012 Kartika 1934November 2013 Kartika 1935December 2014 Pausa 1936January 2015 Pausa 1937January 2017 Pausa 1938January 2018 Magha 1939January 2019 Magha 1940August 2019 Bhadrapada 1941July 2021 Asadha 1943November 2021 Agrahayana 1943PD 180T RSPŠ National Council of EducationalResearch and Training, 2006` 195.00Printed on 80 GSM paper with NCERTwatermarkPublished at the Publication Division by theSecretary, National Council of EducationalResearch and Training, Sri Aurobindo Marg,New Delhi 110 016 and printed atS.K. Offset (P.) Ltd., 10, Sports ComplexEnclave, Delhi Road, Meerut - 250 002 (U.P.)ALL RIGHTS RESERVEDq No part of this publication may be reproduced, stored in a retrieval systemor transmitted, in any form or by any means, electronic, mechanical,photocopying, recording or otherwise without the prior permission of thepublisher.q This book is sold subject to the condition that it shall not, by way of trade,be lent, re-sold, hired out or otherwise disposed of without the publisherâsconsent, in any form of binding or cover other than that in which it ispublished.q The correct price of this publication is the price printed on this page, Anyrevised price indicated by a rubber stamp or by a sticker or by any othermeans is incorrect and should be unacceptable.Publication TeamHead, Publication : Anup Kumar RajputDivisionChief Editor : Shveta UppalChief Production : Arun ChitkaraOfficerChief Business : Vipin DewanManagerAssistant Editor : Shashi ChadhaProduction Assistant : Prakash Veer SinghCover and LayoutShweta RaoIllustrationsLalit MauryaOFFICES OF THE PUBLICATIONDIVISION, NCERTNCERT CampusSri Aurobindo MargNew Delhi 110 016 Phone : 011-26562708108, 100 Feet RoadHosdakere Halli ExtensionBanashankari III StageBengaluru 560 085 Phone : 080-26725740Navjivan Trust BuildingP.O.NavjivanAhmedabad 380 014 Phone : 079-27541446CWC CampusOpp. Dhankal Bus StopPanihatiKolkata 700 114 Phone : 033-25530454CWC ComplexMaligaonGuwahati 781 021 Phone : 0361-2674869ISBN 81-7450-639-X12083 â BIOLOGYTextbook for Class XII2022-23IIIFOREWORDThe National Curriculum Framework (NCF) 2005, recommends that childrenâs life at schoolmust be linked to their life outside the school. This principle marks a departure from thelegacy of bookish learning which continues to shape our system and causes a gap betweenthe school, home and community. The syllabi and textbooks developed on the basis of NCFsignify an attempt to implement this basic idea. They also attempt to discourage rote learningand the maintenance of sharp boundaries between different subject areas. We hope thesemeasures will take us significantly further in the direction of a child-centred system ofeducation outlined in the National Policy on Education (1986).The success of this effort depends on the steps that school principals and teachers willtake to encourage children to reflect on their own learning and to pursue imaginative activitiesand questions. We must recognise that, given space, time and freedom, children generatenew knowledge by engaging with the information passed on to them by adults. Treating theprescribed textbook as the sole basis of examination is one of the key reasons why otherresources and sites of learning are ignored. Inculcating creativity and initiative is possibleif we perceive and treat children as participants in learning, not as receivers of a fixed bodyof knowledge.These aims imply considerable change in school routines and mode of functioning.Flexibility in the daily time-table is as necessary as rigour in implementing the annualcalendar so that the required number of teaching days are actually devoted to teaching. Themethods used for teaching and evaluation will also determine how effective this textbookproves for making childrenâs life at school a happy experience, rather than a source ofstress or boredom. Syllabus designers have tried to address the problem of curricular burdenby restructuring and reorienting knowledge at different stages with greater considerationfor child psychology and the time available for teaching. The textbook attempts to enhancethis endeavour by giving higher priority and space to opportunities for contemplation andwondering, discussion in small groups, and activities requiring hands-on experience.The National Council of Educational Research and Training (NCERT) appreciates thehard work done by the textbook development committee responsible for this book. Wewish to thank the Chairperson of the advisory group in science and mathematics,Professor J.V. Narlikar and the Chief Advisor for this book, Professor K. Muralidhar,Department of Zoology, University of Delhi, Delhi for guiding the work of this committee.Several teachers contributed to the development of this textbook. We are grateful to theirprincipals for making this possible. We are indebted to the institutions and organisationswhich have generously permitted us to draw upon their resources, material and personnel.We are especially grateful to the members of the National Monitoring Committee, appointed2022-23IVby the Department of Secondary and Higher Education, Ministry of Human ResourceDevelopment under the Chairmanship of Professor Mrinal Miri and Professor G.P. Deshpande,for their valuable time and contribution.As an organisation committed to systemic reform and continuous improvement in thequality of its products, NCERT welcomes comments and suggestions which will enable us toundertake further revision and refinement.DirectorNew Delhi National Council of Educational20 November 2006 Research and Training2022-23VBiology is the study of life in its entirety. The growth of biology as a natural science duringthe last 1000 years is interesting from many points of view. One feature of this growth ischanging emphasis. Initially it was description of life forms. Identification, nomenclature,classification of all recorded living forms enjoyed the attention of scientists for a long time.Description of their habitats and (in the case of animals) their behaviour was included inthis study. In later years, the focus was physiology and internal morphology or anatomy.Darwinian ideas of evolution by natural selection changed the perception completely. Classicaldescriptive and clueless biology found a theoretical framework in the evolutionary theory ofDarwin.In the nineteenth and twentieth centuries, Physics and Chemistry were applied to Biologyand the new science of Biochemistry soon became the dominant face of biology. On one handBiochemistry was integrating with Physiology, becoming almost synonymous with it. On theother hand it gave rise to Structural Biology (structure of biomacromolecules), originally calledMolecular Biology. The work of Bernal, Pauling, Watson and Crick, Hodgkins, Perutz and Kendrew,Delbruck, Luria, Monod, Beadle and Tatum, Lederberg, Brenner, Benzer, Nirenberg, Khorana,Mclintock, Sanger, Cohen, Boyer, Kornbergs (father and son), Leder, Chambon and scores ofothers brought in and established a modern version of Molecular Biology dealing with lifeprocesses at molecular level.Physics and Chemistry dominated public perception of science for a long time.Daytoday life of man was influenced by developments in Physics, Chemistry and their respectivemanufacturing industries. Slowly and steadily, Biology, not to be left behind, demonstratedits utility for human welfare. Medical practice, especially diagnostics, green revolution andthe newly emerging biotechnology and its success stories made the presence of biology feltby the common man. Patent laws brought biology into political domain and commercial valueof biology became obvious.For more than a century, classical and so-called reductionist biology fought artificalbattles. The fact is both are important. Ecology brought in synthesis of both approachesand emphasised integrated understanding of biology. Form and process are both equallyimportant. Systems biology, using mathematical tools, is bringing about a modern synthesisof both the aspects of Biology.The Class XI and XII textbooks in biology essentially were to reflect these threads ofbiological thought. While the Class XI book dealt with morphology, taxonomy, molecular andcellular aspects of physiology, the Class XII book deals with the physiological process ofreproduction in flowering plants and humans, the principles of inheritance, the nature ofgenetic material and its function, the contributions of biology to human welfare, basicprinciples of biotechnological processes and their applications and achievements. TheClass XII book also relates genes to evolution on one hand and presents ecological interactions,behaviour of populations and ecosystems on the other. Most important, the guidelines underNCF-2005 have been followed in letter and spirit. The total learning load has been reducedPREFACE2022-23VIconsiderably and themes like environmental issues, adolescent problems and reproductivehealth have been dealt with in some detail. Studied together, the class XI and class XIItextbooks in Biology would enable the student to â(i) become familiar with the diversity of biological material.(ii) appreciate and believe in the Darwinian evolutionary process exhibited by the livingworld.(iii) understand the dynamic state of constituents of living bodies, i.e., metabolicbasis of all physiological processes in plants, animals and microbes.(iv) realise the structure and function of genetic material in directing the inheritedphenotype pattern as well as a mediator of evolutionary process.(v) appreciate the profound contributions of biology to human welfare.(vi) reflect on the physico-chemical basis of living processes and at the same timerealise the limitation of reductionism in understanding behaviour of organisms.(vii) experience the humbling effect of this realisation that all living organisms arerelated to each other by virtue of shared genetic material.(viii) realise that biology is the story of the struggle of living organisms for existenceand survival.One may notice a perceptible change in the writing style. Most of the chapters are writtenin an easy dialogue style engaging the student constantly while some chapters are in theform of critical comments on the subject matter. A number of questions have been providedat the end of each chapter though answers to some may not be found in the text. Studentshave to read supplementary material, upon advise from the teacher, to answer such questions.I am thankful to Professor Krishna Kumar, Director NCERT; Professor G. Ravindra, JointDirector, NCERT and Professor Hukum Singh, Head, DESM, NCERT for constant support. Imust place on record my deep appreciation for Dr B.K. Tripathi, Reader, DESM, NCERT forhis relentless efforts as coordinator in bringing out the Biology textbook for both theClass XI and XII. All the members of the development team, the experts and reviewers, andthe school teachers have contributed enormously in the preparation of this book. I thankthem all. I am indeed highly thankful to the members of monitoring committee constitutedby Ministry of Human Resource Development for their valuable observation that helped inthe improvement of the book at the final stage. The book is prepared keeping in mind theguidelines of the NCF-2005 especially the emphasis on reducing the learning load. We hope thatthe book would meet the expectations of all the stakeholders. All suggestions for furtherimprovement are always welcome.K. MURALIDHARDepartment of Zoology Chief AdvisorUniversity of Delhi Biology Textbook for Class XII2022-23VIITEXTBOOK DEVELOPMENT COMMITTEECHAIRPERSON, ADVISORY GROUP FOR TEXTBOOKS IN SCIENCE AND MATHEMATICSJ.V. Narlikar, Emeritus Professor, Inter University Centre for Astronomy and Astrophysics (IUCAA),Pune University, PuneCHIEF ADVISORK. Muralidhar, Professor, Department of Zoology, University of Delhi, DelhiMEMBERSAjit Kumar Kavathekar, Reader (Botany), Sri Venkateswara College, University of Delhi, DelhiB.B.P. Gupta, Professor, Department of Zoology, North-Eastern Hill University, ShillongB.N. Pandey, Principal, Ordinance Factory Higher Secondary School, DehradunC.V. Shimray, Lecturer, Department of Education in Science and Mathematics, NCERT, New DelhiDinesh Kumar, Reader, Department of Education in Science and Mathematics, NCERT, New DelhiJ.P. Gaur, Professor, Department of Botany, Banaras Hindu University, VaranasiJ.S. Virdi, Reader, Department of Microbiology, University of Delhi, South Campus, New DelhiK. Sarath Chandran, Reader (Zoology), Sri Venkateswara College, University of Delhi, DelhiL.C. Rai, Professor, Department of Botany, Banaras Hindu University, VaranasiM.M. Chaturvedi, Professor, Department of Zoology, University of Delhi, DelhiN.V.S.R.K. Prasad, Reader (Botany), Sri Venkateswara College, University of Delhi, DelhiSangeeta Sharma, PGT (Biology), Kendriya Vidyalaya, JNU, New DelhiSavithri Singh, Principal, Acharya Narendra Dev College, University of Delhi, DelhiShanti Chandrashekaran, Principal Scientist, Division of Genetics, I.A.R.I., New DelhiShardendu, Reader, Department of Botany, Science College, Patna University, PatnaSimminder K. Thukral, Assistant Professor, NIIT Institute of Information Technology, New DelhiSunaina Sharma, Lecturer (Biology), Rajkiya Pratibha Vikas Vidyalaya, Dwarka, New DelhiT.R. Rao, Professor (Retd.) School of Enviornmental Studies, University of Delhi, DelhiV.K. Kakaria, Reader, Regional Institute of Education, BhopalV.V. Anand, Reader, Regional Institute of Education, MysoreMEMBER-COORDINATORB.K. Tripathi, Reader, Department of Education in Science and Mathematics, NCERT, New Delhi2022-23National Council of Educational Research and Training (NCERT) gratefully acknowledgesthe valuable contribution of K.R. Shivanna, Professor (Retd.), Department of Botany,University of Delhi, Delhi; S.K. Saidapur, Professor, Department of Zoology, KarnatakaUniversity Dharwad; Vani Brahmachari, Professor, Ambedkar Centre for BiomedicalResearch, University of Delhi, Delhi; A.N. Lahiri Majumdar, Professor, Bose Institute, Kolkata;Anil Tripathi, Professor, Department of Biotechnology, Banaras Hindu University, Varanasi;J.L. Jain, Senior Physician, WUS Health Centre, University of Delhi, Delhi, in the developmentof the Biology textbook for Class XII. NCERT is also grateful to K.R. Shivanna andT.Subramanyam, IIT, Kanpur for some of the photographs used in the book.NCERT sincerely acknowledges the contributions of the members who participated in thereview of the manuscripts â A.S. Dixit, Reader, Department of Zoology, North-Eastern HillUniversity, Shillong; S.L. Varte, Lecturer, Department of Education in Science and Mathematics,NCERT, New Delhi; Sushma Jairath, Reader, Department of Womenâs Education, NCERT,New Delhi; Mona Yadav, Lecturer, Department of Womenâs Education, NCERT, New Delhi;Poonam A. Kant, Reader (Zoology), Acharya Narendra Dev College, New Delhi; Mrs. SuvarnaFonseca è Antao, Gr. I Teacher (Biology), Carmel Higher Secondary School, Nuvem, Goa;Rashmi Mishra, PGT (Biology), Carmel Convent Senior Secondary School, BHEL, Bhopal;Ishwant Kaur, PGT (Biology), D.M. School, RIE, Bhopal; A.K. Singh, PGT (Biology), KendriyaVidyalaya, Cantt, Varanasi; R.P. Singh, Lecturer (Biology), Rajkiya Pratibha Vikash Vidyalaya,Kishan ganj, Delhi; M.K. Tiwari, PGT (Biology), Kendriya Vidyalaya, Mandsaur, Madhya Pradesh;A.K. Ganguly, PGT (Biology), Jawahar Navodaya Vidyalaya, Roshnabad, Haridwar; ChaitaliDixit, PGT (Biology), St. Anthonyâs Higher Secondary School (Don Bosco), Shillong and AbhishekChari, Acharya Narendra Dev College, New Delhi.Special thanks are due to Rita Sharma, Retd. Professor, RIE Bhopal, A.K. MohapatraProfessor, RIE Bhubaneswar, J.S. Gill, Retd. Professor, DESM, NIE, G.V. Gopal, Professor, RIEMysore, Jaydeep Mandal, Professor, RIE Bhopal, C. Padmija, Professor, RIE Mysore,Dr. Pushplata Verma, Associate Professor, DESM, NIE, Ishwant Kaur, Vice Principal, DM SchoolAjmer for their valuable contribution in review and updation of the textbook.The Council is highly thankful to Hukum Singh, Professor and Head, Department ofEducation in Science and Mathematics, NCERT for his valuable support throughout themaking of this book.The contributions of Deepak Kapoor, Incharge, Computer Station; Seema Mehmi andArvind Sharma, DTP operators; Deepti Sharma, Copy Editor; Rachna Dogra and AbhimanuMohanty, Proof Readers and APC office and administrative staff of Department of Educationin Science and Mathematics, NCERT also acknowledged.The efforts of the Publication Department, NCERT, in bringing out this publication arehighly appreciated.ACKNOWLEDGEMENTS2022-23CONTENTSFOREWORD iiiPREFACE vUNIT VIREPRODUCTION 1-66Chapter 1 : Reproduction in Organisms 3Chapter 2 : Sexual Reproduction in Flowering Plants 19Chapter 3 : Human Reproduction 42Chapter 4 : Reproductive Health 57UNIT VIIGENETICS AND EVOLUTION 67-142Chapter 5 : Principles of Inheritance and Variation 69Chapter 6 : Molecular Basis of Inheritance 95Chapter 7 : Evolution 126UNIT VIIIBIOLOGY IN HUMAN WELFARE 143-190Chapter 8 : Human Health and Disease 145Chapter 9 : Strategies for Enhancement in 165Food ProductionChapter 10 : Microbes in Human Welfare 1792022-23XUNIT IXBIOTECHNOLOGY 191-216Chapter 11 : Biotechnology : Principles and Processes 193Chapter 12 : Biotechnology and its Applications 207UNIT XECOLOGY 217-286Chapter 13 : Organisms and Populations 219Chapter 14 : Ecosystem 241Chapter 15 : Biodiversity and Conservation 258Chapter 16 : Environmental Issues 2702022-23Biology in essence is the story of life on earth. While individualorganisms die without fail, species continue to live throughmillions of years unless threatened by natural or anthropogenicextinction. Reproduction becomes a vital process withoutwhich species cannot survive for long. Each individual leavesits progeny by asexual or sexual means. Sexual mode ofreproduction enables creation of new variants, so that survivaladvantage is enhanced. This unit examines the generalprinciples underlying reproductive processes in living organismsand then explains the details of this process in flowering plantsand humans as easy to relate representative examples. A relatedperspective on human reproductive health and howreproductive ill health can be avoided is also presented tocomplete our understanding of biology of reproduction.Chapter 1Reproduction in OrganismsChapter 2Sexual Reproduction inflowering PlantsChapter 3Human ReproductionChapter 4Reproductive Health2022-23PANCHANAN MAHESHWARI(1904-1966)Born in November 1904 in Jaipur (Rajasthan) Panchanan Maheshwarirose to become one of the most distinguished botanists not only of Indiabut of the entire world. He moved to Allahabad for higher educationwhere he obtained his D.Sc. During his college days, he was inspiredby Dr W. Dudgeon, an American missionary teacher, to develop interestin Botany and especially morphology. His teacher once expressed thatif his student progresses ahead of him, it will give him a great satisfaction.These words encouraged Panchanan to enquire what he could do forhis teacher in return.He worked on embryological aspects and popularised the use ofembryological characters in taxonomy. He established the Departmentof Botany, University of Delhi as an important centre of research inembryology and tissue culture. He also emphasised the need for initiationof work on artificial culture of immature embryos. These days, tissueculture has become a landmark in science. His work on test tubefertilisation and intra-ovarian pollination won worldwide acclaim.He was honoured with fellowship of Royal Society of London (FRS),Indian National Science Academy and several other institutions ofexcellence. He encouraged general education and made a significantcontribution to school education by his leadership in bringing out thevery first textbooks of Biology for Higher Secondary Schools publishedby NCERT in 1964.2022-23Each and every organism can live only for a certain periodof time. The period from birth to the natural death of anorganism represents its life span. Life spans of a feworganisms are given in Figure 1.1. Several other organismsare drawn for which you should find out their life spansand write in the spaces provided. Examine the life spansof organisms represented in the Figure 1.1. Isnât it bothinteresting and intriguing to note that it may be as shortas a few days or as long as a few thousand years? Betweenthese two extremes are the life spans of most other livingorganisms. You may note that life spans of organisms arenot necessarily correlated with their sizes; the sizes ofcrows and parrots are not very different yet their life spansshow a wide difference. Similarly, a mango tree has a muchshorter life span as compared to a peepal tree. Whateverbe the life span, death of every individual organism is acertainty, i.e., no individual is immortal, except single-celledorganisms. Why do we say there is no natural death insingle-celled organisms? Given this reality, have you everwondered how vast number of plant and animal specieshave existed on earth for several thousands of years? Theremust be some processes in living organisms that ensurethis continuity. Yes, we are talking about reproduction,something that we take for granted.CHAPTER 1REPRODUCTION IN ORGANISMS1.1 AsexualReproduction1.2 SexualReproduction2022-234BIOLOGYFigure 1.1 Approximate life spans of some organisms2022-235REPRODUCTION IN ORGANISMSReproduction is defined as a biological process in which anorganism gives rise to young ones (offspring) similar to itself. The offspringgrow, mature and in turn produce new offspring. Thus, there is a cycleof birth, growth and death. Reproduction enables the continuity of thespecies, generation after generation. You will study later in Chapter 5(Principles of Inheritance and Variation) how genetic variation is createdand inherited during reproduction.There is a large diversity in the biological world and each organismhas evolved its own mechanism to multiply and produce offspring.The organismâs habitat, its internal physiology and several other factorsare collectively responsible for how it reproduces. Based on whetherthere is participation of one organism or two in the process ofreproduction, it is of two types. When offspring is produced by a singleparent with or without the involvement of gamete formation, thereproduction is asexual. When two parents (opposite sex) participate inthe reproductive process and also involve fusion of male and femalegametes, it is called sexual reproduction.1.1 ASEXUAL REPRODUCTIONIn this method, a single individual (parent) is capable of producingoffspring. As a result, the offspring that are produced are not onlyidentical to one another but are also exact copies of their parent.Are these offspring likely to be genetically identical or different?The term clone is used to describe such morphologically andgenetically similar individuals.Figure 1.2 Cell division in unicellular organism: (a) Budding inyeast; (b) Binary fission in Amoeba(a) (b)Let us see how widespread asexual reproduction is, among differentgroups of organisms. Asexual reproduction is common amongsingle-celled organisms, and in plants and animals with relatively simpleorganisations. In Protists and Monerans, the organism or the parentcell divides by mitosis into two to give rise to new individuals (Figure1.2).Thus, in these organisms cell division is itself a mode of reproduction.2022-236BIOLOGYMany single-celled organisms reproduce by binary fission, where acell divides into two halves and each rapidly grows into an adult (e.g.,Amoeba, Paramecium). In yeast, the division is unequal and smallbuds are produced that remain attached initially to the parent cellwhich, eventually gets separated and mature into new yeastorganisms (cells). Under unfavourable condition the Amoeba withdrawsits pseudopodia and secretes a three-layered hard covering or cystaround itself. This phenomenon is termed as encystation. Whenfavourable conditions return, the encysted Amoeba divides by multiplefission and produces many minute amoeba or pseudopodiospores;the cyst wall bursts out, and the spores are liberated in the surroundingmedium to grow up into many amoebae. This phenomenon isknown as sporulation.Figure1.3 Asexual reproductive structures: (a) Zoospores of Chlamydomonas; (b) Conidia ofPenicillium; (c) Buds in Hydra; (d) Gemmules in sponge(d)(a)(b)(c)2022-237REPRODUCTION IN ORGANISMSMembers of the Kingdom Fungi and simple plants such as algaereproduce through special asexual reproductive structures (Figure 1.3).The most common of these structures are zoospores that usually aremicroscopic motile structures. Other common asexual reproductivestructures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).You have learnt about vegetative reproduction in plants in Class XI.What do you think â Is vegetative reproduction also a type of asexualreproduction? Why do you say so? Is the term clone applicable to theoffspring formed by vegetative reproduction?While in animals and other simple organisms the term asexual is usedunambiguously, in plants, the term vegetative reproduction is frequentlyused. In plants, the units of vegetative propagation such as runner,rhizome, sucker, tuber, offset, bulb are all capable of giving rise to newoffspring (Figure1.4). These structures are called vegetative propagules.Figure 1.4 Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger;(c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e) Offset of water hyacinthAdventitiousBuds(a)(c) (d) (e)(b)BudsNodesAdventitiousRoot2022-238BIOLOGYObviously, since the formation of these structures does not involve twoparents, the process involved is asexual. In some organisms, if the bodybreaks into distinct pieces (fragments) each fragment grows into anadult capable of producing offspring (e.g., Hydra). This is also a modeof asexual reproduction called fragmentation.You must have heard about the scourge of the water bodies or aboutthe âterror of Bengalâ. This is nothing but the aquatic plant âwater hyacinthâwhich is one of the most invasive weeds found growing wherever there isstanding water. It drains oxygen from the water, which leads to death offishes. You will learn more about it in Chapters 13 and 14. You may findit interesting to know that this plant was introduced in India because ofits beautiful flowers and shape of leaves. Since it can propagate vegetativelyat a phenomenal rate and spread all over the water body in a short periodof time, it is very difficult to get rid off them.Are you aware how plants like potato, sugarcane, banana, ginger,dahlia are cultivated? Have you seen small plants emerging from thebuds (called eyes) of the potato tuber, from the rhizomes of banana andginger? When you carefully try to determine the site of origin of the newplantlets in the plants listed above, you will notice that they invariablyarise from the nodes present in the modified stems of these plants. Whenthe nodes come in contact with damp soil or water, they produce rootsand new plants. Similarly, adventitious buds arise from the notchespresent at margins of leaves of Bryophyllum. This ability is fully exploitedby gardeners and farmers for commercial propagation of such plants.It is interesting to note that asexual reproduction is the common methodof reproduction in organisms that have a relatively simple organisation,like algae and fungi and that they shift to sexual method of reproductionjust before the onset of adverse conditions. Find out how sexualreproduction enables these organisms to survive during unfavourableconditions? Why is sexual reproduction favoured under such conditions?Asexual (vegetative) as well as sexual modes of reproduction are exhibitedby the higher plants. On the other hand, only sexual mode of reproductionis present in most of the animals.1.2 SEXUAL REPRODUCTIONSexual reproduction involves formation of the male and female gametes,either by the same individual or by different individuals of the oppositesex. These gametes fuse to form the zygote which develops to form thenew organism. It is an elaborate, complex and slow process as comparedto asexual reproduction. Because of the fusion of male and female gametes,sexual reproduction results in offspring that are not identical to the parentsor amongst themselves.A study of diverse organismsâplants, animals or fungiâshow thatthough they differ so greatly in external morphology, internal structure2022-239REPRODUCTION IN ORGANISMSand physiology, when it comes to sexual mode of reproduction,surprisingly, they share a similar pattern. Let us first discuss what featuresare common to these diverse organisms.All organisms have to reach a certain stage of growth and maturity intheir life, before they can reproduce sexually. That period of growth iscalled the juvenile phase. It is known as vegetative phase in plants.This phase is of variable durations in different organisms.The end of juvenile/vegetative phase which marks the beginning ofthe reproductive phase can be seen easily in the higher plants when theycome to flower. How long does it take for marigold/rice/wheat/coconut/mango plants to come to flower? In some plants, where flowering occursmore than once, what would you call the inter-flowering period â juvenileor mature?Observe a few trees in your area. Do they flower during the samemonth year after year? Why do you think the availability of fruits likemango, apple, jackfruit, etc., is seasonal? Are there some plants that flowerthroughout the year and some others that show seasonal flowering?Plantsâthe annual and biennial types, show clear cut vegetative,reproductive and senescent phases, but in the perennial species it is verydifficult to clearly define these phases. A few plants exhibit unusualflowering phenomenon; some of them such as bamboo species flower onlyonce in their life time, generally after 50-100 years, produce large numberof fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji),flowers once in 12 years. As many of you would know, this plant floweredduring September-October 2006. Its mass flowering transformed largetracks of hilly areas in Kerala, Karnataka and Tamil Nadu into bluestretches and attracted a large number of tourists. In animals, the juvenilephase is followed by morphological and physiological changes prior toactive reproductive behaviour. The reproductive phase is also of variableduration in different organisms.Can you list the changes seen in human beings that are indicativeof reproductive maturity?Among animals, for example birds, do they lay eggs all through theyear? Or is it a seasonal phenomenon? What about other animals likefrogs and lizards? You will notice that, birds living in nature lay eggs onlyseasonally. However, birds in captivity (as in poultry farms) can be madeto lay eggs throughout the year. In this case, laying eggs is not related toreproduction but is a commercial exploitation for human welfare. Thefemales of placental mammals exhibit cyclical changes in the activities ofovaries and accessory ducts as well as hormones during the reproductivephase. In non-primate mammals like cows, sheep, rats, deers, dogs, tiger,etc., such cyclical changes during reproduction are called oestrus cyclewhere as in primates (monkeys, apes, and humans) it is called menstrualcycle. Many mammals, especially those living in natural, wild conditionsexhibit such cycles only during favourable seasons in their reproductive2022-2310BIOLOGY(a) (b) (c)Figure 1.5 Types of gametes: (a) Isogametes of Cladophora (an alga); (b) Heterogametes ofFucus (an alga); (c) Heterogametes of Homo sapiens (Human beings)phase and are therefore called seasonal breeders. Many other mammalsare reproductively active throughout their reproductive phase and henceare called continuous breeders.That we all grow old (if we live long enough), is something that werecognise. But what is meant by growing old? The end of reproductivephase can be considered as one of the parameters of senescence or oldage. There are concomitant changes in the body (like slowing ofmetabolism, etc.) during this last phase of life span. Old age ultimatelyleads to death.In both plants and animals, hormones are responsible for thetransitions between the three phases. Interaction between hormones andcertain environmental factors regulate the reproductive processes andthe associated behavioural expressions of organisms.Events in sexual reproduction : After attainment of maturity, all sexuallyreproducing organisms exhibit events and processes that have remarkablefundamental similarity, even though the structures associated with sexualreproduction are indeed very different. The events of sexual reproductionthough elaborate and complex, follow a regular sequence. Sexualreproduction is characterised by the fusion (or fertilisation) of the male andfemale gametes, the formation of zygote and embryogenesis. For conveniencethese sequential events may be grouped into three distinct stages namely,the pre-fertilisation, fertilisation and the post-fertilisation events.1.2.1 Pre-fertilisation EventsThese include all the events of sexual reproduction prior to the fusion ofgametes. The two main pre-fertilisation events are gametogenesis andgamete transfer.1.2.1.1 GametogenesisAs you are already aware, gametogenesis refers to the process of formationof the two types of gametes â male and female. Gametes are haploid cells.2022-2311REPRODUCTION IN ORGANISMSIn some algae the two gametes are so similar in appearancethat it is not possible to categorise them into male and female gametes.They are hence called homogametes (isogametes) (Figure 1.5a).However, in a majority of sexually reproducing organisms the gametesproduced are of two morphologically distinct types (heterogametes). Insuch organisms the male gamete is called the antherozoid or spermand the female gamete is called the egg or ovum (Figure1.5 b, c).Sexuality in organisms: Sexual reproduction in organisms generallyinvolves the fusion of gametes from two different individuals. But thisis not always true. From your recollection of examples studied inClass XI, can you identify cases where self-fertilisation is observed? Ofcourse, citing such examples in plants is easy.Plants may have both male and female reproductive structures in thesame plant (bisexual) (Figure 1.6 c, e) or on different plants (unisexual)(Figure 1.6d). In several fungi and plants, terms such as homothallicand monoecious are used to denote the bisexual condition andheterothallic and dioecious are the terms used to describe unisexualcondition. In flowering plants, the unisexual male flower is staminate,i.e., bearing stamens, while the female is pistillate or bearing pistils. Insome flowering plants, both male and female flowers may be present onthe same individual (monoecious) or on separate individuals (dioecious).Some examples of monoecious plants are cucurbits and coconuts and ofdioecious plants are papaya and date palm. Name the type of gametesthat are formed in staminate and pistillate flowers.But what about animals? Are individuals of all species either male orfemale (unisexual)? Or are there species which possess both thereproductive organs (bisexual)? You probably can make a listof several unisexual animal species. Earthworms, (Figure 1.6a) sponge,tapeworm and leech, typical examples of bisexual animals that possessboth male and female reproductive organs, are hermaphrodites.Cockroach (Figure 1.6b) is an example of a unisexual species.Cell division during gamete formation : Gametes in all heterogameticspecies are of two types namely, male and female. Gametes are haploidthough the parent plant body from which they arise may be either haploidor diploid. A haploid parent produces gametes by mitotic division. Doesthis mean that meiosis never occurs in organisms that are haploid?Carefully examine the flow charts of life cycles of algae that you havestudied in Class XI (Chapter 3) to get a suitable answer.Several organisms belonging to monera, fungi, algae and bryophyteshave haploid plant body, but in organisms belonging to pteridophytes,gymnosperms, angiosperms and most of the animals including humanbeings, the parental body is diploid. It is obvious that meiosis, the reductiondivision, has to occur if a diploid body has to produce haploid gametes.2022-2312BIOLOGYFigure 1.6 Diversity of sexuality in organisms (a) Bisexual animal (Earthworm); (b) Unisexualanimal (Cockroach); (c) Monoecious plant (Chara); (d) Dioecious plant (Marchantia);(e) Bisexual flower (sweet potato)(b)MaleFemaleTestis sacwith testisOvaryClitellum(a)TestisOvaryFemale thallusAntheridiophoreMale thallusArchegoniophore(c)(d) (e)2022-2313REPRODUCTION IN ORGANISMSTable 1.1: Chromosome Numbers in Meiocytes (diploid, 2n) and Gametes(haploid, n) of Some Organisms. Fill in the Blank Spaces.Name of organism Chromosome number Chromosome numberin meiocyte (2n) in gamete (n)Human beings 46 23House fly 12 âRat â 21Dog 78 âCat â 19Fruit fly 8 âOphioglossum (a fern) â 630Apple 34 âRice â 12Maize 20 âPotato â 24Butterfly 380 âOnion â 8In diploid organisms, specialised cells called meiocytes (gamete mothercell) undergo meiosis. At the end of meiosis, only one set of chromosomesgets incorporated into each gamete. Carefully study Table 1.1 and fill inthe diploid and haploid chromosome numbers of organisms. Is there anyrelationship in the number of chromosomes of meiocytes and gametes?1.2.1.2 Gamete TransferAfter their formation, male and female gametes must be physicallybrought together to facilitate fusion (fertilisation). Have you everwondered how the gametes meet? In a majority of organisms, malegamete is motile and the female gamete is stationary. Exceptions are afew fungi and algae in which both types of gametes are motile(Figure1.7a). There is a need for a medium through which the malegametes move. In several simple plants like algae, bryophytes andpteridophytes, water is the medium through which this gamete transfertakes place. A large number of the male gametes, however, fail to reachthe female gametes. To compensate this loss of male gametes duringtransport, the number of male gametes produced is several thousandtimes the number of female gametes produced.2022-2314BIOLOGYIn seed plants, pollen grains are the carriers of malegametes and ovule have the egg. Pollen grainsproduced in anthers therefore, have to be transferredto the stigma before it can lead to fertilisation (Figure1.7b). In bisexual, self-fertilising plants, e.g., peas,transfer of pollen grains to the stigma is relatively easyas anthers and stigma are located close to each other;pollen grains soon after they are shed, come in contactwith the stigma. But in cross pollinating plants(including dioecious plants), a specialised event calledpollination facilitates transfer of pollen grains to thestigma. Pollen grains germinate on the stigma and thepollen tubes carrying the male gametes reach the ovuleand discharge male gametes near the egg. In dioeciousanimals, since male and female gametes are formed indifferent individuals, the organism must evolve aspecial mechanism for gamete transfer. Successfultransfer and coming together of gametes is essentialfor the most critical event in sexual reproduction, thefertilisation.1.2.2 FertilisationThe most vital event of sexual reproduction is perhapsthe fusion of gametes. This process called syngamyresults in the formation of a diploid zygote. The termfertilisation is also often used for this process. Theterms syngamy and fertilisation are frequently usedthough , interchangeably.What would happen if syngamy does not occur?However, it has to be mentioned here that in someorganisms like rotifers, honeybees and even some lizardsand birds (turkey), the female gamete undergoesdevelopment to form new organisms without fertilisation. Thisphenomenon is called parthenogenesis.Where does syngamy occur? In most aquatic organisms, such as amajority of algae and fishes as well as amphibians, syngamy occurs inthe external medium (water), i.e., outside the body of the organism. Thistype of gametic fusion is called external fertilisation. Organismsexhibiting external fertilisation show great synchrony between the sexesand release a large number of gametes into the surrounding medium(water) in order to enhance the chances of syngamy. This happens in thebony fishes and frogs where a large number of offspring are produced. Amajor disadvantage is that the offspring are extremely vulnerable topredators threatening their survival up to adulthood.Figure 1.7 (a) Homogametic contact inalga; (b) Germinating pollengrains on the stigma of a flower(a)(b)2022-2315REPRODUCTION IN ORGANISMSIn many terrestrial organisms, belonging to fungi, higher animals suchas reptiles, birds, mammals and in a majority of plants (bryophytes,pteridophytes, gymnosperms and angiosperms), syngamy occurs insidethe body of the organism, hence the process is called internal fertilisation.In all these organisms, egg is formed inside the female body where theyfuse with the male gamete. In organisms exhibiting internal fertilisation,the male gamete is motile and has to reach the egg in order to fuse with it.In these even though the number of sperms produced is very large, thereis a significant reduction in the number of eggs produced. In seed plants,however, the non-motile male gametes are carried to female gamete bypollen tubes.1.2.3 Post-fertilisation EventsEvents in sexual reproduction after the formation of zygote are calledpost-fertilisation events.1.2.3.1 The ZygoteFormation of the diploid zygote is universal in all sexually reproducingorganisms. In organisms with external fertilisation, zygote is formed inthe external medium (usually water), whereas in those exhibiting internalfertilisation, zygote is formed inside the body of the organism.Further development of the zygote depends on the type of life cyclethe organism has and the environment it is exposed to. In organismsbelonging to fungi and algae, zygote develops a thick wall that is resistantto dessication and damage. It undergoes a period of rest beforegermination. In organisms with haplontic life cycle (As you have readin Class XI), zygote divides by meiosis to form haploid spores that growinto haploid individuals. Consult your Class XI book and find out whatkind of development takes place in the zygote in organisms with diplonticand haplo-diplontic life cycles.Zygote is the vital link that ensures continuity of speciesbetween organisms of one generation and the next. Every sexuallyreproducing organism, including human beings begin life as a singlecellâthe zygote.1.2.3.2 EmbryogenesisEmbryogenesis refers to the process of development of embryo from thezygote. During embryogenesis, zygote undergoes cell division (mitosis)and cell differentiation. While cell divisions increase the number of cellsin the developing embryo; cell differentiation helps groups of cells toundergo certain modifications to form specialised tissues and organs toform an organism. You have studied about the process of cell divisionand differentiation in the previous class.2022-2316BIOLOGYAnimals are categorised into oviparous and viviparous based onwhether the development of the zygote takes place outside the body ofthe female parent or inside, i.e., whether they lay fertilised/unfertilisedeggs or give birth to young ones. In oviparous animals like reptiles andbirds, the fertilised eggs covered by hard calcareous shell are laid in asafe place in the environment; after a period of incubation young oneshatch out. On the other hand, in viviparous animals (majority of mammalsincluding human beings), the zygote develops into a young one insidethe body of the female organism. After attaining a certain stage of growth,the young ones are delivered out of the body of the female organism.Because of proper embryonic care and protection, the chances of survivalof young ones is greater in viviparous organisms.In flowering plants, the zygote is formed inside the ovule. Afterfertilisation the sepals, petals and stamens of the flower wither and falloff. Can you name a plant in which the sepals remain attached? Thepistil however, remains attached to the plant. The zygote develops intothe embryo and the ovules develop into the seed. The ovary develops intothe fruit which develops a thick wall called pericarp that is protective infunction (Figure 1.8). After dispersal, seeds germinate under favourableconditions to produce new plants.SUMMARYReproduction enables a species to live generation after generation.Reproduction in organisms can be broadly classified into asexual andsexual reproduction. Asexual reproduction does not involve the fusionof gametes. It is common in organisms that have a relatively simpleorganisation such as the fungi, algae and some invertebrate animals.The offspring formed by asexual reproduction are identical and can bereferred to as clones. Zoospores, conidia, etc., are the most commonasexual structures formed in several algae and fungi. Buddingand gemmule formation are the common asexual methods seen inlower animals.Prokaryotes and unicellular organisms reproduce asexually bycell division or binary fission of the parent cell. In several aquatic andFigure 1.8 A few kinds of fruit showing seeds (S) andprotective pericarp (P)2022-2317REPRODUCTION IN ORGANISMSterrestrial species of angiosperms, structures such as runners,rhizomes, suckers, tubers, offsets, etc., are capable of giving rise tonew offspring. This method of asexual reproduction is generallyreferred to as vegetative propagation.Sexual reproduction involves the formation and fusion of gametes.It is a complex and slower process as compared to asexual reproduction.Most of the higher animals reproduce almost entirely by sexual method.Events of sexual reproduction may be categorised into pre-fertilisation,fertilisation and post-fertilisation events. Pre-fertilisation events includegametogenesis and gamete transfer while post-fertilisation eventsinclude the formation of zygote and embryogenesis.Organisms may be bisexual or unisexual. Sexuality in plants isvaried, particularly in angiosperms, due to the production of diversetypes of flowers. Plants are defined as monoecious and dioecious.Flowers may be bisexual or unisexual flowers.Gametes are haploid in nature and usually a direct product of meioticdivision except in haploid organisms where gametes are formed by mitosis.Transfer of male gametes is an essential event in sexual reproduction.It is relatively easy in bisexual organisms. In unisexual animals it occursby copulation or simultaneous release. In angiosperms, a special processcalled pollination ensures transfer of pollen grains which carry the pollengrains to the stigma.Syngamy (fertilisation) occurs between the male and female gametes.Syngamy may occur either externally, outside the body of organisms orinternally, inside the body. Syngamy leads to formation of a specialisedcell called zygote.The process of development of embryo from the zygote is calledembryogenesis. In animals, the zygote starts developing soon after itsformation. Animals may be either oviparous or viviparous. Embryonalprotection and care are better in viviparous organisms.In flowering plants, after fertilisation, ovary develops into fruit andovules mature into seeds. Inside the mature seed is the progenitor ofthe next generation, the embryo.EXERCISES1. Why is reproduction essential for organisms?2. Which is a better mode of reproduction: sexual or asexual? Why?3. Why is the offspring formed by asexual reproduction referred to as clone?4. Offspring formed due to sexual reproduction have better chances ofsurvival. Why? Is this statement always true?5. How does the progeny formed from asexual reproduction differ fromthose formed by sexual reproduction?6. Distinguish between asexual and sexual reproduction. Why is vegetativereproduction also considered as a type of asexual reproduction?2022-2318BIOLOGY7. What is vegetative propagation? Give two suitable examples.8. Define(a) Juvenile phase,(b) Reproductive phase,(c) Senescent phase.9. Higher organisms have resorted to sexual reproduction in spite of itscomplexity. Why?10. Explain why meiosis and gametogenesis are always interlinked?11. Identify each part in a flowering plant and write whether it is haploid(n) or diploid (2n).(a) Ovary âââââââââââ(b) Anther âââââââââââ(c) Egg âââââââââââ(d) Pollen âââââââââââ(e) Male gamete âââââââââââ(f ) Zygote âââââââââââ12. Define external fertilisation. Mention its disadvantages.13. Differentiate between a zoospore and a zygote.14. Differentiate between gametogenesis from embryogenesis.15. Describe the post-fertilisation changes in a flower.16. What is a bisexual flower? Collect five bisexual flowers from yourneighbourhood and with the help of your teacher find out their commonand scientific names.17. Examine a few flowers of any cucurbit plant and try to identify thestaminate and pistillate flowers. Do you know any other plant thatbears unisexual flowers?18. Why are offspring of oviparous animals at a greater risk as comparedto offspring of viviparous animals?2022-23Are we not lucky that plants reproduce sexually? Themyriads of flowers that we enjoy gazing at, the scents andthe perfumes that we swoon over, the rich colours thatattract us, are all there as an aid to sexual reproduction.Flowers do not exist only for us to be used for our ownselfishness. All flowering plants show sexual reproduction.A look at the diversity of structures of the inflorescences,flowers and floral parts, shows an amazing range ofadaptations to ensure formation of the end products ofsexual reproduction, the fruits and seeds. In this chapter,let us understand the morphology, structure and theprocesses of sexual reproduction in flowering plants(angiosperms).2.1 FLOWER â A FASCINATING ORGAN OFANGIOSPERMSHuman beings have had an intimate relationship withflowers since time immemorial. Flowers are objects ofaesthetic, ornamental, social, religious and cultural valueâ they have always been used as symbols for conveyingimportant human feelings such as love, affection,happiness, grief, mourning, etc. List at least five flowersof ornamental value that are commonly cultivated at2.1 Flower â A FascinatingOrgan of Angiosperms2.2 Pre-fertilisation : Structuresand Events2.3 Double Fertilisation2.4 Post-fertilisation: Structuresand Events2.5 Apomixis andPolyembryonyCHAPTER 2SEXUAL REPRODUCTION INFLOWERING PLANTS2022-2320BIOLOGYhomes and in gardens. Find out the names of five more flowers that areused in social and cultural celebrations in your family. Have you heardof floriculture â what does it refer to?To a biologist, flowers are morphological and embryological marvelsand the sites of sexual reproduction. In class XI, you have read the variousparts of a flower. Figure 2.1 will help you recall the parts of a typicalflower. Can you name the two parts in a flower in which the two mostimportant units of sexual reproduction develop?2.2 PRE-FERTILISATION: STRUCTURES AND EVENTSMuch before the actual flower is seen on a plant, the decision that the plantis going to flower has taken place. Several hormonal and structural changesare initiated which lead to the differentiation and further development ofthe floral primordium. Inflorescences are formed which bear the floral budsand then the flowers. In the flower the male and female reproductivestructures, the androecium and the gynoecium differentiate and develop.You would recollect that the androecium consists of a whorl of stamensrepresenting the male reproductive organ and the gynoecium representsthe female reproductive organ.Figure 2.1 A diagrammatic representation of L.S. of a flower2022-2321HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTS2.2.1 Stamen, Microsporangium and Pollen GrainFigure 2.2a shows the two parts of a typical stamen â the long and slenderstalk called the filament, and the terminal generally bilobed structurecalled the anther. The proximal end of the filamentis attached to the thalamus or the petal of the flower.The number and length of stamens are variable inflowers of different species. If you were to collect astamen each from ten flowers (each from differentspecies) and arrange them on a slide, you wouldbe able to appreciate the large variation in size seenin nature. Careful observation of each stamenunder a dissecting microscope and making neatdiagrams would elucidate the range in shape andattachment of anthers in different flowers.A typical angiosperm anther is bilobed witheach lobe having two theca, i.e., they are dithecous(Figure 2.2 b). Often a longitudinal groove runslengthwise separating the theca. Let usunderstand the various types of tissues and theirorganisation in the transverse section of an anther(Figure 2.3 a). The bilobed nature of an anther isvery distinct in the transverse section of the anther.The anther is a four-sided (tetragonal) structureconsisting of four microsporangia located at thecorners, two in each lobe.The microsporangia develop further andbecome pollen sacs. They extend longitudinallyall through the length of an anther and are packedwith pollen grains.Structure of microsporangium: In a transversesection, a typical microsporangium appears nearcircular in outline. It is generally surrounded by four wall layers (Figure2.3 b)â the epidermis, endothecium, middle layers and the tapetum. Theouter three wall layers perform the function of protection and help indehiscence of anther to release the pollen. The innermost wall layer isthe tapetum. It nourishes the developing pollen grains. Cells of thetapetum possess dense cytoplasm and generally have more than onenucleus. Can you think of how tapetal cells could become bi-nucleate?When the anther is young, a group of compactly arranged homogenouscells called the sporogenous tissue occupies the centre of eachmicrosporangium.Microsporogenesis : As the anther develops, the cells of the sporogenoustissue undergo meiotic divisions to form microspore tetrads. What wouldbe the ploidy of the cells of the tetrad?Figure 2.2 (a) A typical stamen;(b) threeâdimensional cut sectionof an anther(b)(a)2022-2322BIOLOGYAs each cell of the sporogenous tissue is capable of giving rise to amicrospore tetrad. Each one is a potential pollen or microspore mothercell. The process of formation of microspores from a pollen mother cell (PMC)through meiosis is called microsporogenesis. The microspores, as theyare formed, are arranged in a cluster of four cellsâthe microspore tetrad(Figure 2.3 a). As the anthers mature and dehydrate, the microsporesdissociate from each other and develop into pollen grains (Figure 2.3 b).Inside each microsporangium several thousands of microspores or pollengrains are formed that are released with the dehiscence of anther(Figure 2.3 c).Pollen grain: The pollen grains represent the male gametophytes. If youtouch the opened anthers of Hibiscus or any other flower you would finddeposition of yellowish powdery pollen grains on your fingers. Sprinklethese grains on a drop of water taken on a glass slide and observe under(c)(a) (b)Figure 2.3 (a) Transverse section of a young anther; (b) Enlarged view of one microsporangiumshowing wall layers; (c) A mature dehisced anther2022-2323HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSa microscope. You will really be amazed at the variety of architecture âsizes, shapes, colours, designs â seen on the pollen grainsfrom different species (Figure 2.4).Pollen grains are generally spherical measuring about25-50 micrometers in diameter. It has a prominent two-layeredwall. The hard outer layer called the exine is made up ofsporopollenin which is one of the most resistant organic materialknown. It can withstand high temperatures and strong acidsand alkali. No enzyme that degrades sporopollenin is so farknown. Pollen grain exine has prominent apertures called germpores where sporopollenin is absent. Pollen grains are wellpreservedas fossils because of the presence of sporopollenin.The exine exhibits a fascinating array of patterns and designs.Why do you think the exine should be hard? What is thefunction of germ pore? The inner wall of the pollen grain iscalled the intine. It is a thin and continuous layer made up ofcellulose and pectin. The cytoplasm of pollen grain issurrounded by a plasma membrane. When the pollen grain ismature it contains two cells, the vegetative cell and generativecell (Figure 2.5b). The vegetative cell is bigger, has abundantfood reserve and a large irregularly shaped nucleus. Thegenerative cell is small and floats in the cytoplasm of thevegetative cell. It is spindle shaped with dense cytoplasm anda nucleus. In over 60 per cent of angiosperms, pollen grainsare shed at this 2-celled stage. In the remaining species, thegenerative cell divides mitotically to give rise to the two malegametes before pollen grains are shed (3-celled stage).Pollen grains of many species cause severe allergies and bronchialafflictions in some people often leading to chronic respiratorydisordersâ asthma, bronchitis, etc. It may be mentioned that Partheniumor carrot grass that came into India as a contaminant with imported wheat,has become ubiquitous in occurrence and causes pollen allergy.Figure 2.5 (a) Enlarged view ofa pollen grain tetrad; (b) stagesof a microspore maturing into apollen grainFigure 2.4 Scanning electron micrographs of a few pollen grains (a)(b)2022-2324BIOLOGYWhen once they are shed, pollen grains have to land on the stigmabefore they lose viability if they have to bring about fertilisation. How longdo you think the pollen grains retain viability? The period for which pollengrains remain viable is highly variable and to some extent depends on theprevailing temperature and humidity. In some cereals such as rice andwheat, pollen grains lose viability within 30 minutes of their release, andin some members of Rosaceae, Leguminoseae and Solanaceae, theymaintain viability for months. You may have heard of storing semen/sperms of many animals including humans for artificial insemination. Itis possible to store pollen grains of a large number of species for years inliquid nitrogen (-1960C). Such stored pollen can be used as pollen banks,similar to seed banks, in crop breeding programmes.2.2.2 The Pistil, Megasporangium (ovule) and Embryo sacThe gynoecium represents the female reproductive part of the flower. Thegynoecium may consist of a single pistil (monocarpellary) or may havemore than one pistil (multicarpellary). When there are more than one,the pistils may be fused together (syncarpous) (Figure 2.7b) or may befree (apocarpous) (Figure 2.7c). Each pistil has three parts (Figure 2.7a),the stigma, style and ovary. The stigma serves as a landing platformfor pollen grains. The style is the elongated slender part beneath thestigma. The basal bulged part of the pistil is the ovary. Inside the ovaryis the ovarian cavity (locule). The placenta is located inside the ovariancavity. Recall the definition and types of placentation that you studied inFigure 2.6 Pollen productsPollen grains are rich in nutrients. It has become a fashion in recentyears to use pollen tablets as food supplements. In western countries, alarge number of pollen products in the form of tablets and syrups areavailable in the market. Pollen consumption has been claimed to increasethe performance of athletes and race horses (Figure 2.6).2022-2325HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSClass XI. Arising from the placenta are the megasporangia, commonlycalled ovules. The number of ovules in an ovary may be one (wheat,paddy, mango) to many (papaya, water melon, orchids).The Megasporangium (Ovule) : Let us familiarise ourselves with thestructure of a typical angiosperm ovule (Figure 2.7d). The ovule is a smallstructure attached to the placenta by means of a stalk called funicle.The body of the ovule fuses with funicle in the region called hilum. Thus,hilum represents the junction between ovule and funicle. Each ovule hasone or two protective envelopes called integuments. Integuments encirclethe nucellus except at the tip where a small opening called the micropyleis organised. Opposite the micropylar end, is the chalaza, representingthe basal part of the ovule.Enclosed within the integuments is a mass of cells called the nucellus.Cells of the nucellus have abundant reserve food materials. Located in thenucellus is the embryo sac or female gametophyte. An ovule generally hasa single embryo sac formed from a megaspore.Megasporogenesis : The process of formation of megaspores from themegaspore mother cell is called megasporogenesis. Ovules generallydifferentiate a single megaspore mother cell (MMC) in the micropylar regionStigmaStyleOvaryThalamusFigure 2.7 (a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed);(b) Multicarpellary, syncarpous pistil of Papaver; (c) A multicarpellary, apocarpousgynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule(a) (b) (c) (d)2022-2326BIOLOGYFigure 2.8 (a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad ofmegaspores; (b) 2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac; (c) Adiagrammatic representation of the mature embryo sac.(a)(b)(c)of the nucellus. It is a large cell containing dense cytoplasm and aprominent nucleus. The MMC undergoes meiotic division. What is theimportance of the MMC undergoing meiosis? Meiosis results in theproduction of four megaspores (Figure 2.8a).Female gametophyte : In a majority of flowering plants, one of themegaspores is functional while the other three degenerate. Only thefunctional megaspore develops into the female gametophyte (embryosac). This method of embryo sac formation from a single megaspore is termedmonosporic development. What will be the ploidy of the cells of the nucellus,MMC, the functional megaspore and female gametophyte?2022-2327HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSLet us study formation of the embryo sac in a little more detail.(Figure 2.8b). The nucleus of the functional megaspore divides mitoticallyto form two nuclei which move to the opposite poles, forming the2-nucleate embryo sac. Two more sequential mitotic nuclear divisionsresult in the formation of the 4-nucleate and later the 8-nucleate stagesof the embryo sac. It is of interest to note that these mitotic divisions arestrictly free nuclear, that is, nuclear divisions are not followed immediatelyby cell wall formation. After the 8-nucleate stage, cell walls are laid downleading to the organisation of the typical female gametophyteor embryo sac. Observe the distribution of cells inside the embryo sac(Figure 2.8b, c). Six of the eight nuclei are surrounded by cell walls andorganised into cells; the remaining two nuclei, called polar nuclei aresituated below the egg apparatus in the large central cell.There is a characteristic distribution of the cells within the embryosac. Three cells are grouped together at the micropylar end and constitutethe egg apparatus. The egg apparatus, in turn, consists of two synergidsand one egg cell. The synergids have special cellular thickenings at themicropylar tip called filiform apparatus, which play an important role inguiding the pollen tubes into the synergid. Three cells are at the chalazalend and are called the antipodals. The large central cell, as mentionedearlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, atmaturity, though 8-nucleate is 7-celled.2.2.3 PollinationIn the preceding sections you have learnt that the male and female gametesin flowering plants are produced in the pollen grain and embryo sac,respectively. As both types of gametes are non-motile, they have to bebrought together for fertilisation to occur. How is this achieved?Pollination is the mechanism to achieve this objective. Transferof pollen grains (shed from the anther) to the stigma of a pistil istermed pollination. Flowering plants have evolved an amazing arrayof adaptations to achieve pollination. They make use of externalagents to achieve pollination. Can you list the possible externalagents?Kinds of Pollination : Depending on the source of pollen, pollinationcan be divided into three types.(i) Autogamy : In this type, pollination is achieved within the sameflower. Transfer of pollen grains from the anther to the stigma of thesame flower (Figure 2.9a). In a normal flower which opens andexposes the anthers and the stigma, complete autogamy is ratherrare. Autogamy in such flowers requires synchrony in pollen releaseand stigma receptivity and also, the anthers and the stigma should2022-2328BIOLOGYlie close to each other so that self-pollinationcan occur. Some plants such as Viola(common pansy), Oxalis, and Commelinaproduce two types of flowers âchasmogamous flowers which are similar toflowers of other species with exposed anthersand stigma, and cleistogamous flowers whichdo not open at all (Figure 2.9c). In such flowers,the anthers and stigma lie close to each other.When anthers dehisce in the flower buds,pollen grains come in contact with the stigmato effect pollination. Thus, cleistogamousflowers are invariably autogamous as there isno chance of cross-pollen landing on thestigma. Cleistogamous flowers produceassured seed-set even in the absence ofpollinators. Do you think that cleistogamy isadvantageous or disadvantageous to theplant? Why?(ii) Geitonogamy â Transfer of pollen grains fromthe anther to the stigma of another flower ofthe same plant. Although geitonogamy isfunctionally cross-pollination involving apollinating agent, genetically it is similar toautogamy since the pollen grains come fromthe same plant.(iii) Xenogamy â Transfer of pollen grains fromanther to the stigma of a different plant (Figure2.9b). This is the only type of pollination whichduring pollination brings genetically differenttypes of pollen grains to the stigma.Agents of Pollination : Plants use two abiotic (windand water) and one biotic (animals) agents to achievepollination. Majority of plants use biotic agents forpollination. Only a small proportion of plants useabiotic agents. Pollen grains coming in contact withthe stigma is a chance factor in both wind and waterpollination. To compensate for this uncertainties andassociated loss of pollen grains, the flowers produceenormous amount of pollen when compared to thenumber of ovules available for pollination.Figure 2.9 (a) Self-pollinated flowers;(b) Cross pollinated flowers;(c) Cleistogamous flowers(a)(c)(b)2022-2329HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSPollination by wind is more commonamongst abiotic pollinations. Wind pollinationalso requires that the pollen grains are lightand non-sticky so that they can betransported in wind currents. They oftenpossess well-exposed stamens (so that thepollens are easily dispersed into wind currents,Figure 2.10) and large often-feathery stigmato easily trap air-borne pollen grains. Windpollinatedflowers often have a single ovule ineach ovary and numerous flowers packed intoan inflorescence; a familiar example is the corncob â the tassels you see are nothing but thestigma and style which wave in the wind totrap pollen grains. Wind-pollination is quitecommon in grasses.Pollination by water is quite rare inflowering plants and is limited to about 30genera, mostly monocotyledons. As againstthis, you would recall that water is a regularmode of transport for the male gametes amongthe lower plant groups such as algae,bryophytes and pteridophytes. It is believed,particularly for some bryophytes andpteridophytes, that their distribution is limitedbecause of the need for water for the transportof male gametes and fertilisation. Someexamples of water pollinated plants are Vallisneria and Hydrilla whichgrow in fresh water and several marine sea-grasses such as Zostera. Notall aquatic plants use water for pollination. In a majority of aquatic plantssuch as water hyacinth and water lily, the flowers emerge above the levelof water and are pollinated by insects or wind as in most of the landplants. In Vallisneria, the female flower reach the surface of water by thelong stalk and the male flowers or pollen grains are released on to thesurface of water. They are carried passively by water currents (Figure2.11a); some of them eventually reach the female flowers and the stigma.In another group of water pollinated plants such as seagrasses, femaleflowers remain submerged in water and the pollen grains are releasedinside the water. Pollen grains in many such species are long, ribbon likeand they are carried passively inside the water; some of them reach thestigma and achieve pollination. In most of the water-pollinated species,pollen grains are protected from wetting by a mucilaginous covering.Both wind and water pollinated flowers are not very colourful and donot produce nectar. What would be the reason for this?Figure 2.10 A wind-pollinated plant showingcompact inflorecence and wellexposedstamens2022-2330BIOLOGYMajority of flowering plants usea range of animals as pollinatingagents. Bees, butterflies, flies,beetles, wasps, ants, moths, birds(sunbirds and humming birds) andbats are the common pollinatingagents. (Figure 2.11b). Among theanimals, insects, particularly beesare the dominant biotic pollinatingagents. Even larger animals suchas some primates (lemurs), arboreal(tree-dwelling) rodents, or evenreptiles (gecko lizard and gardenlizard) have also been reported aspollinators in some species.Often flowers of animalpollinatedplants are specificallyadapted for a particular species ofanimal.Majority of insect-pollinatedflowers are large, colourful, fragrantand rich in nectar. When the flowersare small, a number of flowers areclustered into an inflorescence tomake them conspicuous. Animalsare attracted to flowers by colourand/or fragrance. The flowerspollinated by flies and beetlessecrete foul odours to attract theseanimals. To sustain animal visits,the flowers have to provide rewardsto the animals. Nectar and pollengrains are the usual floral rewards.For harvesting the reward(s) fromthe flower the animal visitor comesin contact with the anthers and thestigma. The body of the animal getsa coating of pollen grains, which aregenerally sticky in animal pollinated flowers. When the animal carryingpollen on its body comes in contact with the stigma, it brings aboutpollination.In some species floral rewards are in providing safe places to lay eggs;an example is that of the tallest flower of Amorphophallus (the floweritself is about 6 feet in height). A similar relationship exists between aspecies of moth and the plant Yucca where both species â moth and the(a)(b)Figure 2.11 (a) Pollination by water in Vallisneria;(b) Insect pollination2022-2331HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSplant â cannot complete their life cycles without each other. The mothdeposits its eggs in the locule of the ovary and the flower, in turn, getspollinated by the moth. The larvae of the moth come out of the eggs asthe seeds start developing.Why donât you observe some flowers of the following plants (or anyothers available to you): Cucumber, Mango, Peepal, Coriander, Papaya,Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try tofind out which animals visit them and whether they could bepollinators.Youâll have to patiently observe the flowers over a few daysand at different times of the day. You could also try to see whether thereis any correlation in the characteristics of a flower to the animal thatvisits it. Carefully observe if any of the visitors come in contact with theanthers and the stigma as only such visitors can bring about pollination.Many insects may consume pollen or the nectar without bringing aboutpollination. Such floral visitors are referred to as pollen/nectar robbers.You may or may not be able to identify the pollinators, but you will surelyenjoy your efforts!Outbreeding Devices : Majority of flowering plants produce hermaphroditeflowers and pollen grains are likely to come in contact with the stigma ofthe same flower. Continued self-pollination result in inbreeding depression.Flowering plants have developed many devices to discourage selfpollinationand to encourage cross-pollination. In some species, pollenrelease and stigma receptivity are not synchronised. Either the pollen isreleased before the stigma becomes receptive or stigma becomes receptivemuch before the release of pollen. In some other species, the anther andstigma are placed at different positions so that the pollen cannot come incontact with the stigma of the same flower. Both these devices preventautogamy. The third device to prevent inbreeding is self-incompatibility.This is a genetic mechanism and prevents self-pollen (from the same floweror other flowers of the same plant) from fertilising the ovules by inhibitingpollen germination or pollen tube growth in the pistil. Another device toprevent self-pollination is the production of unisexual flowers. If both maleand female flowers are present on the same plant such as castor and maize(monoecious), it prevents autogamy but not geitonogamy. In several speciessuch as papaya, male and female flowers are present on different plants,that is each plant is either male or female (dioecy). This condition preventsboth autogamy and geitonogamy.Pollen-pistil Interaction : Pollination does not guarantee the transferof the right type of pollen (compatible pollen of the same species as thestigma). Often, pollen of the wrong type, either from other species or fromthe same plant (if it is self-incompatible), also land on the stigma. Thepistil has the ability to recognise the pollen, whether it is of the right type(compatible) or of the wrong type (incompatible). If it is of the right type,the pistil accepts the pollen and promotes post-pollination events that2022-2332BIOLOGYleads to fertilisation. If the pollen is of the wrong type, the pistil rejects thepollen by preventing pollen germination on the stigma or the pollen tubegrowth in the style. The ability of the pistil to recognise the pollen followedby its acceptance or rejection is the result of a continuous dialoguebetween pollen grain and the pistil. This dialogue is mediated by chemicalcomponents of the pollen interacting with those of the pistil. It is only inrecent years that botanists have been able to identify some of the pollenand pistil components and the interactions leading to the recognition,followed by acceptance or rejection.As mentioned earlier, following compatible pollination, the pollen graingerminates on the stigma to produce a pollen tube through one of thegerm pores (Figure 2.12a). The contents of the pollen grain move into theFigure 2.12 (a) Pollen grains germinating on the stigma; (b) Pollen tubes growing through thestyle; (c) L.S. of pistil showing path of pollen tube growth; (d) enlarged view of anegg apparatus showing entry of pollen tube into a synergid; (e) Discharge of malegametes into a synergid and the movements of the sperms, one into the egg andthe other into the central cell(d) (e)(a) (b) (c)2022-2333HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSpollen tube. Pollen tube grows through the tissues of the stigma andstyle and reaches the ovary (Figure 2.12b, c). You would recall that insome plants, pollen grains are shed at two-celled condition (a vegetativecell and a generative cell). In such plants, the generative cell divides andforms the two male gametes during the growth of pollen tube in the stigma.In plants which shed pollen in the three-celled condition, pollen tubescarry the two male gametes from the beginning. Pollen tube, after reachingthe ovary, enters the ovule through the micropyle and then enters one ofthe synergids through the filiform apparatus (Figure 2.12d, e). Many recentstudies have shown that filiform apparatus present at the micropylar partof the synergids guides the entry of pollen tube. All these eventsâfrompollen deposition on the stigma until pollen tubes enter the ovuleâaretogether referred to as pollen-pistil interaction. As pointed out earlier,pollen-pistil interaction is a dynamic process involving pollen recognitionfollowed by promotion or inhibition of the pollen. The knowledge gainedin this area would help the plant breeder in manipulating pollen-pistilinteraction, even in incompatible pollinations, to get desired hybrids.You can easily study pollen germination by dusting some pollen fromflowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slidecontaining a drop of sugar solution (about 10 per cent). After about 15â30minutes, observe the slide under the low power lens of the microscope. Youare likely to see pollen tubes coming out of the pollen grains.As you shall learn in the chapter on plant breeding (Chapter 9), abreeder is interested in crossing different species and often genera tocombine desirable characters to produce commercially âsuperiorâ varieties.Artificial hybridisation is one of the major approaches of cropimprovement programme. In such crossing experiments it is importantto make sure that only the desired pollen grains are used for pollinationand the stigma is protected from contamination (from unwanted pollen).This is achieved by emasculation and bagging techniques.If the female parent bears bisexual flowers, removal of anthers fromthe flower bud before the anther dehisces using a pair of forceps isnecessary. This step is referred to as emasculation. Emasculated flowershave to be covered with a bag of suitable size, generally made up of butterpaper, to prevent contamination of its stigma with unwanted pollen. Thisprocess is called bagging. When the stigma of bagged flower attainsreceptivity, mature pollen grains collected from anthers of the male parentare dusted on the stigma, and the flowers are rebagged, and the fruitsallowed to develop.If the female parent produces unisexual flowers, there is no need foremasculation. The female flower buds are bagged before the flowers open.When the stigma becomes receptive, pollination is carried out using thedesired pollen and the flower rebagged.2022-2334BIOLOGY2.4 POST-FERTILISATION : STRUCTURES AND EVENTSFollowing double fertilisation, events of endosperm and embryodevelopment, maturation of ovule(s) into seed(s) and ovary into fruit, arecollectively termed post-fertilisation events.2.4.1 EndospermEndosperm development precedes embryo development. Why? Theprimary endosperm cell divides repeatedly and forms a triploidFigure 2.13 (a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN);(b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)](a) (b)2.3 DOUBLE FERTILISATIONAfter entering one of the synergids, the pollen tube releases the two malegametes into the cytoplasm of the synergid. One of the male gametesmoves towards the egg cell and fuses with its nucleus thus completing thesyngamy. This results in the formation of a diploid cell, the zygote. Theother male gamete moves towards the two polar nuclei located in the centralcell and fuses with them to produce a triploid primary endosperm nucleus(PEN) (Figure 2.13a). As this involves the fusion of three haploid nuclei itis termed triple fusion. Since two types of fusions, syngamy and triplefusion take place in an embryo sac the phenomenon is termed doublefertilisation, an event unique to flowering plants. The central cell aftertriple fusion becomes the primary endosperm cell (PEC) and developsinto the endosperm while the zygote develops into an embryo.2022-2335HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSendosperm tissue. The cells of this tissue are filled withreserve food materials and are used for the nutrition ofthe developing embryo. In the most common type ofendosperm development, the PEN undergoes successivenuclear divisions to give rise to free nuclei. This stage ofendosperm development is called free-nuclear endosperm.Subsequently cell wall formation occurs and theendosperm becomes cellular. The number of free nucleiformed before cellularisation varies greatly. The coconutwater from tender coconut that you are familiar with, isnothing but free-nuclear endosperm (made up ofthousands of nuclei) and the surrounding white kernel isthe cellular endosperm.Endosperm may either be completely consumed by thedeveloping embryo (e.g., pea, groundnut, beans) before seedmaturation or it may persist in the mature seed (e.g. castorand coconut) and be used up during seed germination. Splitopen some seeds of castor, peas, beans, groundnut, fruit ofcoconut and look for the endosperm in each case. Find outwhether the endosperm is persistent in cereals â wheat, riceand maize.2.4.2 EmbryoEmbryo develops at the micropylar end of the embryo sac wherethe zygote is situated. Most zygotes divide only after certainamount of endosperm is formed. This is an adaptation toprovide assured nutrition to the developing embryo. Thoughthe seeds differ greatly, the early stages of embryo development(embryogeny) are similar in both monocotyledons anddicotyledons. Figure 2.13 depicts the stages of embryogeny ina dicotyledonous embryo. The zygote gives rise to theproembryo and subsequently to the globular, heart-shapedand mature embryo.A typical dicotyledonous embryo (Figure 2.14a), consistsof an embryonal axis and two cotyledons. The portion ofembryonal axis above the level of cotyledons is the epicotyl,which terminates with the plumule or stem tip. The cylindricalportion below the level of cotyledons is hypocotyl thatterminates at its lower end in the radicle or root tip. The roottip is covered with a root cap.Embryos of monocotyledons (Figure 2.14 b) possess onlyone cotyledon. In the grass family the cotyledon is calledscutellum that is situated towards one side (lateral) of theembryonal axis. At its lower end, the embryonal axis has theFigure 2.14 (a) A typical dicotembryo; (b) L.S. of anembryo of grass(a)(b)2022-2336BIOLOGYradical and root cap enclosed in an undifferentiated sheath calledcoleorrhiza. The portion of the embryonal axis above the level ofattachment of scutellum is the epicotyl. Epicotyl has a shoot apex and afew leaf primordia enclosed in a hollow foliar structure, the coleoptile.Soak a few seeds in water (say of wheat, maize, peas, chickpeas,ground nut) overnight. Then split the seeds and observe the variousparts of the embryo and the seed.2.4.3 SeedIn angiosperms, the seed is the final product of sexual reproduction. It isoften described as a fertilised ovule. Seeds are formed inside fruits. Aseed typically consists of seed coat(s), cotyledon(s) and an embryo axis.The cotyledons (Figure 2.15a) of the embryo are simple structures,generally thick and swollen due to storage of food reserves (as in legumes).Mature seeds may be non-albuminous or ex-albuminous. Nonalbuminousseeds have no residual endosperm as it is completelyconsumed during embryo development (e.g., pea, groundnut).Albuminous seeds retain a part of endosperm as it is not completely usedup during embryo development (e.g., wheat, maize, barley, castor).Occasionally, in some seeds such as black pepper and beet, remnants ofnucellus are also persistent. This residual, persistent nucellus is theperisperm.Integuments of ovules harden as tough protective seed coats(Figure 2.15a). The micropyle remains as a small pore in the seed coat.This facilitates entry of oxygen and water into the seed during germination.As the seed matures, its water content is reduced and seeds becomerelatively dry (10-15 per cent moisture by mass). The general metabolicactivity of the embryo slows down. The embryo may enter a state ofinactivity called dormancy, or if favourable conditions are available(adequate moisture, oxygen and suitable temperature), they germinate.As ovules mature into seeds, the ovary develops into a fruit, i.e., thetransformation of ovules into seeds and ovary into fruit proceedssimultaneously. The wall of the ovary develops into the wall of fruit calledpericarp. The fruits may be fleshy as in guava, orange, mango, etc., ormay be dry, as in groundnut, and mustard, etc. Many fruits have evolvedmechanisms for dispersal of seeds. Recall the classification of fruits andtheir dispersal mechanisms that you have studied in an earlier class. Isthere any relationship between number of ovules in an ovary and thenumber of seeds present in a fruit?In most plants, by the time the fruit develops from the ovary, otherfloral parts degenerate and fall off. However, in a few species such as apple,strawberry, cashew, etc., the thalamus also contributes to fruit formation.Such fruits are called false fruits (Figure 2.15b). Most fruits howeverdevelop only from the ovary and are called true fruits. Although in mostof the species, fruits are the results of fertilisation, there are a few species2022-2337HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSin which fruits develop without fertilisation. Such fruits are calledparthenocarpic fruits. Banana is one such example. Parthenocarpy canbe induced through the application of growth hormones and such fruitsare seedless.Seeds offer several advantages to angiosperms. Firstly, sincereproductive processes such as pollination and fertilisation areindependent of water, seed formation is more dependable. Also seeds havebetter adaptive strategies for dispersal to new habitats and help the speciesFigure 2.15 (a) Structure of some seeds. (b) False fruits of apple and strawberry(b)(a)2022-2338BIOLOGYto colonise in other areas. As they have sufficient food reserves, youngseedlings are nourished until they are capable of photosynthesis on theirown. The hard seed coat provides protection to the young embryo. Beingproducts of sexual reproduction, they generate new genetic combinationsleading to variations.Seed is the basis of our agriculture. Dehydration and dormancy ofmature seeds are crucial for storage of seeds which can be used as foodthroughout the year and also to raise crop in the next season. Can youimagine agriculture in the absence of seeds, or in the presence of seedswhich germinate straight away soon after formation and cannot be stored?How long do the seeds remain alive after they are dispersed? Thisperiod again varies greatly. In a few species the seeds lose viability withina few months. Seeds of a large number of species live for several years.Some seeds can remain alive for hundreds of years. There are severalrecords of very old yet viable seeds. The oldest is that of a lupine, Lupinusarcticus excavated from Arctic Tundra. The seed germinated and floweredafter an estimated record of 10,000 years of dormancy. A recent record of2000 years old viable seed is of the date palm, Phoenix dactyliferadiscovered during the archeological excavation at King Herodâs palacenear the Dead Sea.After completing a brief account of sexual reproduction of floweringplants it would be worth attempting to comprehend the enormousreproductive capacity of some flowering plants by asking the followingquestions: How many eggs are present in an embryo sac? How manyembryo sacs are present in an ovule? How many ovules are present inan ovary? How many ovaries are present in a typical flower? How manyflowers are present on a tree? And so on...Can you think of some plants in which fruits contain very largenumber of seeds. Orchid fruits are one such category and each fruitcontain thousands of tiny seeds. Similar is the case in fruits of someparasitic species such as Orobanche and Striga. Have you seen a tinyseed of Ficus? How large is the tree of Ficus developed from that tinyseed. How many billions of seeds does each Ficus tree produce? Canyou imagine any other example in which such a tiny structure canproduce such a large biomass over the years?2.5 APOMIXIS AND POLYEMBRYONYAlthough seeds, in general are the products of fertilisation, a few floweringplants such as some species of Asteraceae and grasses, have evolved aspecial mechanism, to produce seeds without fertilisation, called apomixis.What is fruit production without fertilisation called? Thus, apomixis is aform of asexual reproduction that mimics sexual reproduction. There areseveral ways of development of apomictic seeds. In some species, thediploid egg cell is formed without reduction division and develops intothe embryo without fertilisation. More often, as in many Citrus and Mango2022-2339HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTSSUMMARYFlowers are the seat of sexual reproduction in angiosperms. In the flower,androecium consisting of stamens represents the male reproductiveorgans and gynoecium consisting of pistils represents the femalereproductive organs.A typical anther is bilobed, dithecous and tetrasporangiate. Pollengrains develop inside the microsporangia. Four wall layers, theepidermis, endothecium, middle layers and the tapetum surround themicrosporangium. Cells of the sporogenous tissue lying in the centre ofthe microsporangium, undergo meiosis (microsporogenesis) to formtetrads of microspores. Individual microspores mature into pollen grains.Pollen grains represents the male gametophytic generation. Thepollen grains have a two-layered wall, the outer exine and inner intine.The exine is made up of sporopollenin and has germ pores. Pollen grainsmay have two cells (a vegetative cell and generative cell) or three cells (avegetative cell and two male gametes) at the time of shedding.The pistil has three parts â the stigma, style and the ovary. Ovulesare present in the ovary. The ovules have a stalk called funicle, protectiveintegument(s), and an opening called micropyle. The central tissue isthe nucellus in which the archesporium differentiates. A cell of thearchesporium, the megaspore mother cell divides meiotically and one ofthe megaspores forms the embryo sac (the female gametophyte). Themature embryo sac is 7-celled and 8-nucleate. At the micropylar end isvarieties some of the nucellar cells surrounding the embryo sac startdividing, protrude into the embryo sac and develop into the embryos. Insuch species each ovule contains many embryos. Occurrence of morethan one embryo in a seed is referred to as polyembryony. Take outsome seeds of orange and squeeze them. Observe the many embryos ofdifferent sizes and shapes from each seed. Count the number of embryosin each seed. What would be the genetic nature of apomictic embryos?Can they be called clones?Hybrid varieties of several of our food and vegetable crops are beingextensively cultivated. Cultivation of hybrids has tremendously increasedproductivity. One of the problems of hybrids is that hybrid seeds haveto be produced every year. If the seeds collected from hybrids are sown,the plants in the progeny will segregate and do not maintain hybridcharacters. Production of hybrid seeds is costly and hence the cost ofhybrid seeds become too expensive for the farmers. If these hybrids aremade into apomicts, there is no segregation of characters in the hybridprogeny. Then the farmers can keep on using the hybrid seeds to raisenew crop year after year and he does not have to buy hybrid seeds everyyear. Because of the importance of apomixis in hybrid seed industry,active research is going on in many laboratories around the world tounderstand the genetics of apomixis and to transfer apomictic genesinto hybrid varieties.2022-2340BIOLOGYEXERCISES1. Name the parts of an angiosperm flower in which development of maleand female gametophyte take place.2. Differentiate between microsporogenesis and megasporogenesis. Whichtype of cell division occurs during these events? Name the structuresformed at the end of these two events.3. Arrange the following terms in the correct developmental sequence:Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell,male gametes.4. With a neat, labelled diagram, describe the parts of a typical angiospermovule.5. What is meant by monosporic development of female gametophyte?6. With a neat diagram explain the 7-celled, 8-nucleate nature of the femalegametophyte.the egg apparatus consisting of two synergids and an egg cell. At thechalazal end are three antipodals. At the centre is a large central cellwith two polar nuclei.Pollination is the mechanism to transfer pollen grains from theanther to the stigma. Pollinating agents are either abiotic (wind andwater) or biotic (animals).Pollen-pistil interaction involves all events from the landing of pollengrains on the stigma until the pollen tube enters the embryo sac (whenthe pollen is compatible) or pollen inhibition (when the pollen isincompatible). Following compatible pollination, pollen grain germinateson the stigma and the resulting pollen tube grow through the style,enter the ovules and finally discharges two male gametes in one of thesynergids. Angiosperms exhibit double fertilisation because two fusionevents occur in each embryo sac, namely syngamy and triple fusion.The products of these fusions are the diploid zygote and the triploidprimary endosperm nucleus (in the primary endosperm cell). Zygotedevelops into the embryo and the primary endosperm cell forms theendosperm tissue. Formation of endosperm always precedesdevelopment of the embryo.The developing embryo passes through different stages such asthe proembryo, globular and heart-shaped stages before maturation.Mature dicotyledonous embryo has two cotyledons and an embryonalaxis with epicotyl and hypocotyl. Embryos of monocotyledons have asingle cotyledon. After fertilisation, ovary develops into fruit and ovulesdevelop into seeds.A phenomenon called apomixis is found in some angiosperms,particularly in grasses. It results in the formation of seeds withoutfertilisation. Apomicts have several advantages in horticulture andagriculture.Some angiosperms produce more than one embryo in their seed.This phenomenon is called polyembryony.2022-2341HUMAN REPRODUCTION SEXUAL REPRODUCTION IN FLOWERING PLANTS7. What are chasmogamous flowers? Can cross-pollination occur incleistogamous flowers? Give reasons for your answer.8. Mention two strategies evolved to prevent self-pollination in flowers.9. What is self-incompatibility? Why does self-pollination not lead to seedformation in self-incompatible species?10. What is bagging technique? How is it useful in a plant breedingprogramme?11. What is triple fusion? Where and how does it take place? Name thenuclei involved in triple fusion.12. Why do you think the zygote is dormant for sometime in a fertilisedovule?13. Differentiate between:(a) hypocotyl and epicotyl;(b) coleoptile and coleorrhiza;(c) integument and testa;(d) perisperm and pericarp.14. Why is apple called a false fruit? Which part(s) of the flower forms thefruit?15. What is meant by emasculation? When and why does a plant breederemploy this technique?16. If one can induce parthenocarpy through the application of growthsubstances, which fruits would you select to induce parthenocarpyand why?17. Explain the role of tapetum in the formation of pollen-grain wall.18. What is apomixis and what is its importance?2022-23As you are aware, humans are sexually reproducing andviviparous. The reproductive events in humans includeformation of gametes (gametogenesis), i.e., sperms in malesand ovum in females, transfer of sperms into the femalegenital tract (insemination) and fusion of male and femalegametes (fertilisation) leading to formation of zygote. Thisis followed by formation and development of blastocystand its attachment to the uterine wall (implantation),embryonic development (gestation) and delivery of thebaby (parturition). You have learnt that these reproductiveevents occur after puberty. There are remarkabledifferences between the reproductive events in the maleand in the female, for example, sperm formation continueseven in old men, but formation of ovum ceases in womenaround the age of fifty years. Let us examine the male andfemale reproductive systems in human.3.1 THE MALE REPRODUCTIVE SYSTEMThe male reproductive system is located in the pelvis region(Figure 3.1a). It includes a pair of testes alongwithaccessory ducts, glands and the external genitalia.CHAPTER 3HUMAN REPRODUCTION3.1 The Male ReproductiveSystem3.2 The Female ReproductiveSystem3.3 Gametogenesis3.4 Menstrual Cycle3.5 Fertilisation andImplantation3.6 Pregnancy andEmbryonic Development3.7 Parturition and Lactation2022-23The testes are situated outside theabdominal cavity within a pouchcalled scrotum. The scrotum helpsin maintaining the low temperatureof the testes (2â2.5o C lower thanthe normal internal bodytemperature) necessary forspermatogenesis. In adults, eachtestis is oval in shape, with a lengthof about 4 to 5 cm and a width ofabout 2 to 3 cm. The testis iscovered by a dense covering. Eachtestis has about 250 compartmentscalled testicular lobules(Figure 3.1b).Each lobule contains one tothree highly coiled seminiferoustubules in which sperms areproduced. Each seminiferous tubuleis lined on its inside by two typesof cells called male germ cells(spermatogonia) and Sertoli cells(Figure 3.2 ). The male germ cellsundergo meiotic divisions finallyleading to sperm formation, whileSertoli cells provide nutrition to thegerm cells. The regions outside theseminiferous tubules calledinterstitial spaces, contain smallblood vessels and interstitial cellsor Leydig cells (Figure 3.2). Leydigcells synthesise and secretetesticular hormones calledandrogens. Other immunologicallycompetent cells are also present.The male sex accessory ducts include rete testis, vasa efferentia,epididymis and vas deferens (Figure 3.1b). The seminiferous tubules ofthe testis open into the vasa efferentia through rete testis. The vasa efferentialeave the testis and open into epididymis located along the posterior surfaceof each testis. The epididymis leads to vas deferens that ascends to theabdomen and loops over the urinary bladder. It receives a duct from seminalvesicle and opens into urethra as the ejaculatory duct (Figure 3.1a). Theseducts store and transport the sperms from the testis to the outside throughurethra. The urethra originates from the urinary bladder and extendsthrough the penis to its external opening called urethral meatus.Figure 3.1(a) Diagrammatic sectional view of male pelvisshowing reproductive systemFigure 3.1(b) Diagrammatic view of male reproductive system(part of testis is open to show inner details)43HUMAN REPRODUCTION2022-2344BIOLOGYThe penis is the male external genitalia (Figure 3.1a, b). It is made upof special tissue that helps in erection of the penis to facilitate insemination.The enlarged end of penis called the glans penis is covered by a loose foldof skin called foreskin.The male accessory glands (Figure 3.1a, b) include paired seminalvesicles, a prostate and paired bulbourethral glands. Secretions of theseglands constitute the seminal plasma which is rich in fructose, calciumand certain enzymes. The secretions of bulbourethral glands also helpsin the lubrication of the penis.3.2 THE FEMALE REPRODUCTIVE SYSTEMThe female reproductive system consists of a pair of ovaries alongwith a pairof oviducts, uterus, cervix, vagina and the external genitalia located inpelvic region (Figure 3.3a). These parts of the system alongwith a pair of themammary glands are integrated structurally and functionally to supportthe processes of ovulation, fertilisation, pregnancy, birth and child care.Ovaries are the primary female sex organs that produce the femalegamete (ovum) and several steroid hormones (ovarian hormones).The ovaries are located one on each side of the lower abdomen(Figure 3.3b). Each ovary is about 2 to 4 cm in length and is connected tothe pelvic wall and uterus by ligaments. Each ovary is covered by a thinepithelium which encloses the ovarian stroma. The stroma is divided intotwo zones â a peripheral cortex and an inner medulla.Figure 3.2 Diagrammatic sectional view of seminiferous tubule2022-2345HUMAN REPRODUCTIONThe oviducts (fallopian tubes), uterus and vagina constitute the femaleaccessory ducts. Each fallopian tube is about 10-12 cm long and extendsfrom the periphery of each ovary to the uterus (Figure 3.3b), the part closerto the ovary is the funnel-shaped infundibulum. The edges of theinfundibulum possess finger-like projections called fimbriae, which help incollection of the ovum after ovulation. The infundibulum leads to a widerFigure 3.3 (b) Diagrammatic sectional view of the female reproductive systemFigure 3.3 (a) Diagrammatic sectional view of female pelvis showingreproductive system2022-2346BIOLOGYpart of the oviduct called ampulla. The last part of the oviduct, isthmus hasa narrow lumen and it joins the uterus.The uterus is single and it is also called womb. The shape of the uterusis like an inverted pear. It is supported by ligaments attached to the pelvicwall. The uterus opens into vagina through a narrow cervix. The cavity ofthe cervix is called cervical canal (Figure 3.3b) which alongwith vaginaforms the birth canal. The wall of the uterus has three layers of tissue. Theexternal thin membranous perimetrium, middle thick layer of smoothmuscle, myometrium and inner glandular layer called endometrium thatlines the uterine cavity. The endometrium undergoes cyclical changes duringmenstrual cycle while the myometrium exhibits strong contraction duringdelivery of the baby.The female external genitalia include mons pubis, labia majora, labiaminora, hymen and clitoris (Figure 3.3a). Mons pubis is a cushion of fattytissue covered by skin and pubic hair. The labia majora are fleshy folds oftissue, which extend down from the mons pubis and surround the vaginalopening. The labia minora are paired folds of tissue under the labia majora.The opening of the vagina is often covered partially by a membrane calledhymen. The clitoris is a tiny finger-like structure which lies at the upperjunction of the two labia minora above the urethral opening. The hymen isoften torn during the first coitus (intercourse). However, it can also be brokenby a sudden fall or jolt, insertion of a vaginal tampon, active participationin some sports like horseback riding, cycling, etc. In some women the hymenpersists even after coitus. In fact, the presence or absence of hymen is nota reliable indicator of virginity or sexual experience.Figure 3.4 A diagrammatic sectional view of Mammary gland2022-2347HUMAN REPRODUCTIONA functional mammary gland is characteristic of all female mammals.The mammary glands are paired structures (breasts) that containglandular tissue and variable amount of fat. The glandular tissue of eachbreast is divided into 15-20 mammary lobes containing clusters of cellscalled alveoli (Figure 3.4). The cells of alveoli secrete milk, which is storedin the cavities (lumens) of alveoli. The alveoli open into mammary tubules.The tubules of each lobe join to form a mammary duct. Several mammaryducts join to form a wider mammary ampulla which is connected tolactiferous duct through which milk is sucked out.3.3 GAMETOGENESISThe primary sex organs â the testis in the males and the ovaries in thefemalesâproduce gametes, i.e, sperms and ovum, respectively, by theprocess called gametogenesis. In testis, the immature male germ cells(spermatogonia) produce sperms by spermatogenesis that begins atpuberty. The spermatogonia (sing. spermatogonium) present on theinside wall of seminiferous tubules multiply by mitotic division andincrease in numbers. Each spermatogonium is diploid and contains 46chromosomes. Some of the spermatogonia called primaryspermatocytes periodically undergo meiosis. A primary spermatocytecompletes the first meiotic division (reduction division) leading toformation of two equal, haploid cells calledsecondary spermatocytes, which have only23 chromosomes each. The secondaryspermatocytes undergo the second meioticdivision to produce four equal, haploidspermatids (Figure 3.5). What would be thenumber of chromosome in the spermatids?The spermatids are transformed intospermatozoa (sperms) by the process calledspermiogenesis. After spermiogenesis,sperm heads become embedded in theSertoli cells, and are finally released fromthe seminiferous tubules by the processcalled spermiation.Spermatogenesis starts at the age ofpuberty due to significant increase in thesecretion of gonadotropin releasing hormone(GnRH). This, if you recall, is a hypothalamic hormone. The increasedlevels of GnRH then acts at the anterior pituitary gland and stimulatessecretion of two gonadotropins â luteinising hormone (LH) and folliclestimulating hormone (FSH). LH acts at the Leydig cells and stimulatessynthesis and secretion of androgens. Androgens, in turn, stimulate theprocess of spermatogenesis. FSH acts on the Sertoli cells and stimulatesFigure 3.5 Diagrammatic sectional view of aseminiferous tubule (enlarged)2022-2348BIOLOGYsecretion of some factors which help in theprocess of spermiogenesis.Let us examine the structure of a sperm. Itis a microscopic structure composed of a head,neck, a middle piece and a tail (Figure 3.6). Aplasma membrane envelops the whole body ofsperm. The sperm head contains an elongatedhaploid nucleus, the anterior portion of whichis covered by a cap-like structure, acrosome.The acrosome is filled with enzymes that helpfertilisation of the ovum. The middle piecepossesses numerous mitochondria, whichproduce energy for the movement of tail thatfacilitate sperm motility essential for fertilisation.The human male ejaculates about 200 to 300million sperms during a coitus of which, fornormal fertility, at least 60 per cent spermsmust have normal shape and size and at least40 per cent of them must show vigorousmotility.Sperms released from the seminiferoustubules, are transported by the accessoryducts. Secretions of epididymis, vas deferens, seminal vesicle andprostate are essential for maturation and motility of sperms. The seminalplasma along with the sperms constitute the semen. The functions ofmale sex accessory ducts and glands are maintained by the testicularhormones (androgens).The process of formation of a mature female gamete is called oogenesiswhich is markedly different from spermatogenesis. Oogenesis is initiatedduring the embryonic development stage when a couple of million gametemother cells (oogonia) are formed within each fetal ovary; no more oogoniaare formed and added after birth. These cells start division and enter intoprophase-I of the meiotic division and get temporarily arrested at that stage,called primary oocytes. Each primary oocyte then gets surrounded by alayer of granulosa cells and is called the primary follicle (Figure 3.7). Alarge number of these follicles degenerate during the phase from birth topuberty. Therefore, at puberty only 60,000-80,000 primary follicles areleft in each ovary. The primary follicles get surrounded by more layers ofgranulosa cells and a new theca and are called secondary follicles.The secondary follicle soon transforms into a tertiary follicle which ischaracterised by a fluid filled cavity called antrum. The theca layer isorganised into an inner theca interna and an outer theca externa. It isimportant to draw your attention that it is at this stage that the primaryoocyte within the tertiary follicle grows in size and completes its first meioticdivision. It is an unequal division resulting in the formation of a largehaploid secondary oocyte and a tiny first polar body (Figure 3.8b). TheFigure 3.6 Structure of a sperm2022-2349HUMAN REPRODUCTIONsecondary oocyte retains bulk of thenutrient rich cytoplasm of the primaryoocyte. Can you think of any advantagefor this? Does the first polar body bornout of first meiotic division divide furtheror degenerate? At present we are notvery certain about this. The tertiaryfollicle further changes into the maturefollicle or Graafian follicle (Figure 3.7).The secondary oocyte forms a newmembrane called zona pellucidasurrounding it. The Graafian follicle nowruptures to release the secondary oocyte(ovum) from the ovary by theprocess called ovulation. Can youidentify major differences betweenspermatogenesis and oogenesis? A diagrammatic representation ofspermatogenesis and oogenesis is given below (Figure 3.8).Figure 3.7 Diagrammatic Section view of ovaryFigure 3.8 Schematic representation of (a) Spermatogenesis; (b) Oogenesis(a) (b)3.4 MENSTRUAL CYCLEThe reproductive cycle in the female primates (e.g. monkeys, apes andhuman beings) is called menstrual cycle. The first menstruation beginsat puberty and is called menarche. In human females, menstruationis repeated at an average interval of about 28/29 days, and the cycle ofevents starting from one menstruation till the next one is called themenstrual cycle. One ovum is released (ovulation) during the middle2022-2350BIOLOGYFigure 3.9 Diagrammatic presentation of various events during a menstrual cycleof each menstrual cycle. The major events of the menstrual cycle areshown in Figure 3.9. The cycle starts with the menstrual phase, whenmenstrual flow occurs and it lasts for 3-5 days. The menstrual flowresults due to breakdown of endometrial lining of the uterus and itsblood vessels which forms liquid that comes out through vagina.Menstruation only occurs if the released ovum is not fertilised. Lack ofmenstruation may be indicative of pregnancy. However, it may also becaused due to some other underlying causes like stress, poor health etc.The menstrual phase is followed by the follicular phase. Duringthis phase, the primary follicles in the ovary grow to become afully mature Graafian follicle and simultaneously the endometriumof uterus regenerates through proliferation. These changes in theovary and the uterus are induced by changes in the levels ofpituitary and ovarian hormones (Figure 3.9). The secretion of2022-2351HUMAN REPRODUCTIONgonadotropins (LH and FSH) increases gradually during the follicularphase, and stimulates follicular development as well as secretion ofestrogens by the growing follicles. Both LH and FSH attain a peak levelin the middle of cycle (about 14th day). Rapid secretion of LH leading toits maximum level during the mid-cycle called LH surge induces ruptureof Graafian follicle and thereby the release of ovum (ovulation). Theovulation (ovulatory phase) is followed by the luteal phase during whichthe remaining parts of the Graafian follicle transform as the corpusluteum (Figure 3.9). The corpus luteum secretes large amounts ofprogesterone which is essential for maintenance of the endometrium.Such an endometrium is necessary for implantation of the fertilisedovum and other events of pregnancy. During pregnancy all events ofthe menstrual cycle stop and there is no menstruation. In the absenceof fertilisation, the corpus luteum degenerates. This causes disintegrationof the endometrium leading to menstruation, marking a new cycle. Inhuman beings, menstrual cycles ceases around 50 years of age; that istermed as menopause. Cyclic menstruation is an indicator of normalreproductive phase and extends between menarche and menopause.3.5 FERTILISATION AND IMPLANTATIONDuring copulation (coitus) semen is released by the penis into the vagina(insemination). The motile sperms swim rapidly, pass through the cervix,enter into the uterus and finally reach the ampullary region of thefallopian tube (Figure 3.11b). The ovum released by the ovary is alsotransported to the ampullary regionwhere fertilisation takes place.Fertilisation can only occur if theovum and sperms are transportedsimultaneously to the ampullaryregion. This is the reason why not allcopulations lead to fertilisation andpregnancy.The process of fusion of a spermwith an ovum is called fertilisation.During fertilisation, a sperm comes incontact with the zona pellucida layerof the ovum (Figure 3.10) and induceschanges in the membrane that blockthe entry of additional sperms. Thus,it ensures that only one sperm canfertilise an ovum. The secretions of theacrosome help the sperm enter into thecytoplasm of the ovum through thezona pellucida and the plasmaFigure 3.10 Ovum surrounded by few sperms51Maintenance ofhygiene and sanitationduring menstruation isvery important. Takebath and clean yourselfregulary. Use sanitarynapkins or cleanhomemade pads.Change sanitarynapkins or homemadepads after every 4â5 hrsas per the requirement.Dispose of the usedsanitary napkinsproperly wrapping itwith a used paper. Donot throw the usednapkins in thedrainpipe of toilets orin the open area. Afterhandling the napkinwash hands with soap.Menstrual Hygiene2022-2352BIOLOGYFigure 3.11 Transport of ovum, fertilisation and passage of growing embryo through fallopian tubemembrane. This induces the completion of the meiotic division of thesecondary oocyte. The second meiotic division is also unequal and resultsin the formation of a second polar body and a haploid ovum (ootid). Soonthe haploid nucleus of the sperms and that of the ovum fuse together toform a diploid zygote. How many chromosomes will be there in the zygote?One has to remember that the sex of the baby has been decided at thisstage itself. Let us see how? As you know the chromosome pattern in thehuman female is XX and that in the male is XY. Therefore, all the haploidgametes (ova) produced by the female have the sex chromosome X whereasin the male gametes (sperms) the sex chromosome could be either X or Y,hence, 50 per cent of sperms carry the X chromosome while the other 50 percent carry the Y. After fusion of the male and female gametes the zygotewould carry either XX or XY depending on whether the sperm carrying Xor Y fertilised the ovum. The zygote carrying XX would develop into a femalebaby and XY would form a male (you will learn more about the chromosomalpatterns in Chapter 5). That is why, scientifically it is correct to say that thesex of the baby is determined by the father and not by the mother!The mitotic division starts as the zygote moves through the isthmusof the oviduct called cleavage towards the uterus (Figure 3.11) and forms2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 162022-2353HUMAN REPRODUCTIONblastomeres is called a morula (Figure 3.11e). The morula continues todivide and transforms into blastocyst (Figure 3.11g) as it moves furtherinto the uterus. The blastomeres in the blastocyst are arranged into anouter layer called trophoblast and an inner group of cells attached totrophoblast called the inner cell mass. The trophoblast layer then getsattached to the endometrium and the inner cell mass gets differentiatedas the embryo. After attachment, the uterine cells divide rapidly and coversthe blastocyst. As a result, the blastocyst becomes embedded in theendometrium of the uterus (Figure 3.11h). This is called implantationand it leads to pregnancy.3.6 PREGNANCY AND EMBRYONIC DEVELOPMENTAfter implantation, finger-like projections appear on the trophoblast calledchorionic villi which are surrounded by the uterine tissue and maternalblood. The chorionic villi and uterine tissue become interdigitated witheach other and jointly form a structural and functional unit betweendeveloping embryo (foetus) and maternal body called placenta (Figure 3.12).The placenta facilitate the supply of oxygen and nutrients to theembryo and also removal of carbon dioxide and excretory/waste materialsproduced by the embryo. The placenta is connected to the embryo throughan umbilical cord which helps in the transport of substances to and fromthe embryo. Placenta also acts as an endocrine tissue and producesseveral hormones like human chorionic gonadotropin (hCG), humanplacental lactogen (hPL), estrogens, progestogens, etc. In the laterphase of pregnancy, a hormone called relaxin is also secreted bythe ovary. Let us rememberthat hCG, hPL and relaxinare produced in womenonly during pregnancy. Inaddition, during pregnancythe levels of other hormoneslike estrogens, progestogens,cortisol, prolactin, thyroxine,etc., are increased severalfoldsin the maternal blood.Increased production of thesehormones is essential forsupporting the fetal growth,metabolic changes in themother and maintenance ofpregnancy.Immediately afterimplantation, the inner cellmass (embryo) differentiates Figure 3.12 The human foetus within the uterus2022-2354BIOLOGYinto an outer layer called ectoderm and an inner layer called endoderm. Amesoderm soon appears between the ectoderm and the endoderm. Thesethree layers give rise to all tissues (organs) in adults. It needs to be mentionedhere that the inner cell mass contains certain cells called stem cells whichhave the potency to give rise to all the tissues and organs.What are the major features of embryonic development at variousmonths of pregnancy? The human pregnancy lasts 9 months. Do youknow for how many months pregnancy last in dogs, elephants, cats?Find out. In human beings, after one month of pregnancy, the embryoâsheart is formed. The first sign of growing foetus may be noticed by listeningto the heart sound carefully through the stethoscope. By the end of thesecond month of pregnancy, the foetus develops limbs and digits. By theend of 12 weeks (first trimester), most of the major organ systems areformed, for example, the limbs and external genital organs are welldeveloped.The first movements of the foetus and appearance of hair onthe head are usually observed during the fifth month. By the end of about24 weeks (end of second trimester), the body is covered with fine hair,eye-lids separate, and eyelashes are formed. By the end of nine monthsof pregnancy, the foetus is fully developed and is ready for delivery.3.7 PARTURITION AND LACTATIONThe average duration of human pregnancy is about 9 monthswhich is called the gestation period. Vigorous contraction of the uterus atthe end of pregnancy causes expulsion/delivery of the foetus. This processof delivery of the foetus (childbirth) is called parturition. Parturition isinduced by a complex neuroendocrine mechanism. The signals forparturition originate from the fully developed foetus and the placentawhich induce mild uterine contractions called foetal ejection reflex. Thistriggers release of oxytocin from the maternal pituitary. Oxytocin acts onthe uterine muscle and causes stronger uterine contractions, which inturn stimulates further secretion of oxytocin. The stimulatory reflex betweenthe uterine contraction and oxytocin secretion continues resulting instronger and stronger contractions. This leads to expulsion of the babyout of the uterus through the birth canal â parturition. Soon after theinfant is delivered, the placenta is also expelled out of the uterus. What doyou think the doctors inject to induce delivery?The mammary glands of the female undergo differentiation duringpregnancy and starts producing milk towards the end of pregnancy bythe process called lactation. This helps the mother in feeding the newborn.The milk produced during the initial few days of lactation is calledcolostrum which contains several antibodies absolutely essential todevelop resistance for the new-born babies. Breast-feeding during theinitial period of infant growth is recommended by doctors for bringing upa healthy baby.2022-2355HUMAN REPRODUCTIONSUMMARYHumans are sexually reproducing and viviparous. The malereproductive system is composed of a pair of testes, the male sexaccessory ducts and the accessory glands and external genitalia. Eachtestis has about 250 compartments called testicular lobules, and eachlobule contains one to three highly coiled seminiferous tubules. Eachseminiferous tubule is lined inside by spermatogonia and Sertoli cells.The spermatogonia undergo meiotic divisions leading to sperm formation,while Sertoli cells provide nutrition to the dividing germ cells. The Leydigcells outside the seminiferous tubules, synthesise and secrete testicularhormones called androgens. The male external genitalia is called penis.The female reproductive system consists of a pair of ovaries, a pairof oviducts, a uterus, a vagina, external genitalia, and a pair ofmammary glands. The ovaries produce the female gamete (ovum) andsome steroid hormones (ovarian hormones). Ovarian follicles in differentstages of development are embedded in the stroma. The oviducts, uterusand vagina are female accessory ducts. The uterus has three layersnamely perimetrium, myometrium and endometrium. The femaleexternal genitalia includes mons pubis, labia majora, labia minora,hymen and clitoris. The mammary glands are one of the femalesecondary sexual characteristics.Spermatogenesis results in the formation of sperms that aretransported by the male sex accessory ducts. A normal human spermis composed of a head, neck, a middle piece and tail. The process offormation of mature female gametes is called oogenesis. Thereproductive cycle of female primates is called menstrual cycle.Menstrual cycle starts only after attaining sexual maturation (puberty).During ovulation only one ovum is released per menstrual cycle. Thecyclical changes in the ovary and the uterus during menstrual cycleare induced by changes in the levels of pituitary and ovarian hormones.After coitus, sperms are transported to the ampulla, where the spermfertilises the ovum leading to formation of a diploid zygote. The presenceof X or Y chromosome in the sperm determines the sex of the embryo.The zygote undergoes repeated mitotic division to form a blastocyst,which is implanted in the uterus resulting in pregnancy. After ninemonths of pregnancy, the fully developed foetus is ready for delivery.The process of childbirth is called parturition which is induced by acomplex neuroendocrine mechanism involving cortisol, estrogens andoxytocin. Mammary glands differentiate during pregnancy and secretemilk after child-birth. The new-born baby is fed milk by the mother(lactation) during the initial few months of growth.EXERCISES1. Fill in the blanks:(a) Humans reproduce _____________ (asexually/sexually)(b) Humans are _____________ (oviparous, viviparous, ovoviviparous)(c) Fertilisation is _____________ in humans (external/internal)(d) Male and female gametes are _____________ (diploid/haploid)(e) Zygote is _____________ (diploid/haploid)2022-2356BIOLOGY(f) The process of release of ovum from a mature follicle is called_____________(g) Ovulation is induced by a hormone called _____________(h) The fusion of male and female gametes is called _____________(i) Fertilisation takes place in _____________(j) Zygote divides to form _____________which is implanted in uterus.(k) The structure which provides vascular connection between foetusand uterus is called _____________2. Draw a labelled diagram of male reproductive system.3. Draw a labelled diagram of female reproductive system.4. Write two major functions each of testis and ovary.5. Describe the structure of a seminiferous tubule.6. What is spermatogenesis? Briefly describe the process of spermatogenesis.7. Name the hormones involved in regulation of spermatogenesis.8. Define spermiogenesis and spermiation.9. Draw a labelled diagram of sperm.10. What are the major components of seminal plasma?11. What are the major functions of male accessory ducts and glands?12. What is oogenesis? Give a brief account of oogenesis.13. Draw a labelled diagram of a section through ovary.14. Draw a labelled diagram of a Graafian follicle?15. Name the functions of the following:(a) Corpus luteum (b) Endometrium(c) Acrosome (d) Sperm tail(e) Fimbriae16. Identify True/False statements. Correct each false statement to makeit true.(a) Androgens are produced by Sertoli cells. (True/False)(b) Spermatozoa get nutrition from Sertoli cells. (True/False)(c) Leydig cells are found in ovary. (True/False)(d) Leydig cells synthesise androgens. (True/False)(e) Oogenesis takes place in corpus luteum. (True/False)(f) Menstrual cycle ceases during pregnancy. (True/False)(g) Presence or absence of hymen is not a reliable indicator of virginityor sexual experience. (True/False)17. What is menstrual cycle? Which hormones regulate menstrual cycle?18. What is parturition? Which hormones are involved in induction of parturition?19. In our society the women are often blamed for giving birth to daughters.Can you explain why this is not correct?20. How many eggs are released by a human ovary in a month? How manyeggs do you think would have been released if the mother gave birth toidentical twins? Would your answer change if the twins born werefraternal?21. How many eggs do you think were released by the ovary of a female dogwhich gave birth to 6 puppies?2022-23You have learnt about human reproductive system and itsfunctions in Chapter 3. Now, letâs discuss a closely relatedtopic â reproductive health. What do we understand bythis term? The term simply refers to healthy reproductiveorgans with normal functions. However, it has a broaderperspective and includes the emotional and social aspectsof reproduction also. According to the World HealthOrganisation (WHO), reproductive health means a totalwell-being in all aspects of reproduction, i.e., physical,emotional, behavioural and social. Therefore, a society withpeople having physically and functionally normalreproductive organs and normal emotional and behaviouralinteractions among them in all sex-related aspects mightbe called reproductively healthy. Why is it significant tomaintain reproductive health and what are the methodstaken up to achieve it? Let us examine them.4.1 REPRODUCTIVE HEALTH â PROBLEMS ANDSTRATEGIESIndia was amongst the first countries in the world toinitiate action plans and programmes at a national levelto attain total reproductive health as a social goal.These programmes called âfamily planningâ wereinitiated in 1951 and were periodically assessed overthe past decades. Improved programmes covering widerCHAPTER 4REPRODUCTIVE HEALTH4.1 Reproductive Health âProblems and Strategies4.2 Population Explosionand Birth Control4.3 Medical Termination ofPregnancy4.4 Sexually TransmittedDiseases4.5 Infertility2022-2358BIOLOGYreproduction-related areas are currently in operation under thepopular name âReproductive and Child Health Care (RCH) programmesâ.Creating awareness among people about various reproduction relatedaspects and providing facilities and support for building up areproductively healthy society are the major tasks under theseprogrammes.With the help of audio-visual and the print-media governmental andnon-governmental agencies have taken various steps to create awarenessamong the people about reproduction-related aspects. Parents, otherclose relatives, teachers and friends, also have a major role in thedissemination of the above information. Introduction of sex educationin schools should also be encouraged to provide right information tothe young so as to discourage children from believing in myths andhaving misconceptions about sex-related aspects. Proper informationabout reproductive organs, adolescence and related changes, safe andhygienic sexual practices, sexually transmitted diseases (STD), AIDS,etc., would help people, especially those in the adolescent age group tolead a reproductively healthy life. Educating people, especially fertilecouples and those in marriageable age group, about available birthcontrol options, care of pregnant mothers, post-natal care of the motherand child, importance of breast feeding, equal opportunities for the maleand the female child, etc., would address the importance of bringing upsocially conscious healthy families of desired size. Awareness of problemsdue to uncontrolled population growth, social evils like sex-abuse andsex-related crimes, etc., need to be created to enable people to thinkand take up necessary steps to prevent them and thereby build up asocially responsible and healthy society.Successful implementation of various action plans to attainreproductive health requires strong infrastructural facilities, professionalexpertise and material support. These are essential to provide medicalassistance and care to people in reproduction-related problems likepregnancy, delivery, STDs, abortions, contraception, menstrual problems,infertility, etc. Implementation of better techniques and new strategiesfrom time to time are also required to provide more efficient careand assistance to people. Statutory ban on amniocentesis forsex-determination to legally check increasing menace of female foeticides,massive child immunisation, etc., are some programmes that meritmention in this connection. In aminocentesis some of the amniotic fluidof the developing foetus is taken to analyse the fetal cells and dissolvedsubstances. This procedure is used to test for the presence of certaingenetic disorders such as, down syndrome, haemoplilia, sickle-cellanemia, etc., determine the survivability of the foetus.Research on various reproduction-related areas are encouraged andsupported by governmental and non-governmental agencies to find outnew methods and/or to improve upon the existing ones. Do you knowthat âSaheliââa new oral contraceptive for the femalesâwas developed2022-2359REPRODUCTIVE HEALTHby scientists at Central Drug Research Institute (CDRI) in Lucknow, India?Better awareness about sex related matters, increased number of medicallyassisted deliveries and better post-natal care leading to decreased maternaland infant mortality rates, increased number of couples with smallfamilies, better detection and cure of STDs and overall increased medicalfacilities for all sex-related problems, etc. all indicate improved reproductivehealth of the society.4.2 POPULATION STABILISATION AND BIRTH CONTROLIn the last century an all-round development in various fields significantlyimproved the quality of life of the people. However, increased healthfacilities along with better living conditions had an explosive impact onthe growth of population. The world population which was around2 billion (2000 million) in 1900 rocketed to about 6 billion by 2000 and7.2 billion in 2011. A similar trend was observed in India too. Ourpopulation which was approximately 350 million at the time of ourindependence reached close to the billion mark by 2000 and crossed1.2 billion in May 2011. A rapid decline in death rate, maternal mortalityrate (MMR) and infant mortality rate (IMR) as well as an increase innumber of people in reproducible age are probable reasons for this.Through our Reproductive Child Health (RCH) programme, though wecould bring down the population growth rate, it was only marginal.According to the 2011 census report, the population growth rate wasless than 2 per cent, i.e., 20/1000/year, a rate at which our populationcould increase rapidly. Such an alarming growth rate could lead to anabsolute scarcity of even the basic requirements, i.e., food, shelter andclothing, in spite of significant progress made in those areas. Therefore,the government was forced to take up serious measures to check thispopulation growth rate.The most important step to overcome this problem is to motivate smallerfamilies by using various contraceptive methods. You might have seenadvertisements in the media as well as posters/bills, etc., showing a happycouple with two children with a slogan Hum Do Hamare Do (we two, ourtwo). Many couples, mostly the young, urban, working ones have evenadopted an âone child normâ. Statutory raising of marriageable age of thefemale to 18 years and that of males to 21 years, and incentives given tocouples with small families are two of the other measures taken to tacklethis problem. Let us describe some of the commonly used contraceptivemethods, which help prevent unwanted pregnancies.An ideal contraceptive should be user-friendly, easily available,effective and reversible with no or least side-effects. It also should in noway interfere with the sexual drive, desire and/or the sexual act of theuser. A wide range of contraceptive methods are presently available whichcould be broadly grouped into the following categories, namelyNatural/Traditional, Barrier, IUDs, Oral contraceptives, Injectables,Implants and Surgical methods.2022-2360BIOLOGYFigure 4.2. Copper T (CuT)Natural methods work on the principle of avoiding chances of ovumand sperms meeting. Periodic abstinence is one such method in whichthe couples avoid or abstain from coitus from day 10 to 17 of the menstrualcycle when ovulation could be expected. As chances of fertilisation arevery high during this period, it is called the fertile period. Therefore, byabstaining from coitus during this period, conception couldbe prevented. Withdrawal or coitus interruptus is anothermethod in which the male partner withdraws his penis fromthe vagina just before ejaculation so as to avoidinsemination. Lactational amenorrhea (absence ofmenstruation) method is based on the fact that ovulationand therefore the cycle do not occur during the period ofintense lactation following parturition. Therefore, as longas the mother breast-feeds the child fully, chances ofconception are almost nil. However, this method has beenreported to be effective only upto a maximum period of sixmonths following parturition. As no medicines or devicesare used in these methods, side effects are almost nil.Chances of failure, though, of this method are also high.In barrier methods, ovum and sperms are preventedfrom physically meeting with the help of barriers. Suchmethods are available for both males and females.Condoms (Figure 4.1 a, b) are barriers made of thin rubber/latex sheath that are used to cover the penis in the male orvagina and cervix in the female, just before coitus so thatthe ejaculated semen would not enter into the femalereproductive tract. This can prevent conception. âNirodhâ isa popular brand of condom for the male. Use of condomshas increased in recent years due to its additional benefit ofprotecting the user from contracting STIs and AIDS. Boththe male and the female condoms are disposable, can beself-inserted and thereby gives privacy to the user.Diaphragms, cervical caps and vaults are also barriersmade of rubber that are inserted into the female reproductivetract to cover the cervix during coitus. They preventconception by blocking the entry of sperms through thecervix. They are reusable. Spermicidal creams, jellies andfoams are usually used alongwith these barriers to increasetheir contraceptive efficiency.Another effective and popular method is the use of Intra UterineDevices (IUDs). These devices are inserted by doctors or expert nursesin the uterus through vagina. These Intra Uterine Devices are presentlyavailable as the non-medicated IUDs (e.g., Lippes loop), copper releasingIUDs (CuT, Cu7, Multiload 375) and the hormone releasing IUDs(Progestasert, LNG-20) (Figure 4.2). IUDs increase phagocytosis of spermswithin the uterus and the Cu ions released suppress sperm motility andthe fertilising capacity of sperms. The hormone releasing IUDs, in addition,Figure 4.1(b) Condom for femaleFigure 4.1(a) Condom for male2022-2361REPRODUCTIVE HEALTHmake the uterus unsuitable for implantation and thecervix hostile to the sperms. IUDs are ideal contraceptivesfor the females who want to delay pregnancy and/or spacechildren. It is one of most widely accepted methods ofcontraception in India.Oral administration of small doses of either progestogensor progestogenâestrogen combinations is anothercontraceptive method used by the females. They are usedin the form of tablets and hence are popularly called thepills. Pills have to be taken daily for a period of 21 daysstarting preferably within the first five days of menstrualcycle. After a gap of 7 days (during which menstruationoccurs) it has to be repeated in the same pattern till the female desires toprevent conception. They inhibit ovulation and implantation as well asalter the quality of cervical mucus to prevent/retard entry of sperms. Pillsare very effective with lesser side effects and are well accepted by the females.Saheli âthe new oral contraceptive for the females contains a non-steroidalpreparation. It is a âonce a weekâ pill with very few side effects and highcontraceptive value.Progestogens alone or in combination with estrogen can also be usedby females as injections or implants under the skin (Figure 4.3). Theirmode of action is similar to that of pills and their effective periods aremuch longer. Administration of progestogens or progestogen-estrogencombinations or IUDs within 72 hours of coitus have been found to bevery effective as emergency contraceptives as they could be used to avoidpossible pregnancy due to rape or casual unprotected intercourse.Surgical methods, also called sterilisation, are generally advised forthe male/female partner as a terminal method to prevent any moreFigure 4.3 ImplantsFigure 4.4 (a) VasectomyFigure 4.4 (b) Tubectomy2022-2362BIOLOGYpregnancies. Surgical intervention blocks gamete transport and therebyprevent conception. Sterilisation procedure in the male is called âvasectomyâand that in the female, âtubectomyâ. In vasectomy, a small part of the vasdeferens is removed or tied up through a small incision on the scrotum(Figure 4.4a) whereas in tubectomy, a small part of the fallopian tube isremoved (Figure 4.4b) or tied up through a small incision in the abdomenor through vagina. These techniques are highly effective but theirreversibility is very poor.It needs to be emphasised that the selection of a suitable contraceptivemethod and its use should always be undertaken in consultation withqualified medical professionals. One must also remember thatcontraceptives are not regular requirements for the maintenance ofreproductive health. In fact, they are practiced against a naturalreproductive event, i.e., conception/pregnancy. One is forced to use thesemethods either to prevent pregnancy or to delay or space pregnancy dueto personal reasons. No doubt, the widespread use of these methods havea significant role in checking uncontrolled growth of population. However,their possible ill-effects like nausea, abdominal pain, breakthroughbleeding, irregular menstrual bleeding or even breast cancer, though notvery significant, should not be totally ignored.4.3 MEDICAL TERMINATION OF PREGNANCY (MTP)Intentional or voluntary termination of pregnancy before full term is calledmedical termination of pregnancy (MTP) or induced abortion. Nearly45 to 50 million MTPs are performed in a year all over the world whichaccounts to 1/5th of the total number of conceived pregnancies in a year.Whether to accept / legalise MTP or not is being debated upon in manycountries due to emotional, ethical, religious and social issues involvedin it. Government of India legalised MTP in 1971 with some strict conditionsto avoid its misuse. Such restrictions are all the more important to checkindiscriminate and illegal female foeticides which are reported to be highin India.Why MTP? Obviously the answer isâto get rid of unwantedpregnancies either due to casual unprotected intercourse or failure of thecontraceptive used during coitus or rapes. MTPs are also essential incertain cases where continuation of the pregnancy could be harmful oreven fatal either to the mother or to the foetus or both.MTPs are considered relatively safe during the first trimester, i.e., upto12 weeks of pregnancy. Second trimester abortions are much more riskier.One disturbing trend observed is that a majority of the MTPs are performedillegally by unqualified quacks which are not only unsafe but could befatal too. Another dangerous trend is the misuse of amniocentesis todetermine the sex of the unborn child. Frequently, if the foetus is foundto be female, it is followed by MTP- this is totally against what is legal.The Medical Terminationof Pregnancy(Amendment) Act, 2017was enacted by thegovernment of India withthe intension ofreducing the incidence ofillegal abortion andconsequent maternalmortality and morbidity.According to this Act, apregnancy may beterminated on certainconsidered groundswithin the first 12 weeksof pregnancy on theopinion of one registeredmedical practitioner. Ifthe pregnancy has lastedmore than 12 weeks, butfewer than 24 weeks, tworegistered medicalpractitioners must be ofthe opinion, formed ingood faith, that therequired ground exist.The grounds for suchtermination ofpregnancies are:(i) The continuation ofthe pregnancy wouldinvolve a risk to thelife of the pregnantwoman or of graveinjury physical ormental health; or(ii There is asubstantial risk thatof the child wereborn, it would sufferfrom such physicalor mentalabnormalities as tobe seriouslyhandicapped.2022-2363REPRODUCTIVE HEALTHSuch practices should be avoided because these are dangerous both forthe young mother and the foetus. Effective counselling on the need toavoid unprotected coitus and the risk factors involved in illegal abortionsas well as providing more health care facilities could reverse the mentionedunhealthy trend.4.4 SEXUALLY TRANSMITTED INFECTIONS (STIS)Infections or diseases which are transmitted through sexual intercourseare collectively called sexually transmitted infections (STI) or venerealdiseases (VD) or reproductive tract infections (RTI). Gonorrhoea, syphilis,genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-Band of course, the most discussed infection in the recent years, HIV leadingto AIDS are some of the common STIs. Among these, HIV infection ismost dangerous and is discussed in detail in Chapter 8.Some of these infections like hepatitisâB and HIV can also betransmitted by sharing of injection needles, surgical instruments, etc.,with infected persons, transfusion of blood, or from an infected mother tothe foetus too. Except for hepatitis-B, genital herpes and HIV infections,other diseases are completely curable if detected early and treatedproperly. Early symptoms of most of these are minor and include itching,fluid discharge, slight pain, swellings, etc., in the genital region. Infectedfemales may often be asymptomatic and hence, may remain undetectedfor long. Absence or less significant symptoms in the early stages ofinfection and the social stigma attached to the STIs, deter the infectedpersons from going for timely detection and proper treatment. This couldlead to complications later, which include pelvic inflammatory diseases(PID), abortions, still births, ectopic pregnancies, infertility or even cancerof the reproductive tract. STIs are a major threat to a healthy society.Therefore, prevention or early detection and cure of these diseases aregiven prime consideration under the reproductive health-careprogrammes. Though all persons are vulnerable to these infections, theirincidences are reported to be very high among persons in the age groupof 15-24 years â the age group to which you also belong. There is noreason to panic because prevention is possible. One could be free of theseinfections by following the simple principles given below:(i) Avoid sex with unknown partners/multiple partners.(ii) Always try to use condoms during coitus.(iii) In case of doubt, one should go to a qualified doctor for earlydetection and get complete treatment if diagnosed with infection.4.5 INFERTILITYA discussion on reproductive health is incomplete without a mention ofinfertility. A large number of couples all over the world including Indiaare infertile, i.e., they are unable to produce children inspite of unprotected2022-2364BIOLOGYsexual co-habitation. The reasons for this could be manyâphysical,congenital, diseases, drugs, immunological or even psychological.In India, often the female is blamed for the couple being childless, butmore often than not, the problem lies in the male partner. Specialisedhealth care units (infertility clinics, etc.) could help in diagnosis andcorrective treatment of some of these disorders and enable these couples tohave children. However, where such corrections are not possible, the couplescould be assisted to have children through certain special techniquescommonly known as assisted reproductive technologies (ART).In vitro fertilisation (IVFâfertilisation outside the body in almostsimilar conditions as that in the body) followed by embryo transfer (ET)is one of such methods. In this method, popularly known as test tubebaby programme, ova from the wife/donor (female) and sperms from thehusband/donor (male) are collected and are induced to form zygote undersimulated conditions in the laboratory. The zygote or early embryos (withupto 8 blastomeres) could then be transferred into the fallopian tube(ZIFTâzygote intra fallopian transfer) and embryos with more than8 blastomeres, into the uterus (IUT â intra uterine transfer), to completeits further development. Embryos formed by in-vivo fertilisation (fusionof gametes within the female) also could be used for such transfer to assistthose females who cannot conceive.Transfer of an ovum collected from a donor into the fallopian tube(GIFT â gamete intra fallopian transfer) of another female who cannotproduce one, but can provide suitable environment for fertilisation andfurther development is another method attempted. Intra cytoplasmicsperm injection (ICSI) is another specialised procedure to form an embryoin the laboratory in which a sperm is directly injected into the ovum.Infertility cases either due to inability of the male partner to inseminatethe female or due to very low sperm counts in the ejaculates, could becorrected by artificial insemination (AI) technique. In this technique,the semen collected either from the husband or a healthy donor is artificiallyintroduced either into the vagina or into the uterus (IUI â intra-uterineinsemination) of the female.Though options are many, all these techniques require extremely highprecision handling by specialised professionals and expensiveinstrumentation. Therefore, these facilities are presently available only invery few centres in the country. Obviously their benefits is affordable toonly a limited number of people. Emotional, religious and social factorsare also deterrents in the adoption of these methods. Since the ultimateaim of all these procedures is to have children, in India we have so manyorphaned and destitute children, who would probably not survive tillmaturity, unless taken care of. Our laws permit legal adoption and it isas yet, one of the best methods for couples looking for parenthood.2022-2365REPRODUCTIVE HEALTHSUMMARYReproductive health refers to a total well-being in all aspects ofreproduction, i.e., physical, emotional, behavioural and social. Ournation was the first nation in the world to initiate various action plansat national level towards attaining a reproductively healthy society.Counselling and creating awareness among people aboutreproductive organs, adolescence and associated changes, safe andhygienic sexual practices, sexually transmitted infections (STIs)including AIDS, etc., is the primary step towards reproductive health.Providing medical facilities and care to the problems like menstrualirregularities, pregnancy related aspects, delivery, medical terminationof pregnancy, STIs, birth control, infertility, post natal child andmaternal management is another important aspect of the Reproductiveand Child Health Care programmes.An overall improvement in reproductive health has taken place inour country as indicated by reduced maternal and infant mortalityrates, early detection and cure of STIs, assistance to infertile couples,etc. Improved health facilities and better living conditions promoted anexplosive growth of population. Such a growth necessitated intensepropagation of contraceptive methods. Various contraceptive optionsare available now such as natural, traditional, barrier, IUDs, pills,injectables, implants and surgical methods. Though contraceptives arenot regular requirements for reproductive health, one is forced to usethem to avoid pregnancy or to delay or space pregnancy.Medical termination of pregnancy is legalised in our country. MTP isgenerally performed to get rid of unwanted pregnancy due to rapes, casualrelationship, etc., as also in cases when the continuation of pregnancycould be harmful or even fatal to either the mother, or the foetus or both.Infections or diseases transmitted through sexual intercourse arecalled Sexually Transmitted Diseases (STIs). Pelvic InflammatoryDiseases (PIDs), still birth, infertility are some of the complications ofthem. Early detection facilitate better cure of these diseases. Avoidingsexual intercourse with unknown/multiple partners, use of condomsduring coitus are some of the simple precautions to avoid contractingSTIs.Inability to conceive or produce children even after 2 years ofunprotected sexual cohabitation is called infertility. Various methodsare now available to help such couples. In Vitro fertilisation followed bytransfer of embryo into the female genital tract is one such method andis commonly known as the âTest Tube Babyâ Programme.2022-2366BIOLOGYEXERCISES1. What do you think is the significance of reproductive health in a society?2. Suggest the aspects of reproductive health which need to be givenspecial attention in the present scenario.3. Is sex education necessary in schools? Why?4. Do you think that reproductive health in our country has improved inthe past 50 years? If yes, mention some such areas of improvement.5. What are the suggested reasons for population explosion?6. Is the use of contraceptives justified? Give reasons.7. Removal of gonads cannot be considered as a contraceptive option. Why?8. Amniocentesis for sex determination is banned in our country. Is thisban necessary? Comment.9. Suggest some methods to assist infertile couples to have children.10. What are the measures one has to take to prevent from contracting STDs?11. State True/False with explanation(a) Abortions could happen spontaneously too. (True/False)(b) Infertility is defined as the inability to produce a viable offspringand is always due to abnormalities/defects in the female partner.(True/False)(c) Complete lactation could help as a natural method ofcontraception. (True/False)(d) Creating awareness about sex related aspects is an effectivemethod to improve reproductive health of the people. (True/False)12. Correct the following statements :(a) Surgical methods of contraception prevent gamete formation.(b) All sexually transmitted diseases are completely curable.(c) Oral pills are very popular contraceptives among the rural women.(d) In E. T. techniques, embryos are always transferred into the uterus.2022-23The work of Mendel and others who followed him gave us anidea of inheritance patterns. However the nature of those âfactorsâwhich determine the phenotype was not very clear. As theseâfactorsâ represent the genetic basis of inheritance, understandingthe structure of genetic material and the structural basis ofgenotype and phenotype conversion became the focus ofattention in biology for the next century. The entire body ofmolecular biology was a consequent development with majorcontributions from Watson, Crick, Nirenberg, Khorana, Kornbergs(father and son), Benzer, Monod, Brenner, etc. A parallel problembeing tackled was the mechanism of evolution. Awareness in theareas of molecular genetics, structural biology and bio informaticshave enriched our understanding of the molecular basis ofevolution. In this unit the structure and function of DNA and thestory and theory of evolution have been examined and explained.Chapter 5Principles of Inheritanceand VariationChapter 6Molecular Basis of InheritanceChapter 7Evolution2022-23James Dewey Watson was born in Chicago on 6 April 1928. In 1947, hereceived B.Sc. degree in Zoology. During these years his interest inbird-watching had matured into a serious desire to learn genetics. Thisbecame possible when he received a Fellowship for graduate study inZoology at Indiana University, Bloomington, where he received his Ph.D.degree in 1950 on a study of the effect of hard X-rays on bacteriophagemultiplication.He met Crick and discovered their common interest in solving theDNA structure. Their first serious effort, was unsatisfactory. Their second effortbased upon more experimental evidence and better appreciation ofthe nucleic acid literature, resulted, early in March 1953, in the proposalof the complementary double-helical configuration.Francis Harry Compton Crick was born on 8 June 1916, at Northampton,England. He studied physics at University College, London and obtaineda B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled âX-rayDiffraction: Polypeptides and Proteinsâ.A critical influence in Crickâs career was his friendship with J. D.Watson, then a young man of 23, leading in 1953 to the proposal ofthe double-helical structure for DNA and the replication scheme. Crickwas made an F.R.S. in 1959.The honours to Watson with Crick include: the John Collins WarrenPrize of the Massachusetts General Hospital, in 1959; the Lasker Award,in 1960; the Research Corporation Prize, in 1962 and above all, theNobel Prize in 1962.JAMES WATSONFRANCIS CRICK2022-23CHAPTER 5Have you ever wondered why an elephant always givesbirth only to a baby elephant and not some other animal?Or why a mango seed forms only a mango plant and notany other plant?Given that they do, are the offspring identical to theirparents? Or do they show differences in some of theircharacteristics? Have you ever wondered why siblingssometimes look so similar to each other? Or sometimeseven so different?These and several related questions are dealt with,scientifically, in a branch of biology known as Genetics.This subject deals with the inheritance, as well as thevariation of characters from parents to offspring.Inheritance is the process by which characters are passedon from parent to progeny; it is the basis of heredity.Variation is the degree by which progeny differ from theirparents.Humans knew from as early as 8000-1000 B.C. thatone of the causes of variation was hidden in sexualreproduction. They exploited the variations that werenaturally present in the wild populations of plants andanimals to selectively breed and select for organisms thatpossessed desirable characters. For example, throughartificial selection and domestication from ancestralPRINCIPLES OF INHERITANCEAND VARIATION5.1 Mendelâs Laws ofInheritance5.2 Inheritance of One Gene5.3 Inheritance of Two Genes5.4 Sex Determination5.5 Mutation5.6 Genetic Disorders2022-2370BIOLOGYwild cows, we have well-known Indianbreeds, e.g., Sahiwal cows in Punjab. Wemust, however, recognise that though ourancestors knew about the inheritance ofcharacters and variation, they had verylittle idea about the scientific basis of thesephenomena.5.1 MENDELâS LAWS OF INHERITANCEIt was during the mid-nineteenth century thatheadway was made in the understanding ofinheritance. Gregor Mendel, conductedhybridisation experiments on garden peas forseven years (1856-1863) and proposed thelaws of inheritance in living organisms. DuringMendelâs investigations into inheritancepatterns it was for the first time that statisticalanalysis and mathematical logic were appliedto problems in biology. His experiments had alarge sampling size, which gave greatercredibility to the data that he collected. Also,the confirmation of his inferences fromexperiments on successive generations of histest plants, proved that his results pointed togeneral rules of inheritance rather than beingunsubstantiated ideas. Mendel investigatedcharacters in the garden pea plant that weremanifested as two opposing traits, e.g., tall ordwarf plants, yellow or green seeds. Thisallowed him to set up a basic framework ofrules governing inheritance, which wasexpanded on by later scientists to account forall the diverse natural observations and thecomplexity inherent in them.Mendel conducted such artificialpollination/cross pollination experimentsusing several true-breeding pea lines. A truebreedingline is one that, having undergonecontinuous self-pollination, shows the stable trait inheritance andexpression for several generations. Mendel selected 14 true-breeding peaplant varieties, as pairs which were similar except for one character withcontrasting traits. Some of the contrasting traits selected were smooth orwrinkled seeds, yellow or green seeds, inflated (full) or constricted greenor yellow pods and tall or dwarf plants (Figure 5.1, Table 5.1).Figure 5.1 Seven pairs of contrasting traits inpea plant studied by Mendel2022-2371PRINCIPLES OF INHERITANCE AND VARIATION5.2 INHERITANCE OF ONE GENELet us take the example of one suchhybridisation experiment carried out byMendel where he crossed tall and dwarf peaplants to study the inheritance of one gene(Figure 5.2). He collected the seeds producedas a result of this cross and grew them togenerate plants of the first hybrid generation.This generation is also called the Filial1progeny or the F1. Mendel observed that allthe F1 progeny plants were tall, like one ofits parents; none were dwarf (Figure 5.3). Hemade similar observations for the other pairsof traits â he found that the F1 alwaysresembled either one of the parents, and thatthe trait of the other parent was not seen inthem.Mendel then self-pollinated the tall F1plants and to his surprise found that in theFilial2 generation some of the offspring wereâdwarf â; the character that was not seen inthe F1 generation was now expressed. Theproportion of plants that were dwarf were1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall anddwarf traits were identical to their parental type and did not show anyblending, that is all the offspring were either tall or dwarf, none were of inbetweenheight (Figure 5.3).Similar results were obtained with the other traits that he studied:only one of the parental traits was expressed in the F1 generation while atthe F2 stage both the traits were expressed in the proportion 3:1. Thecontrasting traits did not show any blending at either F1 or F2 stage.Figure 5.2 Steps in making a cross in peaTable 5.1: Contrasting Traits Studied byMendel in PeaS.No. Characters Contrasting Traits1. Stem height Tall/dwarf2. Flower colour Violet/white3. Flower position Axial/terminal4. Pod shape Inflated/constricted5. Pod colour Green/yellow6. Seed shape Round/wrinkled7. Seed colour Yellow/green2022-2372BIOLOGYBased on these observations,Mendel proposed that somethingwas being stably passed down,unchanged, from parent to offspringthrough the gametes, oversuccessive generations. He calledthese things as âfactorsâ. Now we callthem as genes. Genes, therefore, arethe units of inheritance. Theycontain the information that isrequired to express a particular traitin an organism. Genes which codefor a pair of contrasting traits areknown as alleles, i.e., they areslightly different forms of the samegene.If we use alphabetical symbolsfor each gene, then the capital letteris used for the trait expressed at theF1 stage and the small alphabet forthe other trait. For example, in caseof the character of height, T is usedfor the Tall trait and t for the âdwarfâ,and T and t are alleles of each other.Hence, in plants the pair of allelesfor height would be TT, Tt or tt.Mendel also proposed that in a truebreeding, tall or dwarf pea varietythe allelic pair of genes for height areidentical or homozygous, TT and tt, respectively. TT and tt are calledthe genotype of the plant while the descriptive terms tall and dwarf arethe phenotype. What then would be the phenotype of a plant that had agenotype Tt?As Mendel found the phenotype of the F1 heterozygote Tt to be exactlylike the TT parent in appearance, he proposed that in a pair of dissimilarfactors, one dominates the other (as in the F1 ) and hence is called thedominant factor while the other factor is recessive . In this case T (fortallness) is dominant over t (for dwarfness), that is recessive. He observedidentical behaviour for all the other characters/trait-pairs that he studied.It is convenient (and logical) to use the capital and lower case of analphabetical symbol to remember this concept of dominance andrecessiveness. (Do not use T for tall and d for dwarf because you willfind it difficult to remember whether T and d are alleles of the samegene/character or not). Alleles can be similar as in the case of homozygotesTT and tt or can be dissimilar as in the case of the heterozygote Tt. SinceFigure 5.3 Diagrammatic representationof monohybrid cross2022-2373PRINCIPLES OF INHERITANCE AND VARIATIONthe Tt plant is heterozygous for genes controllingone character (height), it is a monohybrid and thecross between TT and tt is a monohybrid cross.From the observation that the recessive parentaltrait is expressed without any blending in the F2generation, we can infer that, when the tall anddwarf plant produce gametes, by the process ofmeiosis, the alleles of the parental pair separate orsegregate from each other and only one allele istransmitted to a gamete. This segregation of allelesis a random process and so there is a 50 per centchance of a gamete containing either allele, as hasbeen verified by the results of the crossings. In thisway the gametes of the tall TT plants have the alleleT and the gametes of the dwarf tt plants have theallele t. During fertilisation the two alleles, T fromone parent say, through the pollen, and t from theother parent, then through the egg, are united toproduce zygotes that have one T allele and one tallele. In other words the hybrids have Tt. Sincethese hybrids contain alleles which expresscontrasting traits, the plants are heterozygous. Theproduction of gametes by the parents, the formationof the zygotes, the F1 and F2 plants can beunderstood from a diagram called Punnett Squareas shown in Figure 5.4. It was developed by a Britishgeneticist, Reginald C. Punnett. It is a graphicalrepresentation to calculate the probability of allpossible genotypes of offspring in a genetic cross.The possible gametes are written on two sides,usually the top row and left columns. All possiblecombinations are represented in boxes below in thesquares, which generates a square output form.The Punnett Square shows the parental tall TT(male) and dwarf tt (female) plants, the gametesproduced by them and, the F1 Tt progeny. The F1plants of genotype Tt are self-pollinated. Thesymbols & and % are used to denote the female(eggs) and male (pollen) of the F1 generation, respectively. The F1 plant ofthe genotype Tt when self-pollinated, produces gametes of the genotypeT and t in equal proportion. When fertilisation takes place, the pollengrains of genotype T have a 50 per cent chance to pollinate eggs of thegenotype T, as well as of genotype t. Also pollen grains of genotype t havea 50 per cent chance of pollinating eggs of genotype T, as well as ofFigure 5.4 A Punnett square used tounderstand a typical monohybridcross conducted by Mendelbetween true-breeding tall plantsand true-breeding dwarf plants2022-2374BIOLOGYgenotype t. As a result of random fertilisation, the resultant zygotes canbe of the genotypes TT, Tt or tt.From the Punnett square it is easily seen that 1/4th of the randomfertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt. Though the F1have a genotype of Tt, but the phenotypic character seen is âtallâ. At F2,3/4th of the plants are tall, where some of them are TT while others areTt. Externally it is not possible to distinguish between the plants withthe genotypes TT and Tt. Hence, within the genopytic pair Tt only onecharacter âTâ tall is expressed. Hence the character T or âtallâ is said todominate over the other allele t or âdwarfâ character. It is thus due to thisdominance of one character over the other that all the F1 are tall (thoughthe genotype is Tt) and in the F2 3/4th of the plants are tall (thoughgenotypically 1/2 are Tt and only 1/4th are TT). This leads to a phenotypicratio of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but agenotypic ratio of 1:2:1.The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensableto the form of the binomial expression (ax +by)2, that has the gametesbearing genes T or t in equal frequency of ÂŊ. The expression is expandedas given below :(1/2T + 1/2 t)2 = (1/2T + 1/2t) X (1/2T + 1/2t) = 1/4 TT + 1/2Tt + 1/4 ttMendel self-pollinated the F2 plants and found that dwarf F2 plantscontinued to generate dwarf plants in F3 and F4 generations. He concludedthat the genotype of the dwarfs was homozygous â tt. What do you thinkhe would have got had he self-pollinated a tall F2 plant?From the preceeding paragraphs it is clear that though the genotypicratios can be calculated using mathematical probability, by simply lookingat the phenotype of a dominant trait, it is not possible to know thegenotypic composition. That is, for example, whether a tall plant from F1or F2 has TT or Tt composition, cannot be predicted. Therefore, to determinethe genotype of a tall plant at F2, Mendel crossed the tall plant from F2with a dwarf plant. This he called a test cross. In a typical test cross anorganism (pea plants here) showing a dominant phenotype (and whosegenotype is to be determined) is crossed with the recessive parent insteadof self-crossing. The progenies of such a cross can easily be analysed topredict the genotype of the test organism. Figure 5.5 shows the results oftypical test cross where violet colour flower (W) is dominant over whitecolour flower (w).Using Punnett square, try to find out the nature of offspring of a test cross.What ratio did you get?Using the genotypes of this cross, can you give a general definition fora test cross?2022-2375PRINCIPLES OF INHERITANCE AND VARIATIONFigure 5.5 Diagrammatic representation of a test crossBased on his observations on monohybrid crosses Mendel proposedtwo general rules to consolidate his understanding of inheritance inmonohybrid crosses. Today these rules are called the Principles orLaws of Inheritance: the First Law or Law of Dominance and theSecond Law or Law of Segregation.5.2.1 Law of Dominance(i) Characters are controlled by discrete units called factors.(ii) Factors occur in pairs.(iii) In a dissimilar pair of factors one member of the pair dominates(dominant) the other (recessive).The law of dominance is used to explain the expression of only one ofthe parental characters in a monohybrid cross in the F1 and the expressionof both in the F2. It also explains the proportion of 3:1 obtained at the F2.5.2.2 Law of SegregationThis law is based on the fact that the alleles do not show any blendingand that both the characters are recovered as such in the F2 generationthough one of these is not seen at the F1 stage. Though the parents containtwo alleles during gamete formation, the factors or alleles of a pair segregatefrom each other such that a gamete receives only one of the two factors.Of course, a homozygous parent produces all gametes that are similarwhile a heterozygous one produces two kinds of gametes each havingone allele with equal proportion.2022-2376BIOLOGY5.2.2.1 Incomplete DominanceWhen experiments on peas were repeated using othertraits in other plants, it was found that sometimesthe F1 had a phenotype that did not resemble eitherof the two parents and was in between the two. Theinheritance of flower colour in the dog flower(snapdragon or Antirrhinum sp.) is a good exampleto understand incomplete dominance. In a crossbetween true-breeding red-flowered (RR) and truebreedingwhite-flowered plants (rr), the F1 (Rr) waspink (Figure 5.6). When the F1 was self-pollinatedthe F2 resulted in the following ratio 1 (RR) Red: 2(Rr) Pink: 1 (rr) White. Here the genotype ratios wereexactly as we would expect in any mendelianmonohybrid cross, but the phenotype ratios hadchanged from the 3:1 dominant : recessive ratio.What happened was that R was not completelydominant over r and this made it possible todistinguish Rr as pink from RR (red) and rr (white) .Explanation of the concept of dominance:What exactly is dominance? Why are some allelesdominant and some recessive? To tackle thesequestions, we must understand what a gene does.Every gene, as you know by now, contains theinformation to express a particular trait. In adiploid organism, there are two copies of eachgene, i.e., as a pair of alleles. Now, these two allelesneed not always be identical, as in a heterozygote.One of them may be different due to some changesthat it has undergone (about which you will readfurther on, and in the next chapter) which modifiesthe information that particular allele contains.Letâs take an example of a gene that containsthe information for producing an enzyme. Nowthere are two copies of this gene, the two allelicforms. Let us assume (as is more common) thatthe normal allele produces the normal enzymethat is needed for the transformation of asubstrate S. Theoretically, the modified allele could be responsible forproduction of â(i) the normal/less efficient enzyme, or(ii) a non-functional enzyme, or(iii) no enzyme at allFigure 5.6 Results of monohybrid cross inthe plant Snapdragon, whereone allele is incompletelydominant over the other allele2022-2377PRINCIPLES OF INHERITANCE AND VARIATIONIn the first case, the modified allele is equivalent to the unmodified allele,i.e., it will produce the same phenotype/trait, i.e., result in the transformationof substrate S. Such equivalent allele pairs are very common. But, if theallele produces a non-functional enzyme or no enzyme, the phenotype maybe effected. The phenotype/trait will only be dependent on the functioningof the unmodified allele. The unmodified (functioning) allele, which representsthe original phenotype is the dominant allele and the modified allele isgenerally the recessive allele. Hence, in the example above the recessive traitis seen due to non-functional enzyme or because no enzyme is produced.5.2.2.2 Co-dominanceTill now we were discussing crosses where the F1 resembled either of thetwo parents (dominance) or was in-between (incomplete dominance). But,in the case of co-dominance the F1 generation resembles both parents. Agood example is different types of red blood cells that determine ABOblood grouping in human beings. ABO blood groups are controlled bythe gene I. The plasma membrane of the red blood cells has sugar polymersthat protrude from its surface and the kind of sugar is controlled by thegene. The gene (I) has three alleles IA, IB and i. The alleles IA and IB producea slightly different form of the sugar while allele i does not produce anysugar. Because humans are diploid organisms, each person possessesany two of the three I gene alleles. IA and IB are completely dominant overi, in other words when IA and i are present only IA expresses (because idoes not produce any sugar), and when IB and i are present IB expresses.But when IA and IB are present together they both express their own typesof sugars: this is because of co-dominance. Hence red blood cells haveboth A and B types of sugars. Since there are three different alleles, thereare six different combinations of these three alleles that are possible, andtherefore, a total of six different genotypes of the human ABO blood types(Table 5.2). How many phenotypes are possible?Table 5.2: Table Showing the Genetic Basis of Blood Groupsin Human PopulationAllele from Allele from Genotype of BloodParent 1 Parent 2 offspring types ofoffspringI A I A I A I A AI A I B I A I B ABI A i I A i AI B I A I A I B ABI B I B I B I B BI B i I B i Bi i i i O2022-2378BIOLOGYDo you realise that the example of ABO blood grouping also providesa good example of multiple alleles? Here you can see that there aremore than two, i.e., three alleles, governing the same character. Since inan individual only two alleles can be present, multiple alleles can be foundonly when population studies are made.Occasionally, a single gene product may produce more than one effect.For example, starch synthesis in pea seeds is controlled by one gene. Ithas two alleles (B and b). Starch is synthesised effectively by BBhomozygotes and therefore, large starch grains are produced. In contrast,bb homozygotes have lesser efficiency in starch synthesis and producesmaller starch grains. After maturation of the seeds, BB seeds are roundand the bb seeds are wrinkled. Heterozygotes produce round seeds, andso B seems to be the dominant allele. But, the starch grains produced areof intermediate size in Bb seeds. So if starch grain size is considered asthe phenotype, then from this angle, the alleles show incompletedominance.Therefore, dominance is not an autonomous feature of a gene or theproduct that it has information for. It depends as much on the geneproduct and the production of a particular phenotype from this productas it does on the particular phenotype that we choose to examine, in casemore than one phenotype is influenced by the same gene.5.3 INHERITANCE OF TWO GENESMendel also worked with and crossed pea plants that differed in twocharacters, as is seen in the cross between a pea plant that has seeds withyellow colour and round shape and one that had seeds of green colourand wrinkled shape (Figure5.7). Mendel found that the seeds resultingfrom the crossing of the parents, had yellow coloured and round shapedseeds. Here can you tell which of the characters in the pairs yellow/green colour and round/wrinkled shape was dominant?Thus, yellow colour was dominant over green and round shapedominant over wrinkled. These results were identical to those that he gotwhen he made separate monohybrid crosses between yellow and greenseeded plants and between round and wrinkled seeded plants.Let us use the genotypic symbols Y for dominant yellow seed colourand y for recessive green seed colour, R for round shaped seeds and r forwrinkled seed shape. The genotype of the parents can then be written asRRYY and rryy. The cross between the two plants can be written downas in Figure 5.7 showing the genotypes of the parent plants. The gametesRY and ry unite on fertilisation to produce the F1 hybrid RrYy. WhenMendel self hybridised the F1 plants he found that 3/4th of F2 plants hadyellow seeds and 1/4th had green. The yellow and green colour segregatedin a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1ratio; just like in a monohybrid cross.2022-2379PRINCIPLES OF INHERITANCE AND VARIATIONFigure 5.7 Results of a dihybrid cross where the two parents differed in two pairs ofcontrasting traits: seed colour and seed shape2022-2380BIOLOGY5.3.1 Law of Independent AssortmentIn the dihybrid cross (Figure 5.7), the phenotypes round, yellow;wrinkled, yellow; round, green and wrinkled, green appeared in theratio 9:3:3:1. Such a ratio was observed for several pairs of charactersthat Mendel studied.The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow:1 green, with 3 round : 1 wrinkled. This derivation can be writtenas follows:(3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, GreenBased upon such observations on dihybrid crosses (crosses betweenplants differing in two traits) Mendel proposed a second set of generalisationsthat we call Mendelâs Law of Independent Assortment. The law states thatâwhen two pairs of traits are combined in a hybrid, segregation of one pairof characters is independent of the other pair of charactersâ.The Punnett square can be effectively used to understand theindependent segregation of the two pairs of genes during meiosis andthe production of eggs and pollen in the F1 RrYy plant. Consider thesegregation of one pair of genes R and r. Fifty per cent of the gameteshave the gene R and the other 50 per cent have r. Now besides eachgamete having either R or r, it should also have the allele Y or y. Theimportant thing to remember here is that segregation of 50 per cent Rand 50 per cent r is independent from the segregation of 50 per centY and 50 per cent y. Therefore, 50 per cent of the r bearing gameteshas Y and the other 50 per cent has y. Similarly, 50 per cent of the Rbearing gametes has Y and the other 50 per cent has y. Thus there arefour genotypes of gametes (four types of pollen and four types of eggs).The four types are RY, Ry, rY and ry each with a frequency of 25 percent or 1/4th of the total gametes produced. When you write down thefour types of eggs and pollen on the two sides of a Punnett square it isvery easy to derive the composition of the zygotes that give rise to theF2 plants (Figure 5.7). Although there are 16 squares how manydifferent types of genotypes and phenotypes are formed? Note themdown in the format given.Can you, using the Punnett square data work out the genotypic ratioat the F2 stage and fill in the format given? Is the genotypic ratioalso 9:3:3:1?S.No. Genotypes found in F2 Their expected Phenotypes5.3.2 Chromosomal Theory of InheritanceMendel published his work on inheritance of characters in 1865but for several reasons, it remained unrecognised till 1900. Firstly,2022-2381PRINCIPLES OF INHERITANCE AND VARIATIONcommunication was not easy (as it is now) in those days and his workcould not be widely publicised. Secondly, his concept of genes (orfactors, in Mendelâs words) as stable and discrete units that controlledthe expression of traits and, of the pair of alleles which did not âblendâwith each other, was not accepted by his contemporaries as anexplanation for the apparently continuous variation seen in nature.Thirdly, Mendelâs approach of using mathematics to explain biologicalphenomena was totally new and unacceptable to many of thebiologists of his time. Finally, though Mendelâs work suggested thatfactors (genes) were discrete units, he could not provide any physicalproof for the existence of factors or say what they were made of.In 1900, three Scientists (de Vries, Correns and von Tschermak)independently rediscovered Mendelâs results on the inheritance ofcharacters. Also, by this time due to advancements in microscopy thatwere taking place, scientists were able to carefully observe cell division.This led to the discovery of structures in the nucleus that appeared todouble and divide just before each cell division. These were calledchromosomes (colored bodies, as they were visualised by staining). By1902, the chromosome movement during meiosis had been worked out.Walter Sutton and Theodore Boveri noted that the behaviour ofchromosomes was parallel to the behaviour of genes and usedchromosome movement (Figure 5.8) to explain Mendelâs laws (Table 5.3).Recall that you have studied the behaviour of chromosomes during mitosis(equational division) and during meiosis (reduction division). Theimportant things to remember are that chromosomes as well as genesoccur in pairs. The two alleles of a gene pair are located on homologoussites on homologous chromosomes.Figure 5.8 Meiosis and germ cell formation in a cell with four chromosomes.Can you see how chromosomes segregate when germ cellsare formed?2022-2382BIOLOGYPossibility I Possibility IIOne long orange and short green One long orange and short redchromosome and long yellow and chromosome and long yellow andshort red chromosome at the short green chromosome at thesame pole same poleFigure 5.9 Independent assortment of chromosomesCan you tell which of these columns A or B represent the chromosomeand which represents the gene? How did you decide?During Anaphase of meiosis I, the two chromosome pairs can align atthe metaphase plate independently of each other (Figure 5.9). Tounderstand this, compare the chromosomes of four different colour inthe left and right columns. In the left column (Possibility I) orange andgreen is segregating together. But in the right hand column (PossibilityII) the orange chromosome is segregating with the red chromosomes.Table 5.3: A Comparison between the Behaviour of Chromosomesand GenesAOccur in pairsSegregate at the time of gameteformation such that only one of eachpair is transmitted to a gameteIndependent pairs segregateindependently of each otherBOccur in pairsSegregate at gamete formation and onlyone of each pair is transmitted to agameteOne pair segregates independently ofanother pair2022-2383PRINCIPLES OF INHERITANCE AND VARIATION(a) (b)Figure 5.10 Drosophilamelanogaster (a) Male(b) FemaleSutton and Boveri argued that the pairing and separation of apair of chromosomes would lead to the segregation of a pair offactors they carried. Sutton united the knowledge of chromosomalsegregation with Mendelian principles and called it thechromosomal theory of inheritance.Following this synthesis of ideas, experimental verification ofthe chromosomal theory of inheritance by Thomas Hunt Morganand his colleagues, led to discovering the basis for the variationthat sexual reproduction produced. Morgan worked with the tinyfruit flies, Drosophila melanogaster (Figure 5.10), which werefound very suitable for such studies. They could be grown onsimple synthetic medium in the laboratory. They complete their lifecycle in about two weeks, and a single mating could produce a largenumber of progeny flies. Also, there was a clear differentiation of thesexes â the male and female flies are easily distinguishable. Also, ithas many types of hereditary variations that can be seen with lowpower microscopes.5.3.3 Linkage and RecombinationMorgan carried out several dihybrid crosses in Drosophila to study genesthat were sex-linked. The crosses were similar to the dihybrid crosses carriedout by Mendel in peas. For example Morgan hybridised yellow-bodied,white-eyed females to brown-bodied, red-eyed males and intercrossed theirF1 progeny. He observed that the two genes did not segregate independentlyof each other and the F2 ratio deviated very significantly from the 9:3:3:1ratio (expected when the two genes are independent).Morgan and his group knew that the genes were located on the Xchromosome (Section 5.4) and saw quickly that when the two genes in adihybrid cross were situated on the same chromosome, the proportionof parental gene combinations were much higher than the non-parentaltype. Morgan attributed this due to the physical association or linkageof the two genes and coined the term linkage to describe this physicalassociation of genes on a chromosome and the term recombination todescribe the generation of non-parental gene combinations (Figure 5.11).Morgan and his group also found that even when genes were groupedon the same chromosome, some genes were very tightly linked (showedvery low recombination) (Figure 5.11, Cross A) while others were looselylinked (showed higher recombination) (Figure 5.11, Cross B). Forexample he found that the genes white and yellow were very tightly linkedand showed only 1.3 per cent recombination while white and miniaturewing showed 37.2 per cent recombination. His student AlfredSturtevant used the frequency of recombination between gene pairson the same chromosome as a measure of the distance between genesand âmappedâ their position on the chromosome. Today genetic maps2022-2384BIOLOGYFigure 5.11 Linkage: Results of two dihybrid crosses conducted by Morgan. Cross A showscrossing between gene y and w; Cross B shows crossing between genes w and m.Here dominant wild type alleles are represented with (+) sign in superscriptNote: The strength of linkage between y and w is higher than w and m.are extensively used as a starting point in the sequencing of wholegenomes as was done in the case of the Human Genome SequencingProject, described later.2022-2385PRINCIPLES OF INHERITANCE AND VARIATION5.4 POLYGENIC INHERITANCEMendelâs studies mainly described those traits that have distinct alternateforms such as flower colour which are either purple or white. But if youlook around you will find that there are many traits which are not sodistinct in their occurrence and are spread across a gradient. For example,in humans we donât just have tall or short people as two distinctalternatives but a whole range of possible heights. Such traits are generallycontrolled by three or more genes and are thus called as polygenic traits.Besides the involvement of multiple genes polygenic inheritance also takesinto account the influence of environment. Human skin colour is anotherclassic example for this. In a polygenic trait the phenotype reflects thecontribution of each allele, i.e., the effect of each allele is additive. Tounderstand this better let us assume that three genes A, B, C control skincolour in human with the dominant forms A, B and C responsible fordark skin colour and the recessive forms a, b and c for light skin colour.The genotype with all the dominant alleles (AABBCC) will have the darkestskin colour and that with all the recessive alleles (aabbcc) will have thelightest skin colour. As expected the genotype with three dominant allelesand three recessive alleles will have an intermediate skin colour. In thismanner the number of each type of alleles in the genotype would determinethe darkness or lightness of the skin in an individual.5.5 PLEIOTROPYWe have so far seen the effect of a gene on a single phenotype or trait.There are however instances where a single gene can exhibit multiplephenotypic expression. Such a gene is called a pleiotropic gene. Theunderlying mechanism of pleiotropy in most cases is the effect of a geneon metabolic pathways which contribute towards different phenotypes.An example of this is the disease phenylketonuria, which occurs inhumans. The disease is caused by mutation in the gene that codes for theenzyme phenyl alanine hydroxylase (single gene mutation). This manifestsitself through phenotypic expression characterised by mentalretardation and a reduction in hair and skin pigmentation.5.6 SEX DETERMINATIONThe mechanism of sex determination has always been a puzzle before thegeneticists. The initial clue about the genetic/chromosomal mechanismof sex determination can be traced back to some of the experiments carriedout in insects. In fact, the cytological observations made in a number ofinsects led to the development of the concept of genetic/chromosomalbasis of sex-determination. Henking (1891) could trace a specific nuclearstructure all through spermatogenesis in a few insects, and it was alsoobserved by him that 50 per cent of the sperm received this structureafter spermatogenesis, whereas the other 50 per cent sperm did not receiveit. Henking gave a name to this structure as the X body but he could notexplain its significance. Further investigations by other scientists led tothe conclusion that the âX bodyâ of Henking was in fact a chromosome2022-2386BIOLOGYand that is why it was given the nameX-chromosome. It was also observed that ina large number of insects the mechanism ofsex determination is of the XO type, i.e., alleggs bear an additional X-chromosomebesides the other chromosomes(autosomes). On the other hand, some of thesperms bear the X-chromosome whereassome do not. Eggs fertilised by sperm havingan X-chromosome become females and,those fertilised by sperms that do not havean X-chromosome become males. Do youthink the number of chromosomes in themale and female are equal? Due to theinvolvement of the X-chromosome in thedetermination of sex, it was designated tobe the sex chromosome, and the rest of thechromosomes were named asautosomes.Grasshopper is an example ofXO type of sex determination in which themales have only one X-chromosome besidesthe autosomes, whereas females have a pairof X-chromosomes.These observations led to theinvestigation of a number of species tounderstand the mechanism of sexdetermination. In a number of other insectsand mammals including man, XY type of sexdetermination is seen where both male andfemale have same number of chromosomes.Among the males an X-chromosome ispresent but its counter part is distinctlysmaller and called the Y-chromosome.Females, however, have a pair of Xchromosomes.Both males and females bearsame number of autosomes. Hence, the males have autosomes plus XY,while female have autosomes plus XX. In human beings and inDrosophila the males have one X and one Y chromosome, whereas femaleshave a pair of X-chromosomes besides autosomes (Figure 5.12 a, b).In the above description you have studied about two types of sexdetermining mechanisms, i.e., XO type and XY type. But in both casesmales produce two different types of gametes, (a) either with or withoutX-chromosome or (b) some gametes with X-chromosome and some withY-chromosome. Such types of sex determination mechanism is designatedto be the example of male heterogamety. In some other organisms, e.g.,birds, a different mechanism of sex determination is observed (Figure5.12 c). In this case the total number of chromosome is same in bothmales and females. But two different types of gametes in terms of the sex(a)(b)(c)Figure 5.12 Determination of sex by chromosomaldifferences: (a,b) Both in humans andin Drosophila, the female has a pair ofXX chromosomes (homogametic) and themale XY (heterogametic) composition;(c) In many birds, female has a pair ofdissimilar chromosomes ZW and maletwo similar ZZ chromosomes2022-2387PRINCIPLES OF INHERITANCE AND VARIATIONchromosomes, are produced by females, i.e., female heterogamety. Inorder to have a distinction with the mechanism of sex determinationdescribed earlier, the two different sex chromosomes of a female bird hasbeen designated to be the Z and W chromosomes. In these organisms thefemales have one Z and one W chromosome, whereas males have a pair ofZ-chromosomes besides the autosomes.5.6.1 Sex Determination in HumansIt has already been mentioned that the sex determining mechanism incase of humans is XY type. Out of 23 pairs of chromosomes present,22 pairs are exactly same in both males and females; these are theautosomes. A pair of X-chromosomes are present in the female, whereasthe presence of an X and Y chromosome are determinant of the malecharacteristic. During spermatogenesis among males, two types ofgametes are produced. 50 per cent of the total sperm produced carrythe X-chromosome and the rest 50 per cent has Y-chromosome besidesthe autosomes. Females, however, produce only one type of ovum withan X-chromosome. There is an equal probability of fertilisation of theovum with the sperm carrying either X or Y chromosome. In case theovum fertilises with a sperm carrying X-chromosome the zygote developsinto a female (XX) and the fertilisation of ovum with Y-chromosomecarrying sperm results into a male offspring. Thus, it is evident that itis the genetic makeup of the sperm that determines the sex of the child.It is also evident that in each pregnancy there is always 50 per centprobability of either a male or a female child. It is unfortunate that inour society women are blamed for giving birth to female children andhave been ostracised and ill-treated because of this false notion.5.6.2 Sex Determination in Honey BeeThe sex determination in honey bee isbased on the number of sets ofchromosomes an individual receives. Anoffspring formed from the union of asperm and an egg develops as a female(queen or worker), and an unfertilisedegg develops as a male (drone) by meansof parthenogenesis. This means that themales have half the number ofchromosomes than that of a female. Thefemales are diploid having 32chromosomes and males are haploid, i.e., having 16 chromosomes.This is called as haplodiploid sex-determination system and has specialcharacteristic features such as the males produce sperms by mitosis(Figure 5.13), they do not have father and thus cannot have sons, buthave a grandfather and can have grandsons.How is the sex-determination mechanism different in the birds?Is the sperm or the egg responsible for the sex of the chicks?Figure 5.13 Sex determination in honey bee2022-2388BIOLOGY5.7 MUTATIONMutation is a phenomenon which results in alteration of DNA sequencesand consequently results in changes in the genotype and the phenotypeof an organism. In addition to recombination, mutation is anotherphenomenon that leads to variation in DNA.As you will learn in Chapter 6, one DNA helix runs continuously fromone end to the other in each chromatid, in a highly supercoiled form.Therefore loss (deletions) or gain (insertion/duplication) of a segment ofDNA, result in alteration in chromosomes. Since genes are known to belocated on chromosomes, alteration in chromosomes results inabnormalities or aberrations. Chromosomalaberrations are commonly observed in cancer cells.In addition to the above, mutation also arise dueto change in a single base pair of DNA. This is knownas point mutation. A classical example of such amutation is sickle cell anemia. Deletions and insertionsof base pairs of DNA, causes frame-shift mutations(see Chapter 6).The mechanism of mutation is beyond the scopeof this discussion, at this level. However, there aremany chemical and physical factors that inducemutations. These are referred to as mutagens. UVradiations can cause mutations in organisms â it is amutagen.5.8 GENETIC DISORDERS5.8.1 Pedigree AnalysisThe idea that disorders are inherited has beenprevailing in the human society since long. This wasbased on the heritability of certain characteristicfeatures in families. After the rediscovery of Mendelâswork the practice of analysing inheritance pattern oftraits in human beings began. Since it is evident thatcontrol crosses that can be performed in pea plant orsome other organisms, are not possible in case ofhuman beings, study of the family history aboutinheritance of a particular trait provides analternative. Such an analysis of traits in a several of generations of a familyis called the pedigree analysis. In the pedigree analysis the inheritanceof a particular trait is represented in the family tree over generations.In human genetics, pedigree study provides a strong tool, which isutilised to trace the inheritance of a specific trait, abnormality or disease.Some of the important standard symbols used in the pedigree analysishave been shown in Figure 5.13.As you have studied in this chapter, each and every feature in anyorganism is controlled by one or the other gene located on the DNA presentFigure 5.13 Symbols used in the humanpedigree analysis2022-2389PRINCIPLES OF INHERITANCE AND VARIATIONin the chromosome. DNA is the carrier of genetic information. It is hencetransmitted from one generation to the other without any change oralteration. However, changes or alteration do take place occasionally. Suchan alteration or change in the genetic material is referred to as mutation.A number of disorders in human beings have been found to be associatedwith the inheritance of changed or altered genes or chromosomes.5.8.2 Mendelian DisordersBroadly, genetic disorders may be grouped into two categories â Mendeliandisorders and Chromosomal disorders. Mendelian disorders are mainlydetermined by alteration or mutation in the single gene. These disordersare transmitted to the offspring on the same lines as we have studied inthe principle of inheritance. The pattern of inheritance of such Mendeliandisorders can be traced in a family by the pedigree analysis. Most commonand prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sicklecellanaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It isimportant to mention here that such Mendelian disorders may bedominant or recessive. By pedigree analysis one can easily understandwhether the trait in question is dominant or recessive. Similarly, the traitmay also be linked to the sex chromosome as in case of haemophilia. It isevident that this X-linked recessive trait shows transmission from carrierfemale to male progeny. A representative pedigree is shown in Figure 5.14for dominant and recessive traits. Discuss with your teacher and designpedigrees for characters linked to both autosomes and sex chromosome.(a) (b)Figure 5.14 Representative pedigree analysis of (a) Autosomal dominant trait (for example:Myotonic dystrophy) (b) Autosomal recessive trait (for example: Sickle-cell anaemia)Colour Blidness : It is a sex-linked recessive disorder due to defect ineither red or green cone of eye resulting in failure to discriminate betweenred and green colour. This defect is due to mutation in certain genespresent in the X chromosome. It occurs in about 8 per cent of males andonly about 0.4 per cent of females. This is because the genes that lead tored-green colour blindness are on the X chromosome. Males have onlyone X chromosome and females have two. The son of a woman who carries2022-2390BIOLOGYthe gene has a 50 per cent chance of being colour blind. The mother isnot herself colour blind because the gene is recessive. That means that itseffect is suppressed by her matching dominant normal gene. A daughterwill not normally be colour blind, unless her mother is a carrier and herfather is colour blind.Haemophilia : This sex linked recessive disease, which shows itstransmission from unaffected carrier female to some of the male progenyhas been widely studied. In this disease, a single protein that is a part ofthe cascade of proteins involved in the clotting of blood is affected. Due tothis, in an affected individual a simple cut will result in non-stop bleeding.The heterozygous female (carrier) for haemophilia may transmit the diseaseto sons. The possibility of a female becoming a haemophilic is extremelyrare because mother of such a female has to be at least carrier and thefather should be haemophilic (unviable in the later stage of life). The familypedigree of Queen Victoria shows a number of haemophilic descendentsas she was a carrier of the disease.Sickle-cell anaemia : This is an autosome linked recessive trait that canbe transmitted from parents to the offspring when both the partners arecarrier for the gene (or heterozygous). The disease is controlled by a singlepair of allele, HbA and HbS. Out of the three possible genotypes onlyhomozygous individuals for HbS (HbSHbS) show the diseased phenotype.Heterozygous (HbAHbS) individuals appear apparently unaffected but theyare carrier of the disease as there is 50 per cent probability of transmissionof the mutant gene to the progeny, thus exhibiting sickle-cell trait(Figure 5.15). The defect is caused by the substitution of Glutamic acidFigure 5.15 Micrograph of the red blood cells and the amino acid composition of the relevantportion of b-chain of haemoglobin: (a) From a normal individual; (b) From an individualwith sickle-cell anaemia2022-2391PRINCIPLES OF INHERITANCE AND VARIATION(Glu) by Valine (Val) at the sixth position of the beta globin chain of thehaemoglobin molecule. The substitution of amino acid in the globinprotein results due to the single base substitution at the sixth codon ofthe beta globin gene from GAG to GUG. The mutant haemoglobin moleculeundergoes polymerisation under low oxygen tension causing the changein the shape of the RBC from biconcave disc to elongated sickle likestructure (Figure 5.15).Phenylketonuria : This inborn error of metabolism is also inherited asthe autosomal recessive trait. The affected individual lacks an enzymethat converts the amino acid phenylalanine into tyrosine. As a result ofthis phenylalanine is accumulated and converted into phenylpyruvic acidand other derivatives. Accumulation of these in brain results in mentalretardation. These are also excreted through urine because of its poorabsorption by kidney.Thalassemia : This is also an autosome-linked recessive blood diseasetransmitted from parents to the offspring when both the partners areunaffected carrier for the gene (or heterozygous). The defect could be dueto either mutation or deletion which ultimately results in reduced rate ofsynthesis of one of the globin chains (a and b chains) that make uphaemoglobin. This causes the formation of abnormal haemoglobinmolecules resulting into anaemia which is characteristic of the disease.Thalassemia can be classified according to which chain of the haemoglobinmolecule is affected. In a Thalassemia, production of a globin chain isaffected while in b Thalassemia, production of b globin chain is affected.a Thalassemia is controlled by two closely linked genes HBA1 and HBA2on chromosome 16 of each parent and it is observed due to mutation ordeletion of one or more of the four genes. The more genes affected, theless alpha globin molecules produced. While b Thalassemia is controlledby a single gene HBB on chromosome 11 of each parent and occurs dueto mutation of one or both the genes. Thalassemia differs from sickle-cellanaemia in that the former is a quantitative problem of synthesising toofew globin molecules while the latter is a qualitative problem ofsynthesising an incorrectly functioning globin.5.8.3 Chromosomal DisordersThe chromosomal disorders on the other hand are caused due to absenceor excess or abnormal arrangement of one or more chromosomes.Failure of segregation of chromatids during cell division cycle resultsin the gain or loss of a chromosome(s), called aneuploidy. For example,Downâs syndrome results in the gain of extra copy of chromosome 21.Similarly, Turnerâs syndrome results due to loss of an X chromosome inhuman females. Failure of cytokinesis after telophase stage of cell divisionresults in an increase in a whole set of chromosomes in an organism and,this phenomenon is known as polyploidy. This condition is often seen inplants.The total number of chromosomes in a normal human cell is 46(23 pairs). Out of these 22 pairs are autosomes and one pair ofchromosomes are sex chromosome. Sometimes, though rarely, either anadditional copy of a chromosome may be included in an individual or an2022-2392BIOLOGYFlat back of headMany âloopsâ onfinger tipsPalm creaseBroad flat faceBig and wrinkledtongueCongenital heartdiseaseindividual may lack one of any one pair ofchromosomes. These situations are known as trisomyor monosomy of a chromosome, respectively. Such asituation leads to very serious consequences in theindividual. Downâs syndrome, Turnerâs syndrome,Klinefelterâs syndrome are common examples ofchromosomal disorders.Downâs Syndrome : The cause of this genetic disorderis the presence of an additional copy of thechromosome number 21 (trisomy of 21). This disorderwas first described by Langdon Down (1866). Theaffected individual is short statured with small roundhead, furrowed tongue and partially open mouth(Figure 5.16). Palm is broad with characteristic palmcrease. Physical, psychomotor and mentaldevelopment is retarded.Klinefelterâs Syndrome : This genetic disorder is alsocaused due to the presence of an additional copy of Xchromosomeresulting into a karyotype of 47, XXY.Such an individual has overall masculine development,however, the feminine development (developmentof breast, i.e., Gynaecomastia) is also expressed(Figure 5.17 a). Such individuals are sterile.Turnerâs Syndrome : Such a disorder is caused dueto the absence of one of the X chromosomes, i.e., 45 with X0, Such femalesare sterile as ovaries are rudimentary besides other features includinglack of other secondary sexual characters (Figure 5.17 b).Figure 5.16 A representative figure showing an individual inflicted with Downâssyndrome and the corresponding chromosomes of the individualTall staturewith feminisedcharacterShort stature andunderdevelopedfeminine character(a)(b)Figure 5.17 Diagrammatic representationof genetic disorders due to sexchromosome composition in humans :(a) Klinefelter Syndrome; (b) TurnerâsSyndrome2022-2393PRINCIPLES OF INHERITANCE AND VARIATIONSUMMARYGenetics is a branch of biology which deals with principles of inheritanceand its practices. Progeny resembling the parents in morphological andphysiological features has attracted the attention of many biologists.Mendel was the first to study this phenomenon systematically. Whilestudying the pattern of inheritance in pea plants of contrastingcharacters, Mendel proposed the principles of inheritance, which aretoday referred to as âMendelâs Laws of Inheritanceâ. He proposed thatthe âfactorsâ (later named as genes) regulating the characters are foundin pairs known as alleles. He observed that the expression of thecharacters in the offspring follow a definite pattern in differentâfirstgenerations (F1), second (F2) and so on. Some characters are dominantover others. The dominant characters are expressed when factors arein heterozygous condition (Law of Dominance). The recessive charactersare only expressed in homozygous conditions. The characters neverblend in heterozygous condition. A recessive character that was notexpressed in heterozygous conditon may be expressed again when itbecomes homozygous. Hence, characters segregate while formation ofgametes (Law of Segregation).Not all characters show true dominance. Some characters showincomplete, and some show co-dominance. When Mendel studied theinheritance of two characters together, it was found that the factorsindependently assort and combine in all permutations andcombinations (Law of Independent Assortment). Different combinationsof gametes are theoretically represented in a square tabular form knownas âPunnett Squareâ. The factors (now known as gene) on chromosomesregulating the characters are called the genotype and the physicalexpression of the chraracters is called phenotype.After knowing that the genes are located on the chromosomes, agood correlation was drawn between Mendelâs laws : segregation andassortment of chromosomes during meiosis. The Mendelâs laws wereextended in the form of âChromosomal Theory of Inheritanceâ. Later, itwas found that Mendelâs law of independent assortment does not holdtrue for the genes that were located on the same chromosomes. Thesegenes were called as âlinked genesâ. Closely located genes assortedtogether, and distantly located genes, due to recombination, assortedindependently. Linkage maps, therefore, corresponded to arrangementof genes on a chromosome.Many genes were linked to sexes also, and called as sex-linkedgenes. The two sexes (male and female) were found to have a set ofchromosomes which were common, and another set which wasdifferent. The chromosomes which were different in two sexes werenamed as sex chromosomes. The remaining set was named asautosomes. In humans, a normal female has 22 pairs of autosomesand a pair of sex chromosomes (XX). A male has 22 pairs of autosomesand a pair of sex chromosome as XY. In chicken, sex chromosomes inmale are ZZ, and in females are ZW.Mutation is defined as change in the genetic material. A pointmutation is a change of a single base pair in DNA. Sickle-cell anemia iscaused due to change of one base in the gene coding for beta-chain ofhemoglobin. Inheritable mutations can be studied by generating apedigree of a family. Some mutations involve changes in whole set of2022-2394BIOLOGYEXERCISES1. Mention the advantages of selecting pea plant for experiment by Mendel.2. Differentiate between the following â(a) Dominance and Recessive(b) Homozygous and Heterozygous(c) Monohybrid and Dihybrid.3. A diploid organism is heterozygous for 4 loci, how many types of gametescan be produced?4. Explain the Law of Dominance using a monohybrid cross.5. Define and design a test-cross.6. Using a Punnett Square, workout the distribution of phenotypic featuresin the first filial generation after a cross between a homozygous femaleand a heterozygous male for a single locus.7. When a cross in made between tall plant with yellow seeds (TtYy) andtall plant with green seed (Ttyy), what proportions of phenotype in theoffspring could be expected to be(a) tall and green.(b) dwarf and green.8. Two heterozygous parents are crossed. If the two loci are linked whatwould be the distribution of phenotypic features in F1 generation for adibybrid cross?9. Briefly mention the contribution of T.H. Morgan in genetics.10. What is pedigree analysis? Suggest how such an analysis, can be useful.11. How is sex determined in human beings?12. A child has blood group O. If the father has blood group A and motherblood group B, work out the genotypes of the parents and the possiblegenotypes of the other offsprings.13. Explain the following terms with example(a) Co-dominance(b) Incomplete dominance14. What is point mutation? Give one example.15. Who had proposed the chromosomal theory of the inheritance?16. Mention any two autosomal genetic disorders with their symptoms.chromosomes (polyploidy) or change in a subset of chromosome number(aneuploidy). This helped in understanding the mutational basis ofgenetic disorders. Downâs syndrome is due to trisomy of chromosome 21,where there is an extra copy of chromosome 21 and consequently thetotal number of chromosome becomes 47. In Turnerâs syndrome, one Xchromosome is missing and the sex chromosome is as XO, and inKlinefelterâs syndrome, the condition is XXY. These can be easily studiedby analysis of Karyotypes.2022-23CHAPTER 6MOLECULAR BASIS OFINHERITANCE6.1 The DNA6.2 The Search for GeneticMaterial6.3 RNA World6.4 Replication6.5 Transcription6.6 Genetic Code6.7 Translation6.8 Regulation of GeneExpression6.9 Human Genome Project6.10 DNA FingerprintingIn the previous chapter, you have learnt the inheritancepatterns and the genetic basis of such patterns. At thetime of Mendel, the nature of those âfactorsâ regulatingthe pattern of inheritance was not clear. Over the nexthundred years, the nature of the putative genetic materialwas investigated culminating in the realisation thatDNA â deoxyribonucleic acid â is the genetic material, atleast for the majority of organisms. In class XI you havelearnt that nucleic acids are polymers of nucleotides.Deoxyribonucleic acid (DNA) and ribonucleic acid(RNA) are the two types of nucleic acids found in livingsystems. DNA acts as the genetic material in most of theorganisms. RNA though it also acts as a genetic materialin some viruses, mostly functions as a messenger. RNAhas additional roles as well. It functions as adapter,structural, and in some cases as a catalytic molecule. InClass XI you have already learnt the structures ofnucleotides and the way these monomer units are linkedto form nucleic acid polymers. In this chapter we are goingto discuss the structure of DNA, its replication, the processof making RNA from DNA (transcription), the genetic codethat determines the sequences of amino acids in proteins,the process of protein synthesis (translation) andelementary basis of their regulation. The determination2022-2396BIOLOGYof complete nucleotide sequence of human genome during last decadehas set in a new era of genomics. In the last section, the essentials ofhuman genome sequencing and its consequences will also be discussed.Let us begin our discussion by first understanding the structure ofthe most interesting molecule in the living system, that is, the DNA. Insubsequent sections, we will understand that why it is the most abundantgenetic material, and what its relationship is with RNA.6.1 THE DNADNA is a long polymer of deoxyribonucleotides. The length of DNA isusually defined as number of nucleotides (or a pair of nucleotide referredto as base pairs) present in it. This also is the characteristic of an organism.For example, a bacteriophage known as f Ã174 has 5386 nucleotides,Bacteriophage lambda has 48502 base pairs (bp), Escherichia coli has4.6 à 106 bp, and haploid content of human DNA is 3.3 à 109 bp. Let usdiscuss the structure of such a long polymer.6.1.1 Structure of Polynucleotide ChainLet us recapitulate the chemical structure of a polynucleotide chain (DNAor RNA). A nucleotide has three components â a nitrogenous base, apentose sugar (ribose in case of RNA, and deoxyribose for DNA), and aphosphate group. There are two types of nitrogenous bases â Purines(Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).Cytosine is common for both DNA and RNA and Thymine is present inDNA. Uracil is present in RNA at the place of Thymine. A nitrogenousbase is linked to the OH of 1' C pentose sugar through a N-glycosidiclinkage to form a nucleoside, such as adenosine or deoxyadenosine,guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine ordeoxythymidine. When a phosphate group is linked to OH of 5' C of anucleoside through phosphoester linkage, a corresponding nucleotide(or deoxynucleotide depending upon the type of sugar present) is formed.Two nucleotides are linked through 3'-5' phosphodiester linkage to forma dinucleotide. More nucleotides can be joined in such a manner to forma polynucleotide chain. A polymer thus formed has at one end a freeFigure 6.1 A Polynucleotide chain2022-2397MOLECULAR BASIS OF INHERITANCEphosphate moiety at 5' -end of sugar, which is referred to as 5â-end ofpolynucleotide chain. Similarly, at the other end of the polymer the sugarhas a free OH of 3'C group which is referred to as 3' -end of thepolynucleotide chain. The backbone of a polynucleotide chain is formeddue to sugar and phosphates. The nitrogenous bases linked to sugarmoiety project from the backbone (Figure 6.1).In RNA, every nucleotide residue has an additional âOH group presentat 2' -position in the ribose. Also, in RNA the uracil is found at the place ofthymine (5-methyl uracil, another chemical name for thymine).DNA as an acidic substance present in nucleus was first identified byFriedrich Meischer in 1869. He named it as âNucleinâ. However, due totechnical limitation in isolating such a long polymer intact, the elucidationof structure of DNA remained elusive for a very long period of time. It wasonly in 1953 that James Watson and Francis Crick, based on the X-raydiffraction data produced by Maurice Wilkins and Rosalind Franklin,proposed a very simple but famous Double Helix model for the structureof DNA. One of the hallmarks of their proposition was base pairing betweenthe two strands of polynucleotide chains. However, this proposition wasalso based on the observation of Erwin Chargaff that for a double strandedDNA, the ratios between Adenine and Thymine and Guanine and Cytosineare constant and equals one.The base pairing confers a very unique property to the polynucleotidechains. They are said to be complementary to each other, and therefore ifthe sequence of bases in one strand is known then the sequence in otherstrand can be predicted. Also, if each strand from a DNA (let us call it as aparental DNA) acts as a template for synthesis of a new strand, the twodouble stranded DNA (let us call them as daughter DNA) thus, producedwould be identical to the parental DNA molecule. Because of this, the geneticimplications of the structure of DNA became very clear.The salient features of the Double-helix structure of DNA are as follows:(i) It is made of two polynucleotide chains, where the backbone isconstituted by sugar-phosphate, and the bases project inside.(ii) The two chains have anti-parallel polarity. It means, if onechain has the polarity 5'à 3', the other has 3'à 5' .(iii) The bases in two strands are paired through hydrogen bond(H-bonds) forming base pairs (bp). Adenine forms two hydrogenbonds with Thymine from opposite strand and vice-versa.Similarly, Guanine is bonded with Cytosine with three H-bonds.As a result, always a purine comes opposite to a pyrimidine. Thisgenerates approximately uniform distance between the twostrands of the helix (Figure 6.2).(iv) The two chains are coiled in a right-handed fashion. The pitchof the helix is 3.4 nm (a nanometre is one billionth of ametre, that is 10-9 m) and there are roughly 10 bp in each2022-2398BIOLOGYFigure 6.2 Double stranded polynucleotide chainFigure 6.3 DNA double helixturn. Consequently, the distancebetween a bp in a helix isapproximately 0.34 nm.(v) The plane of one base pair stacksover the other in double helix. This,in addition to H-bonds, confersstability of the helical structure(Figure 6.3).Compare the structure of purines andpyrimidines. Can you find out why thedistance between two polynucleotidechains in DNA remains almost constant?The proposition of a double helixstructure for DNA and its simplicity inexplaining the genetic implication becamerevolutionary. Very soon, Francis Crickproposed the Central dogma in molecularbiology, which states that the geneticinformation flows from DNAà RNAà Protein.Central dogma2022-2399MOLECULAR BASIS OF INHERITANCEFigure 6.4a NucleosomeFigure 6.4b EM picture - âBeads-on-StringâIn some viruses the flow of information is in reverse direction, that is,from RNA to DNA. Can you suggest a simple name to the process?6.1.2 Packaging of DNA HelixTaken the distance between two consecutive base pairsas 0.34 nm (0.34Ã10â9 m), if the length of DNA doublehelix in a typical mammalian cell is calculated (simplyby multiplying the total number of bp with distancebetween two consecutive bp, that is, 6.6 à 109 bp Ã0.34 à 10-9m/bp), it comes out to be approximately2.2 metres. A length that is far greater than thedimension of a typical nucleus (approximately 10â6 m).How is such a long polymer packaged in a cell?If the length of E. coli DNA is 1.36 mm, can youcalculate the number of base pairs in E.coli?In prokaryotes, such as, E. coli, though they donot have a defined nucleus, the DNA is not scatteredthroughout the cell. DNA (being negatively charged)is held with some proteins (that have positivecharges) in a region termed as ânucleoidâ. The DNAin nucleoid is organised in large loops held byproteins.In eukaryotes, this organisation is much morecomplex. There is a set of positively charged, basicproteins called histones. A protein acquires chargedepending upon the abundance of amino acidsresidues with charged side chains. Histones are richin the basic amino acid residues lysine and arginine.Both the amino acid residues carry positive chargesin their side chains. Histones are organised to forma unit of eight molecules called histone octamer.The negatively charged DNA is wrapped around the positively chargedhistone octamer to form a structure called nucleosome (Figure 6.4 a). Atypical nucleosome contains 200 bp of DNA helix. Nucleosomes constitutethe repeating unit of a structure in nucleus called chromatin, threadlikestained (coloured) bodies seen in nucleus. The nucleosomes inchromatin are seen as âbeads-on-stringâ structure when viewed underelectron microscope (EM) (Figure 6.4 b).Theoretically, how many such beads (nucleosomes) do you imagineare present in a mammalian cell?The beads-on-string structure in chromatin is packaged to formchromatin fibers that are further coiled and condensed at metaphase stageof cell division to form chromosomes. The packaging of chromatin at higherlevel requires additional set of proteins that collectively are referred to as2022-23100BIOLOGYNon-histone Chromosomal (NHC) proteins. In a typical nucleus, someregion of chromatin are loosely packed (and stains light) and are referred toas euchromatin. The chromatin that is more densely packed and stainsdark are called as Heterochromatin. Euchromatin is said to betranscriptionally active chromatin, whereas heterochromatin is inactive.6.2 THE SEARCH FOR GENETIC MATERIALEven though the discovery of nuclein by Meischer and the propositionfor principles of inheritance by Mendel were almost at the same time, butthat the DNA acts as a genetic material took long to be discovered andproven. By 1926, the quest to determine the mechanism for geneticinheritance had reached the molecular level. Previous discoveries byGregor Mendel, Walter Sutton, Thomas Hunt Morgan and numerous otherscientists had narrowed the search to the chromosomes located in thenucleus of most cells. But the question of what molecule was actually thegenetic material, had not been answered.Transforming PrincipleIn 1928, Frederick Griffith, in a series of experiments with Streptococcuspneumoniae (bacterium responsible for pneumonia), witnessed amiraculous transformation in the bacteria. During the course of hisexperiment, a living organism (bacteria) had changed in physical form.When Streptococcus pneumoniae (pneumococcus) bacteria are grownon a culture plate, some produce smooth shiny colonies (S) while othersproduce rough colonies (R). This is because the S strain bacteria have amucous (polysaccharide) coat, while R strain does not. Mice infected withthe S strain (virulent) die from pneumonia infection but mice infectedwith the R strain do not develop pneumonia.Griffith was able to kill bacteria by heating them. He observed thatheat-killed S strain bacteria injected into mice did not kill them. When he2022-23101MOLECULAR BASIS OF INHERITANCEinjected a mixture of heat-killed S and live R bacteria, the mice died.Moreover, he recovered living S bacteria from the dead mice.He concluded that the R strain bacteria had somehow beentransformed by the heat-killed S strain bacteria. Some âtransformingprincipleâ, transferred from the heat-killed S strain, had enabled theR strain to synthesise a smooth polysaccharide coat and become virulent.This must be due to the transfer of the genetic material. However, thebiochemical nature of genetic material was not defined from hisexperiments.Biochemical Characterisation of Transforming PrinciplePrior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty(1933-44), the genetic material was thought to be a protein. They workedto determine the biochemical nature of âtransforming principleâ in Griffith'sexperiment.They purified biochemicals (proteins, DNA, RNA, etc.) from theheat-killed S cells to see which ones could transform live R cells intoS cells. They discovered that DNA alone from S bacteria caused R bacteriato become transformed.They also discovered that protein-digesting enzymes (proteases) andRNA-digesting enzymes (RNases) did not affect transformation, so thetransforming substance was not a protein or RNA. Digestion with DNasedid inhibit transformation, suggesting that the DNA caused thetransformation. They concluded that DNA is the hereditary material, butnot all biologists were convinced.Can you think of any difference between DNAs and DNase?6.2.1 The Genetic Material is DNAThe unequivocal proof that DNA is the genetic material came from theexperiments of Alfred Hershey and Martha Chase (1952). They workedwith viruses that infect bacteria called bacteriophages.The bacteriophage attaches to the bacteria and its genetic materialthen enters the bacterial cell. The bacterial cell treats the viral geneticmaterial as if it was its own and subsequently manufactures more virusparticles. Hershey and Chase worked to discover whether it was proteinor DNA from the viruses that entered the bacteria.They grew some viruses on a medium that contained radioactivephosphorus and some others on medium that contained radioactive sulfur.Viruses grown in the presence of radioactive phosphorus containedradioactive DNA but not radioactive protein because DNA containsphosphorus but protein does not. Similarly, viruses grown on radioactivesulfur contained radioactive protein but not radioactive DNA becauseDNA does not contain sulfur.2022-23102BIOLOGYRadioactive phages were allowed to attach to E. coli bacteria. Then, asthe infection proceeded, the viral coats were removed from the bacteria byagitating them in a blender. The virus particles were separated from thebacteria by spinning them in a centrifuge.Bacteria which was infected with viruses that had radioactive DNAwere radioactive, indicating that DNA was the material that passed fromthe virus to the bacteria. Bacteria that were infected with viruses that hadradioactive proteins were not radioactive. This indicates that proteins didnot enter the bacteria from the viruses. DNA is therefore the geneticmaterial that is passed from virus to bacteria (Figure 6.5).Figure 6.5 The Hershey-Chase experiment6.2.2 Properties of Genetic Material (DNA versus RNA)From the foregoing discussion, it is clear that the debate between proteinsversus DNA as the genetic material was unequivocally resolved fromHershey-Chase experiment. It became an established fact that it is DNAthat acts as genetic material. However, it subsequently became clear that2022-23103MOLECULAR BASIS OF INHERITANCEin some viruses, RNA is the genetic material (for example, Tobacco Mosaicviruses, QB bacteriophage, etc.). Answer to some of the questions such as,why DNA is the predominant genetic material, whereas RNA performsdynamic functions of messenger and adapter has to be found from thedifferences between chemical structures of the two nucleic acid molecules.Can you recall the two chemical differences between DNA and RNA?A molecule that can act as a genetic material must fulfill the followingcriteria:(i) It should be able to generate its replica (Replication).(ii) It should be stable chemically and structurally.(iii) It should provide the scope for slow changes (mutation) thatare required for evolution.(iv) It should be able to express itself in the form of 'MendelianCharactersâ.If one examines each requirement one by one, because of rule of basepairing and complementarity, both the nucleic acids (DNA and RNA) havethe ability to direct their duplications. The other molecules in the livingsystem, such as proteins fail to fulfill first criteria itself.The genetic material should be stable enough not to change withdifferent stages of life cycle, age or with change in physiology of theorganism. Stability as one of the properties of genetic material was veryevident in Griffithâs âtransforming principleâ itself that heat, which killedthe bacteria, at least did not destroy some of the properties of geneticmaterial. This now can easily be explained in light of the DNA that thetwo strands being complementary if separated by heating come together,when appropriate conditions are provided. Further, 2'-OH group presentat every nucleotide in RNA is a reactive group and makes RNA labile andeasily degradable. RNA is also now known to be catalytic, hence reactive.Therefore, DNA chemically is less reactive and structurally more stablewhen compared to RNA. Therefore, among the two nucleic acids, the DNAis a better genetic material.In fact, the presence of thymine at the place of uracil also confersadditional stability to DNA. (Detailed discussion about this requiresunderstanding of the process of repair in DNA, and you will study theseprocesses in higher classes.)Both DNA and RNA are able to mutate. In fact, RNA being unstable,mutate at a faster rate. Consequently, viruses having RNA genome andhaving shorter life span mutate and evolve faster.RNA can directly code for the synthesis of proteins, hence can easilyexpress the characters. DNA, however, is dependent on RNA for synthesisof proteins. The protein synthesising machinery has evolved around RNA.The above discussion indicate that both RNA and DNA can function as2022-23104BIOLOGYgenetic material, but DNA being more stable is preferred for storage ofgenetic information. For the transmission of genetic information, RNAis better.6.3 RNA WORLDFrom foregoing discussion, an immediate question becomes evident âwhich is the first genetic material? It shall be discussed in detail in thechapter on chemical evolution, but briefly, we shall highlight some of thefacts and points.RNA was the first genetic material. There is now enough evidence tosuggest that essential life processes (such as metabolism, translation,splicing, etc.), evolved around RNA. RNA used to act asa genetic material as well as a catalyst (there are someimportant biochemical reactions in living systems thatare catalysed by RNA catalysts and not by proteinenzymes). But, RNA being a catalyst was reactive andhence unstable. Therefore, DNA has evolved from RNAwith chemical modifications that make it more stable.DNA being double stranded and having complementarystrand further resists changes by evolving a process ofrepair.6.4 REPLICATIONWhile proposing the double helical structure for DNA,Watson and Crick had immediately proposed a schemefor replication of DNA. To quote their original statementthat is as follows:ââIt has not escaped our notice that the specificpairing we have postulated immediately suggests apossible copying mechanism for the genetic materialââ(Watson and Crick, 1953).The scheme suggested that the two strands wouldseparate and act as a template for the synthesis of newcomplementary strands. After the completion ofreplication, each DNA molecule would have oneparental and one newly synthesised strand. Thisscheme was termed as semiconservative DNAreplication (Figure 6.6).6.4.1 The Experimental ProofIt is now proven that DNA replicates semiconservatively. It was shown first inEscherichia coli and subsequently in higher organisms, such as plantsFigure 6.6 Watson-Crick model forsemiconservative DNAreplication2022-23105MOLECULAR BASIS OF INHERITANCEFigure 6.7 Meselson and Stahlâs Experimentand human cells. Matthew Meselson and Franklin Stahl performed thefollowing experiment in 1958:(i) They grew E. coli in a medium containing 15NH4Cl (15N is the heavyisotope of nitrogen) as the only nitrogen source for manygenerations. The result was that 15N was incorporated into newlysynthesised DNA (as well as other nitrogen containing compounds).This heavy DNA molecule could be distinguished from the normalDNA by centrifugation in a cesium chloride (CsCl) density gradient(Please note that 15N is not a radioactive isotope, and it can beseparated from 14N only based on densities).(ii) Then they transferred the cells into a medium with normal14NH4Cl and took samples at various definite time intervals asthe cells multiplied, and extracted the DNA that remained asdouble-stranded helices. The various samples were separatedindependently on CsCl gradients to measure the densities ofDNA (Figure 6.7).Can you recall what centrifugal force is, and think why amolecule with higher mass/density would sediment faster?The results are shown in Figure 6.7.(iii) Thus, the DNA that was extracted from the culture onegeneration after the transfer from 15N to 14N medium [that isafter 20 minutes; E. coli divides in 20 minutes] had a hybrid orintermediate density. DNA extracted from the culture afteranother generation [that is after 40 minutes, II generation] was2022-23106BIOLOGYcomposed of equal amounts of this hybrid DNA and of âlightâDNA.If E. coli was allowed to grow for 80 minutes then what would be theproportions of light and hybrid densities DNA molecule?Very similar experiments involving use of radioactive thymidine todetect distribution of newly synthesised DNA in the chromosomes wasperformed on Vicia faba (faba beans) by Taylor and colleagues in 1958.The experiments proved that the DNA in chromosomes also replicatesemiconservatively.6.4.2 The Machinery and the EnzymesIn living cells, such as E. coli, the process of replication requires a set ofcatalysts (enzymes). The main enzyme is referred to as DNA-dependentDNA polymerase, since it uses a DNA template to catalyse thepolymerisation of deoxynucleotides. These enzymes are highly efficientenzymes as they have to catalyse polymerisation of a large number ofnucleotides in a very short time. E. coli that has only 4.6 Ã106 bp (compareit with human whose diploid content is 6.6 à 109 bp), completes theprocess of replication within 18 minutes; that means the average rate ofpolymerisation has to be approximately 2000 bp per second. Not only dothese polymerases have to be fast, but they also have to catalyse the reactionwith high degree of accuracy. Any mistake during replication would resultinto mutations. Furthermore, energetically replication is a very expensiveprocess. Deoxyribonucleoside triphosphates serve dual purposes. Inaddition to acting as substrates, they provide energy for polymerisationreaction (the two terminal phosphates in a deoxynucleoside triphosphatesare high-energy phosphates, same as in case of ATP).In addition to DNA-dependent DNA polymerases, many additionalenzymes are required to complete the process of replication with highdegree of accuracy. For long DNA molecules, since the two strands ofDNA cannot be separated in its entire length (due to very high energyrequirement), the replication occur within a small opening of the DNAhelix, referred to as replication fork. The DNA-dependent DNApolymerases catalyse polymerisation only in one direction, that is 5'à 3' .This creates some additional complications at the replicating fork.Consequently, on one strand (the template with polarity 3'à 5' ), thereplication is continuous, while on the other (the template withpolarity 5'à 3' ), it is discontinuous. The discontinuously synthesisedfragments are later joined by the enzyme DNA ligase (Figure 6.8).The DNA polymerases on their own cannot initiate the process ofreplication. Also the replication does not initiate randomly at any placein DNA. There is a definite region in E. coli DNA where the replicationoriginates. Such regions are termed as origin of replication. It is2022-23107MOLECULAR BASIS OF INHERITANCEbecause of the requirement of the origin ofreplication that a piece of DNA if needed to bepropagated during recombinant DNA procedures,requires a vector. The vectors provide the origin ofreplication.Further, not every detail of replication isunderstood well. In eukaryotes, the replication ofDNA takes place at S-phase of the cell-cycle. Thereplication of DNA and cell division cycle should behighly coordinated. A failure in cell division afterDNA replication results into polyploidy(achromosomal anomaly). You will learn the detailednature of origin and the processes occurring at thissite, in higher classes.6.5 TRANSCRIPTIONThe process of copying genetic information from onestrand of the DNA into RNA is termed astranscription. Here also, the principle ofcomplementarity governs the process of transcription, except the adenosinecomplements now forms base pair with uracil instead of thymine. However,unlike in the process of replication, which once set in, the total DNA of anorganism gets duplicated, in transcription only a segment of DNA andonly one of the strands is copied into RNA. This necessitates defining theboundaries that would demarcate the region and the strand of DNA thatwould be transcribed.Why both the strands are not copied during transcription has thesimple answer. First, if both strands act as a template, they would codefor RNA molecule with different sequences (Remember complementaritydoes not mean identical), and in turn, if they code for proteins, the sequenceof amino acids in the proteins would be different. Hence, one segment ofthe DNA would be coding for two different proteins, and this wouldcomplicate the genetic information transfer machinery. Second, the twoRNA molecules if produced simultaneously would be complementary toeach other, hence would form a double stranded RNA. This would preventRNA from being translated into protein and the exercise of transcriptionwould become a futile one.6.5.1 Transcription UnitA transcription unit in DNA is defined primarily by the three regions inthe DNA:(i) A Promoter(ii) The Structural gene(iii) A TerminatorFigure 6.8 Replicating Fork2022-23108BIOLOGYThere is a convention in defining the two strands of the DNA in thestructural gene of a transcription unit. Since the two strands have oppositepolarity and the DNA-dependent RNA polymerase also catalyse thepolymerisation in only one direction, that is, 5'ÂŽ3' , the strand that hasthe polarity 3'ÂŽ5' acts as a template, and is also referred to as templatestrand. The other strand which has the polarity (5'ÂŽ3') and the sequencesame as RNA (except thymine at the place of uracil), is displaced duringtranscription. Strangely, this strand (which does not code for anything)is referred to as coding strand. All the reference point while defining atranscription unit is made with coding strand. To explain the point, ahypothetical sequence from a transcription unit is represented below:3'-ATGCATGCATGCATGCATGCATGC-5' Template Strand5'-TACGTACGTACGTACGTACGTACG-3' Coding StrandCan you now write the sequence of RNA transcribed from the above DNA?Figure 6.9 Schematic structure of a transcription unitThe promoter and terminator flank the structural gene in atranscription unit. The promoter is said to be located towards 5'-end(upstream) of the structural gene (the reference is made with respect tothe polarity of coding strand). It is a DNA sequence that provides bindingsite for RNA polymerase, and it is the presence of a promoter in atranscription unit that also defines the template and coding strands. Byswitching its position with terminator, the definition of coding and templatestrands could be reversed. The terminator is located towards 3'-end(downstream) of the coding strand and it usually defines the end of theprocess of transcription (Figure 6.9). There are additional regulatorysequences that may be present further upstream or downstream to thepromoter. Some of the properties of these sequences shall be discussedwhile dealing with regulation of gene expression.6.5.2 Transcription Unit and the GeneA gene is defined as the functional unit of inheritance. Though there is noambiguity that the genes are located on the DNA, it is difficult to literally2022-23109MOLECULAR BASIS OF INHERITANCEdefine a gene in terms of DNA sequence. The DNA sequence coding fortRNA or rRNA molecule also define a gene. However by defining a cistronas a segment of DNA coding for a polypeptide, the structural gene in atranscription unit could be said as monocistronic (mostly in eukaryotes)or polycistronic (mostly in bacteria or prokaryotes). In eukaryotes, themonocistronic structural genes have interrupted coding sequences â thegenes in eukaryotes are split. The coding sequences or expressedsequences are defined as exons. Exons are said to be those sequencethat appear in mature or processed RNA. The exons are interrupted byintrons. Introns or intervening sequences do not appear in mature orprocessed RNA. The split-gene arrangement further complicates thedefinition of a gene in terms of a DNA segment.Inheritance of a character is also affected by promoter and regulatorysequences of a structural gene. Hence, sometime the regulatory sequencesare loosely defined as regulatory genes, even though these sequences donot code for any RNA or protein.6.5.3 Types of RNA and the process of TranscriptionIn bacteria, there are three major types of RNAs: mRNA (messenger RNA),tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs areneeded to synthesise a protein in a cell. The mRNA provides the template,tRNA brings aminoacids and reads the genetic code, and rRNAs playstructural and catalytic role during translation. There is singleDNA-dependent RNA polymerase that catalyses transcription of all typesof RNA in bacteria. RNA polymerase binds to promoter and initiatestranscription (Initiation). It uses nucleoside triphosphates as substrateFigure 6.10 Process of Transcription in Bacteria2022-23110BIOLOGYand polymerises in a template depended fashion following the rule ofcomplementarity. It somehow also facilitates opening of the helix andcontinues elongation. Only a short stretch of RNA remains bound to theenzyme. Once the polymerases reaches the terminator region, the nascentRNA falls off, so also the RNA polymerase. This results in termination oftranscription.An intriguing question is that how is the RNA polymerases ableto catalyse all the three steps, which are initiation, elongation andtermination. The RNA polymerase is only capable of catalysing theprocess of elongation. It associates transiently with initiation-factor (s)and termination-factor (r) to initiate and terminate the transcription,respectively. Association with these factors alter the specificity of theRNA polymerase to either initiate or terminate (Figure 6.10).In bacteria, since the mRNA does not require any processing to becomeactive, and also since transcription and translation take place in the samecompartment (there is no separation of cytosol and nucleus in bacteria),many times the translation can begin much before the mRNA is fullytranscribed. Consequently, the transcription and translation can be coupledin bacteria.In eukaryotes, there are two additional complexities â(i) There are at least three RNA polymerases in the nucleus (in additionto the RNA polymerase found in the organelles). There is a clearcut division of labour. The RNA polymerase I transcribes rRNAsFigure 6.11 Process of Transcription in Eukaryotes2022-23111MOLECULAR BASIS OF INHERITANCE(28S, 18S, and 5.8S), whereas the RNA polymerase III is responsiblefor transcription of tRNA, 5srRNA, and snRNAs (small nuclearRNAs). The RNA polymerase II transcribes precursor of mRNA, theheterogeneous nuclear RNA (hnRNA).(ii) The second complexity is that the primary transcripts contain boththe exons and the introns and are non-functional. Hence, it issubjected to a process called splicing where the introns are removedand exons are joined in a defined order. hnRNA undergoesadditional processing called as capping and tailing. In capping anunusual nucleotide (methyl guanosine triphosphate) is added tothe 5'-end of hnRNA. In tailing, adenylate residues (200-300) areadded at 3'-end in a template independent manner. It is the fullyprocessed hnRNA, now called mRNA, that is transported out of thenucleus for translation (Figure 6.11).The significance of such complexities is now beginning to beunderstood. The split-gene arrangements represent probably an ancientfeature of the genome. The presence of introns is reminiscent of antiquity,and the process of splicing represents the dominance of RNA-world. Inrecent times, the understanding of RNA and RNA-dependent processesin the living system have assumed more importance.6.6 GENETIC CODEDuring replication and transcription a nucleic acid was copied to formanother nucleic acid. Hence, these processes are easy to conceptualiseon the basis of complementarity. The process of translation requirestransfer of genetic information from a polymer of nucleotides to synthesisea polymer of amino acids. Neither does any complementarity exist betweennucleotides and amino acids, nor could any be drawn theoretically. Thereexisted ample evidences, though, to support the notion that change innucleic acids (genetic material) were responsible for change in amino acidsin proteins. This led to the proposition of a genetic code that could directthe sequence of amino acids during synthesis of proteins.If determining the biochemical nature of genetic material and thestructure of DNA was very exciting, the proposition and deciphering ofgenetic code were most challenging. In a very true sense, it requiredinvolvement of scientists from several disciplines â physicists, organicchemists, biochemists and geneticists. It was George Gamow, a physicist,who argued that since there are only 4 bases and if they have to code for20 amino acids, the code should constitute a combination of bases. Hesuggested that in order to code for all the 20 amino acids, the code shouldbe made up of three nucleotides. This was a very bold proposition, becausea permutation combination of 43 (4 à 4 à 4) would generate 64 codons;generating many more codons than required.Providing proof that the codon was a triplet, was a more dauntingtask. The chemical method developed by Har Gobind Khorana was2022-23112BIOLOGYinstrumental in synthesising RNA molecules with defined combinationsof bases (homopolymers and copolymers). Marshall Nirenbergâs cell-freesystem for protein synthesis finally helped the code to be deciphered.Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpfulin polymerising RNA with defined sequences in a template independentmanner (enzymatic synthesis of RNA). Finally a checker-board for geneticcode was prepared which is given in Table 6.1.Table 6.1: The Codons for the Various Amino AcidsThe salient features of genetic code are as follows:(i) The codon is triplet. 61 codons code for amino acids and 3 codons donot code for any amino acids, hence they function as stop codons.(ii) Some amino acids are coded by more than one codon, hencethe code is degenerate.(iii) The codon is read in mRNA in a contiguous fashion. There areno punctuations.(iv) The code is nearly universal: for example, from bacteria to humanUUU would code for Phenylalanine (phe). Some exceptions to thisrule have been found in mitochondrial codons, and in someprotozoans.(v) AUG has dual functions. It codes for Methionine (met) , and italso act as initiator codon.(vi) UAA, UAG, UGA are stop terminator codons.If following is the sequence of nucleotides in mRNA, predict thesequence of amino acid coded by it (take help of the checkerboard):-AUG UUU UUC UUC UUU UUU UUC-2022-23113MOLECULAR BASIS OF INHERITANCENow try the opposite. Following is the sequence of amino acids codedby an mRNA. Predict the nucleotide sequence in the RNA:Met-Phe-Phe-Phe-Phe-Phe-PheDo you face any difficulty in predicting the opposite?Can you now correlate which two properties of genetic code you havelearnt?6.6.1 Mutations and Genetic CodeThe relationships between genes and DNA are best understood by mutationstudies. You have studied about mutation and its effect in Chapter 5. Effectsof large deletions and rearrangements in a segment of DNA are easy tocomprehend. It may result in loss or gain of a gene and so a function. Theeffect of point mutations will be explained here. A classical example ofpoint mutation is a change of single base pair in the gene for beta globinchain that results in the change of amino acid residue glutamate to valine.It results into a diseased condition called as sickle cell anemia. Effect ofpoint mutations that inserts or deletes a base in structural gene can bebetter understood by following simple example.Consider a statement that is made up of the following words eachhaving three letters like genetic code.RAM HAS RED CAPIf we insert a letter B in between HAS and RED and rearrange thestatement, it would read as follows:RAM HAS BRE DCA PSimilarly, if we now insert two letters at the same place, say BI'. Now itwould read,RAM HAS BIR EDC APNow we insert three letters together, say BIG, the statement would readRAM HAS BIG RED CAPThe same exercise can be repeated, by deleting the letters R, E and D,one by one and rearranging the statement to make a triplet word.RAM HAS EDC APRAM HAS DCA PRAM HAS CAPThe conclusion from the above exercise is very obvious. Insertion ordeletion of one or two bases changes the reading frame from the point ofinsertion or deletion. However, such mutations are referred to as2022-23114BIOLOGYframeshift insertion or deletion mutations. Insertion or deletion ofthree or its multiple bases insert or delete in one or multiple codon henceone or multiple amino acids, and reading frame remains unaltered fromthat point onwards.6.6.2 tRNAâ the Adapter MoleculeFrom the very beginning of the proposition of code, it was clear to FrancisCrick that there has to be a mechanism to read the code and also to link itto the amino acids, because amino acids have no structural specialities toread the code uniquely. He postulated the presence of an adapter moleculethat would on one hand read the code and on other hand would bindto specific amino acids. The tRNA, then called sRNA (soluble RNA),was known before the genetic code was postulated. However, its roleas an adapter molecule was assigned much later.tRNA has ananticodon loopthat has basescomplementary tothe code, and it alsohas an amino acidacceptor end towhich it binds toamino acids.tRNAs are specificfor each amino acid(Figure 6.12). Forinitiation, there isanother specific tRNA that is referred to as initiator tRNA. There are notRNAs for stop codons. In figure 6.12, the secondary structure of tRNAhas been depicted that looks like a clover-leaf. In actual structure, thetRNA is a compact molecule which looks like inverted L.6.7 TRANSLATIONTranslation refers to the process of polymerisation of amino acids toform a polypeptide (Figure 6.13). The order and sequence of amino acidsare defined by the sequence of bases in the mRNA. The amino acids arejoined by a bond which is known as a peptide bond. Formation of apeptide bond requires energy. Therefore, in the first phase itself aminoacids are activated in the presence of ATP and linked to their cognatetRNAâa process commonly called as charging of tRNA oraminoacylation of tRNA to be more specific. If two such charged tRNAsare brought close enough, the formation of peptide bond between themFigure 6.12 tRNA - the adapter molecule2022-23115MOLECULAR BASIS OF INHERITANCEwould be favoured energetically. Thepresence of a catalyst would enhancethe rate of peptide bond formation.The cellular factory responsible forsynthesising proteins is the ribosome.The ribosome consists of structuralRNAs and about 80 different proteins.In its inactive state, it exists as twosubunits; a large subunit and a smallsubunit. When the small subunitencounters an mRNA, the process oftranslation of the mRNA to proteinbegins. There are two sites in the largesubunit, for subsequent amino acidsto bind to and thus, be close enoughto each other for the formation of apeptide bond. The ribosome also acts as a catalyst (23S rRNA in bacteriais the enzyme- ribozyme) for the formation of peptide bond.A translational unit in mRNA is the sequence of RNA that is flankedby the start codon (AUG) and the stop codon and codes for a polypeptide.An mRNA also has some additional sequences that are not translatedand are referred as untranslated regions (UTR). The UTRs are presentat both 5' -end (before start codon) and at 3' -end (after stop codon). Theyare required for efficient translation process.For initiation, the ribosome binds to the mRNA at the start codon (AUG)that is recognised only by the initiator tRNA. The ribosome proceeds to theelongation phase of protein synthesis. During this stage, complexescomposed of an amino acid linked to tRNA, sequentially bind to theappropriate codon in mRNA by forming complementary base pairs withthe tRNA anticodon. The ribosome moves from codon to codon along themRNA. Amino acids are added one by one, translated into Polypeptidesequences dictated by DNA and represented by mRNA. At the end, a releasefactor binds to the stop codon, terminating translation and releasing thecomplete polypeptide from the ribosome.6.8 REGULATION OF GENE EXPRESSIONRegulation of gene expression refers to a very broad term that may occurat various levels. Considering that gene expression results in the formationof a polypeptide, it can be regulated at several levels. In eukaryotes, theregulation could be exerted at(i) transcriptional level (formation of primary transcript),(ii) processing level (regulation of splicing),(iii) transport of mRNA from nucleus to the cytoplasm,(iv) translational level.Figure 6.13 Translation2022-23116BIOLOGYThe genes in a cell are expressed to perform a particular function or aset of functions. For example, if an enzyme called beta-galactosidase issynthesised by E. coli, it is used to catalyse the hydrolysis of adisaccharide, lactose into galactose and glucose; the bacteria use themas a source of energy. Hence, if the bacteria do not have lactose aroundthem to be utilised for energy source, they would no longer require thesynthesis of the enzyme beta-galactosidase. Therefore, in simple terms,it is the metabolic, physiological or environmental conditions that regulatethe expression of genes. The development and differentiation of embryointo adult organisms are also a result of the coordinated regulation ofexpression of several sets of genes.In prokaryotes, control of the rate of transcriptional initiation is thepredominant site for control of gene expression. In a transcription unit,the activity of RNA polymerase at a given promoter is in turn regulatedby interaction with accessory proteins, which affect its ability to recognisestart sites. These regulatory proteins can act both positively (activators)and negatively (repressors). The accessibility of promoter regions ofprokaryotic DNA is in many cases regulated by the interaction of proteinswith sequences termed operators. The operator region is adjacent to thepromoter elements in most operons and in most cases the sequences ofthe operator bind a repressor protein. Each operon has its specificoperator and specific repressor. For example, lac operator is presentonly in the lac operon and it interacts specifically with lac repressor only.6.8.1 The Lac operonThe elucidation of the lac operon was also a result of a close associationbetween a geneticist, Francois Jacob and a biochemist, Jacque Monod. Theywere the first to elucidate a transcriptionally regulated system. In lac operon(here lac refers to lactose), a polycistronic structural gene is regulated by acommon promoter and regulatory genes. Such arrangement is very commonin bacteria and is referred to as operon. To name few such examples, lacoperon, trp operon, ara operon, his operon, val operon, etc.The lac operon consists of one regulatory gene (the i gene â here theterm i does not refer to inducer, rather it is derived from the word inhibitor)and three structural genes (z, y, and a). The i gene codes for the repressorof the lac operon. The z gene codes for beta-galactosidase (b-gal), whichis primarily responsible for the hydrolysis of the disaccharide, lactoseinto its monomeric units, galactose and glucose. The y gene codes forpermease, which increases permeability of the cell to b-galactosides. Thea gene encodes a transacetylase. Hence, all the three gene products inlac operon are required for metabolism of lactose. In most other operonsas well, the genes present in the operon are needed together to functionin the same or related metabolic pathway (Figure 6.14).2022-23117MOLECULAR BASIS OF INHERITANCELactose is the substrate for the enzyme beta-galactosidase and itregulates switching on and off of the operon. Hence, it is termed as inducer.In the absence of a preferred carbon source such as glucose, if lactose isprovided in the growth medium of the bacteria, the lactose is transportedinto the cells through the action of permease (Remember, a very low levelof expression of lac operon has to be present in the cell all the time,otherwise lactose cannot enter the cells). The lactose then induces theoperon in the following manner.The repressor of the operon is synthesised (all-the-time â constitutively)from the i gene. The repressor protein binds to the operator region of theoperon and prevents RNA polymerase from transcribing the operon. Inthe presence of an inducer, such as lactose or allolactose, the repressor isinactivated by interaction with the inducer. This allows RNA polymeraseaccess to the promoter and transcription proceeds (Figure 6.14).Essentially, regulation of lac operon can also be visualised as regulationof enzyme synthesis by its substrate.Remember, glucose or galactose cannot act as inducers for lacoperon. Can you think for how long the lac operon would be expressedin the presence of lactose?Regulation of lac operon by repressor is referred to as negativeregulation. Lac operon is under control of positive regulation as well,but it is beyond the scope of discussion at this level.Figure 6.14 The lac Operon2022-23118BIOLOGY6.9 HUMAN GENOME PROJECTIn the preceding sections you have learnt that it is the sequence of bases inDNA that determines the genetic information of a given organism. In otherwords, genetic make-up of an organism or an individual lies in the DNAsequences. If two individuals differ, then their DNA sequences should alsobe different, at least at some places. These assumptions led to the quest offinding out the complete DNA sequence of human genome. With theestablishment of genetic engineering techniques where it was possible toisolate and clone any piece of DNA and availability of simple and fasttechniques for determining DNA sequences, a very ambitious project ofsequencing human genome was launched in the year 1990.Human Genome Project (HGP) was called a mega project. You canimagine the magnitude and the requirements for the project if we simplydefine the aims of the project as follows:Human genome is said to have approximately 3 x 109 bp, and if thecost of sequencing required is US $ 3 per bp (the estimated cost in thebeginning), the total estimated cost of the project would be approximately9 billion US dollars. Further, if the obtained sequences were to be storedin typed form in books, and if each page of the book contained 1000letters and each book contained 1000 pages, then 3300 such books wouldbe required to store the information of DNA sequence from a single humancell. The enormous amount of data expected to be generated alsonecessitated the use of high speed computational devices for data storageand retrieval, and analysis. HGP was closely associated with the rapiddevelopment of a new area in biology called Bioinformatics.Goals of HGPSome of the important goals of HGP were as follows:(i) Identify all the approximately 20,000-25,000 genes in human DNA;(ii) Determine the sequences of the 3 billion chemical base pairs thatmake up human DNA;(iiii) Store this information in databases;(iv) Improve tools for data analysis;(v) Transfer related technologies to other sectors, such as industries;(vi) Address the ethical, legal, and social issues (ELSI) that may arisefrom the project.The Human Genome Project was a 13-year project coordinated bythe U.S. Department of Energy and the National Institute of Health. Duringthe early years of the HGP, the Wellcome Trust (U.K.) became a majorpartner; additional contributions came from Japan, France, Germany,China and others. The project was completed in 2003. Knowledge aboutthe effects of DNA variations among individuals can lead to revolutionarynew ways to diagnose, treat and someday prevent the thousands of2022-23119MOLECULAR BASIS OF INHERITANCEdisorders that affect human beings. Besides providing clues tounderstanding human biology, learning about non-human organismsDNA sequences can lead to an understanding of their natural capabilitiesthat can be applied toward solving challenges in health care, agriculture,energy production, environmental remediation. Many non-human modelorganisms, such as bacteria, yeast, Caenorhabditis elegans (a free livingnon-pathogenic nematode), Drosophila (the fruit fly), plants (rice andArabidopsis), etc., have also been sequenced.Methodologies : The methods involved two major approaches. Oneapproach focused on identifying all the genes that are expressed asRNA (referred to as Expressed Sequence Tags (ESTs). The other tookthe blind approach of simply sequencing the whole set of genome thatcontained all the coding and non-coding sequence, and later assigningdifferent regions in the sequence with functions (a term referred to asSequence Annotation). For sequencing, the total DNA from a cell isisolated and converted into random fragments of relatively smaller sizes(recall DNA is a very long polymer, and there are technical limitations insequencing very long pieces of DNA) and cloned in suitable host usingspecialised vectors. The cloning resulted into amplification of each pieceof DNA fragment so that it subsequently could be sequenced with ease.The commonly used hosts were bacteria and yeast, and the vectors werecalled as BAC (bacterial artificial chromosomes), and YAC (yeast artificialchromosomes).The fragments were sequenced using automated DNA sequencers thatworked on the principle of a method developed by Frederick Sanger.(Remember, Sanger is also credited for developing method fordetermination of amino acidsequences in proteins). Thesesequences were then arranged basedon some overlapping regionspresent in them. This requiredgeneration of overlapping fragmentsfor sequencing. Alignment of thesesequences was humanly notpossible. Therefore, specialisedcomputer based programs weredeveloped (Figure 6.15). Thesesequences were subsequentlyannotated and were assigned to eachchromosome. The sequence ofchromosome 1 was completed onlyin May 2006 (this was the last of the24 human chromosomes â 22autosomes and X and Y â to beFigure 6.15 A representative diagram of humangenome project2022-23120BIOLOGYsequenced). Another challenging task was assigning the genetic andphysical maps on the genome. This was generated using information onpolymorphism of restriction endonuclease recognition sites, and somerepetitive DNA sequences known as microsatellites (one of the applicationsof polymorphism in repetitive DNA sequences shall be explained in nextsection of DNA fingerprinting).6.9.1 Salient Features of Human GenomeSome of the salient observations drawn from human genome project areas follows:(i) The human genome contains 3164.7 million bp.(ii) The average gene consists of 3000 bases, but sizes vary greatly, withthe largest known human gene being dystrophin at 2.4 million bases.(iii) The total number of genes is estimated at 30,000âmuch lowerthan previous estimates of 80,000 to 1,40,000 genes. Almost all(99.9 per cent) nucleotide bases are exactly the same in all people.(iv) The functions are unknown for over 50 per cent of the discoveredgenes.(v) Less than 2 per cent of the genome codes for proteins.(vi) Repeated sequences make up very large portion of the human genome.(vii) Repetitive sequences are stretches of DNA sequences that arerepeated many times, sometimes hundred to thousand times. Theyare thought to have no direct coding functions, but they shed lighton chromosome structure, dynamics and evolution.(viii) Chromosome 1 has most genes (2968), and the Y has the fewest (231).(ix) Scientists have identified about 1.4 million locations where singlebaseDNA differences (SNPs â single nucleotide polymorphism,pronounced as âsnipsâ) occur in humans. This information promisesto revolutionise the processes of finding chromosomal locations fordisease-associated sequences and tracing human history.6.9.2 Applications and Future ChallengesDeriving meaningful knowledge from the DNA sequences will defineresearch through the coming decades leading to our understanding ofbiological systems. This enormous task will require the expertise andcreativity of tens of thousands of scientists from varied disciplines in boththe public and private sectors worldwide. One of the greatest impacts ofhaving the HG sequence may well be enabling a radically new approachto biological research. In the past, researchers studied one or a few genesat a time. With whole-genome sequences and new high-throughputtechnologies, we can approach questions systematically and on a much2022-23121MOLECULAR BASIS OF INHERITANCEbroader scale. They can study all the genes in a genome, for example, allthe transcripts in a particular tissue or organ or tumor, or how tens ofthousands of genes and proteins work together in interconnected networksto orchestrate the chemistry of life.6.10 DNA FINGERPRINTINGAs stated in the preceding section, 99.9 per cent of base sequence amonghumans is the same. Assuming human genome as 3 à 109 bp, in howmany base sequences would there be differences? It is these differencesin sequence of DNA which make every individual unique in theirphenotypic appearance. If one aims to find out genetic differencesbetween two individuals or among individuals of a population,sequencing the DNA every time would be a daunting and expensivetask. Imagine trying to compare two sets of 3 à 106base pairs. DNAfingerprinting is a very quick way to compare the DNA sequences of anytwo individuals.DNA fingerprinting involves identifying differences in some specificregions in DNA sequence called as repetitive DNA, because in thesesequences, a small stretch of DNA is repeated many times. These repetitiveDNA are separated from bulk genomic DNA as different peaks duringdensity gradient centrifugation. The bulk DNA forms a major peak andthe other small peaks are referred to as satellite DNA. Depending onbase composition (A : T rich or G:C rich), length of segment, and numberof repetitive units, the satellite DNA is classified into many categories,such as micro-satellites, mini-satellites etc. These sequences normallydo not code for any proteins, but they form a large portion of humangenome. These sequence show high degree of polymorphism and formthe basis of DNA fingerprinting. Since DNA from every tissue (such asblood, hair-follicle, skin, bone, saliva, sperm etc.), from an individualshow the same degree of polymorphism, they become very usefulidentification tool in forensic applications. Further, as the polymorphismsare inheritable from parents to children, DNA fingerprinting is the basisof paternity testing, in case of disputes.As polymorphism in DNA sequence is the basis of genetic mappingof human genome as well as of DNA fingerprinting, it is essential that weunderstand what DNA polymorphism means in simple terms.Polymorphism (variation at genetic level) arises due to mutations. (Recalldifferent kind of mutations and their effects that you have alreadystudied in Chapter 5, and in the preceding sections in this chapter.)New mutations may arise in an individual either in somatic cells or inthe germ cells (cells that generate gametes in sexually reproducingorganisms). If a germ cell mutation does not seriously impair individualâsability to have offspring who can transmit the mutation, it can spread to2022-23122BIOLOGYthe other members of population (through sexual reproduction). Allelic(again recall the definition of alleles from Chapter 5) sequence variationhas traditionally been described as a DNA polymorphism if more thanone variant (allele) at a locus occurs in human population with afrequency greater than 0.01. In simple terms, if an inheritable mutationis observed in a population at high frequency, it is referred to as DNApolymorphism. The probability of such variation to be observed in noncodingDNA sequence would be higher as mutations in these sequencesmay not have any immediate effect/impact in an individualâsreproductive ability. These mutations keep on accumulating generationafter generation, and form one of the basis of variability/polymorphism.There is a variety of different types of polymorphisms ranging from singlenucleotide change to very large scale changes. For evolution andspeciation, such polymorphisms play very important role, and you willstudy these in details at higher classes.The technique of DNA Fingerprinting was initially developed by AlecJeffreys. He used a satellite DNA as probe that shows very high degreeof polymorphism. It was called as Variable Number of Tandem Repeats(VNTR). The technique, as used earlier, involved Southern blothybridisation using radiolabelled VNTR as a probe. It included(i) isolation of DNA,(ii) digestion of DNA by restriction endonucleases,(iii) separation of DNA fragments by electrophoresis,(iv) transferring (blotting) of separated DNA fragments to syntheticmembranes, such as nitrocellulose or nylon,(v) hybridisation using labelled VNTR probe, and(vi) detection of hybridised DNA fragments by autoradiography. A schematicrepresentation of DNA fingerprinting is shown in Figure 6.16.The VNTR belongs to a class of satellite DNA referred to as mini-satellite.A small DNA sequence is arranged tandemly in many copy numbers. Thecopy number varies from chromosome to chromosome in an individual.The numbers of repeat show very high degree of polymorphism. As aresult the size of VNTR varies in size from 0.1 to20 kb. Consequently, after hybridisation with VNTR probe, theautoradiogram gives many bands of differing sizes. These bands give acharacteristic pattern for an individual DNA (Figure 6.16). It differs fromindividual to individual in a population except in the case of monozygotic(identical) twins. The sensitivity of the technique has been increased byuse of polymerase chain reaction (PCRâyou will study about it inChapter 11). Consequently, DNA from a single cell is enough to performDNA fingerprinting analysis. In addition to application in forensic2022-23123MOLECULAR BASIS OF INHERITANCEFigure 6.16 Schematic representation of DNA fingerprinting: Few representative chromosomeshave been shown to contain different copy number of VNTR. For the sake ofunderstanding different colour schemes have been used to trace the origin of eachband in the gel. The two alleles (paternal and maternal) of a chromosome alsocontain different copy numbers of VNTR. It is clear that the banding pattern of DNAfrom crime scene matches with individual B, and not with A.science, it has much wider application, such as in determiningpopulation and genetic diversities. Currently, many different probesare used to generate DNA fingerprints.2022-23124BIOLOGYSUMMARYNucleic acids are long polymers of nucleotides. While DNA stores geneticinformation, RNA mostly helps in transfer and expression of information.Though DNA and RNA both function as genetic material, but DNA beingchemically and structurally more stable is a better genetic material.However, RNA is the first to evolve and DNA was derived from RNA. Thehallmark of the double stranded helical structure of DNA is the hydrogenbonding between the bases from opposite strands. The rule is thatAdenine pairs with Thymine through two H-bonds, and Guanine withCytosine through three H-bonds. This makes one strandcomplementary to the other. The DNA replicates semiconservatively,the process is guided by the complementary H-bonding. A segment ofDNA that codes for RNA may in a simplistic term can be referred asgene. During transcription also, one of the strands of DNA acts atemplate to direct the synthesis of complementary RNA. In bacteria,the transcribed mRNA is functional, hence can directly be translated.In eukaryotes, the gene is split. The coding sequences, exons, areinterrupted by non-coding sequences, introns. Introns are removedand exons are joined to produce functional RNA by splicing. Themessenger RNA contains the base sequences that are read in acombination of three (to make triplet genetic code) to code for an aminoacid. The genetic code is read again on the principle of complementarityby tRNA that acts as an adapter molecule. There are specific tRNAs forevery amino acid. The tRNA binds to specific amino acid at one endand pairs through H-bonding with codes on mRNA through itsanticodons. The site of translation (protein synthesis) is ribosomes,which bind to mRNA and provide platform for joining of amino acids.One of the rRNA acts as a catalyst for peptide bond formation, which isan example of RNA enzyme (ribozyme). Translation is a process thathas evolved around RNA, indicating that life began around RNA. Since,transcription and translation are energetically very expensiveprocesses, these have to be tightly regulated. Regulation of transcriptionis the primary step for regulation of gene expression. In bacteria, morethan one gene is arranged together and regulated in units called asoperons. Lac operon is the prototype operon in bacteria, which codesfor genes responsible for metabolism of lactose. The operon is regulatedby the amount of lactose in the medium where the bacteria are grown.Therefore, this regulation can also be viewed as regulation of enzymesynthesis by its substrate.Human genome project was a mega project that aimed to sequenceevery base in human genome. This project has yielded much newinformation. Many new areas and avenues have opened up as aconsequence of the project. DNA Fingerprinting is a technique to findout variations in individuals of a population at DNA level. It works onthe principle of polymorphism in DNA sequences. It has immenseapplications in the field of forensic science, genetic biodiversity andevolutionary biology.2022-23125MOLECULAR BASIS OF INHERITANCEEXERCISES1 Group the following as nitrogenous bases and nucleosides:Adenine, Cytidine, Thymine, Guanosine, Uracil and Cytosine.2. If a double stranded DNA has 20 per cent of cytosine, calculate the percent of adenine in the DNA.3. If the sequence of one strand of DNA is written as follows:5'-ATGCATGCATGCATGCATGCATGCATGC-3'Write down the sequence of complementary strand in 5'ÂŽ3' direction.4. If the sequence of the coding strand in a transcription unit is writtenas follows:5'-ATGCATGCATGCATGCATGCATGCATGC-3'Write down the sequence of mRNA.5. Which property of DNA double helix led Watson and Crick to hypothesisesemi-conservative mode of DNA replication? Explain.6. Depending upon the chemical nature of the template (DNA or RNA)and the nature of nucleic acids synthesised from it (DNA or RNA), listthe types of nucleic acid polymerases.7. How did Hershey and Chase differentiate between DNA and protein intheir experiment while proving that DNA is the genetic material?8. Differentiate between the followings:(a) Repetitive DNA and Satellite DNA(b) mRNA and tRNA(c) Template strand and Coding strand9. List two essential roles of ribosome during translation.10. In the medium where E. coli was growing, lactose was added, whichinduced the lac operon. Then, why does lac operon shut down sometime after addition of lactose in the medium?11. Explain (in one or two lines) the function of the followings:(a) Promoter(b) tRNA(c) Exons12. Why is the Human Genome project called a mega project?13. What is DNA fingerprinting? Mention its application.14. Briefly describe the following:(a) Transcription(b) Polymorphism(c) Translation(d) Bioinformatics2022-23126BIOLOGYEvolutionary Biology is the study of history of life formson earth. What exactly is evolution? To understand thechanges in flora and fauna that have occurred over millionsof years on earth, we must have an understanding of thecontext of origin of life, i.e., evolution of earth, of stars andindeed of the universe itself. What follows is the longest ofall the construed and conjectured stories. This is the storyof origin of life and evolution of life forms or biodiversity onplanet earth in the context of evolution of earth and againstthe background of evolution of universe itself.7.1 ORIGIN OF LIFEWhen we look at stars on a clear night sky we are, in away, looking back in time. Stellar distances are measuredin light years. What we see today is an object whose emittedlight started its journey millions of year back and fromtrillions of kilometres away and reaching our eyes now.However, when we see objects in our immediatesurroundings we see them instantly and hence in thepresent time. Therefore, when we see stars we apparentlyare peeping into the past.The origin of life is considered a unique event in thehistory of universe. The universe is vast. Relatively speakingthe earth itself is almost only a speck. The universe is veryCHAPTER 7EVOLUTION7.1 Origin of Life7.2 Evolution of Life Forms - ATheory7.3 What are the Evidencesfor Evolution?7.4 What is AdaptiveRadiation?7.5 Biological Evolution7.6 Mechanism of Evolution7.7 Hardy - WeinbergPrinciple7.8 A Brief Account ofEvolution7.9 Origin and Evolution ofMan2022-23127EVOLUTIONold â almost 20 billion years old. Huge clusters of galaxies comprise theuniverse. Galaxies contain stars and clouds of gas and dust. Consideringthe size of universe, earth is indeed a speck. The Big Bang theory attemptsto explain to us the origin of universe. It talks of a singular huge explosionunimaginable in physical terms. The universe expanded and hence, thetemperature came down. Hydrogen and Helium formed sometime later.The gases condensed under gravitation and formed the galaxies of thepresent day universe. In the solar system of the milky way galaxy, earthwas supposed to have been formed about 4.5 billion years back. Therewas no atmosphere on early earth. Water vapour, methane, carbondioxideand ammonia released from molten mass covered the surface. The UV raysfrom the sun brokeup water into Hydrogen and Oxygen and the lighter H2escaped. Oxygen combined with ammonia and methane to form water,CO2 and others. The ozone layer was formed. As it cooled, the water vaporfell as rain, to fill all the depressions and form oceans. Life appeared 500million years after the formation of earth, i.e., almost four billion years back.Did life come from outerspace? Some scientists believe that it camefrom outside. Early Greek thinkers thought units of life called sporeswere transferred to different planets including earth. âPanspermiaâ is stilla favourite idea for some astronomers. For a long time it was also believedthat life came out of decaying and rotting matter like straw, mud, etc.This was the theory of spontaneous generation. Louis Pasteur by carefulexperimentation demonstrated that life comes only from pre-existing life.He showed that in pre-sterilised flasks, life did not come from killed yeastwhile in another flask open to air, new living organisms arose from âkilledyeastâ. Spontaneous generation theory was dismissed once and for all.However, this did not answer how the first life form came on earth.Oparin of Russia and Haldane of England proposed that the first formof life could have come from pre-existing non-living organic molecules(e.g. RNA, protein, etc.) and that formation of life was preceded by chemicalevolution, i.e., formation of diverse organic molecules from inorganicconstituents. The conditions on earth were â high temperature, volcanicstorms, reducing atmosphere containing CH4, NH3, etc. In 1953, S.L. Miller,an American scientist created similar conditions in a laboratory scale(Figure 7.1). He created electric discharge in a closed flask containingCH4, H2, NH3 and water vapour at 8000C. He observed formation of aminoacids. In similar experiments others observed, formation of sugars,nitrogen bases, pigment and fats. Analysis of meteorite content alsorevealed similar compounds indicating that similar processes areoccurring elsewhere in space. With this limited evidence, the first part ofthe conjectured story, i.e., chemical evolution was more or less accepted.We have no idea about how the first self replicating metabolic capsuleof life arose. The first non-cellular forms of life could have originated3 billion years back. They would have been giant molecules (RNA, Protein,2022-23128BIOLOGYPolysaccharides, etc.). These capsules reproduced their molecules perhaps.The first cellular form of life did not possibly originate till about 2000million years ago. These were probably single-cells. All life forms were inwater environment only. This version of a biogenesis, i.e., the first form oflife arose slowly through evolutionary forces from non-living molecules isaccepted by majority. However, once formed, how the first cellular formsof life could have evolved into the complex biodiversity of today is thefascinating story that will be discussed below.7.2 EVOLUTION OF LIFE FORMS â A THEORYConventional religious literature tells us about the theory of specialcreation. This theory has three connotations. One, that all living organisms(species or types) that we see today were created as such. Two, that thediversity was always the same since creation and will be the same in futurealso. Three, that earth is about 4000 years old. All these ideas werestrongly challenged during the nineteenth century. Based on observationsmade during a sea voyage in a sail ship called H.M.S. Beagle round theworld, Charles Darwin concluded that existing living forms sharesimilarities to varying degrees not only among themselves but also withlife forms that existed millions of years ago. Many such life forms do notexist any more. There had been extinctions of different life forms in theyears gone by just as new forms of life arose at different periods of historyof earth. There has been gradual evolution of life forms. Any populationFigure 7.1 Diagrammatic representation of Millerâsexperiment2022-23129EVOLUTIONhas built in variation in characteristics. Those characteristics which enablesome to survive better in natural conditions (climate, food, physical factors,etc.) would outbreed others that are less-endowed to survive under suchnatural conditions. Another word used is fitness of the individual orpopulation. The fitness, according to Darwin, refers ultimately and onlyto reproductive fitness. Hence, those who are better fit in an environment,leave more progeny than others. These, therefore, will survive more andhence are selected by nature. He called it natural selection and implied itas a mechanism of evolution. Let us also remember that Alfred Wallace, anaturalist who worked in Malay Archipelago had also come to similarconclusions around the same time. In due course of time, apparently newtypes of organisms are recognisable. All the existing life forms sharesimilarities and share common ancestors. However, these ancestors werepresent at different periods in the history of earth (epochs, periods anderas). The geological history of earth closely correlates with the biologicalhistory of earth. A common permissible conclusion is that earth is veryold, not thousand of years as was thought earlier but billions of years old.7.3 WHAT ARE THE EVIDENCES FOR EVOLUTION?Evidence that evolution of life forms has indeed taken place on earth hascome from many quarters. Fossils are remains of hard parts oflife-forms found in rocks. Rocks form sediments and a cross-section ofearth's crust indicates the arrangement of sediments one over the otherduring the long history of earth. Different-aged rock sediments containfossils of different life-forms who probably died during the formation ofthe particular sediment. Some of them appear similar to modernorganisms (Figure 7.2). They represent extinct organisms (e.g., Dinosaurs).A study of fossils in different sedimentary layers indicates the geologicalperiod in which they existed. The study showed that life-forms variedover time and certain life forms are restricted to certain geological timespans.Hence, new forms of life have arisen at different times in the historyof earth. All this is called paleontological evidence. Do you rememberhow the ages of the fossils are calculated? Do you recollect the methodof radioactive-dating and the principles behind the procedure?Embryological support for evolution was also proposed by ErnstHeckel based upon the observation of certain features during embryonicstage common to all vertebrates that are absent in adult. For example,the embryos of all vertebrates including human develop a row of vestigialgill slit just behind the head but it is a functional organ only in fish andnot found in any other adult vertebrates. However, this proposal wasdisapproved on careful study performed by Karl Ernst von Baer. He notedthat embryos never pass through the adult stages of other animals.Comparative anatomy and morphology shows similarities anddifferences among organisms of today and those that existed years ago.2022-23130BIOLOGYFigure 7.2 A family tree of dinosaurs and their living modern day counterpart organisms likecrocodiles and birdsSuch similarities can be interpreted to understand whether commonancestors were shared or not. For example whales, bats, Cheetah andhuman (all mammals) share similarities in the pattern of bones of forelimbs(Figure 7.3b). Though these forelimbs perform different functions in theseanimals, they have similar anatomical structure â all of them havehumerus, radius, ulna, carpals, metacarpals and phalanges in theirforelimbs. Hence, in these animals, the same structure developed alongdifferent directions due to adaptations to different needs. This is divergentevolution and these structures are homologous. Homology indicatescommon ancestry. Other examples are vertebrate hearts or brains. In2022-23131EVOLUTIONplants also, the thorn and tendrils ofBougainvillea and Cucurbita representhomology (Figure 7.3a). Homology isbased on divergent evolution whereasanalogy refers to a situation exactlyopposite. Wings of butterfly and of birdslook alike. They are not anatomicallysimilar structures though they performsimilar functions. Hence, analogousstructures are a result of convergentevolution - different structures evolvingfor the same function and hence havingsimilarity. Other examples of analogy arethe eye of the octopus and of mammalsor the flippers of Penguins and Dolphins.One can say that it is the similar habitatthat has resulted in selection of similaradaptive features in different groups oforganisms but toward the same function:Sweet potato (root modification) andpotato (stem modification) is anotherexample for analogy.In the same line of argument,similarities in proteins and genesperforming a given function among diverseorganisms give clues to common ancestry.These biochemical similarities point to thesame shared ancestry as structuralsimilarities among diverse organisms.Man has bred selected plants andanimals for agriculture, horticulture, sportor security. Man has domesticated manywild animals and crops. This intensivebreeding programme has created breedsthat differ from other breeds (e.g., dogs) butstill are of the same group. It is argued thatif within hundreds of years, man could create new breeds, could not naturehave done the same over millions of years?Another interesting observation supporting evolution by naturalselection comes from England. In a collection of moths made in 1850s,i.e., before industrialisation set in, it was observed that there were morewhite-winged moths on trees than dark-winged or melanised moths.However, in the collection carried out from the same area, but afterindustrialisation, i.e., in 1920, there were more dark-winged moths inthe same area, i.e., the proportion was reversed.(b)Figure 7.3 Example of homologous organs in(a) Plants and (b) Animals(a)Tendril2022-23132BIOLOGYThe explanation put forth for this observation was that âpredators willspot a moth against a contrasting backgroundâ. During postindustrialisationperiod, the tree trunks became dark due to industrialsmoke and soots. Under this condition the white-winged moth did notsurvive due to predators, dark-winged or melanised moth survived. Beforeindustrialisation set in, thick growth of almost white-coloured lichencovered the trees - in that background the white winged moth survivedbut the dark-coloured moth were picked out by predators. Do you knowthat lichens can be used as industrial pollution indicators? They willnot grow in areas that are polluted. Hence, moths that were able tocamouflage themselves, i.e., hide in the background, survived(Figure 7.4). This understanding is supported by the fact that in areaswhere industrialisation did not occur e.g., in rural areas, the count ofmelanic moths was low. This showed that in a mixed population, thosethat can better-adapt, survive and increase in population size. Rememberthat no variant is completely wiped out.Similarly, excess use of herbicides, pesticides, etc., has only resulted inselection of resistant varieties in a much lesser time scale. This is also true formicrobes against which we employ antibiotics or drugs against eukaryoticorganisms/cell. Hence, resistant organisms/cells are appearing in a timescale of months or years and not centuries. These are examples of evolutionby anthropogenic action. This also tells us that evolution is not a directedprocess in the sense of determinism. It is a stochastic process based onchance events in nature and chance mutation in the organisms.7.4 WHAT IS ADAPTIVE RADIATION?During his journey Darwin went to Galapagos Islands. There he observedan amazing diversity of creatures. Of particular interest, small black birdslater called Darwinâs Finches amazed him. He realised that there were manyFigure 7.4 Figure showing white - winged moth and dark - winged moth (melanised)on a tree trunk (a) In unpolluted area (b) In polluted area(a) (b)2022-23133EVOLUTIONvarieties of finches in the same island. All the varieties, he conjectured,evolved on the island itself. From the original seed-eating features, manyother forms with altered beaks arose, enabling them to become insectivorousand vegetarian finches (Figure 7.5). This process of evolution of differentspecies in a given geographical area starting from a point and literallyradiating to other areas of geography (habitats) is called adaptive radiation.Darwinâs finches represent one of the best examples of this phenomenon.Another example is Australian marsupials. A number of marsupials, eachdifferent from the other (Figure 7.6) evolved from an ancestral stock, but allwithin the Australian island continent. When more than one adaptive radiationappeared to have occurred in an isolated geographical area (representingFigure 7.6 Adaptive radiation of marsupials of AustraliaFigure 7.5 Variety of beaks of finches that Darwin found in Galapagos Island2022-23134BIOLOGYdifferent habitats), one can call this convergentevolution. Placental mammals in Australia alsoexhibit adaptive radiation in evolving intovarieties of such placental mammals each ofwhich appears to be âsimilarâ to a correspondingmarsupial (e.g., Placental wolf and Tasmanianwolf-marsupial). (Figure 7.7).7.5 BIOLOGICAL EVOLUTIONEvolution by natural selection, in a true sensewould have started when cellular forms of lifewith differences in metabolic capabilityoriginated on earth.The essence of Darwinian theory aboutevolution is natural selection. The rate ofappearance of new forms is linked to the life cycleor the life span. Microbes that divide fast havethe ability to multiply and become millions ofindividuals within hours. A colony of bacteria(say A) growing on a given medium has built-invariation in terms of ability to utilise a feedcomponent. A change in the mediumcomposition would bring out only that part ofthe population (say B) that can survive underthe new conditions. In due course of time thisvariant population outgrows the others andappears as new species. This would happenwithin days. For the same thing to happen in afish or fowl would take million of years as lifespans of these animals are in years. Here we saythat fitness of B is better than that of A underthe new conditions. Nature selects for fitness.One must remember that the so-called fitness isbased on characteristics which are inherited.Hence, there must be a genetic basis for getting selected and to evolve.Another way of saying the same thing is that some organisms are betteradapted to survive in an otherwise hostile environment. Adaptive ability isinherited. It has a genetic basis. Fitness is the end result of the ability toadapt and get selected by nature.Branching descent and natural selection are the two key conceptsof Darwinian Theory of Evolution (Figures 7.7 and 7.8).Even before Darwin, a French naturalist Lamarck had said thatevolution of life forms had occurred but driven by use and disuse ofFigure 7.7 Picture showing convergent evolutionof Australian Marsupials andplacental mammals2022-23135EVOLUTIONorgans. He gave the examples of Giraffes who in an attempt to forageleaves on tall trees had to adapt by elongation of their necks. As theypassed on this acquired character of elongated neck to succeedinggenerations, Giraffes, slowly, over the years, came to acquire long necks.Nobody believes this conjecture any more.Is evolution a process or the result of a process? The world we see,inanimate and animate, is only the success stories of evolution. When wedescribe the story of this world we describe evolution as a process. On theother hand when we describe the story of life on earth, we treat evolutionas a consequence of a process called natural selection. We are still notvery clear whether to regard evolution and natural selection as processesor end result of unknown processes.It is possible that the work of Thomas Malthus on populationsinfluenced Darwin. Natural selection is based on certain observationswhich are factual. For example, natural resources are limited, populationsare stable in size except for seasonal fluctuation, members of a populationvary in characteristics (infact no two individuals are alike) even thoughthey look superficially similar, most of variations are inherited etc. Thefact that theoretically population size will grow exponentially if everybodyreproduced maximally (this fact can be seen in a growing bacterialpopulation) and the fact that population sizes in reality are limited, meansthat there had been competition for resources. Only some survived andgrew at the cost of others that could not flourish. The novelty and brilliantinsight of Darwin was this: he asserted that variations, which are heritableand which make resource utilisation better for few (adapted to habitatbetter) will enable only those to reproduce and leave more progeny. Hencefor a period of time, over many generations, survivors will leave moreprogeny and there would be a change in population characteristic andhence new forms appear to arise.7.6 MECHANISM OF EVOLUTIONWhat is the origin of this variation and how does speciation occur? Eventhough Mendel had talked of inheritable 'factors' influencing phenotype,Darwin either ignored these observations or kept silence. In the first decadeof twentieth century, Hugo deVries based on his work on evening primrosebrought forth the idea of mutations â large difference arising suddenly ina population. He believed that it is mutation which causes evolution andnot the minor variations (heritable) that Darwin talked about. Mutationsare random and directionless while Darwinian variations are small anddirectional. Evolution for Darwin was gradual while deVries believedmutation caused speciation and hence called it saltation (single steplarge mutation). Studies in population genetics, later, brought outsome clarity.2022-23136BIOLOGYFigure 7.8 Diagrammatic representation of the operation of natural selection on differenttraits : (a) Stabilising (b) Directional and (c) Disruptive(a)(b)(c)7.7 HARDY-WEINBERG PRINCIPLEIn a given population one can find out the frequency of occurrence ofalleles of a gene or a locus. This frequency is supposed to remain fixedand even remain the same through generations. Hardy-Weinberg principlestated it using algebraic equations.This principle says that allele frequencies in a population are stableand is constant from generation to generation. The gene pool (total genesand their alleles in a population) remains a constant. This is calledgenetic equilibrium. Sum total of all the allelic frequencies is 1. Individual2022-23137EVOLUTIONfrequencies, for example, can be named p, q, etc. In a diploid, p and qrepresent the frequency of allele A and allele a. The frequency of AAindividuals in a population is simply p2. This is simply stated in anotherways, i.e., the probability that an allele A with a frequency of p appear onboth the chromosomes of a diploid individual is simply the productof the probabilities, i.e., p2. Similarly of aa is q2, of Aa 2pq. Hence,p2+2pq+q2=1. This is a binomial expansion of (p+q)2. When frequencymeasured, differs from expected values, the difference (direction) indicatesthe extent of evolutionary change. Disturbance in genetic equilibrium, orHardy- Weinberg equilibrium, i.e., change of frequency of alleles in apopulation would then be interpreted as resulting in evolution.Five factors are known to affect Hardy-Weinberg equilibrium. Theseare gene migration or gene flow, genetic drift, mutation, geneticrecombination and natural selection. When migration of a section ofpopulation to another place and population occurs, gene frequencieschange in the original as well as in the new population. New genes/allelesare added to the new population and these are lost from the old population.There would be a gene flow if this gene migration, happens multiple times.If the same change occurs by chance, it is called genetic drift. Sometimesthe change in allele frequency is so different in the new sample of populationthat they become a different species. The original drifted populationbecomes founders and the effect is called founder effect.Microbial experiments show that pre-existing advantageousmutations when selected will result in observation of new phenotypes.Over few generations, this would result in Speciation. Natural selection isa process in which heritable variations enabling better survival are enabledto reproduce and leave greater number of progeny. A critical analysismakes us believe that variation due to mutation or variation due torecombination during gametogenesis, or due to gene flow or genetic driftresults in changed frequency of genes and alleles in future generation.Coupled to enhance reproductive success, natural selection makes it looklike different population. Natural selection can lead to stabilisation (inwhich more individuals acquire mean character value), directional change(more individuals acquire value other than the mean character value) ordisruption (more individuals acquire peripheral character value at bothends of the distribution curve) (Figure 7.8).7.8 A BRIEF ACCOUNT OF EVOLUTIONAbout 2000 million years ago (mya) the first cellular forms of life appearedon earth. The mechanism of how non-cellular aggregates of giantmacromolecules could evolve into cells with membranous envelop is notknown. Some of these cells had the ability to release O2. The reaction2022-23138BIOLOGYFigure 7.9 A sketch of the evolution of plant forms through geological periodscould have been similar to the light reaction in photosynthesis where wateris split with the help of solar energy captured and channelised byappropriate light harvesting pigments. Slowly single-celled organismsbecame multi-cellular life forms. By the time of 500 mya, invertebrateswere formed and active. Jawless fish probably evolved around 350 mya.Sea weeds and few plants existed probably around 320 mya. We are toldthat the first organisms that invaded land were plants. They werewidespread on land when animals invaded land. Fish with stout and strongfins could move on land and go back to water. This was about 350 mya. In1938, a fish caught in South Africa happened to be a Coelacanth which wasthought to be extinct. These animals called lobefins evolved into the2022-23139EVOLUTIONfirst amphibians that lived on both land and water. There are no specimensof these left with us. However, these were ancestors of modern day frogsand salamanders. The amphibians evolved into reptiles. They lay thickshelledeggs which do not dry up in sun unlike those of amphibians.Again we only see their modern day descendents, the turtles, tortoisesand crocodiles. In the next 200 millions years or so, reptiles of differentFigure 7.10 Representative evolutionary history of vertebrates through geological periods2022-23140BIOLOGYshapes and sizes dominated on earth. Giant ferns (pteridophytes) werepresent but they all fell to form coal deposits slowly. Some of these landreptiles went back into water to evolve into fish like reptiles probably 200mya (e.g. Ichthyosaurs). The land reptiles were, of course, the dinosaurs.The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in heightand had huge fearsome dagger like teeth. About 65 mya, the dinosaurssuddenly disappeared from the earth. We do not know the true reason.Some say climatic changes killed them. Some say most of them evolvedinto birds. The truth may live in between. Small sized reptiles of that erastill exist today.The first mammals were like shrews. Their fossils are small sized.Mammals were viviparous and protected their unborn young inside themotherâs body. Mammals were more intelligent in sensing and avoidingdanger at least. When reptiles came down mammals took over this earth.There were in South America mammals resembling horse, hippopotamus,bear, rabbit, etc. Due to continental drift, when South America joinedNorth America, these animals were overridden by North American fauna.Due to the same continental drift pouched mammals of Australia survivedbecause of lack of competition from any other mammal.Lest we forget, some mammals live wholly in water. Whales, dolphins,seals and sea cows are some examples. Evolution of horse, elephant, dog,etc., are special stories of evolution. You will learn about these in higherclasses. The most successful story is the evolution of man with languageskills and self-consciousness.A rough sketch of the evolution of life forms, their times on a geologicalscale are indicated in (Figure 7.9 and 7.10).7.9 ORIGIN AND EVOLUTION OF MANAbout 15 mya, primates called Dryopithecus and Ramapithecus wereexisting. They were hairy and walked like gorillas and chimpanzees.Ramapithecus was more man-like while Dryopithecus was moreape-like. Few fossils of man-like bones have been discovered in Ethiopiaand Tanzania (Figure 7.11). These revealed hominid features leading tothe belief that about 3-4 mya, man-like primates walked in eastern Africa.They were probably not taller than 4 feet but walked up right. Two mya,Australopithecines probably lived in East African grasslands. Evidenceshows they hunted with stone weapons but essentially ate fruit. Some ofthe bones among the bones discovered were different. This creature wascalled the first human-like being the hominid and was called Homo habilis.The brain capacities were between 650-800cc. They probably did not eatmeat. Fossils discovered in Java in 1891 revealed the next stage, i.e., Homoerectus about 1.5 mya. Homo erectus had a large brain around 900cc.2022-23141EVOLUTIONFigure 7.11 A comparison of the skulls of adult modern human being, baby chimpanzee andadult chimpanzee. The skull of baby chimpanzee is more like adult human skullthan adult chimpanzee skullHomo erectus probably ate meat. The Neanderthal man with a brain sizeof 1400cc lived in near east and central Asia between 1,00,000-40,000years back. They used hides to protect their body and buried their dead.Homo sapiens arose in Africa and moved across continents and developedinto distinct races. During ice age between 75,000-10,000 years agomodern Homo sapiens arose. Pre-historic cave art developed about18,000 years ago. One such cave paintings by Pre-historic humans canbe seen at Bhimbetka rock shelter in Raisen district of Madhya Pradesh.Agriculture came around 10,000 years back and human settlementsstarted. The rest of what happened is part of human history of growthand decline of civilisations.2022-23142BIOLOGYEXERCISES1. Explain antibiotic resistance observed in bacteria in light of Darwinianselection theory.2. Find out from newspapers and popular science articles any new fossildiscoveries or controversies about evolution.3. Attempt giving a clear definition of the term species.4. Try to trace the various components of human evolution (hint: brainsize and function, skeletal structure, dietary preference, etc.)5. Find out through internet and popular science articles whether animalsother than man has self-consciousness.6. List 10 modern-day animals and using the internet resources link it toa corresponding ancient fossil. Name both.7. Practise drawing various animals and plants.8. Describe one example of adaptive radiation.9. Can we call human evolution as adaptive radiation?10. Using various resources such as your school Library or the internetand discussions with your teacher, trace the evolutionary stages ofany one animal, say horse.SUMMARYThe origin of life on earth can be understood only against thebackground of origin of universe especially earth. Most scientistsbelieve chemical evolution, i.e., formation of biomolecules precededthe appearance of the first cellular forms of life. The subsequent eventsas to what happened to the first form of life is a conjectured storybased on Darwinian ideas of organic evolution by natural selection.Diversity of life forms on earth has been changing over millions ofyears. It is generally believed that variations in a population result invariable fitness. Other phenomena like habitat fragmentation andgenetic drift may accentuate these variations leading to appearanceof new species and hence evolution. Homology is accounted for by theidea of branching descent. Study of comparative anatomy, fossils andcomparative biochemistry provides evidence for evolution. Among thestories of evolution of individual species, the story of evolution ofmodern man is most interesting and appears to parallel evolution ofhuman brain and language.2022-23Chapter 8Human Health and DiseaseChapter 9Strategies for Enhancement inFood ProductionChapter 10Microbes in Human WelfareBiology is the youngest of the formalised disciplines of naturalscience. Progress in physics and chemistry proceeded muchfaster than in Biology. Applications of physics and chemistry inour daily life also have a higher visibility than those of biology.However, twentieth century and certainly twenty-first centuryhas demonstrated the utility of biological knowledge infurthering human welfare, be it in health sector or agriculture.The discovery of antibiotics, and synthetic plant-derived drugs,anaesthetics have changed medical practice on one handand human health on the other hand. Life expectancy ofhuman beings have dramatically changed over the years.Agricultural practices, food processing and diagnostics havebrought socio-cultural changes in human communities. Theseare briefly described in the following three chapters of this unit.2022-23Born in August 1925 in Kumbakonam in Tamil Nadu, Monkambu SambasivanSwaminathan did his graduation and post-graduation in Botany fromMadras University. He worked in different capacities in large number ofinstitutions in India and abroad and developed his expertise in geneticsand plant breeding.The School of Cytogenetics and Radiation Research established at theIndian Agricultural Research Institute (IARI) enabled Swaminathan and histeam to develop short-duration high-yielding varieties of rice including scentedBasmati. He is also known for the development of the concept of cropcafeteria, crop scheduling and genetically improving the yield and quality.Swaminathan initiated collaboration with Norman Borlaug, whichculminated in the âGreen Revolutionâ through introduction of Mexicanvarieties of wheat in India. This was highly recognised and appreciated. Heis also the initiator of âLab-to-Landâ, food security and several otherenvironmental programmes. He has been honoured with Padma Bhushanand several other prestigious awards, medals and fellowships by institutionsof excellence.M.S. SWAMINATHAN(1925)2022-23Health, for a long time, was considered as a state of bodyand mind where there was a balance of certain âhumorsâ.This is what early Greeks like Hippocrates as well asIndian Ayurveda system of medicine asserted. It wasthought that persons with âblackbileâ belonged to hotpersonality and would have fevers. This idea was arrivedat by pure reflective thought. The discovery of bloodcirculation by William Harvey using experimental methodand the demonstration of normal body temperature inpersons with blackbile using thermometer disproved theâgood humorâ hypothesis of health. In later years, biologystated that mind influences, through neural system andendocrine system, our immune system and that ourimmune system maintains our health. Hence, mind andmental state can affect our health. Of course, health isaffected by â(i) genetic disorders â deficiencies with which a child isborn and deficiencies/defects which the child inheritsfrom parents from birth;(ii) infections and(iii) life style including food and water we take, rest andexercise we give to our bodies, habits that we have orlack etc.CHAPTER 8HUMAN HEALTH AND DISEASE8.1 Common Diseases inHumans8.2 Immunity8.3 AIDS8.4 Cancer8.5 Drugs and Alcohol Abuse2022-23146BIOLOGYThe term health is very frequently used by everybody. How do wedefine it? Health does not simply mean âabsence of diseaseâ or âphysicalfitnessâ. It could be defined as a state of complete physical, mental andsocial well-being. When people are healthy, they are more efficient atwork. This increases productivity and brings economic prosperity. Healthalso increases longevity of people and reduces infant and maternalmortality.Balanced diet, personal hygiene and regular exercise are very importantto maintain good health. Yoga has been practised since time immemorialto achieve physical and mental health. Awareness about diseases andtheir effect on different bodily functions, vaccination (immunisation)against infectious diseases, proper disposal of wastes, control of vectorsand maintenance of hygiene in food and water resources are necessaryfor achieving good health.When the functioning of one or more organs or systems of the body isadversely affected, characterised by appearance of various signs andsymptoms, we say that we are not healthy, i.e., we have a disease. Diseasescan be broadly grouped into infectious and non-infectious. Diseaseswhich are easily transmitted from one person to another, are calledinfectious diseases. Infectious diseases are very common and everyone of us suffers from these at sometime or other. Some of the infectiousdiseases like AIDS are fatal. Among non-infectious diseases, cancer is themajor cause of death. Drug and alcohol abuse also affect our health adversely.8.1 COMMON DISEASES IN HUMANSA wide range of organisms belonging to bacteria, viruses, fungi,protozoans, helminths, etc., could cause diseases in man. Such diseasecausingorganisms are called pathogens. Most parasites are thereforepathogens as they cause harm to the host by living in (or on) them. Thepathogens can enter our body by various means, multiply and interferewith normal vital activities, resulting in morphological and functionaldamage. Pathogens have to adapt to life within the environment of thehost. For example, the pathogens that enter the gut must know a way ofsurviving in the stomach at low pH and resisting the various digestiveenzymes. A few representative members from different groups ofpathogenic organisms are discussed here alongwith the diseases causedby them. Preventive and control measures against these diseases in general,are also briefly described.Salmonella typhi is a pathogenic bacterium which causes typhoidfever in human beings. These pathogens generally enter the small intestinethrough food and water contaminated with them and migrate to otherorgans through blood. Sustained high fever (39° to 40°C), weakness,stomach pain, constipation, headache and loss of appetite are some ofthe common symptoms of this disease. Intestinal perforation and deathmay occur in severe cases. Typhoid fever could be confirmed by2022-23HUMAN HEALTH AND DISEASE147Widal test : A classic case in medicine, that of Mary Mallon nicknamedTyphoid Mary, is worth mentioning here. She was a cook by professionand was a typhoid carrier who continued to spread typhoid for severalyears through the food she prepared.Bacteria like Streptococcus pneumoniae and Haemophilus influenzaeare responsible for the disease pneumonia in humans which infects thealveoli (air filled sacs) of the lungs. As a result of the infection, the alveoliget filled with fluid leading to severe problems in respiration. The symptomsof pneumonia include fever, chills, cough and headache. In severe cases,the lips and finger nails may turn gray to bluish in colour. A healthyperson acquires the infection by inhaling the droplets/aerosols releasedby an infected person or even by sharing glasses and utensils with aninfected person. Dysentery, plague, diphtheria, etc., are some of the otherbacterial diseases in man.Many viruses also cause diseases in human beings. Rhino virusesrepresent one such group of viruses which cause one of the most infectioushuman ailments â the common cold. They infect the nose and respiratorypassage but not the lungs. The common cold is characterised by nasalcongestion and discharge, sore throat, hoarseness, cough, headache,tiredness, etc., which usually last for 3-7 days. Droplets resulting fromcough or sneezes of an infected person are either inhaled directly ortransmitted through contaminated objects such as pens, books, cups,doorknobs, computer keyboard or mouse, etc., and cause infection in ahealthy person.Some of the human diseases are caused by protozoans too. You mighthave heard about malaria, a disease man has been fighting since manyyears. Plasmodium, a tiny protozoan is responsible for this disease. Differentspecies of Plasmodium (P. vivax, P. malaria and P. falciparum) areresponsible for different types of malaria. Of these, malignant malaria causedby Plasmodium falciparum is the most serious one and can even be fatal.Let us take a glance at the life cycle of Plasmodium (Figure 8.1).Plasmodium enters the human body as sporozoites (infectious form)through the bite of infected female Anopheles mosquito. The parasitesinitially multiply within the liver cells and then attack the red blood cells(RBCs) resulting in their rupture. The rupture of RBCs is associated withrelease of a toxic substance, haemozoin, which is responsible for the chilland high fever recurring every three to four days. When a female Anophelesmosquito bites an infected person, these parasites enter the mosquitoâsbody and undergo further development. The parasites multiply withinthem to form sporozoites that are stored in their salivary glands. Whenthese mosquitoes bite a human, the sporozoites are introduced into his/her body, thereby initiating the events mentioned above. It is interestingto note that the malarial parasite requires two hosts â human andmosquitoes â to complete its life cycle (Figure 8.1); the female Anophelesmosquito is the vector (transmitting agent) too.2022-23148BIOLOGYEntamoeba histolytica is a protozoan parasite in the large intestine ofhuman which causes amoebiasis (amoebic dysentery). Symptoms ofthis disease include constipation, abdominal pain and cramps, stoolswith excess mucous and blood clots. Houseflies act as mechanical carriersand serve to transmit the parasite from faeces of infected person to foodFigure 8.1 Stages in the life cycle of Plasmodium2022-23HUMAN HEALTH AND DISEASE149and food products, thereby contaminating them.Drinking water and food contaminated by the faecalmatter are the main source of infection.Ascaris, the common round worm and Wuchereria,the filarial worm, are some of the helminths which areknown to be pathogenic to man. Ascaris, an intestinalparasite causes ascariasis. Symptoms of these diseaseinclude internal bleeding, muscular pain, fever, anemiaand blockage of the intestinal passage. The eggs of theparasite are excreted along with the faeces of infectedpersons which contaminate soil, water, plants, etc. Ahealthy person acquires this infection throughcontaminated water, vegetables, fruits, etc.Wuchereria (W. bancrofti and W. malayi), the filarialworms cause a slowly developing chronic inflammationof the organs in which they live for many years, usuallythe lymphatic vessels of the lower limbs and the diseaseis called elephantiasis or filariasis (Figure 8.2). Thegenital organs are also often affected, resulting in grossdeformities. The pathogens are transmitted to a healthyperson through the bite by the female mosquito vectors.Many fungi belonging to the genera Microsporum,Trichophyton and Epidermophyton areresponsible for ringworms which is one ofthe most common infectious diseases in man.Appearance of dry, scaly lesions on variousparts of the body such as skin, nails andscalp (Figure 8.3) are the main symptoms ofthe disease. These lesions are accompaniedby intense itching. Heat and moisture helpthese fungi to grow, which makes them thrivein skin folds such as those in the groin orbetween the toes. Ringworms are generallyacquired from soil or by using towels, clothesor even the comb of infected individuals.Figure 8.2 Diagram showinginflammation in oneof the lower limbs dueto elephantiasisFigure 8.3 Diagram showing ringwormaffected area of the skinMaintenance of personal and public hygiene is very important forprevention and control of many infectious diseases. Measures for personalhygiene include keeping the body clean; consumption of clean drinkingwater, food, vegetables, fruits, etc. Public hygiene includes proper disposalof waste and excreta; periodic cleaning and disinfection of water reservoirs,pools, cesspools and tanks and observing standard practices of hygienein public catering. These measures are particularly essential where theinfectious agents are transmitted through food and water such as typhoid,amoebiasis and ascariasis. In cases of air-borne diseases such aspneumonia and common cold, in addition to the above measures, close2022-23150BIOLOGYcontact with the infected persons or their belongings should be avoided.For diseases such as malaria and filariasis that are transmitted throughinsect vectors, the most important measure is to control or eliminate thevectors and their breeding places. This can be achieved by avoidingstagnation of water in and around residential areas, regular cleaning ofhousehold coolers, use of mosquito nets, introducing fishes like Gambusiain ponds that feed on mosquito larvae, spraying of insecticides in ditches,drainage areas and swamps, etc. In addition, doors and windows shouldbe provided with wire mesh to prevent the entry of mosquitoes. Suchprecautions have become more important especially in the light of recentwidespread incidences of the vector-borne (Aedes mosquitoes) diseaseslike dengue and chikungunya in many parts of India.The advancements made in biological science have armed us toeffectively deal with many infectious diseases. The use of vaccines andimmunisation programmes have enabled us to completely eradicate adeadly disease like smallpox. A large number of other infectious diseaseslike polio, diphtheria, pneumonia and tetanus have been controlled to alarge extent by the use of vaccines. Biotechnology (about which you willread more in Chapter 12) is at the verge of making available newer andsafer vaccines. Discovery of antibiotics and various other drugs has alsoenabled us to effectively treat infectious diseases.8.2 IMMUNITYEveryday we are exposed to large number of infectious agents. However,only a few of these exposures result in disease. Why? This is due to thefact that the body is able to defend itself from most of these foreign agents.This overall ability of the host to fight the disease-causing organisms,conferred by the immune system is called immunity.Immunity is of two types: (i) Innate immunity and (ii) Acquiredimmunity.8.2.1 Innate ImmunityInnate immunity is non-specific type of defence, that is present at thetime of birth. This is accomplished by providing different types of barriersto the entry of the foreign agents into our body. Innate immunity consistof four types of barriers. These are â(i) Physical barriers : Skin on our body is the main barrier whichprevents entry of the micro-organisms. Mucus coating of theepithelium lining the respiratory, gastrointestinal and urogenitaltracts also help in trapping microbes entering our body.(ii) Physiological barriers : Acid in the stomach, saliva in the mouth,tears from eyesâall prevent microbial growth.(iii) Cellular barriers : Certain types of leukocytes (WBC) of our bodylike polymorpho-nuclear leukocytes (PMNL-neutrophils) and2022-23HUMAN HEALTH AND DISEASE151monocytes and natural killer (type of lymphocytes) in the blood aswell as macrophages in tissues can phagocytose and destroymicrobes.(iv) Cytokine barriers : Virus-infected cells secrete proteins calledinterferons which protect non-infected cells from further viralinfection.8.2.2 Acquired ImmunityAcquired immunity, on the other hand is pathogen specific. It ischaracterised by memory. This means when our body encounters apathogen for the first time it produces a response called primaryresponse which is of low intensity. Subsequent encounter with the samepathogen elicits a highly intensified secondary or anamnestic response.This is ascribed to the fact that our body appears to have memory of thefirst encounter.The primary and secondaryimmune responses arecarried out with the helpof two special types oflymphocytes present in ourblood, i.e., B-lymphocytes andT-lymphocytes.The B-lymphocytes produce anarmy of proteins in response topathogens into our blood to fightwith them. These proteins arecalled antibodies. TheT-cells themselves do not secreteantibodies but help B cells toproduce them. Each antibodymolecule has four peptide chains,two small called light chains andtwo longer called heavy chains.Hence, an antibody is representedas H2L2. Different types of antibodies are produced in our body. IgA, IgM,IgE, IgG are some of them. A cartoon of an antibody is given in Figure8.4. Because these antibodies are found in the blood, the response is alsocalled as humoral immune response. This is one of the two types ofour acquired immune response â antibody mediated. The second type iscalled cell-mediated immune response or cell-mediated immunity(CMI). The T-lymphocytes mediate CMI. Very often, when some humanorgans like heart, eye, liver, kidney fail to function satisfactorily,transplantation is the only remedy to enable the patient to live a normallife. Then a search begins â to find a suitable donor. Why is it that theorgans cannot be taken from just anybody? What is it that the doctorsFigure 8.4 Structure of an antibody molecule2022-23152BIOLOGYcheck? Grafts from just any source â an animal, another primate, or anyhuman beings cannot be made since the grafts would be rejected sooneror later. Tissue matching, blood group matching are essential beforeundertaking any graft/transplant and even after this the patient has totake immunoâsuppresants all his/her life. The body is able to differentiateâself â and ânonselfâ and the cell-mediated immune response is responsiblefor the graft rejection.8.2.3 Active and Passive ImmunityWhen a host is exposed to antigens, which may be in the form of livingor dead microbes or other proteins, antibodies are produced in the hostbody. This type of immunity is called active immunity. Active immunityis slow and takes time to give its full effective response. Injecting themicrobes deliberately during immunisation or infectious organismsgaining access into body during natural infection induce activeimmunity. When ready-made antibodies are directly given to protectthe body against foreign agents, it is called passive immunity. Do youknow why motherâs milk is considered very essential for the newborninfant? The yellowish fluid colostrum secreted by mother duringthe initial days of lactation has abundant antibodies (IgA) to protect theinfant. The foetus also receives some antibodies from their mother,through the placenta during pregnancy. These are some examples ofpassive immunity.8.2.4 Vaccination and ImmunisationThe principle of immunisation or vaccination is based on the property ofâmemoryâ of the immune system. In vaccination, a preparation of antigenicproteins of pathogen or inactivated/weakened pathogen (vaccine) areintroduced into the body. The antibodies produced in the body againstthese antigens would neutralise the pathogenic agents during actualinfection. The vaccines also generate memory â B and T-cells that recognisethe pathogen quickly on subsequent exposure and overwhelm theinvaders with a massive production of antibodies. If a person is infectedwith some deadly microbes to which quick immune response is requiredas in tetanus, we need to directly inject the preformed antibodies, orantitoxin (a preparation containing antibodies to the toxin). Even in casesof snakebites, the injection which is given to the patients, contain preformedantibodies against the snake venom. This type of immunisation is calledpassive immunisation.Recombinant DNA technology has allowed the production of antigenicpolypeptides of pathogen in bacteria or yeast. Vaccines produced usingthis approach allow large scale production and hence greater availabilityfor immunisation, e.g., hepatitis B vaccine produced from yeast.2022-23HUMAN HEALTH AND DISEASE1538.2.5 AllergiesWhen you have gone to a new place and suddenly you started sneezing,wheezing for no explained reason, and when you went away, yoursymptoms dissappeared. Did this happen to you? Some of us are sensitiveto some particles in the environment. The above-mentioned reaction couldbe because of allergy to pollen, mites, etc., which are different in differentplaces.The exaggerated response of the immune system to certain antigenspresent in the environment is called allergy. The substances to whichsuch an immune response is produced are called allergens. The antibodiesproduced to these are of IgE type. Common examples of allergens aremites in dust, pollens, animal dander, etc. Symptoms of allergic reactionsinclude sneezing, watery eyes, running nose and difficulty in breathing.Allergy is due to the release of chemicals like histamine and serotoninfrom the mast cells. For determining the cause of allergy, the patient isexposed to or injected with very small doses of possible allergens, and thereactions studied. The use of drugs like anti-histamine, adrenalin andsteroids quickly reduce the symptoms of allergy. Somehow, modern-daylife style has resulted in lowering of immunity and more sensitivity toallergens â more and more children in metro cities of India suffer fromallergies and asthma due to sensitivity to the environment. This could bebecause of the protected environment provided early in life.8.2.6 Auto ImmunityMemory-based acquired immunity evolved in higher vertebrates basedon the ability to differentiate foreign organisms (e.g., pathogens) from selfcells.While we still do not understand the basis of this, two corollaries ofthis ability have to be understood. One, higher vertebrates can distinguishforeign molecules as well as foreign organisms. Most of the experimentalimmunology deals with this aspect. Two, sometimes, due to genetic andother unknown reasons, the body attacks self-cells. This results in damageto the body and is called auto-immune disease. Rheumatoid arthritiswhich affects many people in our society is an auto-immune disease.8.2.7 Immune System in the BodyThe human immune system consists of lymphoid organs, tissues, cellsand soluble molecules like antibodies. As you have read, immune systemis unique in the sense that it recognises foreign antigens, responds tothese and remembers them. The immune system also plays an importantrole in allergic reactions, auto-immune diseases and organtransplantation.Lymphoid organs: These are the organs where origin and/or maturationand proliferation of lymphocytes occur. The primary lymphoid organsare bone marrow and thymus where immature lymphocytes differentiate2022-23154BIOLOGYinto antigen-sensitive lymphocytes. After maturation thelymphocytes migrate to secondary lymphoid organs like spleen,lymph nodes, tonsils, Peyerâs patches of small intestine andappendix. The secondary lymphoid organs provide the sites forinteraction of lymphocytes with the antigen, which then proliferateto become effector cells. The location of various lymphoid organsin the human body is shown in Figure 8.5.The bone marrow is the main lymphoid organ where allblood cells including lymphocytes are produced. The thymusis a lobed organ located near the heart and beneath thebreastbone. The thymus is quite large at the time of birth butkeeps reducing in size with age and by the time puberty isattained it reduces to a very small size. Both bone-marrowand thymus provide micro-environments for the developmentand maturation of T-lymphocytes. The spleen is a large beanshapedorgan. It mainly contains lymphocytes and phagocytes.It acts as a filter of the blood by trapping blood-borne microorganisms.Spleen also has a large reservoir of erythrocytes.The lymph nodes are small solid structures located at differentpoints along the lymphatic system. Lymph nodes serve to trap themicro-organisms or other antigens, which happen to get into the lymphand tissue fluid. Antigens trapped in the lymph nodes are responsible forthe activation of lymphocytes present there and cause the immuneresponse.There is lymphoid tissue also located within the lining of the majortracts (respiratory, digestive and urogenital tracts) called mucosaassociatedlymphoid tissue (MALT). It constitutes about 50 per cent ofthe lymphoid tissue in human body.8.3 AIDSThe word AIDS stands for Acquired Immuno Deficiency Syndrome.This means deficiency of immune system, acquired during the lifetime ofan individual indicating that it is not a congenital disease. âSyndromeâmeans a group of symptoms. AIDS was first reported in 1981 and in thelast twenty-five years or so, it has spread all over the world killing morethan 25 million persons.AIDS is caused by the Human Immuno deficiency Virus (HIV), amember of a group of viruses called retrovirus, which have an envelopeenclosing the RNA genome (Figure 8.6). Transmission of HIV-infectiongenerally occurs by (a) sexual contact with infected person, (b) bytransfusion of contaminated blood and blood products, (c) by sharinginfected needles as in the case of intravenous drug abusers and (d) frominfected mother to her child through placenta. So, people who are at highrisk of getting this infection includes - individuals who have multipleFigure 8.5 Diagrammaticrepresentationof Lymph nodes2022-23HUMAN HEALTH AND DISEASE155Figure 8.6 Replication of retrovirussexual partners, drug addicts who take drugs intravenously, individualswho require repeated blood transfusions and children born to an HIVinfected mother. Do you knowâwhen do people need repeated bloodtransfusion? Find out and make a list of such conditions. It is importantto note that HIV/AIDS is not spread by mere touch or physical contact; itspreads only through body fluids. It is, hence, imperative, for the physicaland psychological well-being, that the HIV/AIDS infected persons arenot isolated from family and society. There is always a time-lag betweenthe infection and appearance of AIDS symptoms. This period may varyfrom a few months to many years (usually 5-10 years).2022-23156BIOLOGYAfter getting into the body of the person, the virus enters into macrophageswhere RNA genome of the virus replicates to form viral DNA with the help ofthe enzyme reverse transcriptase. This viral DNA gets incorporated into hostcellâs DNA and directs the infected cells to produce virus particles (Figure 8.6).The macrophages continue to produce virus and in this way acts like a HIVfactory. Simultaneously, HIV enters into helperT-lymphocytes (TH), replicates and produce progeny viruses. The progenyviruses released in the blood attack other helper T-lymphocytes. This isrepeated leading to a progressive decrease in the number of helper Tlymphocytesin the body of the infected person. During this period, the personsuffers from bouts of fever, diarrhoea and weight loss. Due to decrease inthe number of helper T lymphocytes, the person starts suffering from infectionsthat could have been otherwise overcome such as those due to bacteriaespecially Mycobacterium, viruses, fungi and even parasites like Toxoplasma.The patient becomes so immuno-deficient that he/she is unable to protecthimself/herself against these infections. A widely used diagnostic test forAIDS is enzyme linked immuno-sorbent assay (ELISA). Treatment of AIDSwith anti-retroviral drugs is only partially effective. They can only prolongthe life of the patient but cannot prevent death, which is inevitable.Prevention of AIDS : As AIDS has no cure, prevention is the best option.Moreover, HIV infection, more often, spreads due to conscious behaviourpatterns and is not something that happens inadvertently, like pneumoniaor typhoid. Of course, infection in blood transfusion patients, new-borns(from mother) etc., may take place due to poor monitoring. The only excusemay be ignorance and it has been rightly said â âdonât die of ignoranceâ.In our country the National AIDS Control Organisation (NACO) and othernon-governmental organisation (NGOs) are doing a lot to educate peopleabout AIDS. WHO has started a number of programmes to prevent thespreading of HIV infection. Making blood (from blood banks) safe fromHIV, ensuring the use of only disposable needles and syringes in publicand private hospitals and clinics, free distribution of condoms, controllingdrug abuse, advocating safe sex and promoting regular check-ups forHIV in susceptible populations, are some such steps taken up.Infection with HIV or having AIDS is something that should not behidden â since then, the infection may spread to many more people.HIV/AIDS-infected people need help and sympathy instead of beingshunned by society. Unless society recognises it as a problem to be dealtwith in a collective manner â the chances of wider spread of the diseaseincrease manifold. It is a malady that can only be tackled, by the societyand medical fraternity acting together, to prevent the spread of the disease.8.4 CANCERCancer is one of the most dreaded diseases of human beings and is a majorcause of death all over the globe. More than a million Indians suffer from2022-23HUMAN HEALTH AND DISEASE157cancer and a large number of them die from it annually. The mechanismsthat underlie development of cancer or oncogenic transformation of cells,its treatment and control have been some of the most intense areas ofresearch in biology and medicine.In our body, cell growth and differentiation is highly controlled andregulated. In cancer cells, there is breakdown of these regulatorymechanisms. Normal cells show a property called contact inhibition byvirtue of which contact with other cells inhibits their uncontrolled growth.Cancer cells appears to have lost this property. As a result of this, cancerouscells just continue to divide giving rise to masses of cells called tumors.Tumors are of two types: benign and malignant. Benign tumors normallyremain confined to their original location and do not spread to other partsof the body and cause little damage. The malignant tumors, on theother hand are a mass of proliferating cells called neoplastic or tumorcells. These cells grow very rapidly, invading and damaging thesurrounding normal tissues. As these cells actively divide and grow theyalso starve the normal cells by competing for vital nutrients. Cells sloughedfrom such tumors reach distant sites through blood, and wherever theyget lodged in the body, they start a new tumor there. This property calledmetastasis is the most feared property of malignant tumors.Causes of cancer : Transformation of normal cells into cancerousneoplastic cells may be induced by physical, chemical or biological agents.These agents are called carcinogens. Ionising radiations like X-rays andgamma rays and non-ionizing radiations like UV cause DNA damageleading to neoplastic transformation. The chemical carcinogens presentin tobacco smoke have been identified as a major cause of lung cancer.Cancer causing viruses called oncogenic viruses have genes called viraloncogenes. Furthermore, several genes called cellular oncogenes(c-onc) or proto oncogenes have been identified in normal cells which,when activated under certain conditions, could lead to oncogenictransformation of the cells.Cancer detection and diagnosis : Early detection of cancers is essentialas it allows the disease to be treated successfully in many cases. Cancerdetection is based on biopsy and histopathological studies of the tissueand blood and bone marrow tests for increased cell counts in the case ofleukemias. In biopsy, a piece of the suspected tissue cut into thin sectionsis stained and examined under microscope (histopathological studies) bya pathologist. Techniques like radiography (use of X-rays), CT (computedtomography) and MRI (magnetic resonance imaging) are very useful todetect cancers of the internal organs. Computed tomography uses X-raysto generate a three-dimensional image of the internals of an object. MRIuses strong magnetic fields and non-ionising radiations to accurately detectpathological and physiological changes in the living tissue.Antibodies against cancer-specific antigens are also used fordetection of certain cancers. Techniques of molecular biology can be2022-23158BIOLOGYFigure 8.7 Chemical structure of Morphine Figure 8.8 Opium poppyapplied to detect genes in individuals with inherited susceptibility tocertain cancers. Identification of such genes, which predispose anindividual to certain cancers, may be very helpful in prevention ofcancers. Such individuals may be advised to avoid exposure toparticular carcinogens to which they are susceptible (e.g., tobaccosmoke in case of lung cancer).Treatment of cancer : The common approaches for treatment of cancerare surgery, radiation therapy and immunotherapy. In radiotherapy,tumor cells are irradiated lethally, taking proper care of the normal tissuessurrounding the tumor mass. Several chemotherapeutic drugs are usedto kill cancerous cells. Some of these are specific for particular tumors.Majority of drugs have side effects like hair loss, anemia, etc. Most cancersare treated by combination of surgery, radiotherapy and chemotherapy.Tumor cells have been shown to avoid detection and destruction byimmune system. Therefore, the patients are given substances calledbiological response modifiers such as a-interferon which activates theirimmune system and helps in destroying the tumor.8.5 DRUGS AND ALCOHOL ABUSESurveys and statistics show that use of drugs and alcohol has been onthe rise especially among the youth. This is really a cause of concern as itcould result in many harmful effects. Proper education and guidancewould enable youth to safeguard themselves against these dangerousbehaviour patterns and follow healthy lifestyles.The drugs, which are commonly abused are opioids, cannabinoidsand coca alkaloids. Majority of these are obtained from flowering plants.Some are obtained from fungi.Opioids are the drugs, which bind to specific opioid receptors presentin our central nervous system and gastrointestinal tract. Heroin(Figure 8.7), commonly called smack is chemically diacetylmorphine whichis a white, odourless, bitter crystalline compound. This is obtained byacetylation of morphine (Figure 8.7), which is extracted from the latex of2022-23HUMAN HEALTH AND DISEASE159poppy plant Papaver somniferum (Figure 8.8). Generally taken by snortingand injection, heroin is a depressant and slows down body functions.Cannabinoids are a group of chemicals (Figure 8.9), which interactwith cannabinoid receptors present principally in the brain. Naturalcannabinoids are obtained from the inflorescences of the plant Cannabissativa (Figure 8.10). The flower tops, leaves and the resin of cannabisplant are used in various combinations to produce marijuana, hashish,charas and ganja. Generally taken by inhalation and oral ingestion, theseare known for their effects on cardiovascular system of the body.Figure 8.9 Skeletal structure ofcannabinoid moleculeFigure 8.10 Leaves of Cannabis sativaCoca alkaloid or cocaine is obtained from cocaplant Erythroxylum coca, native to South America. Itinterferes with the transport of the neuro-transmitterdopamine. Cocaine, commonly called coke or crack isusually snorted. It has a potent stimulating action oncentral nervous system, producing a sense of euphoriaand increased energy. Excessive dosage of cocainecauses hallucinations. Other well-known plants withhallucinogenic properties are Atropa belladona andDatura (Figure 8.11). These days cannabinoids are alsobeing abused by some sportspersons.Drugs like barbiturates, amphetamines,benzodiazepines, and other similar drugs, that arenormally used as medicines to help patients cope withmental illnesses like depression and insomnia, are oftenabused. Morphine is a very effective sedative and painkiller, and is very usefulin patients who have undergone surgery. Several plants, fruits and seedshaving hallucinogenic properties have been used for hundreds of years infolk-medicine, religious ceremonies and rituals all over the globe. When theseare taken for a purpose other than medicinal use or in amounts/frequencythat impairs oneâs physical, physiological or psychological functions, itconstitutes drug abuse.Figure 8.11 Flowering branch of Datura2022-23160BIOLOGYSmoking also paves the way to hard drugs. Tobacco has been usedby human beings for more than 400 years. It is smoked, chewed or usedas a snuff. Tobacco contains a large number of chemical substancesincluding nicotine, an alkaloid. Nicotine stimulates adrenal gland torelease adrenaline and nor-adrenaline into blood circulation, both ofwhich raise blood pressure and increase heart rate. Smoking is associatedwith increased incidence of cancers of lung, urinary bladder and throat,bronchitis, emphysema, coronary heart disease, gastric ulcer, etc. Tobaccochewing is associated with increased risk of cancer of the oral cavity.Smoking increases carbon monoxide (CO) content in blood and reducesthe concentration of haembound oxygen. This causes oxygen deficiencyin the body.When one buys packets of cigarettes one cannot miss the statutorywarning that is present on the packing which warns against smokingand says how it is injurious to health. Yet, smoking is very prevalent insociety, both among young and old. Knowing the dangers of smokingand chewing tobacco, and its addictive nature, the youth and old need toavoid these habits. Any addict requires counselling and medical help toget rid of the habit.8.5.1 Adolescence and Drug/Alcohol AbuseAdolescence means both âa periodâ and âa processâ during which a childbecomes mature in terms of his/her attitudes and beliefs for effectiveparticipation in society. The period between 12-18 years of age maybe thought of as adolescence period. In other words, adolescence is abridge linking childhood and adulthood. Adolescence is accompaniedby several biological and behavioural changes. Adolescence, thus is avery vulnerable phase of mental and psychological development of anindividual.Curiosity, need for adventure and excitement, and experimentation,constitute common causes, which motivate youngsters towards drugand alcohol use. A childâs natural curiosity motivates him/her toexperiment. This is complicated further by effects that might be perceivedas benefits, of alcohol or drug use. Thus, the first use of drugs or alcoholmay be out of curiosity or experimentation, but later the child startsusing these to escape facing problems. Of late, stress, from pressures toexcel in academics or examinations, has played a significant role inpersuading the youngsters to try alcohol and drugs. The perceptionamong youth that it is âcoolâ or progressive to smoke, use drugs oralcohol, is also in a way a major cause for youth to start these habits.Television, movies, newspapers, internet also help to promote thisperception. Other factors that have been seen to be associated with drugand alcohol abuse among adolescents are unstable or unsupportivefamily structures and peer pressure.2022-23HUMAN HEALTH AND DISEASE1618.5.2 Addiction and DependenceBecause of the perceived benefits, drugs are frequently used repeatedly.The most important thing, which one fails to realise, is the inherentaddictive nature of alcohol and drugs. Addiction is a psychologicalattachment to certain effects âsuch as euphoria and a temporary feelingof well-being âassociated with drugs and alcohol. These drive people totake them even when these are not needed, or even when their use becomesself-destructive. With repeated use of drugs, the tolerance level of thereceptors present in our body increases. Consequently the receptorsrespond only to higher doses of drugs or alcohol leading to greater intakeand addiction. However, it should be clearly borne in mind that use ofthese drugs even once, can be a fore-runner to addiction. Thus, theaddictive potential of drugs and alcohol, pull the user into a vicious circleleading to their regular use (abuse) from which he/she may not be ableto get out. In the absence of any guidance or counselling, the person getsaddicted and becomes dependent on their use.Dependence is the tendency of the body to manifest a characteristicand unpleasant withdrawal syndrome if regular dose of drugs/alcoholis abruptly discontinued. This is characterised by anxiety, shakiness,nausea and sweating, which may be relieved when use is resumed again.In some cases, withdrawal symptoms can be severe and even lifethreatening and the person may need medical supervision.Dependence leads the patient to ignore all social norms in order toget sufficient funds to satiate his/her needs. These result in many socialadjustment problems.8.5.3 Effects of Drug/Alcohol AbuseThe immediate adverse effects of drugs and alcohol abuse are manifestedin the form of reckless behaviour, vandalism and violence. Excessivedoses of drugs may lead to coma and death due to respiratory failure,heart failure or cerebral hemorrhage. A combination of drugs or theirintake along with alcohol generally results in overdosing and evendeaths. The most common warning signs of drug and alcohol abuseamong youth include drop in academic performance, unexplainedabsence from school/college, lack of interest in personal hygiene,withdrawal, isolation, depression, fatigue, aggressive and rebelliousbehaviour, deteriorating relationships with family and friends, loss ofinterest in hobbies, change in sleeping and eating habits, fluctuationsin weight, appetite, etc.There may even be some far-reaching implications of drug/alcoholabuse. If an abuser is unable to get money to buy drugs/alcohol he/shemay turn to stealing. The adverse effects are just not restricted to theperson who is using drugs or alcohol. At times, a drug/alcohol addictbecomes the cause of mental and financial distress to his/her entire familyand friends.2022-23162BIOLOGYThose who take drugs intravenously (direct injection into the veinusing a needle and syringe), are much more likely to acquire seriousinfections like AIDS and Hepatitis B. The viruses, which are responsiblefor these diseases, are transferred from one person to another by sharingof infected needles and syringes. Both AIDS and Hepatitis B infectionsare chronic infections and ultimately fatal. Both can be transmittedthrough sexual contact or infected blood.The use of alcohol during adolescence may also have long-term effects.It could lead to heavy drinking in adulthood. The chronic use of drugs andalcohol damages nervous system and liver (cirrhosis). The use of drugsand alcohol during pregnancy is also known to adversely affect the foetus.Another misuse of drugs is what certain sportspersons do to enhancetheir performance. They (mis)use narcotic analgesics, anabolic steroids,diuretics and certain hormones in sports to increase muscle strength andbulk and to promote aggressiveness and as a result increase athleticperformance. The side-effects of the use of anabolic steroids in femalesinclude masculinisation (features like males), increased aggressiveness,mood swings, depression, abnormal menstrual cycles, excessive hairgrowth on the face and body, enlargement of clitoris, deepening of voice.In males it includes acne, increased aggressiveness, mood swings,depression, reduction of size of the testicles, decreased sperm production,potential for kidney and liver dysfunction, breast enlargement, prematurebaldness, enlargement of the prostate gland. These effects may bepermanent with prolonged use. In the adolescent male or female, severefacial and body acne, and premature closure of the growth centres of thelong bones may result in stunted growth.8.5.4 Prevention and ControlThe age-old adage of âprevention is better than cureâ holds true here also.It is also true that habits such as smoking, taking drug or alcohol aremore likely to be taken up at a young age, more during adolescence.Hence, it is best to identify the situations that may push an adolescenttowards use of drugs or alcohol, and to take remedial measures well intime. In this regard, the parents and the teachers have a specialresponsibility. Parenting that combines with high levels of nurturanceand consistent discipline, has been associated with lowered risk ofsubstance (alcohol/drugs/tobacco) abuse. Some of the measuresmentioned here would be particularly useful for prevention and controlof alcohol and drugs abuse among adolescents(i) Avoid undue peer pressure - Every child has his/her own choiceand personality, which should be respected and nurtured. A childshould not be pushed unduly to perform beyond his/her thresholdlimits; be it studies, sports or other activities.2022-23HUMAN HEALTH AND DISEASE163SUMMARYHealth is not just the absence of disease. It is a state of complete physical,mental, social and psychological well-being. Diseases like typhoid,cholera, pneumonia, fungal infections of skin, malaria and many othersare a major cause of distress to human beings. Vector-borne diseaseslike malaria especially one caused by Plasmodium falciparum, if nottreated, may prove fatal. Besides personal cleanliness and hygiene,public health measures like proper disposal of waste, decontaminationof drinking water, control of vectors like mosquitoes and immunisationare very helpful in preventing these diseases. Our immune system playsthe major role in preventing these diseases when we are exposed todisease-causing agents. The innate defences of our body like skin,mucous membranes, antimicrobial substances present in our tears,saliva and the phagocytic cells help to block the entry of pathogensinto our body. If the pathogens succeed in gaining entry to our body,specific antibodies (humoral immune response) and cells (cell mediatedimmune response) serve to kill these pathogens. Immune system hasmemory. On subsequent exposure to same pathogen, the immuneresponse is rapid and more intense. This forms the basis of protection(ii) Education and counselling - Educating and counselling him/her to face problems and stresses, and to accept disappointmentsand failures as a part of life. It would also be worthwhile to channelisethe childâs energy into healthy pursuits like sports, reading, music,yoga and other extracurricular activities.(iii) Seeking help from parents and peers - Help from parents andpeers should be sought immediately so that they can guideappropriately. Help may even be sought from close and trustedfriends. Besides getting proper advise to sort out their problems,this would help young to vent their feelings of anxiety and guilt.(iv) Looking for danger signs - Alert parents and teachers need tolook for and identify the danger signs discussed above. Even friends,if they find someone using drugs or alcohol, should not hesitate tobring this to the notice of parents or teacher in the best interests ofthe person concerned. Appropriate measures would then be requiredto diagnose the malady and the underlying causes. This would helpin initiating proper remedial steps or treatment.(v) Seeking professional and medical help - A lot of help is availablein the form of highly qualified psychologists, psychiatrists, and deaddictionand rehabilitation programmes to help individuals whohave unfortunately got in the quagmire of drug/alcohol abuse. Withsuch help, the affected individual with sufficient efforts and will power,can get rid of the problem completely and lead a perfectly normaland healthy life.2022-23164BIOLOGYafforded by vaccination and immunisation. Among other diseases, AIDSand cancer kill a large number of individuals worldwide. AIDS causedby the human immuno-deficiency virus (HIV) is fatal but can beprevented if certain precautions are taken. Many cancers are curable ifdetected early and appropriate therapeutic measures are taken. Of late,drug and alcohol abuse among youth and adolescents is becominganother cause of concern. Because of the addictive nature of alcoholand drugs, and their perceived benefits like relief from stress, a personmay try taking these in the face of peer pressure, examinations-relatedand competition-related stresses. In doing so, he/she may get addictedto them. Education about their harmful effects, counselling and seekingimmediate professional and medical help would totally relieve theindividual from these evils.EXERCISES1. What are the various public health measures, which you would suggestas safeguard against infectious diseases?2. In which way has the study of biology helped us to control infectiousdiseases?3. How does the transmission of each of the following diseases take place?(a) Amoebiasis (b) Malaria (c) Ascariasis (d) Pneumonia4. What measure would you take to prevent water-borne diseases?5. Discuss with your teacher what does âa suitable geneâ means, in thecontext of DNA vaccines.6. Name the primary and secondary lymphoid organs.7. The following are some well-known abbreviations, which have beenused in this chapter. Expand each one to its full form:(a) MALT (b) CMI (c) AIDS (d) NACO(e) HIV8. Differentiate the following and give examples of each:(a) Innate and acquired immunity (b) Active and passive immunity9. Draw a well-labelled diagram of an antibody molecule.10. What are the various routes by which transmission of human immunodeficiencyvirus takes place?11. What is the mechanism by which the AIDS virus causes deficiency ofimmune system of the infected person?12. How is a cancerous cell different from a normal cell?13. Explain what is meant by metastasis.14. List the harmful effects caused by alcohol/drug abuse.15. Do you think that friends can influence one to take alcohol/drugs? Ifyes, how may one protect himself/herself from such an influence?16. Why is that once a person starts taking alcohol or drugs, it is difficultto get rid of this habit? Discuss it with your teacher.17. In your view what motivates youngsters to take to alcohol or drugs andhow can this be avoided?2022-23With ever -increasing population of the world,enhancement of food production is a major necessity.Biological principles as applied to animal husbandry andplant breeding have a major role in our efforts to increasefood production. Several new techniques like embryotransfer technology and tissue culture techniques are goingto play a pivotal role in further enhancing food production.9.1 ANIMAL HUSBANDRYAnimal husbandry is the agricultural practice of breedingand raising livestock. As such it is a vital skill for farmersand is as much science as it is art. Animal husbandrydeals with the care and breeding of livestock like buffaloes,cows, pigs, horses, cattle, sheep, camels, goats, etc., thatare useful to humans. Extended, it includes poultryfarming and fisheries. Fisheries include rearing, catching,selling, etc., of fish, molluscs (shell-fish) and crustaceans(prawns, crabs, etc.). Since time immemorial, animals likebees, silk-worm, prawns, crabs, fishes, birds, pigs, cattle,sheep and camels have been used by humans for productslike milk, eggs, meat, wool, silk, honey, etc.It is estimated that more then 70 per cent of the worldlivestock population is in India and China. However, it isCHAPTER 9STRATEGIES FOR ENHANCEMENTIN FOOD PRODUCTION9.1 Animal Husbandry9.2 Plant Breeding9.3 Single Cell Proteins9.4 Tissue Culture2022-23166BIOLOGYsurprising to note that the contribution to the world farm produce is only25 per cent, i.e., the productivity per unit is very low. Hence, in additionto conventional practices of animal breeding and care, newer technologiesalso have to be applied to achieve improvement in quality and productivity.9.1.1 Management of Farms and Farm AnimalsA professional approach to what have been traditional practices of farmmanagement gives the much needed boost to our food production. Let usdiscuss some of the management procedures, employed in various animalfarm systems.9.1.1.1 Dairy Farm ManagementDairying is the management of animals for milk and its products forhuman consumption. Can you list the animals that you would expectto find in a dairy? What are different kinds of products that can bemade with milk from a dairy farm? In dairy farm management, we dealwith processes and systems that increase yield and improve quality ofmilk. Milk yield is primarily dependent on the quality of breeds in thefarm. Selection of good breeds having high yielding potential (under theclimatic conditions of the area), combined with resistance to diseases isvery important. For the yield potential to be realised the cattle have to bewell looked after â they have to be housed well, should have adequatewater and be maintained disease free. The feeding of cattle should becarried out in a scientific manner â with special emphasis on the qualityand quantity of fodder. Besides, stringent cleanliness and hygiene (bothof the cattle and the handlers) are of paramount importance while milking,storage and transport of the milk and its products. Nowadays, of course,much of these processes have become mechanised, which reduces chanceof direct contact of the produce with the handler. Ensuring these stringentmeasures would of course, require regular inspections, with proper recordkeeping. It would also help to identify and rectify the problems as earlyas possible. Regular visits by a veterinary doctor would be mandatory.You would probably find it interesting if you were to prepare aquestionnaire on diverse aspects of dairy keeping and then follow it up witha visit to a dairy farm in your locality and seek answers to the questions.9.1.1.2 Poultry Farm ManagementPoultry is the class of domesticated fowl (birds) used for food or for theireggs. They typically include chicken and ducks, and sometimes turkey andgeese. The word poultry is often used to refer to the meat of only these birds,but in a more general sense it may refer to the meat of other birds too.As in dairy farming, selection of disease free and suitable breeds,proper and safe farm conditions, proper feed and water, and hygiene andhealth care are important components of poultry farm management.2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION167You may have seen TV news or read newspaperâreports about the âbird flu virusâ which created a scare inthe country and drastically affected egg and chickenconsumption. Find out more about it and discuss whetherthe panic reaction was justified. How can we prevent thespread of the flu in case some chicken are infected?9.1.2 Animal BreedingBreeding of animals is an important aspect of animalhusbandry. Animal breeding aims at increasing the yieldof animals and improving the desirable qualities of theproduce. For what kind of characters would we breedanimals? Would the selection of characters differ withthe choice of animals?What do we understand by the term âbreedâ? A groupof animals related by descent and similar in most characterslike general appearance, features, size, configuration, etc.,are said to belong to a breed. Find out the names of somecommon breeds of cattle and poultry in the farms of yourarea.When breeding is between animals of the same breed itis called inbreeding, while crosses between different breedsare called outbreeding.Inbreeding : Inbreeding refers to the mating of moreclosely related individuals within the same breed for 4-6 generations. The breeding strategy is as follows â superior males andsuperior females of the same breed are identified and mated in pairs.The progeny obtained from such matings are evaluated and superiormales and females among them are identified for further mating. Asuperior female, in the case of cattle, is the cow or buffalo that producesmore milk per lactation. On the other hand, a superior male is the bull,which gives rise to superior progeny as compared to those of othermales.Try to recollect the homozygous purelines developed by Mendel asdiscussed in Chapter 5. A similar strategy is used for developing purelinesin cattle as was used in case of peas. Inbreeding increases homozygosity.Thus inbreeding is necessary if we want to evolve a pureline in any animal.Inbreeding exposes harmful recessive genes that are eliminated by selection.It also helps in accumulation of superior genes and elimination of lessdesirable genes. Therefore, this approach, where there is selection at eachstep, increases the productivity of inbred population. However, continuedinbreeding, especially close inbreeding, usually reduces fertility and evenproductivity. This is called inbreeding depression. Whenever this becomesa problem, selected animals of the breeding population should be matedFigure 9.1 Improved breed ofcattle and chickens(a) Jersey (b) Leghorn(a)(b)2022-23168BIOLOGYwith unrelated superior animals of the same breed. This usually helpsrestore fertility and yield.Out-breeding : Out-breeding is the breeding of the unrelated animals,which may be between individuals of the same breed but having nocommon ancestors for 4-6 generations (out-crossing) or betweendifferent breeds (cross-breeding) or different species (inter-specifichybridisation).Out-crossing: This is the practice of mating of animals within the samebreed, but having no common ancestors on either side of their pedigreeup to 4-6 generations. The offspring of such a mating is known as anout-cross. It is the best breeding method for animals that are belowaverage in productivity in milk production, growth rate in beef cattle,etc. A single outcross often helps to overcome inbreeding depression.Cross-breeding: In this method, superior males of one breed are matedwith superior females of another breed. Cross-breeding allows thedesirable qualities of two different breeds to be combined. The progenyhybrid animals may themselves be used for commercial production.Alternatively, they may be subjected to some form of inbreeding andselection to develop new stable breeds that may be superior to the existingbreeds. Many new animal breeds have been developed by this approach.Hisardale is a new breed of sheep developed in Punjab by crossingBikaneri ewes and Marino rams.Interspecific hybridisation: In this method, male and female animalsof two different related species are mated. In some cases, the progenymay combine desirable features of both the parents, and may be ofconsiderable economic value, e.g., the mule (Figure 9.2). Do you knowwhat cross leads to the production of the mule?Controlled breeding experiments are carried out using artificialinsemination. The semen is collected from the male thatis chosen as a parent and injected into the reproductivetract of the selected female by the breeder. The semenmay be used immediately or can be frozen and used at alater date. It can also be transported in a frozen form towhere the female is housed. In this way desirable matingsare carried. Artificial insemination helps us overcomeseveral problems of normal matings. Can you discussand list some of them?Often, the success rate of crossing mature male andfemale animals is fairly low even though artificialinsemination is carried out. To improve chances ofsuccessful production of hybrids, other means are also used. MultipleOvulation Embryo Transfer Technology (MOET) is one suchprogramme for herd improvement. In this method, a cow is administeredhormones, with FSH-like activity, to induce follicular maturation and superovulation â instead of one egg, which they normally yield per cycle, theyFigure 9.2 Mule2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION169produce 6-8 eggs. The animal is either mated with an elite bull orartificially inseminated. The fertilised eggs at 8â32 cells stages, arerecovered non-surgically and transferred to surrogate mothers. The geneticmother is available for another round of super ovulation. This technologyhas been demonstrated for cattle, sheep, rabbits, buffaloes, mares, etc.High milk-yielding breeds of females and high quality (lean meat withless lipid) meat-yielding bulls have been bred successfully to increaseherd size in a short time.9.1.3 Bee-keepingBee-keeping or apiculture is the maintenance of hives of honeybees forthe production of honey. It has been an age-old cottage industry. Honeyis a food of high nutritive value and also finds use in the indigenoussystems of medicine. Honeybee also produces beeswax, which finds manyuses in industry, such as in the preparation of cosmetics and polishes ofvarious kinds. The increased demand of honey has led to large-scale beekeepingpractices; it has become an established income generatingindustry, whether practiced on a small or on a large scale.Bee-keeping can be practiced in any area where there are sufficientbee pastures of some wild shrubs, fruit orchards and cultivated crops.There are several species of honeybees which can be reared. Of these, themost common species is Apis indica. Beehives can be kept in oneâscourtyard, on the verandah of the house or even on the roof. Bee-keepingis not labour-intensive.Bee-keeping though relatively easy does require some specialisedknowledge and there are several organisations that teach bee-keeping.The following points are important for successful bee-keeping:(i) Knowledge of the nature and habits of bees,(ii) Selection of suitable location for keeping the beehives,(iii) Catching and hiving of swarms (group of bees),(iv) Management of beehives during different seasons, and(v) Handling and collection of honey and of beeswax. Bees are the pollinatorsof many of our crop species (see chapter 2) such as sunflower, Brassica,apple and pear. Keeping beehives in crop fields during flowering periodincreases pollination efficiency and improves the yieldâbeneficial bothfrom the point of view of crop yield and honey yield.9.1.4 FisheriesFishery is an industry devoted to the catching, processing or selling of fish,shellfish or other aquatic animals. A large number of our population isdependent on fish, fish products and other aquatic animals such as prawn,crab, lobster, edible oyster, etc., for food. Some of the freshwater fishes whichare very common include Catla, Rohu and common carp. Some of the marinefishes that are eaten include â Hilsa, Sardines, Mackerel and Pomfrets.Find out what fishes are commonly eaten in your area.2022-23170BIOLOGYFisheries has an important place in Indian economy. It provides incomeand employment to millions of fishermen and farmers, particularly in thecoastal states. For many, it is the only source of their livelihood. In orderto meet the increasing demands on fisheries, different techniques havebeen employed to increase production. For example, through aquacultureand pisciculture we have been able to increase the production of aquaticplants and animals, both fresh-water and marine. Find out the differencebetween pisciculture and aquaculture. This has led to the developmentand flourishing of the fishery industry, and it has brought a lot of incometo the farmers in particular and the country in general. We now talk aboutâBlue Revolutionâ as being implemented along the same lines as âGreenRevolutionâ.9.2 PLANT BREEDINGTraditional farming can only yield a limited biomass, as food for humansand animals. Better management practices and increase in acreage canincrease yield, but only to a limited extent. Plant breeding as a technologyhas helped increase yields to a very large extent. Who in India has notheard of Green Revolution which was responsible for our country tonot merely meet the national requirements in food production but alsohelped us even to export it? Green revolution was dependent to a largeextent on plant breeding techniques for development of high-yielding anddisease resistant varieties in wheat, rice, maize, etc.9.2.1 What is Plant Breeding?Plant breeding is the purposeful manipulation of plant species in order tocreate desired plant types that are better suited for cultivation, give betteryields and are disease resistant. Conventional plant breeding has beenpracticed for thousands of years, since the beginning of human civilisation;recorded evidence of plant breeding dates back to 9,000-11,000 years ago.Many present-day crops are the result of domestication in ancient times.Today, all our major food crops are derived from domesticated varieties.Classical plant breeding involves crossing or hybridisation of pure lines,followed by artificial selection to produce plants with desirable traits of higheryield, nutrition and resistance to diseases. With advancements in genetics,molecular biology and tissue culture, plant breeding is now increasinglybeing carried out by using molecular genetic tools.If we were to list the traits or characters that the breeders have tried toincorporate into crop plants, the first we would list would be increasedcrop yield and improved quality. Increased tolerance to environmentalstresses (salinity, extreme temperatures, drought), resistance to pathogens(viruses, fungi and bacteria) and increased tolerance to insect pests wouldbe on our list too.2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION171Plant breeding programmes are carried out in a systematic wayworldwideâin government institutions and commercial companies. Themain steps in breeding a new genetic variety of a crop are â(i) Collection of variability: Genetic variability is the root of anybreeding programme. In many crops pre-existing genetic variabilityis available from wild relatives of the crop. Collection and preservationof all the different wild varieties, species and relatives of the cultivatedspecies (followed by their evaluation for their characteristics) is apre-requisite for effective exploitation of natural genes available inthe populations. The entire collection (of plants/seeds) having allthe diverse alleles for all genes in a given crop is called germplasmcollection.(ii) Evaluation and selection of parents: The germplasm is evaluatedso as to identify plants with desirable combination of characters.The selected plants are multiplied and used in the process ofhybridisation. Purelines are created wherever desirable and possible.(iii) Cross hybridisation among the selected parents: The desiredcharacters have very often to be combined from two different plants(parents), for example high protein quality of one parent may needto be combined with disease resistance from another parent. This ispossible by cross hybridising the two parents to produce hybridsthat genetically combine the desired characters in one plant. This isa very time-consuming and tedious process since the pollengrains from the desirable plant chosen as male parent have to becollected and placed on the stigma of the flowers selected as femaleparent (In chapter 2 details on how to make crosses have beendescribed). Also, it is not necessary that the hybrids do combine thedesirable characters; usually only one in few hundred to a thousandcrosses shows the desirable combination.(iv) Selection and testing of superior recombinants: This stepconsists of selecting, among the progeny of the hybrids, those plantsthat have the desired character combination. The selection process iscrucial to the success of the breeding objective and requires carefulscientific evaluation of the progeny. This step yields plants that aresuperior to both of the parents (very often more than one superiorprogeny plant may become available). These are self-pollinated forseveral generations till they reach a state of uniformity (homozygosity),so that the characters will not segregate in the progeny.(v) Testing, release and commercialisation of new cultivars: Thenewly selected lines are evaluated for their yield and other agronomictraits of quality, disease resistance, etc. This evaluation is done bygrowing these in the research fields and recording their performanceunder ideal fertiliser application, irrigation, and other cropmanagement practices. The evaluation in research fields is followed2022-23172BIOLOGYby testing the materials in farmersâ fields, for at least three growingseasons at several locations in the country, representing all theagroclimatic zones where the crop is usually grown. The material isevaluated in comparison to the best available local crop cultivar â acheck or reference cultivar.India is mainly an agricultural country. Agriculture accountsfor approximately 33 per cent of Indiaâs GDP and employs nearly62 per cent of the population. After Indiaâs independence, one of the mainchallenges facing the country was that of producing enough food for theincreasing population. As only limited land is fit for cultivation, India hasto strive to increase yields per unit area from existing farm land. Thedevelopment of several high yielding varieties of wheat and rice in themid-1960s, as a result of various plant breeding techniques led to dramaticincrease in food production in our country. This phase is often referredto as the Green Revolution. Figure 9.3 represents some Indian hybridcrops of high yeilding varieties.(c)(a) (b)Figure 9.3 Some Indian hybrid crops: (a) Maize; (b) Wheat; (c) Garden peas2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION173Wheat and Rice: During the period 1960 to 2000, wheat productionincreased from 11 million tonnes to 75 million tonnes while rice productionwent up from 35 million tonnes to 89.5 million tonnes. This was due to thedevelopment of semi-dwarf varieties of wheat and rice. Nobel laureateNorman E. Borlaug, at International Centre for Wheat and MaizeImprovement in Mexico, developed semi-dwarf wheat. In 1963, severalvarieties such as Sonalika and Kalyan Sona, which were high yielding anddisease resistant, were introduced all over the wheat-growing belt of India.Semi-dwarf rice varieties were derived from IR-8, (developed at InternationalRice Research Institute (IRRI), Philippines) and Taichung Native-1 (fromTaiwan). The derivatives were introduced in 1966. Later better-yielding semidwarfvarieties Jaya and Ratna were developed in India.Sugar cane: Saccharum barberi was originally grown in north India, buthad poor sugar content and yield. Tropical canes grown in south IndiaSaccharum officinarum had thicker stems and higher sugar content butdid not grow well in north India. These two species were successfullycrossed to get sugar cane varieties combining the desirable qualities ofhigh yield, thick stems, high sugar and ability to grow in the sugar caneareas of north India.Millets: Hybrid maize, jowar and bajra have been successfully developedin India. Hybrid breeding have led to the development of several highyielding varieties resistant to water stress.9.2.2 Plant Breeding for Disease ResistanceA wide range of fungal, bacterial and viral pathogens, affect the yield ofcultivated crop species, especially in tropical climates. Crop losses canoften be significant, up to 20-30 per cent, or sometimes even total. In thissituation, breeding and development of cultivars resistant to diseaseenhances food production. This also helps reduce the dependence onuse of fungicides and bacteriocides. Resistance of the host plant is theability to prevent the pathogen from causing disease and is determinedby the genetic constitution of the host plant. Before breeding isundertaken, it is important to know about the causative organism andthe mode of transmission. Some of the diseases caused by fungi are rusts,e.g., brown rust of wheat, red rot of sugarcane and late blight of potato;by bacteriaâ black rot of crucifers; and by viruses â tobacco mosaic,turnip mosaic, etc.Methods of breeding for disease resistance: Breeding is carriedout by the conventional breeding techniques (described earlier) or bymutation breeding. The conventional method of breeding for diseaseresistance is that of hybridisation and selection. Itâs steps are essentiallyidentical to those for breeding for any other agronomic characters suchas high yield. The various sequential steps are : screening germplasm2022-23174BIOLOGYfor resistance sources, hybridisation of selected parents, selection andevaluation of the hybrids and testing and release of new varieties.Some crop varieties bred by hybridisation and selection, fordisease resistance to fungi, bacteria and viral diseases are released(Table 9.1).Table 9.1Crop Variety Resistance to diseasesWheat Himgiri Leaf and stripe rust, hill buntBrassica Pusa swarnim White rust(Karan rai)Cauliflower Pusa Shubhra, Black rot and CurlPusa Snowball K-1 blight black rotCowpea Pusa Komal Bacterial blightChilli Pusa Sadabahar Chilly mosaic virus,Tobacco mosaic virusand Leaf curlConventional breeding is often constrained by the availability of limitednumber of disease resistance genes that are present and identified in variouscrop varieties or wild relatives. Inducing mutations in plants through diversemeans and then screening the plant materials for resistance sometimesleads to desirable genes being identified. Plants having these desirablecharacters can then be either multiplied directly or can be used in breeding.Other breeding methods that are used are selection amongst somaclonalvariants and genetic engineering.Mutation is the process by which genetic variations are createdthrough changes in the base sequence within genes (see Chapter 5)resulting in the creation of a new character or trait not found in the parentaltype. It is possible to induce mutations artificially through use of chemicalsor radiations (like gamma radiations), and selecting and using the plantsthat have the desirable character as a source in breeding â this process iscalled mutation breeding. In mung bean, resistance to yellow mosaicvirus and powdery mildew were induced by mutations.Several wild relatives of different cultivated species of plants have beenshown to have certain resistant characters but have very low yield. Hence,there is a need to introduce the resistant genes into the high-yieldingcultivated varieties. Resistance to yellow mosaic virus in bhindi(Abelmoschus esculentus) was transferred from a wild species andresulted in a new variety of A. esculentus called Parbhani kranti.2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION175All the above examples involve sources of resistance genes that are inthe same crop species, which has to be bred for disease resistance, or in arelated wild species. Transfer of resistance genes is achieved by sexualhybridisation between the target and the source plant followed byselection.9.2.3 Plant Breeding for Developing Resistanceto Insect PestsAnother major cause for large scale destruction of crop plant and cropproduce is insect and pest infestation. Insect resistance in host crop plantsmay be due to morphological, biochemical or physiological characteristics.Hairy leaves in several plants are associated with resistance to insect pests,e.g, resistance to jassids in cotton and cereal leaf beetle in wheat. In wheat,solid stems lead to non-preference by the stem sawfly and smooth leavedand nectar-less cotton varieties do not attract bollworms. High asparticacid, low nitrogen and sugar content in maize leads to resistance to maizestem borers.Breeding methods for insect pest resistance involve the same steps asthose for any other agronomic trait such as yield or quality and are asdiscussed earlier. Sources of resistance genes may be cultivated varieties,germplasm collections of the crop or wild relatives.Some released crop varieties bred by hybridisation and selection, forinsect pest resistance are given in Table 9.2.Table 9.2Crop Variety Insect PestsBrassica Pusa Gaurav Aphids(rapeseed mustard)Flat bean Pusa Sem 2, Jassids, aphids andPusa Sem 3 fruit borerOkra (Bhindi) Pusa Sawani Shoot and Fruit borerPusa A-49.2.4 Plant Breeding for Improved Food QualityMore than 840 million people in the world do not have adequate food tomeet their daily food and nutritional requirements. A far greater numberâthree billion people â suffer from micronutrient, protein and vitamindeficiencies or âhidden hungerâ because they cannot afford to buy enoughfruits, vegetables, legumes, fish and meat. Diets lacking essentialmicronutrients â particularly iron, vitamin A, iodine and zinc â increasethe risk for disease, reduce lifespan and reduce mental abilities.2022-23176BIOLOGYBiofortification â breeding crops with higher levels of vitamins andminerals, or higher protein and healthier fats â is the most practicalmeans to improve public health.Breeding for improved nutritional quality is undertaken with theobjectives of improving â(i) Protein content and quality;(ii) Oil content and quality;(iii) Vitamin content; and(iv) Micronutrient and mineral content.In 2000, maize hybrids that had twice the amount of the amino acids,lysine and tryptophan, compared to existing maize hybrids weredeveloped. Wheat variety, Atlas 66, having a high protein content, hasbeen used as a donor for improving cultivated wheat. It has been possibleto develop an iron-fortified rice variety containing over five times as muchiron as in commonly consumed varieties.The Indian Agricultural Research Institute, New Delhi has also releasedseveral vegetable crops that are rich in vitamins and minerals, e.g., vitaminA enriched carrots, spinach, pumpkin; vitamin C enriched bitter gourd,bathua, mustard, tomato; iron and calcium enriched spinach and bathua;and protein enriched beans â broad, lablab, French and garden peas.9.3 SINGLE CELL PROTEIN (SCP)Conventional agricultural production of cereals, pulses, vegetables, fruits,etc., may not be able to meet the demand of food at the rate at whichhuman and animal population is increasing. The shift from grain to meatdiets also creates more demand for cereals as it takes 3-10 Kg of grain toproduce 1 Kg of meat by animal farming. Can you explain this statementin the light of your knowledge of food chains? More than 25 per cent ofhuman population is suffering from hunger and malnutrition. One of thealternate sources of proteins for animal and human nutrition is SingleCell Protein (SCP).Microbes are being grown on an industrial scale as source of goodprotein. Blue-green algae like Spirulina can be grown easily on materialslike waste water from potato processing plants (containing starch), straw,molasses, animal manure and even sewage, to produce large quantitiesand can serve as food rich in protein, minerals, fats, carbohydrate andvitamins. Incidentally such utilisation also reduces environmentalpollution.Certain bacterial species like Methylophilus methylotrophus, becauseof its high rate of biomass production and growth, can be expected toproduce 25 tonnes of protein. The fact that edible mushrooms are eatenby many people and large scale mushroom culture is a growing industry2022-23STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION177makes it believable that microscopic fungi too would becomeacceptable as food.9.4 TISSUE CULTUREAs traditional breeding techniques failed to keep pace with demand andto provide sufficiently fast and efficient systems for crop improvement,another technology called tissue culture got developed. What does tissueculture mean? It was learnt by scientists, during 1950s, that wholeplants could be regenerated from explants, i.e., any part of a plant takenout and grown in a test tube, under sterile conditions in special nutrientmedia. This capacity to generate a whole plant from any cell/explant iscalled totipotency. You will learn how to accomplish this in higherclasses. It is important to stress here that the nutrient medium mustprovide a carbon source such as sucrose and also inorganic salts,vitamins, amino acids and growth regulators like auxins, cytokininsetc. By application of these methods it is possible to achieve propagationof a large number of plants in very short durations. This method ofproducing thousands of plants through tissue culture is called micropropagation.Each of these plants will be genetically identical to theoriginal plant from which they were grown, i.e., they are somaclones.Many important food plants like tomato, banana, apple, etc., have beenproduced on commercial scale using this method. Try to visit a tissueculture laboratory with your teacher to better understand and appreciatethe process.Another important application of the method is the recovery ofhealthy plants from diseased plants. Even if the plant is infected with avirus, the meristem (apical and axillary) is free of virus. Hence, onecan remove the meristem and grow it in vitro to obtain virus-free plants.Scientists have succeeded in culturing meristems of banana, sugarcane,potato, etc.Scientists have even isolated single cells from plants and afterdigesting their cell walls have been able to isolate naked protoplasts(surrounded by plasma membranes). Isolated protoplasts from twodifferent varieties of plants â each having a desirable character â can befused to get hybrid protoplasts, which can be further grown to form anew plant. These hybrids are called somatic hybrids while the processis called somatic hybridisation. Imagine a situation when a protoplastof tomato is fused with that of potato, and then they are grown â to formnew hybrid plants combining tomato and potato characteristics. Well,this has been achieved â resulting in formation of pomato; unfortunatelythis plant did not have all the desired combination of characteristics forits commercial utilisation.2022-23178BIOLOGYEXERCISES1. Explain in brief the role of animal husbandry in human welfare.2. If your family owned a dairy farm, what measures would you undertaketo improve the quality and quantity of milk production?3. What is meant by the term âbreedâ? What are the objectives of animalbreeding?4. Name the methods employed in animal breeding. According to you whichof the methods is best? Why?5. What is apiculture? How is it important in our lives?6. Discuss the role of fishery in enhancement of food production.7. Briefly describe various steps involved in plant breeding.8. Explain what is meant by biofortification.9. Which part of the plant is best suited for making virus-free plants andwhy?10. What is the major advantage of producing plants by micropropagation?11. Find out what the various components of the medium used forpropagation of an explant in vitro are?12. Name any five hybrid varieties of crop plants which have been developedin India.SUMMARYAnimal husbandry is the practice of taking care and breeding domesticanimals by applying scientific principles. The ever-increasing demandof food from animals and animal products both in terms of quality andquantity has been met by good animal husbandry practices. Thesepractices include (i) management of farm and farm animals, and (ii)animal breeding. In view of the high nutritive value of honey and itsmedicinal importance, there has been a remarkable growth in the practiceof bee-keeping or apiculture. Fishery is another flourishing industrymeeting the ever-increasing demand for fish, fish products and otheraquatic foods.Plant breeding may be used to create varieties, which are resistantto pathogens and to insect pests. This increases the yield of the food.This method has also been used to increase the protein content of theplant foods and thereby enhance the quality of food. In India, severalvarieties of different crop plants have been produced. All these measuresenhance the production of food. Techniques of tissue culture andsomatic hybridisation offer vast potential for manipulation of plants invitro to produce new varieties.2022-23Besides macroscopic plants and animals, microbes arethe major components of biological systems on this earth.You have studied about the diversity of living organismsin Class XI. Do you remember which Kingdoms amongthe living organisms contain micro-organisms? Which arethe ones that are only microscopic? Microbes are presenteverywhere â in soil, water, air, inside our bodies and thatof other animals and plants. They are present even at siteswhere no other life-form could possibly existâsites suchas deep inside the geysers (thermal vents) where thetemperature may be as high as 1000C, deep in the soil,under the layers of snow several metres thick, and in highlyacidic environments. Microbes are diverseâprotozoa,bacteria, fungi and microscopic animal and plant viruses,viroids and also prions that are proteinacious infectiousagents. Some of the microbes are shown in Figures 10.1and 10.2.Microbes like bacteria and many fungi can be grownon nutritive media to form colonies (Figure 10.3), that canbe seen with the naked eyes. Such cultures are useful instudies on micro-organisms.CHAPTER 10MICROBES IN HUMAN WELFARE10.1 Microbes in HouseholdProducts10.2 Microbes in IndustrialProducts10.3 Microbes in SewageTreatment10.4 Microbes in Production ofBiogas10.5 Microbes as BiocontrolAgents10.6 Microbes as Biofertilisers2022-23180BIOLOGY(a)(b)(c)(a) (b)Figure 10.3 (a) Colonies of bacteria growing in a petri dish;(b) Fungal colony growing in a petri dish(a) (b)(c)Figure10.1 Bacteria: (a) Rod-shaped,magnified 1500X; (b) Sphericalshaped, magnified1500X; (c) A rodshapedbacterium showing flagella,magnified 50,000XFigure 10.2 Viruses: (a) A bacteriophage; (b)Adenovirus which causes respiratoryinfections; (c) Rod-shaped TobaccoMosaic Virus (TMV). Magnified about1,00,000â1,50,000X2022-23181MICROBES IN HUMAN WELFAREIn chapter 8, you have read that microbes cause a large number ofdiseases in human beings. They also cause diseases in animals and plants.But this should not make you think that all microbes are harmful; severalmicrobes are useful to man in diverse ways. Some of the most importantcontributions of microbes to human welfare are discussed in this chapter.10.1 MICROBES IN HOUSEHOLD PRODUCTSYou would be surprised to know that we use microbes or productsderived from them everyday. A common example is the production ofcurd from milk. Micro-organisms such as Lactobacillus and otherscommonly called lactic acid bacteria (LAB) grow in milk and convert itto curd. During growth, the LAB produce acids that coagulate andpartially digest the milk proteins. A small amount of curd added to thefresh milk as inoculum or starter contain millions of LAB, which atsuitable temperatures multiply, thus converting milk to curd, whichalso improves its nutritional quality by increasing vitamin B12. In ourstomach too, the LAB play very beneficial role in checking diseasecausingmicrobes.The dough, which is used for making foods such as dosa and idli isalso fermented by bacteria. The puffed-up appearance of dough is due tothe production of CO2 gas. Can you tell which metabolic pathway istaking place resulting in the formation of CO2? Where do you think thebacteria for these fermentations come from? Similarly the dough, whichis used for making bread, is fermented using bakerâs yeast(Saccharomyces cerevisiae). A number of traditional drinks and foodsare also made by fermentation by the microbes. âToddyâ, a traditionaldrink of some parts of southern India is made by fermenting sap frompalms. Microbes are also used to ferment fish, soyabean and bambooshootsto make foods. Cheese, is one of the oldest food items in whichmicrobes were used. Different varieties of cheese are known by theircharacteristic texture, flavour and taste, the specificity coming from themicrobes used. For example, the large holes in âSwiss cheeseâ are due toproduction of a large amount of CO2 by a bacterium namedPropionibacterium sharmanii. The âRoquefort cheeseâ are ripened bygrowing a specific fungi on them, which gives them a particular flavour.10.2 MICROBES IN INDUSTRIAL PRODUCTSEven in industry, microbes are used to synthesise a number of productsvaluable to human beings. Beverages and antibiotics are some examples.Production on an industrial scale, requires growing microbes in very largevessels called fermentors (Figure 10.4).2022-23182BIOLOGY10.2.1 Fermented BeveragesMicrobes especially yeasts have been used fromtime immemorial for the production of beverageslike wine, beer, whisky, brandy or rum. For thispurpose the same yeast Saccharomycescerevisiae used for bread-making andcommonly called brewerâs yeast, is used forfermenting malted cereals and fruit juices, toproduce ethanol. Do you recollect the metabolicreactions, which result in the production ofethanol by yeast? Depending on the type of theraw material used for fermentation and the typeof processing (with or without distillation)different types of alcoholic drinks are obtained.Wine and beer are produced without distillationwhereas whisky, brandy and rum are producedby distillation of the fermented broth. Thephotograph of a fermentation plant is shown inFigure 10.5.10.2.2 AntibioticsAntibiotics produced by microbes are regardedas one of the most significant discoveries of thetwentieth century and have greatly contributedtowards the welfare of the human society. Anti isa Greek word that means âagainstâ, and bio meansâlifeâ, together they mean âagainst lifeâ (in thecontext of disease causing organisms); whereas with reference to humanbeings, they are âpro lifeâ and not against. Antibiotics are chemicalsubstances, which are produced by some microbes and can kill or retardthe growth of other (disease-causing) microbes.You are familiar with the commonly used antibiotic Penicillin. Do youknow that Penicillin was the first antibiotic to be discovered, and it was achance discovery? Alexander Fleming while working on Staphylococcibacteria, once observed a mould growing in one of his unwashed cultureplates around which Staphylococci could not grow. He found out that itwas due to a chemical produced by the mould and he named it Penicillinafter the mould Penicillium notatum. However, its full potential as aneffective antibiotic was established much later by Ernest Chain andHoward Florey. This antibiotic was extensively used to treat Americansoldiers wounded in World War II. Fleming, Chain and Florey were awardedthe Nobel Prize in 1945, for this discovery.Figure 10.5 Fermentation PlantFigure 10.4 Fermentors2022-23183MICROBES IN HUMAN WELFAREAfter Penicillin, other antibiotics were also purified from othermicrobes. Can you name some other antibiotics and find out theirsources? Antibiotics have greatly improved our capacity to treat deadlydiseases such as plague, whooping cough (kali khansi ), diphtheria (galghotu) and leprosy (kusht rog), which used to kill millions all over theglobe. Today, we cannot imagine a world without antibiotics.10.2.3 Chemicals, Enzymes and other Bioactive MoleculesMicrobes are also used for commercial and industrial production ofcertain chemicals like organic acids, alcohols and enzymes. Examples ofacid producers are Aspergillus niger (a fungus) of citric acid, Acetobacteraceti (a bacterium) of acetic acid; Clostridium butylicum (a bacterium) ofbutyric acid and Lactobacillus (a bacterium) of lactic acid.Yeast (Saccharomyces cerevisiae) is used for commercial productionof ethanol. Microbes are also used for production of enzymes. Lipases areused in detergent formulations and are helpful in removing oily stainsfrom the laundry. You must have noticed that bottled fruit juices boughtfrom the market are clearer as compared to those made at home. This isbecause the bottled juices are clarified by the use of pectinases andproteases. Streptokinase produced by the bacterium Streptococcus andmodified by genetic engineering is used as a âclot busterâ for removingclots from the blood vessels of patients who have undergone myocardialinfarction leading to heart attack.Another bioactive molecule, cyclosporin A, that is used as animmunosuppressive agent in organ-transplant patients, is produced bythe fungus Trichoderma polysporum. Statins produced by the yeastMonascus purpureus have been commercialised as blood-cholesterollowering agents. It acts by competitively inhibiting the enzyme responsiblefor synthesis of cholesterol.10.3 MICROBES IN SEWAGE TREATMENTWe know that large quantities of waste water are generated everyday incities and towns. A major component of this waste water is human excreta.This municipal waste-water is also called sewage. It contains largeamounts of organic matter and microbes. Many of which are pathogenic.Have you ever wondered where this huge quantity of sewage or urbanwaste water is disposed off daily? This cannot be discharged into naturalwater bodies like rivers and streams directly â you can understand why.Before disposal, hence, sewage is treated in sewage treatment plants (STPs)to make it less polluting. Treatment of waste water is done by the2022-23184BIOLOGYheterotrophic microbes naturally present inthe sewage. This treatment is carried out intwo stages:Primary treatment : These treatmentsteps basically involve physical removal ofparticles â large and small â from the sewagethrough filtration and sedimentation. Theseare removed in stages; initially, floating debrisis removed by sequential filtration. Then thegrit (soil and small pebbles) are removed bysedimentation. All solids that settle form theprimary sludge, and the supernatant formsthe effluent. The effluent from the primarysettling tank is taken for secondary treatment.Secondary treatment or Biological treatment : The primaryeffluent is passed into large aeration tanks (Figure 10.6) where it isconstantly agitated mechanically and air is pumped into it. This allowsvigorous growth of useful aerobic microbes into flocs (masses ofbacteria associated with fungal filaments to form mesh likestructures). While growing, these microbes consume the major partof the organic matter in the effluent. This significantly reduces theBOD (biochemical oxygen demand) of the effluent. BOD refers tothe amount of the oxygen that would be consumed if all the organicmatter in one liter of water were oxidised by bacteria. The sewagewater is treated till the BOD is reduced. The BOD test measures therate of uptake of oxygen by micro-organisms in a sample of waterand thus, indirectly, BOD is a measure of the organic matter presentin the water. The greater the BOD of waste water, more is its pollutingpotential.Once the BOD of sewage or waste water is reduced significantly, theeffluent is then passed into a settling tank where the bacterial âflocsâ areallowed to sediment. This sediment is called activated sludge. A smallpart of the activated sludge is pumped back into the aeration tank toserve as the inoculum. The remaining major part of the sludge is pumpedinto large tanks called anaerobic sludge digesters. Here, other kindsof bacteria, which grow anaerobically, digest the bacteria and the fungiin the sludge. During this digestion, bacteria produce a mixture of gasessuch as methane, hydrogen sulphide and carbon dioxide. These gasesform biogas and can be used as source of energy as it is inflammable.The effluent from the secondary treatment plant is generally releasedinto natural water bodies like rivers and streams. An aerial view of sucha plant is shown in Figure 10.7.Figure 10.6 Secondary treatment2022-23185MICROBES IN HUMAN WELFAREYou can appreciate how microbes play a majorrole in treating millions of gallons of waste watereveryday across the globe. This methodology hasbeen practiced for more than a century now, inalmost all parts of the world. Till date, no manmadetechnology has been able to rival themicrobial treatment of sewage.You are aware that due to increasingurbanisation, sewage is being produced in muchlarger quantities than ever before. However thenumber of sewage treatment plants has notincreased enough to treat such large quantities.So the untreated sewage is often discharged directly into rivers leading totheir pollution and increase in water-borne diseases.The Ministry of Environment and Forests has initiated Ganga ActionPlan and Yamuna Action Plan to save these major rivers of our countryfrom pollution. Under these plans, it is proposed to build a large numberof sewage treatment plants so that only treated sewage may be dischargedin the rivers. A visit to a sewage treatment plant situated in any placenear you would be a very interesting and educating experience.10.4 MICROBES IN PRODUCTION OF BIOGASBiogas is a mixture of gases (containing predominantly methane) producedby the microbial activity and which may be used as fuel. You have learntthat microbes produce different types of gaseous end-products duringgrowth and metabolism. The type of the gas produced depends upon themicrobes and the organic substrates they utilise. In the examples cited inrelation to fermentation of dough, cheese making and production ofbeverages, the main gas produced was CO2.. However, certain bacteria,which grow anaerobically on cellulosic material, produce large amountof methane along with CO2 and H2. These bacteria are collectively calledmethanogens, and one such common bacterium is Methanobacterium.These bacteria are commonly found in the anaerobic sludge duringsewage treatment. These bacteria are also present in the rumen (a part ofstomach) of cattle. A lot of cellulosic material present in the food of cattleis also present in the rumen. In rumen, these bacteria help in thebreakdown of cellulose and play an important role in the nutrition ofcattle. Do you think we, human beings, are able to digest the celluosepresent in our foods? Thus, the excreta (dung) of cattle, commonly calledgobar, is rich in these bacteria. Dung can be used for generation of biogas,commonly called gobar gas.The biogas plant consists of a concrete tank (10-15 feet deep) in whichbio-wastes are collected and a slurry of dung is fed. A floating cover isFigure 10.7 An aerial view of a sewage plant2022-23186BIOLOGYplaced over the slurry, whichkeeps on rising as the gas isproduced in the tank due to themicrobial activity. The biogasplant has an outlet, which isconnected to a pipe to supplybiogas to nearby houses. Thespent slurry is removed throughanother outlet and may be usedas fertiliser. Cattle dung isavailable in large quantities inrural areas where cattle are usedfor a variety of purposes. Sobiogas plants are more oftenbuilt in rural areas. The biogasthus produced is used forcooking and lighting. Thepicture of a biogas plant isshown in Figure 10.8. Thetechnology of biogas productionwas developed in India mainlydue to the efforts of Indian Agricultural Research Institute (IARI) andKhadi and Village Industries Commission (KVIC). If your school is situatedin a village or near a village, it would be very interesting to enquire if thereare any biogas plants nearby. Visit the biogas plant and learn more aboutit from the people who are actually managing it.10.5 MICROBES AS BIOCONTROL AGENTSBiocontrol refers to the use of biological methods for controlling plantdiseases and pests. In modern society, these problems have been tackledincreasingly by the use of chemicals â by use of insecticides and pesticides.These chemicals are toxic and extremely harmful, to human beings andanimals alike, and have been polluting our environment (soil, groundwater), fruits, vegetables and crop plants. Our soil is also polluted throughour use of weedicides to remove weeds.Biological control of pests and diseases: In agriculture, there is amethod of controlling pests that relies on natural predation rather thanintroduced chemicals. A key belief of the organic farmer is that biodiversityfurthers health. The more variety a landscape has, the more sustainableit is. The organic farmer, therefore, works to create a system where theinsects that are sometimes called pests are not eradicated, but insteadare kept at manageable levels by a complex system of checks and balanceswithin a living and vibrant ecosystem. Contrary to the âconventionalâfarming practices which often use chemical methods to kill both usefulFigure 10.8 A typical biogas plant2022-23187MICROBES IN HUMAN WELFAREand harmful life forms indiscriminately, this is a holistic approach thatseeks to develop an understanding of the webs of interaction between themyriad of organisms that constitute the field fauna and flora. The organicfarmer holds the view that the eradication of the creatures that are oftendescribed as pests is not only possible, but also undesirable, for withoutthem the beneficial predatory and parasitic insects which depend uponthem as food or hosts would not be able to survive. Thus, the use ofbiocontrol measures will greatly reduce our dependence on toxic chemicalsand pesticides. An important part of the biological farming approach isto become familiar with the various life forms that inhabit the field,predators as well as pests, and also their life cycles, patterns of feedingand the habitats that they prefer. This will help develop appropriate meansof biocontrol.The very familiar beetle with red and black markings â the Ladybird,and Dragonflies are useful to get rid of aphids and mosquitoes,respectively. An example of microbial biocontrol agents that can beintroduced in order to control butterfly caterpillars is the bacteria Bacillusthuringiensis (often written as Bt ). These are available in sachets as driedspores which are mixed with water and sprayed onto vulnerable plantssuch as brassicas and fruit trees, where these are eaten by the insectlarvae. In the gut of the larvae, the toxin is released and the larvae getkilled. The bacterial disease will kill the caterpillars, but leave other insectsunharmed. Because of the development of methods of genetic engineeringin the last decade or so, the scientists have introduced B. thuringiensistoxin genes into plants. Such plants are resistant to attack by insect pests.Bt-cotton is one such example, which is being cultivated in some statesof our country. You will learn more about this in chapter 12.A biological control being developed for use in the treatment of plantdisease is the fungus Trichoderma. Trichoderma species are free-livingfungi that are very common in the root ecosystems. They are effectivebiocontrol agents of several plant pathogens.Baculoviruses are pathogens that attack insects and other arthropods.The majority of baculoviruses used as biological control agents are in thegenus Nucleopolyhedrovirus. These viruses are excellent candidates forspecies-specific, narrow spectrum insecticidal applications. They havebeen shown to have no negative impacts on plants, mammals, birds, fishor even on non-target insects. This is especially desirable when beneficialinsects are being conserved to aid in an overall integrated pestmanagement (IPM) programme, or when an ecologically sensitive area isbeing treated.10.6 MICROBES AS BIOFERTILISERSWith our present day life styles environmental pollution is a major causeof concern. The use of the chemical fertilisers to meet the ever-increasing2022-23188BIOLOGYdemand of agricultural produce has contributed significantly tothis pollution. Of course, we have now realised that there are problemsassociated with the overuse of chemical fertilisers and there is alarge pressure to switch to organic farming â the use of biofertilisers.Biofertilisers are organisms that enrich the nutrient quality of the soil.The main sources of biofertilisers are bacteria, fungi and cyanobacteria.You have studied about the nodules on the roots of leguminous plantsformed by the symbiotic association of Rhizobium. These bacteria fixatmospheric nitrogen into organic forms, which is used by the plant asnutrient. Other bacteria can fix atmospheric nitrogen while free-living inthe soil (examples Azospirillum and Azotobacter), thus enriching thenitrogen content of the soil.Fungi are also known to form symbiotic associations with plants(mycorrhiza). Many members of the genus Glomus form mycorrhiza.The fungal symbiont in these associations absorbs phosphorus fromsoil and passes it to the plant. Plants having such associations showother benefits also, such as resistance to root-borne pathogens, toleranceto salinity and drought, and an overall increase in plant growth anddevelopment. Can you tell what advantage the fungus derives fromthis association?Cyanobacteria are autotrophic microbes widely distributed in aquaticand terrestrial environments many of which can fix atmospheric nitrogen,e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteriaserve as an important biofertiliser. Blue green algae also add organic matterto the soil and increase its fertility. Currently, in our country, a numberof biofertilisers are available commercially in the market and farmers usethese regularly in their fields to replenish soil nutrients and to reducedependence on chemical fertilisers.SUMMARYMicrobes are a very important component of life on earth. Not allmicrobes are pathogenic. Many microbes are very useful to humanbeings. We use microbes and microbially derived products almost everyday. Bacteria called lactic acid bacteria (LAB) grow in milk to convert itinto curd. The dough, which is used to make bread, is fermented byyeast called Saccharomyces cerevisiae. Certain dishes such as idli anddosa, are made from dough fermented by microbes. Bacteria and fungiare used to impart particular texture, taste and flavor to cheese. Microbesare used to produce industrial products like lactic acid, acetic acidand alcohol, which are used in a variety of processes in the industry.Antibiotics like penicillins produced by useful microbes are used to killdisease-causing harmful microbes. Antibiotics have played a major rolein controlling infectious diseases like diphtheria, whooping cough and2022-23189MICROBES IN HUMAN WELFAREEXERCISES1. Bacteria cannot be seen with the naked eyes, but these can be seenwith the help of a microscope. If you have to carry a sample from yourhome to your biology laboratory to demonstrate the presence of microbeswith the help of a microscope, which sample would you carry and why?2. Give examples to prove that microbes release gases during metabolism.3. In which food would you find lactic acid bacteria? Mention some oftheir useful applications.4. Name some traditional Indian foods made of wheat, rice and Bengalgram (or their products) which involve use of microbes.5. In which way have microbes played a major role in controlling diseasescaused by harmful bacteria?6. Name any two species of fungus, which are used in the production ofthe antibiotics.7. What is sewage? In which way can sewage be harmful to us?8. What is the key difference between primary and secondary sewagetreatment?9. Do you think microbes can also be used as source of energy? If yes, how?10. Microbes can be used to decrease the use of chemical fertilisers andpesticides. Explain how this can be accomplished.11. Three water samples namely river water, untreated sewage water andsecondary effluent discharged from a sewage treatment plant weresubjected to BOD test. The samples were labelled A, B and C; but thelaboratory attendant did not note which was which. The BOD valuesof the three samples A, B and C were recorded as 20mg/L, 8mg/L and400mg/L, respectively. Which sample of the water is most polluted?Can you assign the correct label to each assuming the river water isrelatively clean?pneumonia. For more than a hundred years, microbes are being usedto treat sewage (waste water) by the process of activated sludge formationand this helps in recycling of water in nature. Methanogens producemethane (biogas) while degrading plant waste. Biogas produced bymicrobes is used as a source of energy in rural areas. Microbes can alsobe used to kill harmful pests, a process called as biocontrol. Thebiocontrol measures help us to avoid heavy use of toxic pesticides forcontrolling pests. There is a need these days to push for use ofbiofertilisers in place of chemical fertilisers. It is clear from the diverseuses human beings have put microbes to that they play an importantrole in the welfare of human society.2022-23190BIOLOGY12. Find out the name of the microbes from which Cyclosporin A (animmunosuppressive drug) and Statins (blood cholesterol loweringagents) are obtained.13. Find out the role of microbes in the following and discuss it with your teacher.(a) Single cell protein (SCP)(b) Soil14. Arrange the following in the decreasing order (most important first) oftheir importance, for the welfare of human society. Give reasons foryour answer.Biogas, Citric acid, Penicillin and Curd15. How do biofertilisers enrich the fertility of the soil?2022-23Ever since the days of Rene Descartes, the French philosopher,mathematician and biologist of seventeenth century, all humanknowledge especially natural sciences were directed to developtechnologies which add to the creature comforts of humanlives, as also value to human life. The whole approach tounderstanding natural phenomena became anthropocentric.Physics and chemistry gave rise to engineering, technologiesand industries which all worked for human comfort and welfare.The major utility of the biological world is as a source of food.Biotechnology, the twentieth century off-shoot of modernbiology, changed our daily life as its products broughtqualitative improvement in health and food production. Thebasic principles underlying biotechnological processes and someapplications are highlighted and discussed in this unit.Chapter 11Biotechnology : Principles andProcessesChapter 12Biotechnology and ItsApplications2022-23Herbert Boyer was born in 1936 and brought up in a corner of westernPennsylvania where railroads and mines were the destiny of most youngmen. He completed graduate work at the University of Pittsburgh, in1963, followed by three years of post-graduate studies at Yale.In 1966, Boyer took over assistant professorship at the University ofCalifornia at San Francisco. By 1969, he performed studies on a coupleof restriction enzymes of the E. coli bacterium with especially usefulproperties. Boyer observed that these enzymes have the capability ofcutting DNA strands in a particular fashion, which left what has becameknown as âsticky endsâ on the strands. These clipped ends made pastingtogether pieces of DNA a precise exercise.This discovery, in turn, led to a rich and rewarding conversation inHawaii with a Stanford scientist named Stanley Cohen. Cohen hadbeen studying small ringlets of DNA called plasmids and which floatabout freely in the cytoplasm of certain bacterial cells and replicateindependently from the coding strand of DNA. Cohen had developeda method of removing these plasmids from the cell and then reinsertingthem in other cells. Combining this process with that of DNA splicingenabled Boyer and Cohen to recombine segments of DNA in desiredconfigurations and insert the DNA in bacterial cells, which could thenact as manufacturing plants for specific proteins. This breakthrough wasthe basis upon which the discipline of biotechnology was founded.HERBERT BOYER(1936 )2022-23Biotechnology deals with techniques of using liveorganisms or enzymes from organisms to produce productsand processes useful to humans. In this sense, makingcurd, bread or wine, which are all microbe-mediatedprocesses, could also be thought as a form ofbiotechnology. However, it is used in a restricted sensetoday, to refer to such of those processes which usegenetically modified organisms to achieve the same on alarger scale. Further, many other processes/techniques arealso included under biotechnology. For example, in vitrofertilisation leading to a âtest-tubeâ baby, synthesising agene and using it, developing a DNA vaccine or correctinga defective gene, are all part of biotechnology.The European Federation of Biotechnology (EFB) hasgiven a definition of biotechnology that encompasses bothtraditional view and modern molecular biotechnology.The definition given by EFB is as follows:âThe integration of natural science and organisms,cells, parts thereof, and molecular analogues for productsand servicesâ.11.1 PRINCIPLES OF BIOTECHNOLOGYAmong many, the two core techniques that enabled birthof modern biotechnology are :(i) Genetic engineering : Techniques to alter thechemistry of genetic material (DNA and RNA),CHAPTER 11BIOTECHNOLOGY : PRINCIPLESAND PROCESSES11.1 Principles of Biotechnology11.2 Tools of Recombinant DNATechnology11.3 Processes of RecombinantDNA Technology2022-23194BIOLOGYto introduce these into host organisms and thus change thephenotype of the host organism.(ii) Bioprocess engineering: Maintenance of sterile (microbialcontamination-free) ambience in chemical engineering processesto enable growth of only the desired microbe/eukaryotic cell inlarge quantities for the manufacture of biotechnological productslike antibiotics, vaccines, enzymes, etc.Let us now understand the conceptual development of the principlesof genetic engineering.You probably appreciate the advantages of sexual reproduction overasexual reproduction. The former provides opportunities for variationsand formulation of unique combinations of genetic setup, some of whichmay be beneficial to the organism as well as the population. Asexualreproduction preserves the genetic information, while sexual reproductionpermits variation. Traditional hybridisation procedures used in plant andanimal breeding, very often lead to inclusion and multiplication ofundesirable genes along with the desired genes. The techniques of geneticengineering which include creation of recombinant DNA, use ofgene cloning and gene transfer, overcome this limitation and allows usto isolate and introduce only one or a set of desirable genes withoutintroducing undesirable genes into the target organism.Do you know the likely fate of a piece of DNA, which is somehowtransferred into an alien organism? Most likely, this piece of DNA wouldnot be able to multiply itself in the progeny cells of the organism. But,when it gets integrated into the genome of the recipient, it may multiplyand be inherited along with the host DNA. This is because the alien pieceof DNA has become part of a chromosome, which has the ability toreplicate. In a chromosome there is a specific DNA sequence called theorigin of replication, which is responsible for initiating replication.Therefore, for the multiplication of any alien piece of DNA in an organismit needs to be a part of a chromosome(s) which has a specific sequenceknown as âorigin of replicationâ. Thus, an alien DNA is linked with theorigin of replication, so that, this alien piece of DNA can replicate andmultiply itself in the host organism. This can also be called as cloning ormaking multiple identical copies of any template DNA.Let us now focus on the first instance of the construction of an artificialrecombinant DNA molecule. The construction of the first recombinantDNA emerged from the possibility of linking a gene encoding antibioticresistance with a native plasmid (autonomously replicating circularextra-chromosomal DNA) of Salmonella typhimurium. Stanley Cohen andHerbert Boyer accomplished this in 1972 by isolating the antibioticresistance gene by cutting out a piece of DNA from a plasmid which wasresponsible for conferring antibiotic resistance. The cutting of DNA atspecific locations became possible with the discovery of the so-called2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES195âmolecular scissorsââ restriction enzymes. The cut piece of DNA wasthen linked with the plasmid DNA. These plasmid DNA act as vectors totransfer the piece of DNA attached to it. You probably know that mosquitoacts as an insect vector to transfer the malarial parasite into human body.In the same way, a plasmid can be used as vector to deliver an alien pieceof DNA into the host organism. The linking of antibiotic resistance genewith the plasmid vector became possible with the enzyme DNA ligase,which acts on cut DNA molecules and joins their ends. This makes a newcombination of circular autonomously replicating DNA created in vitroand is known as recombinant DNA. When this DNA is transferred intoEscherichia coli, a bacterium closely related to Salmonella, it couldreplicate using the new hostâs DNA polymerase enzyme and make multiplecopies. The ability to multiply copies of antibiotic resistance gene inE. coli was called cloning of antibiotic resistance gene in E. coli.You can hence infer that there are three basic steps in geneticallymodifying an organism â(i) identification of DNA with desirable genes;(ii) introduction of the identified DNA into the host;(iii) maintenance of introduced DNA in the host and transfer of the DNAto its progeny.11.2 TOOLS OF RECOMBINANT DNA TECHNOLOGYNow we know from the foregoing discussion that genetic engineering orrecombinant DNA technology can be accomplished only if we have thekey tools, i.e., restriction enzymes, polymerase enzymes, ligases, vectorsand the host organism. Let us try to understand some of these in detail.11.2.1 Restriction EnzymesIn the year 1963, the two enzymes responsible for restricting the growthof bacteriophage in Escherichia coli were isolated. One of these addedmethyl groups to DNA, while the other cut DNA. The later was calledrestriction endonuclease.The first restriction endonucleaseâHind II, whose functioningdepended on a specific DNA nucleotide sequence was isolated andcharacterised five years later. It was found that Hind II always cut DNAmolecules at a particular point by recognising a specific sequence ofsix base pairs. This specific base sequence is known as therecognition sequence for Hind II. Besides Hind II, today we know morethan 900 restriction enzymes that have been isolated from over 230 strainsof bacteria each of which recognise different recognition sequences.The convention for naming these enzymes is the first letter of the namecomes from the genus and the second two letters come from the species ofthe prokaryotic cell from which they were isolated, e.g., EcoRI comes fromEscherichia coli RY 13. In EcoRI, the letter âRâ is derived from the name of2022-23196BIOLOGYstrain. Roman numbers following the names indicate the order in whichthe enzymes were isolated from that strain of bacteria.Restriction enzymes belong to a larger class of enzymes callednucleases. These are of two kinds; exonucleases and endonucleases.Exonucleases remove nucleotides from the ends of the DNA whereas,endonucleases make cuts at specific positions within the DNA.Each restriction endonuclease functions by âinspectingâ the length ofa DNA sequence. Once it finds its specific recognition sequence, itwill bind to the DNA and cut each of the two strands of the doublehelix at specific points in their sugar -phosphate backbones(Figure 11.1). Each restriction endonuclease recognises a specificpalindromic nucleotide sequences in the DNA.Figure 11.1 Steps in formation of recombinant DNA by action of restriction endonucleaseenzyme - EcoRIDo you know what palindromes are? These are groups of lettersthat form the same words when read both forward and backward,e.g., âMALAYALAMâ. As against a word-palindrome where the sameword is read in both directions, the palindrome in DNA is a sequenceof base pairs that reads same on the two strands when orientation of2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES197reading is kept the same. For example, the following sequences readsthe same on the two strands in 5' à 3' direction. This is also true ifread in the 3' à 5' direction.5' ââ GAATTC ââ 3'3' ââ CTTAAG ââ 5'Restriction enzymes cut the strand of DNA a little away from the centreof the palindrome sites, but between the same two bases on the oppositestrands. This leaves single stranded portions at the ends. There areoverhanging stretches called sticky ends on each strand (Figure 11.1).These are named so because they form hydrogen bonds with theircomplementary cut counterparts. This stickiness of the ends facilitatesthe action of the enzyme DNA ligase.Restriction endonucleases are used in genetic engineering to formârecombinantâ molecules of DNA, which are composed of DNA fromdifferent sources/genomes.When cut by the same restriction enzyme, the resultant DNA fragmentshave the same kind of âsticky-endsâ and, these can be joined together(end-to-end) using DNA ligases (Figure 11.2).Figure 11.2 Diagrammatic representation of recombinant DNA technologyRecombinant DNAMolecule(Cloning Host)2022-23198BIOLOGYYou may have realised that normally, unless one cuts the vector andthe source DNA with the same restriction enzyme, the recombinant vectormolecule cannot be created.Separation and isolation of DNA fragments : The cutting of DNA byrestriction endonucleases results in the fragments of DNA. These fragmentscan be separated by a technique known as gel electrophoresis. SinceDNA fragments are negatively charged molecules they can be separatedby forcing them to move towards the anode under an electric field througha medium/matrix. Nowadays the most commonly used matrix is agarosewhich is a natural polymer extracted from sea weeds. The DNA fragmentsseparate (resolve) according to their size through sieving effect providedby the agarose gel. Hence, the smaller the fragment size, the farther itmoves. Look at the Figure 11.3 and guess at which end of the gel thesample was loaded.The separated DNA fragments can bevisualised only after staining the DNAwith a compound known as ethidiumbromide followed by exposure to UVradiation (you cannot see pure DNAfragments in the visible light andwithout staining). You can see brightorange coloured bands of DNA in aethidium bromide stained gelexposed to UV light (Figure 11.3). Theseparated bands of DNA are cut outfrom the agarose gel and extractedfrom the gel piece. This step is knownas elution. The DNA fragmentspurified in this way are used inconstructing recombinant DNA byjoining them with cloning vectors.11.2.2 Cloning VectorsYou know that plasmids and bacteriophages have the ability to replicatewithin bacterial cells independent of the control of chromosomal DNA.Bacteriophages because of their high number per cell, have very highcopy numbers of their genome within the bacterial cells. Some plasmidsmay have only one or two copies per cell whereas others may have15-100 copies per cell. Their numbers can go even higher. If we are ableto link an alien piece of DNA with bacteriophage or plasmid DNA, we canmultiply its numbers equal to the copy number of the plasmid orbacteriophage. Vectors used at present, are engineered in such a waythat they help easy linking of foreign DNA and selection of recombinantsfrom non-recombinants.Figure 11.3 A typical agarose gelelectrophoresis showingmigration of undigested(lane 1) and digested set ofDNA fragments (lane 2 to 4)2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES199The following are the features that are required to facilitate cloninginto a vector.(i) Origin of replication (ori) : This is a sequence from wherereplication starts and any piece of DNA when linked to this sequencecan be made to replicate within the host cells. This sequence is alsoresponsible for controlling the copy number of the linked DNA. So,if one wants to recover many copies of the target DNA it should becloned in a vector whose origin support high copy number.(ii) Selectable marker : In addition to âoriâ, the vector requires aselectable marker, which helps in identifying and eliminating nontransformantsand selectively permitting the growth of thetransformants. Transformation is a procedure through which apiece of DNA is introduced in a host bacterium (you will study theprocess in subsequent section). Normally, the genes encodingresistance to antibiotics such as ampicillin, chloramphenicol,tetracycline or kanamycin, etc., are considered useful selectablemarkers for E. coli. The normal E. coli cells do not carry resistanceagainst any of these antibiotics.(iii) Cloning sites: In order to link thealien DNA, the vector needs to havevery few, preferably single,recognition sites for the commonlyused restriction enzymes. Presence ofmore than one recognition sites withinthe vector will generate severalfragments, which will complicate thegene cloning (Figure 11.4). Theligation of alien DNA is carried out ata restriction site present in one of thetwo antibiotic resistance genes. Forexample, you can ligate a foreign DNAat the BamH I site of tetracyclineresistance gene in the vector pBR322.The recombinant plasmids will losetetracycline resistance due to insertionof foreign DNA but can still be selectedout from non-recombinant ones byplating the transformants ontetracycline containing medium. The transformants growing onampicillin containing medium are then transferred on a mediumcontaining tetracycline. The recombinants will grow in ampicillincontaining medium but not on that containing tetracycline. But,non- recombinants will grow on the medium containing both theantibiotics. In this case, one antibiotic resistance gene helps inselecting the transformants, whereas the other antibiotic resistanceFigure 11.4 E. coli cloning vector pBR322showing restriction sites(Hind III, EcoR I, BamH I, Sal I,Pvu II, Pst I, Cla I), ori andantibiotic resistance genes(ampR and tetR). rop codes forthe proteins involved in thereplication of the plasmid.2022-23200BIOLOGYgene gets âinactivated due to insertionâ of alien DNA, and helps inselection of recombinants.Selection of recombinants due to inactivation of antibiotics is acumbersome procedure because it requires simultaneous platingon two plates having different antibiotics. Therefore, alternativeselectable markers have been developed which differentiaterecombinants from non-recombinants on the basis of their abilityto produce colour in the presence of a chromogenic substrate. Inthis, a recombinant DNA is inserted within the coding sequence ofan enzyme, b-galactosidase. This results into inactivation of thegene for synthesis of this enzyme, which is referred to as insertionalinactivation. The presence of a chromogenic substrate gives bluecoloured colonies if the plasmid in the bacteria does not have aninsert. Presence of insert results into insertional inactivation of theb-galactosidase gene and the colonies do not produce any colour,these are identified as recombinant colonies.(iv) Vectors for cloning genes in plants and animals : You may besurprised to know that we have learnt the lesson of transferring genesinto plants and animals from bacteria and viruses which have knownthis for ages â how to deliver genes to transform eukaryotic cells andforce them to do what the bacteria or viruses want. For example,Agrobacterium tumifaciens, a pathogen of several dicot plants is ableto deliver a piece of DNA known as âT-DNAâ to transform normalplant cells into a tumor and direct these tumor cells to produce thechemicals required by the pathogen. Similarly, retroviruses in animalshave the ability to transform normal cells into cancerous cells. Abetter understanding of the art of delivering genes by pathogens intheir eukaryotic hosts has generated knowledge to transform thesetools of pathogens into useful vectors for delivering genes of interestto humans. The tumor inducing (Ti) plasmid of Agrobacteriumtumifaciens has now been modified into a cloning vector which is nomore pathogenic to the plants but is still able to use the mechanismsto deliver genes of our interest into a variety of plants. Similarly,retroviruses have also been disarmed and are now used to deliverdesirable genes into animal cells. So, once a gene or a DNA fragmenthas been ligated into a suitable vector it is transferred into a bacterial,plant or animal host (where it multiplies).11.2.3 Competent Host (For Transformation withRecombinant DNA)Since DNA is a hydrophilic molecule, it cannot pass through cellmembranes. Why? In order to force bacteria to take up the plasmid, thebacterial cells must first be made âcompetentâ to take up DNA. This isdone by treating them with a specific concentration of a divalent cation,such as calcium, which increases the efficiency with which DNA enters2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES201the bacterium through pores in its cell wall. Recombinant DNA can thenbe forced into such cells by incubating the cells with recombinant DNAon ice, followed by placing them briefly at 420C (heat shock), and thenputting them back on ice. This enables the bacteria to take up therecombinant DNA.This is not the only way to introduce alien DNA into host cells. In amethod known as micro-injection, recombinant DNA is directly injectedinto the nucleus of an animal cell. In another method, suitable for plants,cells are bombarded with high velocity micro-particles of gold or tungstencoated with DNA in a method known as biolistics or gene gun. And thelast method uses âdisarmed pathogenâ vectors, which when allowed toinfect the cell, transfer the recombinant DNA into the host.Now that we have learnt about the tools for constructing recombinantDNA, let us discuss the processes facilitating recombinant DNA technology.11.3 PROCESSES OF RECOMBINANT DNA TECHNOLOGYRecombinant DNA technology involves several steps in specificsequence such as isolation of DNA, fragmentation of DNA byrestriction endonucleases, isolation of a desired DNA fragment,ligation of the DNA fragment into a vector, transferring therecombinant DNA into the host, culturing the host cells in amedium at large scale and extraction of the desired product.Let us examine each of these steps in some details.11.3.1 Isolation of the Genetic Material (DNA)Recall that nucleic acid is the genetic material of all organismswithout exception. In majority of organisms this isdeoxyribonucleic acid or DNA. In order to cut the DNA withrestriction enzymes, it needs to be in pure form, free from othermacro-molecules. Since the DNA is enclosed within themembranes, we have to break the cell open to release DNA alongwith other macromolecules such as RNA, proteins,polysaccharides and also lipids. This can be achieved by treatingthe bacterial cells/plant or animal tissue with enzymes such aslysozyme (bacteria), cellulase (plant cells), chitinase (fungus).You know that genes are located on long molecules of DNAinterwined with proteins such as histones. The RNA can be removed bytreatment with ribonuclease whereas proteins can be removed bytreatment with protease. Other molecules can be removed by appropriatetreatments and purified DNA ultimately precipitates out after the additionof chilled ethanol. This can be seen as collection of fine threads in thesuspension (Figure 11.5).Figure 11.5 DNA thatseparates out can beremoved by spooling2022-23202BIOLOGY11.3.2 Cutting of DNA at Specific LocationsRestriction enzyme digestions are performed by incubating purified DNAmolecules with the restriction enzyme, at the optimal conditions for thatspecific enzyme. Agarose gel electrophoresis is employed to check theprogression of a restriction enzyme digestion. DNA is a negatively chargedmolecule, hence it moves towards the positive electrode (anode)(Figure 11.3). The process is repeated with the vector DNA also.The joining of DNA involves several processes. After having cut thesource DNA as well as the vector DNA with a specific restriction enzyme,the cut out âgene of interestâ from the source DNA and the cut vector withspace are mixed and ligase is added. This results in the preparation ofrecombinant DNA.11.3.3 Amplification of Gene of Interest using PCRPCR stands for Polymerase Chain Reaction. In this reaction, multiplecopies of the gene (or DNA) of interest is synthesised in vitro using twoFigure 11.6 Polymerase chain reaction (PCR) : Each cycle has three steps: (i) Denaturation;(ii) Primer annealing; and (iii) Extension of primers2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES203sets of primers (small chemically synthesised oligonucleotides that arecomplementary to the regions of DNA) and the enzyme DNA polymerase.The enzyme extends the primers using the nucleotides provided in thereaction and the genomic DNA as template. If the process of replicationof DNA is repeated many times, the segment of DNA can be amplifiedto approximately billion times, i.e., 1 billion copies are made. Suchrepeated amplification is achieved by the use of a thermostable DNApolymerase (isolated from a bacterium, Thermus aquaticus), whichremain active during the high temperature induced denaturation ofdouble stranded DNA. The amplified fragment if desired can now beused to ligate with a vector for further cloning (Figure11.6).11.3.4 Insertion of Recombinant DNA into the HostCell/OrganismThere are several methods of introducing the ligated DNA into recipientcells. Recipient cells after making them âcompetentâ to receive, take upDNA present in its surrounding. So, if a recombinant DNA bearing genefor resistance to an antibiotic (e.g., ampicillin) is transferred into E. colicells, the host cells become transformed into ampicillin-resistant cells. Ifwe spread the transformed cells on agar plates containing ampicillin, onlytransformants will grow, untransformed recipient cells will die. Since, dueto ampicillin resistance gene, one is able to select a transformed cell in thepresence of ampicillin. The ampicillin resistance gene in this case is calleda selectable marker.11.3.5 Obtaining the Foreign Gene ProductWhen you insert a piece of alien DNA into a cloning vector and transfer itinto a bacterial, plant or animal cell, the alien DNA gets multiplied. Inalmost all recombinant technologies, the ultimate aim is to produce adesirable protein. Hence, there is a need for the recombinant DNA to beexpressed. The foreign gene gets expressed under appropriate conditions.The expression of foreign genes in host cells involve understanding manytechnical details.After having cloned the gene of interest and having optimised theconditions to induce the expression of the target protein, one has toconsider producing it on a large scale. Can you think of any reasonwhy there is a need for large-scale production? If any protein encodinggene is expressed in a heterologous host, it is called a recombinantprotein. The cells harbouring cloned genes of interest may be grownon a small scale in the laboratory. The cultures may be used forextracting the desired protein and then purifying it by using differentseparation techniques.The cells can also be multiplied in a continuous culture system whereinthe used medium is drained out from one side while fresh medium isadded from the other to maintain the cells in their physiologically most2022-23204BIOLOGYA stirred-tank reactor is usually cylindrical or with a curved base tofacilitate the mixing of the reactor contents. The stirrer facilitates evenmixing and oxygen availability throughout the bioreactor. Alternativelyair can be bubbled through the reactor. If you look at the figure closelyyou will see that the bioreactor has an agitator system, an oxygen deliverysystem and a foam control system, a temperature control system, pHcontrol system and sampling ports so that small volumes of the culturecan be withdrawn periodically.11.3.6 Downstream ProcessingAfter completion of the biosynthetic stage, the product has to be subjectedthrough a series of processes before it is ready for marketing as a finishedactive log/exponential phase. This type of culturing method produces alarger biomass leading to higher yields of desired protein.Small volume cultures cannot yield appreciable quantities of products.To produce in large quantities, the development of bioreactors, wherelarge volumes (100-1000 litres) of culture can be processed, was required.Thus, bioreactors can be thought of as vessels in which raw materials arebiologically converted into specific products, individual enzymes, etc.,using microbial plant, animal or human cells. A bioreactor provides theoptimal conditions for achieving the desired product by providingoptimum growth conditions (temperature, pH, substrate, salts, vitamins,oxygen).The most commonly used bioreactors are of stirring type, which areshown in Figure 11.7.Figure 11.7 (a) Simple stirred-tank bioreactor; (b) Sparged stirred-tank bioreactor through whichsterile air bubbles are sparged(a) (b)2022-23BIOTECHNOLOGY : PRINCIPLES AND PROCESSES205SUMMARYBiotechnology deals with large scale production and marketing ofproducts and processes using live organisms, cells or enzymes.Modern biotechnology using genetically modified organisms wasmade possible only when man learnt to alter the chemistry of DNAand construct recombinant DNA. This key process is calledrecombinant DNA technology or genetic engineering. This processinvolves the use of restriction endonucleases, DNA ligase,appropriate plasmid or viral vectors to isolate and ferry the foreignDNA into host organisms, expression of the foreign gene, purificationof the gene product, i.e., the functional protein and finally making asuitable formulation for marketing. Large scale production involvesuse of bioreactors.EXERCISES1. Can you list 10 recombinant proteins which are used in medicalpractice? Find out where they are used as therapeutics (use the internet).2. Make a chart (with diagrammatic representation) showing a restrictionenzyme, the substrate DNA on which it acts, the site at which it cutsDNA and the product it produces.3. From what you have learnt, can you tell whether enzymes are bigger orDNA is bigger in molecular size? How did you know?4. What would be the molar concentration of human DNA in a humancell? Consult your teacher.5. Do eukaryotic cells have restriction endonucleases? Justify your answer.6. Besides better aeration and mixing properties, what other advantagesdo stirred tank bioreactors have over shake flasks?7. Collect 5 examples of palindromic DNA sequences by consulting your teacher.Better try to create a palindromic sequence by following base-pair rules.8. Can you recall meiosis and indicate at what stage a recombinant DNAis made?9. Can you think and answer how a reporter enzyme can be used to monitortransformation of host cells by foreign DNA in addition to a selectablemarker?product. The processes include separation and purification, which arecollectively referred to as downstream processing. The product has to beformulated with suitable preservatives. Such formulation has to undergothorough clinical trials as in case of drugs. Strict quality control testingfor each product is also required. The downstream processing and qualitycontrol testing vary from product to product.2022-23206BIOLOGY10. Describe briefly the following:(a) Origin of replication(b) Bioreactors(c) Downstream processing11. Explain briefly(a) PCR(b) Restriction enzymes and DNA(c) Chitinase12. Discuss with your teacher and find out how to distinguish between(a) Plasmid DNA and Chromosomal DNA(b) RNA and DNA(c) Exonuclease and Endonuclease2022-23Biotechnology, as you would have learnt from theprevious chapter, essentially deals with industrial scaleproduction of biopharmaceuticals and biologicals usinggenetically modified microbes, fungi, plants and animals.The applications of biotechnology include therapeutics,diagnostics, genetically modified crops for agriculture,processed food, bioremediation, waste treatment, andenergy production. Three critical research areas ofbiotechnology are:(i) Providing the best catalyst in the form of improvedorganism usually a microbe or pure enzyme.(ii) Creating optimal conditions through engineering fora catalyst to act, and(iii) Downstream processing technologies to purify theprotein/organic compound.Let us now learn how human beings have usedbiotechnology to improve the quality of human life,especially in the field of food production and health.12.1 BIOTECHNOLOGICAL APPLICATIONS INAGRICULTURELet us take a look at the three options that can be thoughtfor increasing food production(i) agro-chemical based agriculture;CHAPTER 12BIOTECHNOLOGY AND ITSAPPLICATIONS12.1 BiotechnologicalApplications inAgriculture12.2 BiotechnologicalApplications inMedicine12.3 Transgenic Animals12.4 Ethical Issues2022-23208BIOLOGY(ii) organic agriculture; and(iii) genetically engineered crop-based agriculture.The Green Revolution succeeded in tripling the food supply but yetit was not enough to feed the growing human population. Increased yieldshave partly been due to the use of improved crop varieties, but mainlydue to the use of better management practices and use of agrochemicals(fertilisers and pesticides). However, for farmers in the developing world,agrochemicals are often too expensive, and further increases in yield withexisting varieties are not possible using conventional breeding. Is thereany alternative path that our understanding of genetics can show so thatfarmers may obtain maximum yield from their fields? Is there a way tominimise the use of fertilisers and chemicals so that their harmful effectson the environment are reduced? Use of genetically modified crops is apossible solution.Plants, bacteria, fungi and animals whose genes have been altered bymanipulation are called Genetically Modified Organisms (GMO). GMplants have been useful in many ways. Genetic modification has:(i) made crops more tolerant to abiotic stresses (cold, drought, salt, heat).(ii) reduced reliance on chemical pesticides (pest-resistant crops).(iii) helped to reduce post harvest losses.(iv) increased efficiency of mineral usage by plants (this prevents earlyexhaustion of fertility of soil).(v) enhanced nutritional value of food, e.g., golden rice, i.e., Vitamin âAâenriched rice.In addition to these uses, GM has been used to create tailor-madeplants to supply alternative resources to industries, in the form of starches,fuels and pharmaceuticals.Some of the applications of biotechnology in agriculture that you willstudy in detail are the production of pest resistant plants, which coulddecrease the amount of pesticide used. Bt toxin is produced by abacterium called Bacillus thuringiensis (Bt for short). Bt toxin gene hasbeen cloned from the bacteria and been expressed in plants to provideresistance to insects without the need for insecticides; in effect created abio-pesticide. Examples are Bt cotton, Bt corn, rice, tomato, potato andsoyabean etc.Bt Cotton: Some strains of Bacillus thuringiensis produce proteins thatkill certain insects such as lepidopterans (tobacco budworm, armyworm),coleopterans (beetles) and dipterans (flies, mosquitoes). B. thuringiensisforms protein crystals during a particular phase of their growth. Thesecrystals contain a toxic insecticidal protein. Why does this toxin not killthe Bacillus? Actually, the Bt toxin protein exist as inactive protoxins butonce an insect ingest the inactive toxin, it is converted into an active formof toxin due to the alkaline pH of the gut which solubilise the crystals.The activated toxin binds to the surface of midgut epithelial cells and2022-23209BIOTECHNOLOGY AND ITS APPLICATIONScreate pores that cause cell swelling and lysis and eventually cause deathof the insect.Specific Bt toxin genes were isolated from Bacillus thuringiensis andincorporated into the several crop plants such as cotton (Figure 12.1).The choice of genes depends upon the crop and the targeted pest, asmost Bt toxins are insect-group specific. The toxin is coded by a genecryIAc named cry. There are a number of them, for example, the proteinsencoded by the genes cryIAc and cryIIAb control the cotton bollworms,that of cryIAb controls corn borer.Figure 12.1 Cotton boll: (a) destroyed by bollworms; (b) a fully maturecotton boll(b)(a)Pest Resistant Plants: Several nematodes parasitise a wide variety ofplants and animals including human beings. A nematode Meloidegyneincognitia infects the roots of tobacco plants and causes a great reductionin yield. A novel strategy was adopted to prevent this infestation whichwas based on the process of RNA interference (RNAi). RNAi takes placein all eukaryotic organisms as a method of cellular defense. This methodinvolves silencing of a specific mRNA due to a complementary dsRNAmolecule that binds to and prevents translation of the mRNA (silencing).The source of this complementary RNA could be from an infection byviruses having RNA genomes or mobile genetic elements (transposons)that replicate via an RNA intermediate.Using Agrobacterium vectors, nematode-specific genes wereintroduced into the host plant (Figure 12.2). The introduction of DNAwas such that it produced both sense and anti-sense RNA in the hostcells. These two RNAâs being complementary to each other formed a doublestranded (dsRNA) that initiated RNAi and thus, silenced the specific mRNA2022-23210BIOLOGYof the nematode. The consequence was that the parasite could not survivein a transgenic host expressing specific interfering RNA. The transgenicplant therefore got itself protected from the parasite (Figure 12.2).Figure 12.2 Host plant-generated dsRNA triggers protection against nematode infestation:(a) Roots of a typical control plants; (b) transgenic plant roots 5 days after deliberateinfection of nematode but protected through novel mechanism.(a) (b)12.2 BIOTECHNOLOGICAL APPLICATIONS IN MEDICINEThe recombinant DNA technological processes have made immense impactin the area of healthcare by enabling mass production of safe and moreeffective therapeutic drugs. Further, the recombinant therapeutics do notinduce unwanted immunological responses as is common in case ofsimilar products isolated from non-human sources. At present, about30 recombinant therapeutics have been approved for human-use theworld over. In India, 12 of these are presently being marketed.12.2.1 Genetically Engineered InsulinManagement of adult-onset diabetes is possible by taking insulin atregular time intervals. What would a diabetic patient do if enoughhuman-insulin was not available? If you discuss this, you would soonrealise that one would have to isolate and use insulin from other animals.Would the insulin isolated from other animals be just as effective asthat secreted by the human body itself and would it not elicit an immuneresponse in the human body? Now, imagine if bacterium were availablethat could make human insulin. Suddenly the whole process becomesso simple. You can easily grow a large quantity of the bacteria and makeas much insulin as you need.Think about whether insulin can be orally administered to diabeticpeople or not. Why?2022-23211BIOTECHNOLOGY AND ITS APPLICATIONSInsulin used for diabetes was earlier extracted frompancreas of slaughtered cattle and pigs. Insulin from ananimal source, though caused some patients to developallergy or other types of reactions to the foreignprotein. Insulin consists of two short polypeptidechains: chain A and chain B, that are linked together bydisulphide bridges (Figure 12.3). In mammals, includinghumans, insulin is synthesised as a pro-hormone (like apro-enzyme, the pro-hormone also needs to be processedbefore it becomes a fully mature and functional hormone)which contains an extra stretch called the C peptide.This C peptide is not present in the mature insulin and isremoved during maturation into insulin.The mainchallenge for production of insulin using rDNA techniqueswas getting insulin assembled into a mature form. In1983, Eli Lilly an American company prepared two DNA sequencescorresponding to A and B, chains of human insulin and introduced themin plasmids of E. coli to produce insulin chains. Chains A and B wereproduced separately, extracted and combined by creating disulfide bondsto form human insulin.12.2.2 Gene TherapyIf a person is born with a hereditary disease, can a corrective therapybe taken for such a disease? Gene therapy is an attempt to do this.Gene therapy is a collection of methods that allows correction of agene defect that has been diagnosed in a child/embryo. Here genesare inserted into a personâs cells and tissues to treat a disease.Correction of a genetic defect involves delivery of a normal gene intothe individual or embryo to take over the function of and compensatefor the non-functional gene.The first clinical gene therapy was given in 1990 to a 4-year old girlwith adenosine deaminase (ADA) deficiency. This enzyme is crucial forthe immune system to function. The disorder is caused due to the deletionof the gene for adenosine deaminase. In some children ADA deficiencycan be cured by bone marrow transplantation; in others it can be treatedby enzyme replacement therapy, in which functional ADA is given to thepatient by injection. But the problem with both of these approaches thatthey are not completely curative. As a first step towards gene therapy,lymphocytes from the blood of the patient are grown in a culture outsidethe body. A functional ADA cDNA (using a retroviral vector) is thenintroduced into these lymphocytes, which are subsequently returned tothe patient. However, as these cells are not immortal, the patient requiresperiodic infusion of such genetically engineered lymphocytes. However, ifthe gene isolate from marrow cells producing ADA is introduced into cellsat early embryonic stages, it could be a permanent cure.Figure 12.3 Maturation ofpro-insulin into insulin(simplified)2022-23212BIOLOGY12.2.3 Molecular DiagnosisYou know that for effective treatment of a disease, early diagnosis andunderstanding its pathophysiology is very important. Using conventionalmethods of diagnosis (serum and urine analysis, etc.) early detection isnot possible. Recombinant DNA technology, Polymerase Chain Reaction(PCR) and Enzyme Linked Immuno-sorbent Assay (ELISA) are some ofthe techniques that serve the purpose of early diagnosis.Presence of a pathogen (bacteria, viruses, etc.) is normally suspectedonly when the pathogen has produced a disease symptom. By this timethe concentration of pathogen is already very high in the body. However,very low concentration of a bacteria or virus (at a time when the symptomsof the disease are not yet visible) can be detected by amplification of theirnucleic acid by PCR. Can you explain how PCR can detect very lowamounts of DNA? PCR is now routinely used to detect HIV in suspectedAIDS patients. It is being used to detect mutations in genes in suspectedcancer patients too. It is a powerful techqnique to identify many othergenetic disorders.A single stranded DNA or RNA, tagged with a radioactive molecule(probe) is allowed to hybridise to its complementary DNA in a clone ofcells followed by detection using autoradiography. The clone having themutated gene will hence not appear on the photographic film, becausethe probe will not have complementarity with the mutated gene.ELISA is based on the principle of antigen-antibody interaction.Infection by pathogen can be detected by the presence of antigens(proteins, glycoproteins, etc.) or by detecting the antibodies synthesisedagainst the pathogen.12.3 TRANSGENIC ANIMALSAnimals that have had their DNA manipulated to possess and express anextra (foreign) gene are known as transgenic animals. Transgenic rats,rabbits, pigs, sheep, cows and fish have been produced, although over95 per cent of all existing transgenic animals are mice. Why are theseanimals being produced? How can man benefit from such modifications?Let us try and explore some of the common reasons:(i) Normal physiology and development: Transgenic animals canbe specifically designed to allow the study of how genes areregulated, and how they affect the normal functions of the bodyand its development, e.g., study of complex factors involved in growthsuch as insulin-like growth factor. By introducing genes from otherspecies that alter the formation of this factor and studying thebiological effects that result, information is obtained about thebiological role of the factor in the body.(ii) Study of disease: Many transgenic animals are designed to increaseour understanding of how genes contribute to the development of2022-23213BIOTECHNOLOGY AND ITS APPLICATIONSdisease. These are specially made to serve as models for humandiseases so that investigation of new treatments for diseases is madepossible. Today transgenic models exist for many human diseasessuch as cancer, cystic fibrosis, rheumatoid arthritis and Alzheimerâs.(iii) Biological products: Medicines required to treat certain humandiseases can contain biological products, but such products areoften expensive to make. Transgenic animals that produce usefulbiological products can be created by the introduction of the portionof DNA (or genes) which codes for a particular product such ashuman protein (a-1-antitrypsin) used to treat emphysema. Similarattempts are being made for treatment of phenylketonuria (PKU)and cystic fibrosis. In 1997, the first transgenic cow, Rosie, producedhuman protein-enriched milk (2.4 grams per litre). The milkcontained the human alpha-lactalbumin and was nutritionally amore balanced product for human babies than natural cow-milk.(iv) Vaccine safety: Transgenic mice are being developed for use intesting the safety of vaccines before they are used on humans.Transgenic mice are being used to test the safety of the polio vaccine.If successful and found to be reliable, they could replace the use ofmonkeys to test the safety of batches of the vaccine.(v) Chemical safety testing: This is known as toxicity/safety testing.The procedure is the same as that used for testing toxicity of drugs.Transgenic animals are made that carry genes which make them moresensitive to toxic substances than non-transgenic animals. They arethen exposed to the toxic substances and the effects studied. Toxicitytesting in such animals will allow us to obtain results in less time.12.4 ETHICAL ISSUESThe manipulation of living organisms by the human race cannot go onany further, without regulation. Some ethical standards are required toevaluate the morality of all human activities that might help or harm livingorganisms.Going beyond the morality of such issues, the biological significanceof such things is also important. Genetic modification of organisms canhave unpredicatable results when such organisms are introduced intothe ecosystem.Therefore, the Indian Government has set up organisations such asGEAC (Genetic Engineering Approval Committee), which will makedecisions regarding the validity of GM research and the safety ofintroducing GM-organisms for public services.The modification/usage of living organisms for public services (as foodand medicine sources, for example) has also created problems with patentsgranted for the same.2022-23214BIOLOGYThere is growing public anger that certain companies are beinggranted patents for products and technologies that make use of thegenetic materials, plants and other biological resources that have longbeen identified, developed and used by farmers and indigenous peopleof a specific region/country.Rice is an important food grain, the presence of which goes backthousands of years in Asiaâs agricultural history. There are an estimated200,000 varieties of rice in India alone. The diversity of rice in India isone of the richest in the world. Basmati rice is distinct for its uniquearoma and flavour and 27 documented varieties of Basmati are grownin India. There is reference to Basmati in ancient texts, folklore andpoetry, as it has been grown for centuries. In 1997, an Americancompany got patent rights on Basmati rice through the US Patent andTrademark Office. This allowed the company to sell a ânewâ variety ofBasmati, in the US and abroad. This ânewâ variety of Basmati had actuallybeen derived from Indian farmerâs varieties. Indian Basmati was crossedwith semi-dwarf varieties and claimed as an invention or a novelty. Thepatent extends to functional equivalents, implying that other peopleselling Basmati rice could be restricted by the patent. Several attemptshave also been made to patent uses, products and processes based onIndian traditional herbal medicines, e.g., turmeric neem. If we are notvigilant and we do not immediately counter these patent applications,other countries/individuals may encash on our rich legacy and we maynot be able to do anything about it.Biopiracy is the term used to refer to the use of bio-resources bymultinational companies and other organisations without properauthorisation from the countries and people concerned withoutcompensatory payment.Most of the industrialised nations are rich financially but poor inbiodiversity and traditional knowledge. In contrast the developing andthe underdeveloped world is rich in biodiversity and traditionalknowledge related to bio-resources. Traditional knowledge related tobio-resources can be exploited to develop modern applications and canalso be used to save time, effort and expenditure during theircommercialisation.There has been growing realisation of the injustice, inadequatecompensation and benefit sharing between developed and developingcountries. Therefore, some nations are developing laws to prevent suchunauthorised exploitation of their bio-resources and traditionalknowledge.The Indian Parliament has recently cleared the second amendmentof the Indian Patents Bill, that takes such issues into consideration,including patent terms emergency provisions and research anddevelopment initiative.2022-23215BIOTECHNOLOGY AND ITS APPLICATIONSEXERCISES1. Crystals of Bt toxin produced by some bacteria do not kill the bacteriathemselves because â(a) bacteria are resistant to the toxin(b) toxin is immature;(c) toxin is inactive;(d) bacteria encloses toxin in a special sac.2. What are transgenic bacteria? Illustrate using any one example.3. Compare and contrast the advantages and disadvantages of productionof genetically modified crops.SUMMARYBiotechnology has given to humans several useful products by usingmicrobes, plant, animals and their metabolic machinery. RecombinantDNA technology has made it possible to engineer microbes, plantsand animals such that they have novel capabilities. GeneticallyModified Organisms have been created by using methods other thannatural methods to transfer one or more genes from one organism toanother, generally using techniques such as recombinant DNAtechnology.GM plants have been useful in increasing crop yields, reduce postharvestlosses and make crops more tolerant of stresses. There areseveral GM crop plants with improved nutritional value of foods andreduced the reliance on chemical pesticides (pest-resistant crops).Recombinant DNA technological processes have made immenseimpact in the area of healthcare by enabling mass production of safeand more effective therapeutics. Since the recombinant therapeuticsare identical to human proteins, they do not induce unwantedimmunological responses and are free from risk of infection as wasobserved in case of similar products isolated from non-human sources.Human insulin is made in bacteria yet its structure is absolutelyidentical to that of the natural molecule.Transgenic animals are also used to understand how genescontribute to the development of a disease by serving as models forhuman diseases, such as cancer, cystic fibrosis, rheumatoid arthritisand Alzheimerâs.Gene therapy is the insertion of genes into an individualâs cellsand tissues to treat diseases especially hereditary diseases. It doesso by replacing a defective mutant allele with a functional one orgene targeting which involves gene amplification. Viruses that attacktheir hosts and introduce their genetic material into the host cell aspart of their replication cycle are used as vectors to transfer healthygenes or more recently portions of genes.The current interest in the manipulation of microbes, plants, andanimals has raised serious ethical questions.2022-23216BIOLOGY4. What are Cry proteins? Name an organism that produce it. How hasman exploited this protein to his benefit?5. What is gene therapy? Illustrate using the example of adenosinedeaminase (ADA) deficiency.6. Digrammatically represent the experimental steps in cloning andexpressing an human gene (say the gene for growth hormone) into abacterium like E. coli ?7. Can you suggest a method to remove oil (hydrocarbon) from seeds basedon your understanding of rDNA technology and chemistry of oil?8. Find out from internet what is golden rice.9. Does our blood have proteases and nucleases?10. Consult internet and find out how to make orally active proteinpharmaceutical. What is the major problem to be encountered?2022-23Diversity is not only a characteristic of living organisms butalso of content in biology textbooks. Biology is presented eitheras botany, zoology and microbiology or as classical andmodern. The latter is a euphemism for molecular aspects ofbiology. Luckily we have many threads which weave thedifferent areas of biological information into a unifyingprinciple. Ecology is one such thread which gives us a holisticperspective to biology. The essence of biological understandingis to know how organisms, while remaining an individual,interact with other organisms and physical habitats as a groupand hence behave like organised wholes, i.e., population,community, ecosystem or even as the whole biosphere.Ecology explains to us all this. A particular aspect of this is thestudy of anthropogenic environmental degradation and thesocio-political issues it has raised. This unit describes as well astakes a critical view of the above aspects.Chapter 13Organisms and PopulationsChapter 14EcosystemChapter 15Biodiversity and ConservationChapter 16Environmental Issues2022-23Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August1908, Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,FRS, from Leeds University in UK. He established teaching and research inecology at the Department of Botany of the Banaras Hindu University,Varanasi. His research laid the foundations for understanding of tropicalcommunities and their succession, environmental responses of plantpopulations and productivity and nutrient cycling in tropical forest andgrassland ecosystems. Misra formulated the first postgraduate course inecology in India. Over 50 scholars obtained Ph. D degree under his supervisionand moved on to other universities and research institutes to initiate ecologyteaching and research across the country.He was honoured with the Fellowships of the Indian National ScienceAcademy and World Academy of Arts and Science, and the prestigious SanjayGandhi Award in Environment and Ecology. Due to his efforts, theGovernment of India established the National Committee for EnvironmentalPlanning and Coordination (1972) which, in later years, paved the wayfor the establishment of the Ministry of Environment and Forests (1984).RAMDEO MISRA(1908-1998)2022-23Our living world is fascinatingly diverse and amazinglycomplex. We can try to understand its complexity byinvestigating processes at various levels of biologicalorganisationâmacromolecules, cells, tissues, organs,individual organisms, population, communities,ecosystems and biomes. At any level of biologicalorganisation we can ask two types of questions â forexample, when we hear the bulbul singing early morningin the garden, we may ask â âHow does the bird sing?âOr, âWhy does the bird sing ?â The âhow-typeâ questionsseek the mechanism behind the process while the âwhytypeâquestions seek the significance of the process. Forthe first question in our example, the answer might be interms of the operation of the voice box and the vibratingbone in the bird, whereas for the second question theanswer may lie in the birdâs need to communicate with itsmate during breeding season. When you observe naturearound you with a scientific frame of mind you willcertainly come up with many interesting questions of bothtypes - Why are night-blooming flowers generally white?How does the bee know which flower has nectar? Whydoes cactus have so many thorns? How does the chickspures recognise her own mother ?, and so on.CHAPTER 13ORGANISMS AND POPULATIONS13.1 Organism and ItsEnvironment13.2 Populations2022-23220BIOLOGYYou have already learnt in previous classes that Ecology is a subjectwhich studies the interactions among organisms and between theorganism and its physical (abiotic) environment.Ecology is basically concerned with four levels of biologicalorganisation â organisms, populations, communities and biomes. In thischapter we explore ecology at organismic and population levels.13.1 ORGANISM AND ITS ENVIRONMENTEcology at the organismic level is essentially physiological ecology whichtries to understand how different organisms are adapted to theirenvironments in terms of not only survival but also reproduction. Youmay have learnt in earlier classes how the rotation of our planet aroundthe Sun and the tilt of its axis cause annual variations in the intensityand duration of temperature, resulting in distinct seasons. Thesevariations together with annual variation in precipitation (rememberprecipitation includes both rain and snow) account for the formation ofmajor biomes such as desert, rain forest and tundra (Figure 13.1).Figure 13.1 Biome distribution with respect to annual temperature and precipitationRegional and local variations within each biome lead to the formation of awide variety of habitats. Major biomes of India are shown in Figure 13.2.On planet Earth, life exists not just in a few favourable habitats but evenin extreme and harsh habitats â scorching Rajasthan desert, rain-soakedMeghalaya forests, deep ocean trenches, torrential streams, permafrost(snow laden) polar regions, high mountain tops, thermal springs, andstinking compost pits, to name a few. Even our intestine is a uniquehabitat for hundreds of species of microbes.2022-23221ORGANISMS AND POPULATIONSWhat are the key elements that lead to so much variation in thephysical and chemical conditions of different habitats? The mostimportant ones are temperature, water, light and soil. We must rememberthat the physico-chemical (abiotic) components alone do not characterisethe habitat of an organism completely; the habitat includes bioticcomponents also â pathogens, parasites, predators and competitors â ofthe organism with which they interact constantly. We assume that over aperiod of time, the organism had through natural selection, evolvedadaptations to optimise its survival and reproduction in its habitat.Each organism has an invariably defined range of conditions that itcan tolerate, diversity in the resources it utilises and a distinct functionalrole in the ecological system, all these together comprise its niche.13.1.1 Major Abiotic FactorsTemperature: Temperature is the most important ecologically relevantenvironmental factor. You are aware that the average temperature onland varies seasonally, decreases progressively from the equator towardsthe poles and from plains to the mountain tops. It ranges from subzerolevels in polar areas and high altitudes to >500C in tropical deserts insummer. There are, however, unique habitats such as thermal springsand deep-sea hydrothermal vents where average temperatures exceed1000 C. It is general knowledge that mango trees do not and cannot growFigure 13.2 Major biomes of India : (a) Tropical rain forest; (b) Deciduous forest;(c) Desert; (d) Sea coast(a) (b)(c) (d)2022-23222BIOLOGYin temperate countries like Canada and Germany, snow leopards are notfound in Kerala forests and tuna fish are rarely caught beyond tropicallatitudes in the ocean. You can appreciate the significance of temperatureto living organisms when you realise that it affects the kinetics of enzymesand through it the metabolic activity and other physiological functions ofthe organism. A few organisms can tolerate and thrive in a wide range oftemperatures (they are called eurythermal ), but, a vast majority of themare restricted to a narrow range of temperatures (such organisms are calledstenothermal ). The levels of thermal tolerance of different species determineto a large extent their geographical distribution. Can you think of a feweurythermal and stenothermal animals and plants?In recent years, there has been a growing concern about the graduallyincreasing average global temperatures (Chapter 16). If this trend continues,would you expect the distributional range of some species to be affected?Water: Water is another the most important factor influencing the life oforganisms. In fact, life on earth originated in water and is unsustainablewithout water. Its availability is so limited in deserts that only specialadaptations make it possible for organisms to live there. The productivityand distribution of plants is also heavily dependent on water. You mightthink that organisms living in oceans, lakes and rivers should not faceany water-related problems, but it is not true. For aquatic organisms thequality (chemical composition, pH) of water becomes important. The saltconcentration (measured as salinity in parts per thousand), is less than5 in inland waters, 30-35 in the sea and > 100 in some hypersalinelagoons. Some organisms are tolerant of a wide range of salinities(euryhaline) but others are restricted to a narrow range (stenohaline).Many freshwater animals cannot live for long in sea water and vice versabecause of the osmotic problems, they would face.Light: Since plants produce food through photosynthesis, a process whichis only possible when sunlight is available as a source of energy, we canquickly understand the importance of light for living organisms,particularly autotrophs. Many species of small plants (herbs and shrubs)growing in forests are adapted to photosynthesise optimally under verylow light conditions because they are constantly overshadowed by tall,canopied trees. Many plants are also dependent on sunlight to meet theirphotoperiodic requirement for flowering. For many animals too, light isimportant in that they use the diurnal and seasonal variations in lightintensity and duration (photoperiod) as cues for timing their foraging,reproductive and migratory activities. The availability of light on land isclosely linked with that of temperature since the sun is the source for both.But, deep (>500m) in the oceans, the environment is dark and its inhabitantsare not aware of the existence of a celestial source of energy called Sun.What, then is their source of energy?. The spectral quality of solar radiationis also important for life. The UV component of the spectrum is harmful tomany organisms while not all the colour components of the visible spectrum2022-23223ORGANISMS AND POPULATIONSare available for marine plants living at different depths of the ocean. Amongthe red, green and brown algae that inhabit the sea, which is likely tobe found in the deepest waters? Why?Soil: The nature and properties of soil in different places vary; it isdependent on the climate, the weathering process, whether soil istransported or sedimentary and how soil development occurred. Variouscharacteristics of the soil such as soil composition, grain size andaggregation determine the percolation and water holding capacity of thesoils. These characteristics along with parameters such as pH, mineralcomposition and topography determine to a large extent the vegetation inany area. This in turn dictates the type of animals that can be supported.Similarly, in the aquatic environment, the sediment-characteristics oftendetermine the type of benthic animals that can thrive there.13.1.2 Responses to Abiotic FactorsHaving realised that the abiotic conditions of many habitats may varydrastically in time, we now askâhow do the organisms living in suchhabitats cope or manage with stressful conditions? But before attemptingto answer this question, we should perhaps ask first why a highly variableexternal environment should bother organisms after all. One would expectthat during the course of millions of years of their existence, many specieswould have evolved a relatively constant internal (within the body)environment that permits all biochemical reactions and physiologicalfunctions to proceed with maximal efficiencyand thus, enhance the overall âfitnessâ of thespecies. This constancy, for example, couldbe in terms of optimal temperature andosmotic concentration of body fluids. Ideallythen, the organism should try to maintainthe constancy of its internal environment (aprocess called homeostasis) despite varyingexternal environmental conditions that tendto upset its homeostasis. Let us take ananalogy to clarify this important concept.Suppose a person is able to perform his/herbest when the temperature is 250C andwishes to maintain it so, even when it isscorchingly hot or freezingly cold outside. Itcould be achieved at home, in the car whiletravelling, and at workplace by using an air conditioner in summer andheater in winter. Then his/her performance would be always maximalregardless of the weather around him/her. Here the personâs homeostasisis accomplished, not through physiological, but artificial means. How doother living organisms cope with the situation? Let us look at variouspossibilities (Figure 13.3).Figure 13.3 Diagrammatic representation oforganismic response2022-23224BIOLOGY(i) Regulate: Some organisms are able to maintain homeostasis byphysiological (sometimes behavioural also) means which ensuresconstant body temperature, constant osmotic concentration, etc.All birds and mammals, and a very few lower vertebrate andinvertebrate species are indeed capable of such regulation(thermoregulation and osmoregulation). Evolutionary biologistsbelieve that the âsuccessâ of mammals is largely due to their abilityto maintain a constant body temperature and thrive whether theylive in Antarctica or in the Sahara desert.The mechanisms used by most mammals to regulate their bodytemperature are similar to the ones that we humans use. We maintaina constant body temperature of 370C. In summer, when outsidetemperature is more than our body temperature, we sweat profusely.The resulting evaporative cooling, similar to what happens with adesert cooler in operation, brings down the body temperature. Inwinter when the temperature is much lower than 370C, we start toshiver, a kind of exercise which produces heat and raises the bodytemperature. Plants, on the other hand, do not have suchmechanisms to maintain internal temperatures.(ii) Conform: An overwhelming majority (99 per cent) of animals andnearly all plants cannot maintain a constant internal environment.Their body temperature changes with the ambient temperature. Inaquatic animals, the osmotic concentration of the body fluidschange with that of the ambient air, water osmotic concentration.These animals and plants are simply conformers. Considering thebenefits of a constant internal environment to the organism, we mustask why these conformers had not evolved to become regulators.Recall the human analogy we used above; much as they like, howmany people can really afford an air conditioner? Many simplyâsweat it outâ and resign themselves to suboptimal performance inhot summer months. Thermoregulation is energetically expensivefor many organisms. This is particularly true for small animals likeshrews and humming birds. Heat loss or heat gain is a function ofsurface area. Since small animals have a larger surface area relativeto their volume, they tend to lose body heat very fast when it is coldoutside; then they have to expend much energy to generate bodyheat through metabolism. This is the main reason why very smallanimals are rarely found in polar regions. During the course ofevolution, the costs and benefits of maintaining a constant internalenvironment are taken into consideration. Some species have evolvedthe ability to regulate, but only over a limited range of environmentalconditions, beyond which they simply conform.If the stressful external conditions are localised or remain onlyfor a short duration, the organism has two other alternatives forsurvival.2022-23225ORGANISMS AND POPULATIONS(iii) Migrate: The organism can move away temporarily from thestressful habitat to a more hospitable area and return when stressfulperiod is over. In human analogy, this strategy is like a personmoving from Delhi to Shimla for the duration of summer. Manyanimals, particularly birds, during winter undertake long-distancemigrations to more hospitable areas. Every winter the famousKeolado National Park (Bharatpur) in Rajasthan host thousands ofmigratory birds coming from Siberia and other extremely coldnorthern regions.(iv) Suspend: In bacteria, fungi and lower plants, various kinds of thickwalledspores are formed which help them to survive unfavourableconditions â these germinate on availability of suitable environment.In higher plants, seeds and some other vegetative reproductivestructures serve as means to tide over periods of stress besides helpingin dispersal â they germinate to form new plants under favourablemoisture and temperature conditions. They do so by reducing theirmetabolic activity and going into a state of âdormancyâ.In animals, the organism, if unable to migrate, might avoid thestress by escaping in time. The familiar case of bears going intohibernation during winter is an example of escape in time. Somesnails and fish go into aestivation to avoid summerârelatedproblems-heat and dessication. Under unfavourable conditionsmany zooplankton species in lakes and ponds are known to enterdiapause, a stage of suspended development.13.1.3 AdaptationsWhile considering the various alternatives available to organisms forcoping with extremes in their environment, we have seen that some areable to respond through certain physiological adjustments while othersdo so behaviourally (migrating temporarily to a less stressful habitat).These responses are also actually, their adaptations. So, we can say thatadaptation is any attribute of the organism (morphological, physiological,behavioural) that enables the organism to survive and reproduce in itshabitat. Many adaptations have evolved over a long evolutionary timeand are genetically fixed. In the absence of an external source of water,the kangaroo rat in North American deserts is capable of meeting all itswater requirements through its internal fat oxidation (in which water isa by product). It also has the ability to concentrate its urine so thatminimal volume of water is used to remove excretory products.Many desert plants have a thick cuticle on their leaf surfaces andhave their stomata arranged in deep pits (sunken) to minimise water lossthrough transpiration. They also have a special photosynthetic pathway(CAM) that enables their stomata to remain closed during day time. Somedesert plants like Opuntia, have no leaves â they are reduced to spinesâand the photosynthetic function is taken over by the flattened stems.2022-23226BIOLOGYMammals from colder climates generally have shorter ears and limbsto minimise heat loss. (This is called the Allenâs Rule.) In the polar seasaquatic mammals like seals have a thick layer of fat (blubber) below theirskin that acts as an insulator and reduces loss of body heat.Some organisms possess adaptations that are physiological whichallow them to respond quickly to a stressful situation. If you had everbeen to any high altitude place (>3,500m Rohtang Pass near Manali andLeh you must have experienced what is called altitude sickness. Itssymptoms include nausea, fatigue and heart palpitations. This is becausein the low atmospheric pressure of high altitudes, the body does not getenough oxygen. But, gradually you get acclimatised and stop experiencingaltitude sickness. How did your body solve this problem? The bodycompensates low oxygen availability by increasing red blood cellproduction, decreasing the binding affinity of hemoglobin and byincreasing breathing rate. Many tribes live in the high altitude ofHimalayas. Find out if they normally have a higher red blood cell count(or total hemoglobin) than people living in the plains.In most animals, the metabolic reactions and hence all thephysiological functions proceed optimally in a narrow temperature range(in humans, it is 370C). But there are microbes (archaebacteria) thatflourish in hot springs and deep sea hydrothermal vents wheretemperatures far exceed 1000C. How is this possible?Many fish thrive in Antarctic waters where the temperature is alwaysbelow zero. How do they manage to prevent their body fluids from freezing?A large variety of marine invertebrates and fish live at great depths inthe ocean where the pressure could be >100 times the normal atmosphericpressure that we experience. How do they live under such high pressuresand do they have any special enzymes? Organisms living in such extremeenvironments show a fascinating array of biochemical adaptations.Some organisms show behavioural responses to cope up withvariations in their environment. Desert lizards lack the physiological abilitythat mammals have to deal with the high temperatures of their habitat,but manage to keep their body temperature fairly constant by behaviouralmeans. They bask in the sun and absorb heat when their bodytemperature drops below the comfort zone, but move into shade whenthe ambient temperature starts increasing. Some species are capable ofburrowing into the soil to hide and escape from the above-ground heat.13.2 POPULATIONS13.2.1 Population AttributesIn nature, we rarely find isolated, single individuals of any species; majorityof them live in groups in a well defined geographical area, share or competefor similar resources, potentially interbreed and thus constitute apopulation. Although the term interbreeding implies sexual reproduction,2022-23227ORGANISMS AND POPULATIONSa group of individuals resulting from even asexual reproduction is alsogenerally considered a population for the purpose of ecological studies.All the cormorants in a wetland, rats in an abandoned dwelling, teakwoodtrees in a forest tract, bacteria in a culture plate and lotus plants in apond, are some examples of a population. In earlier chapters you havelearnt that although an individual organism is the one that has to copewith a changed environment, it is at the population level that naturalselection operates to evolve the desired traits. Population ecology is,therefore, an important area because it links ecology to population geneticsand evolution.A population has certain attributes whereas, an individual organismdoes not. An individual may have births and deaths, but a population hasbirth rates and death rates. In a population these rates refer to per capitabirths and deaths. The rates, hence, expressed are change in numbers(increase or decrease) with respect to members of the population. Here is anexample. If in a pond there were 20 lotus plants last year and throughreproduction 8 new plants are added, taking the current population to 28,we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4individuals in a laboratory population of 40 fruitflies died during a specifiedtime interval, say a week, the death rate in the population during that periodis 4/40 = 0.1 individuals per fruitfly per week.Another attribute characteristic of a population is sex ratio. Anindividual is either a male or a female but a population has a sex ratio(e.g., 60 per cent of the population are females and 40 per cent males).A population at any given time is composed of individuals ofdifferent ages. If the age distribution (per cent individuals of a givenage or age group) is plotted for the population, the resulting structureis called an age pyramid (Figure 13.4). For human population, theage pyramids generally show age distribution of males and females ina diagram. The shape of the pyramids reflects the growth status ofthe population - (a) whether it is growing, (b) stable or (c) declining.The size of the population tells us a lot about its status in the habitat.Whatever ecological processes we wish to investigate in a population, beit the outcome of competition with another species, the impact of aFigure 13.4 Representation of age pyramids for human population2022-23228BIOLOGYpredator or the effect of a pesticide application, we always evaluate themin terms of any change in the population size. The size, in nature, couldbe as low as <10 (Siberian cranes at Bharatpur wetlands in any year) orgo into millions (Chlamydomonas in a pond). Population size, technicallycalled population density (designated as N), need not necessarily bemeasured in numbers only. Although total number is generally the mostappropriate measure of population density, it is in some cases eithermeaningless or difficult to determine. In an area, if there are 200 carrotgrass (Parthenium hysterophorus) plants but only a single huge banyantree with a large canopy, stating that the population density of banyan islow relative to that of carrot grass amounts to underestimating theenormous role of the Banyan in that community. In such cases, the percent cover or biomass is a more meaningful measure of the populationsize. Total number is again not an easily adoptable measure if thepopulation is huge and counting is impossible or very time-consuming.If you have a dense laboratory culture of bacteria in a petri dish what isthe best measure to report its density? Sometimes, for certain ecologicalinvestigations, there is no need to know the absolute population densities;relative densities serve the purpose equally well. For instance, the numberof fish caught per trap is good enough measure of its total populationdensity in the lake. We are mostly obliged to estimate population sizesindirectly, without actually counting them or seeing them. The tiger censusin our national parks and tiger reserves is often based on pug marks andfecal pellets.13.2.2 Population GrowthThe size of a population for any species is not a static parameter. It keepschanging with time, depending on various factors including foodavailability, predation pressure and adverse weather. In fact, it is thesechanges in population density that give us some idea of what is happeningto the population â whether it is flourishing or declining. Whatever mightbe the ultimate reasons, the density of a population in a given habitatduring a given period, fluctuates due to changes in four basic processes,two of which (natality and immigration) contribute to an increase inpopulation density and two (mortality and emigration) to a decrease.(i) Natality refers to the number of births during a given period in thepopulation that are added to the initial density.(ii) Mortality is the number of deaths in the population during a givenperiod.(iii) Immigration is the number of individuals of the same species thathave come into the habitat from elsewhere during the time periodunder consideration.(iv) Emigration is the number of individuals of the population wholeft the habitat and gone elsewhere during the time period underconsideration.2022-23229ORGANISMS AND POPULATIONSSo, if N is the population density at time t, then its density at time t +1 isNt+1 = Nt + [(B + I) â (D + E)]You can see from the above equation (Fig. 13.5) that populationdensity will increase if the number of births plus the number ofimmigrants (B + I) is more than the number of deaths plus the numberof emigrants (D + E). Under normal conditions, births and deaths arethe most important factors influencing population density, the othertwo factors assuming importance only under special conditions. Forinstance, if a new habitat is just being colonised, immigration maycontribute more significantly to population growth than birth rates.Growth Models : Does the growth of a population with time show anyspecific and predictable pattern? We have been concerned aboutunbridled human population growth and problems created by it in ourcountry and it is therefore natural for us to be curious if different animalpopulations in nature behave the same way or show some restraints ongrowth. Perhaps we can learn a lesson or two from nature on how tocontrol population growth.(i) Exponential growth: Resource (food and space) availability isobviously essential for the unimpeded growth of a population.Ideally, when resources in the habitat are unlimited, each specieshas the ability to realise fully its innate potential to grow in number,as Darwin observed while developing his theory of naturalselection. Then the population grows in an exponential orFigure 13.52022-23230BIOLOGYgeometric fashion. If in a population of size N, the birth rates (nottotal number but per capita births) are represented as b and deathrates (again, per capita death rates) as d, then the increase ordecrease in N during a unit time period t (dN/dt) will bedN/dt = (b â d) à NLet (bâd) = r, thendN/dt = rNThe r in this equation is called the âintrinsic rate of natural increaseâand is a very important parameter chosen for assessing impacts ofany biotic or abiotic factor on population growth.To give you some idea about the magnitude of r values, for theNorway rat the r is 0.015, and for the flour beetle it is 0.12. In1981, the r value for human population in India was 0.0205. Findout what the current r value is. For calculating it, you need toknow the birth rates and death rates.The above equation describes the exponential or geometric growthpattern of a population (Figure 13.6) and results in a J-shaped curvewhen we plot N in relation to time. If you are familiar with basiccalculus, you can derive the integral form of theexponential growth equation asNt = N0 ertwhereNt = Population density after time tN0 = Population density at time zeror = intrinsic rate of natural increasee = the base of natural logarithms (2.71828)Any species growing exponentially under unlimitedresource conditions can reach enormous populationdensities in a short time. Darwin showed how evena slow growing animal like elephant could reachenormous numbers in the absence of checks. Thefollowing is an anecdote popularly narrated todemonstrate dramatically how fast a hugepopulation could build up when growingexponentially.The king and the minister sat for a chess game. The king, confidentof winning the game, was ready to accept any bet proposed by theminister. The minister humbly said that if he won, he wanted onlysome wheat grains, the quantity of which is to be calculated by placingon the chess board one grain in Square 1, then two in Square 2,then four in Square 3, and eight in Square 4, and so on, doubling eachtime the previous quantity of wheat on the next square until all the 64squares were filled. The king accepted the seemingly silly bet and startedFigure 13.6 Population growth curvea when responses are notlimiting the growth, plot isexponential,b when responses are limitingthe growth, plot is logistic,K is carrying capacity2022-23231ORGANISMS AND POPULATIONSthe game, but unluckily for him, the minister won. The king felt that fulfillingthe ministerâs bet was so easy. He started with a single grain onthe first square and proceeded to fill the other squares followingministerâs suggested procedure, but by the time he covered half thechess board, the king realised to his dismay that all the wheatproduced in his entire kingdom pooled together would still beinadequate to cover all the 64 squares. Now think of a tinyParamecium starting with just one individual and through binaryfission, doubling in numbers every day, and imagine what a mindbogglingpopulation size it would reach in 64 days. (provided foodand space remain unlimited)(ii) Logistic growth: No population of any species in nature has at itsdisposal unlimited resources to permit exponential growth. Thisleads to competition between individuals for limited resources.Eventually, the âfittestâ individual will survive and reproduce. Thegovernments of many countries have also realised this fact andintroduced various restraints with a view to limit human populationgrowth. In nature, a given habitat has enough resources to supporta maximum possible number, beyond which no further growth ispossible. Let us call this limit as natureâs carrying capacity (K) forthat species in that habitat.A population growing in a habitat with limited resources showinitially a lag phase, followed by phases of acceleration anddeceleration and finally an asymptote, when the population densityreaches the carrying capacity. A plot of N in relation to time (t)results in a sigmoid curve. This type of population growth is calledVerhulst-Pearl Logistic Growth (Figure 13.6) and is described bythe following equation:dN/dt =K NrNKâ Where N = Population density at time tr = Intrinsic rate of natural increaseK = Carrying capacitySince resources for growth for most animal populations are finiteand become limiting sooner or later, the logistic growth model isconsidered a more realistic one.Gather from Government Census data the population figuresfor India for the last 100 years, plot them and check which growthpattern is evident.13.2.3 Life History VariationPopulations evolve to maximise their reproductive fitness, also calledDarwinian fitness (high r value), in the habitat in which they live. Under2022-23232BIOLOGYa particular set of selection pressures, organisms evolve towards the mostefficient reproductive strategy. Some organisms breed only once in theirlifetime (Pacific salmon fish, bamboo) while others breed many timesduring their lifetime (most birds and mammals). Some produce a largenumber of small-sized offspring (Oysters, pelagic fishes) while othersproduce a small number of large-sized offspring (birds, mammals). So,which is desirable for maximising fitness? Ecologists suggest that lifehistory traits of organisms have evolved in relation to the constraintsimposed by the abiotic and biotic components of the habitat in whichthey live. Evolution of life history traits in different species is currently animportant area of research being conducted by ecologists.13.2.4 Population InteractionsCan you think of any natural habitat on earth that is inhabited just by asingle species? There is no such habitat and such a situation is eveninconceivable. For any species, the minimal requirement is one morespecies on which it can feed. Even a plant species, which makes its ownfood, cannot survive alone; it needs soil microbes to break down the organicmatter in soil and return the inorganic nutrients for absorption. And then,how will the plant manage pollination without an animal agent? It isobvious that in nature, animals, plants and microbes do not and cannotlive in isolation but interact in various ways to form a biologicalcommunity. Even in minimal communities, many interactive linkagesexist, although all may not be readily apparent.Interspecific interactions arise from the interaction of populations oftwo different species. They could be beneficial, detrimental or neutral(neither harm nor benefit) to one of the species or both. Assigning a â+âsign for beneficial interaction, â-â sign for detrimental and 0 for neutralinteraction, let us look at all the possible outcomes of interspecificinteractions (Table13.1).Both the species benefit in mutualism and both lose in competition intheir interactions with each other. In both parasitism and predation onlyone species benefits (parasite and predator, respectively) and the interactionSpecies A Species B Name of Interaction+ + Mutualismâ â Competition+ â Predation+ â Parasitism+ 0 Commensalismâ 0 AmensalismTable 13.1 : Population Interactions2022-23233ORGANISMS AND POPULATIONSis detrimental to the other species (host and prey, respectively).The interaction where one species is benefitted and the other is neitherbenefitted nor harmed is called commensalism. In amensalism onthe other hand one species is harmed whereas the other isunaffected. Predation, parasitism and commensalism share a commoncharacteristicâ the interacting species live closely together.(i) Predation: What would happen to all the energy fixed byautotrophic organisms if the community has no animals to eat theplants? You can think of predation as natureâs way of transferringto higher trophic levels the energy fixed by plants. When we thinkof predator and prey, most probably it is the tiger and the deer thatreadily come to our mind, but a sparrow eating any seed is no lessa predator. Although animals eating plants are categorisedseparately as herbivores, they are, in a broad ecological context,not very different from predators.Besides acting as âconduitsâ for energy transfer across trophiclevels, predators play other important roles. They keep preypopulations under control. But for predators, prey species couldachieve very high population densities and cause ecosysteminstability. When certain exotic species are introduced into ageographical area, they become invasive and start spreading fastbecause the invaded land does not have its natural predators. Theprickly pear cactus introduced into Australia in the early 1920âscaused havoc by spreading rapidly into millions of hectares ofrangeland. Finally, the invasive cactus was brought under controlonly after a cactus-feeding predator (a moth) from its natural habitatwas introduced into the country. Biological control methods adoptedin agricultural pest control are based on the ability of the predatorto regulate prey population. Predators also help in maintainingspecies diversity in a community, by reducing the intensity ofcompetition among competing prey species. In the rocky intertidalcommunities of the American Pacific Coast the starfish Pisaster isan important predator. In a field experiment, when all the starfishwere removed from an enclosed intertidal area, more than 10 speciesof invertebrates became extinct within a year, because of interspecificcompetition.If a predator is too efficient and overexploits its prey, then theprey might become extinct and following it, the predator will alsobecome extinct for lack of food. This is the reason why predators innature are âprudentâ. Prey species have evolved various defenses tolessen the impact of predation. Some species of insects and frogsare cryptically-coloured (camouflaged) to avoid being detected easilyby the predator. Some are poisonous and therefore avoided by the2022-23234BIOLOGYpredators. The Monarch butterfly is highly distasteful to its predator(bird) because of a special chemical present in its body.Interestingly, the butterfly acquires this chemical during itscaterpillar stage by feeding on a poisonous weed.For plants, herbivores are the predators. Nearly 25 per cent ofall insects are known to be phytophagous (feeding on plant sapand other parts of plants). The problem is particularly severe forplants because, unlike animals, they cannot run away from theirpredators. Plants therefore have evolved an astonishing variety ofmorphological and chemical defences against herbivores. Thorns(Acacia, Cactus) are the most common morphological means ofdefence. Many plants produce and store chemicals that make theherbivore sick when they are eaten, inhibit feeding or digestion,disrupt its reproduction or even kill it. You must have seen theweed Calotropis growing in abandoned fields. The plant produceshighly poisonous cardiac glycosides and that is why you never seeany cattle or goats browsing on this plant. A wide variety of chemicalsubstances that we extract from plants on a commercial scale(nicotine, caffeine, quinine, strychnine, opium, etc.,) are producedby them actually as defences against grazers and browsers.(ii) Competition: When Darwin spoke of the struggle for existence andsurvival of the fittest in nature, he was convinced that interspecificcompetition is a potent force in organic evolution. It is generallybelieved that competition occurs when closely related speciescompete for the same resources that are limiting, but this is notentirely true. Firstly, totally unrelated species could also competefor the same resource. For instance, in some shallow SouthAmerican lakes, visiting flamingoes and resident fishes compete fortheir common food, the zooplankton in the lake. Secondly,resources need not be limiting for competition to occur; ininterference competition, the feeding efficiency of one species mightbe reduced due to the interfering and inhibitory presence of theother species, even if resources (food and space) are abundant.Therefore, competition is best defined as a process in which thefitness of one species (measured in terms of its ârâ the intrinsic rateof increase) is significantly lower in the presence of another species.It is relatively easy to demonstrate in laboratory experiments, asGause and other experimental ecologists did, when resources arelimited the competitively superior species will eventually eliminatethe other species, but evidence for such competitive exclusionoccurring in nature is not always conclusive. Strong and persuasivecircumstantial evidence does exist however in some cases. TheAbingdon tortoise in Galapagos Islands became extinct within adecade after goats were introduced on the island, apparently dueto the greater browsing efficiency of the goats. Another evidence for2022-23235ORGANISMS AND POPULATIONSthe occurrence of competition in nature comes from what is calledâcompetitive releaseâ. A species whose distribution is restricted to asmall geographical area because of the presence of a competitivelysuperior species, is found to expand its distributional rangedramatically when the competing species is experimentally removed.Connellâs elegant field experiments showed that on the rocky seacoasts of Scotland, the larger and competitively superior barnacleBalanus dominates the intertidal area, and excludes the smallerbarnacle Chathamalus from that zone. In general, herbivores andplants appear to be more adversely affected by competition thancarnivores.Gauseâs âCompetitive Exclusion Principleâ states that twoclosely related species competing for the same resources cannotco-exist indefinitely and the competitively inferior one will beeliminated eventually. This may be true if resources are limiting,but not otherwise. More recent studies do not support such grossgeneralisations about competition. While they do not rule out theoccurrence of interspecific competition in nature, they point outthat species facing competition might evolve mechanisms thatpromote co-existence rather than exclusion. One such mechanismis âresource partitioningâ. If two species compete for the sameresource, they could avoid competition by choosing, for instance,different times for feeding or different foraging patterns. MacArthurshowed that five closely related species of warblers living on thesame tree were able to avoid competition and co-exist due tobehavioural differences in their foraging activities.(iii) Parasitism: Considering that the parasitic mode of life ensuresfree lodging and meals, it is not surprising that parasitism hasevolved in so many taxonomic groups from plants to highervertebrates. Many parasites have evolved to be host-specific (theycan parasitise only a single species of host) in such a way that bothhost and the parasite tend to co-evolve; that is, if the host evolvesspecial mechanisms for rejecting or resisting the parasite, theparasite has to evolve mechanisms to counteract and neutralisethem, in order to be successful with the same host species. Inaccordance with their life styles, parasites evolved specialadaptations such as the loss of unnecessary sense organs, presenceof adhesive organs or suckers to cling on to the host, loss of digestivesystem and high reproductive capacity. The life cycles of parasitesare often complex, involving one or two intermediate hosts or vectorsto facilitate parasitisation of its primary host. The human liver fluke(a trematode parasite) depends on two intermediate hosts (a snailand a fish) to complete its life cycle. The malarial parasite needs a2022-23236BIOLOGYvector (mosquito) to spread to other hosts. Majority of the parasitesharm the host; they may reduce the survival, growth andreproduction of the host and reduce its population density. Theymight render the host more vulnerable to predation by making itphysically weak. Do you believe that an ideal parasite should beable to thrive within the host without harming it? Then why didnâtnatural selection lead to the evolution of such totally harmlessparasites?Parasites that feed on the external surface of the host organismare called ectoparasites. The most familiar examples of this groupare the lice on humans and ticks on dogs. Many marine fish areinfested with ectoparasitic copepods. Cuscuta, a parasitic plant thatis commonly found growing on hedge plants, has lost its chlorophylland leaves in the course of evolution. It derives its nutrition fromthe host plant which it parasitises. The female mosquito is notconsidered a parasite, although it needs our blood for reproduction.Can you explain why?In contrast, endoparasites are those that live inside the hostbody at different sites (liver, kidney, lungs, red blood cells, etc.).The life cycles of endoparasites are more complex because of theirextreme specialisation. Their morphological and anatomical featuresare greatly simplified while emphasising their reproductive potential.Brood parasitism in birds is a fascinating example of parasitismin which the parasitic bird lays its eggs in the nest of its host andlets the host incubate them. During the course of evolution, theeggs of the parasitic bird have evolved to resemble the hostâs egg insize and colour to reduce the chances of the host bird detecting theforeign eggs and ejecting them from the nest. Try to follow themovements of the cuckoo (koel) and the crow in your neighborhoodpark during the breeding season (spring to summer) and watchbrood parasitism in action.(iv) Commensalism: This is the interaction in which one species benefitsand the other is neither harmed nor benefited. An orchid growingas an epiphyte on a mango branch, and barnacles growing on theback of a whale benefit while neither the mango tree nor the whalederives any apparent benefit. The cattle egret and grazing cattle inclose association, a sight you are most likely to catch if you live infarmed rural areas, is a classic example of commensalism. Theegrets always forage close to where the cattle are grazing becausethe cattle, as they move, stir up and flush out insects from thevegetation that otherwise might be difficult for the egrets to findand catch. Another example of commensalism is the interaction2022-23237ORGANISMS AND POPULATIONSbetween sea anemone that has stinging tentacles and the clownfish that lives among them. The fish gets protection from predatorswhich stay away from the stinging tentacles. The anemone doesnot appear to derive any benefit by hosting the clown fish.(v) Mutualism: This interaction confers benefits on both the interactingspecies. Lichens represent an intimate mutualistic relationshipbetween a fungus and photosynthesising algae or cyanobacteria.Similarly, the mycorrhizae are associations between fungi and theroots of higher plants. The fungi help the plant in the absorption ofessential nutrients from the soil while the plant in turn provides thefungi with energy-yielding carbohydrates.The most spectacular and evolutionarily fascinating examplesof mutualism are found in plant-animal relationships. Plants needthe help of animals for pollinating their flowers and dispersing theirseeds. Animals obviously have to be paid âfeesâ for the services thatplants expect from them. Plants offer rewards or fees in the form ofpollen and nectar for pollinators and juicy and nutritious fruits forseed dispersers. But the mutually beneficial system should alsobe safeguarded against âcheatersâ, for example, animals that try tosteal nectar without aiding in pollination. Now you can see whyplant-animal interactions often involve co-evolution of themutualists, that is, the evolutions of the flower and its pollinatorspecies are tightly linked with one another. In many species of figtrees, there is a tight one-to-one relationship with the pollinatorspecies of wasp (Figure 13.7). It means that a given fig species canbe pollinated only by its âpartnerâ wasp species and no other species.The female wasp uses the fruit not only as an oviposition (egg-laying)site but uses the developing seeds within the fruit for nourishingFigure 13.7 Mutual relationship between fig tree and wasp: (a) Fig flower is pollinatedby wasp; (b) Wasp laying eggs in a fig fruit(a) (b)2022-23238BIOLOGYits larvae. The wasp pollinates the fig inflorescence whilesearching for suitable egg-laying sites. In return for thefavour of pollination the fig offers the wasp some of itsdeveloping seeds, as food for the developing wasp larvae.Orchids show a bewildering diversity of floralpatterns many of which have evolved to attract the rightpollinator insect (bees and bumblebees) and ensureguaranteed pollination by it (Figure 13.8). Not allorchids offer rewards. The Mediterranean orchidOphrys employs âsexual deceitâ to get pollination doneby a species of bee. One petal of its flower bears anuncanny resemblance to the female of the bee in size,colour and markings. The male bee is attracted to whatit perceives as a female, âpseudocopulatesâ with theflower, and during that process is dusted with pollenfrom the flower. When this same bee âpseudocopulatesâwith another flower, it transfers pollen to it and thus,pollinates the flower. Here you can see how co-evolutionoperates. If the female beeâs colour patterns change even slightly for anyreason during evolution, pollination success will be reduced unless theorchid flower co-evolves to maintain the resemblance of its petal to thefemale bee.Figure 13.8 Showing bee-a pollinatoron orchid flowerSUMMARYAs a branch of biology, Ecology is the study of the relationships ofliving organisms with the abiotic (physico-chemical factors) and bioticcomponents (other species) of their environment. It is concernedwith four levels of biological organisation-organisms, populations,communities and biomes.Temperature, light, water and soil are the most importantphysical factors of the environment to which the organisms areadapted in various ways. Maintenance of a constant internalenvironment (homeostasis) by the organisms contributes to optimalperformance, but only some organisms (regulators) are capable ofhomeostasis in the face of changing external environment. Otherseither partially regulate their internal environment or simplyconform. A few other species have evolved adaptations to avoidunfavourable conditions in space (migration) or in time (aestivation,hibernation, and diapause).Evolutionary changes through natural selection take place atthe population level and hence, population ecology is an importantarea of ecology. A population is a group of individuals of a givenspecies sharing or competing for similar resources in a definedgeographical area. Populations have attributes that individualorganisms do not- birth rates and death rates, sex ratio and age2022-23239ORGANISMS AND POPULATIONSdistribution. The proportion of different age groups of males andfemales in a population is often presented graphically as age pyramid;its shape indicates whether a population is stationary, growing ordeclining.Ecological effects of any factors on a population are generallyreflected in its size (population density), which may be expressed indifferent ways (numbers, biomass, per cent cover, etc.,) dependingon the species.Populations grow through births and immigration and declinethrough deaths and emigration. When resources are unlimited, thegrowth is usually exponential but when resources becomeprogressively limiting, the growth pattern turns logistic. In eithercase, growth is ultimately limited by the carrying capacity of theenvironment. The intrinsic rate of natural increase (r) is a measureof the inherent potential of a population to grow.In nature populations of different species in a habitat do not livein isolation but interact in many ways. Depending on the outcome,these interactions between two species are classified as competition(both species suffer), predation and parasitism (one benefits and theother suffers), commensalism (one benefits and the other isunaffected), amensalism (one is harmed, other unaffected) andmutualism (both species benefit). Predation is a very importantprocess through which trophic energy transfer is facilitated and somepredators help in controlling their prey populations. Plants haveevolved diverse morphological and chemical defenses againstherbivory. In competition, it is presumed that the superior competitoreliminates the inferior one (the Competitive Exclusion Principle), butmany closely related species have evolved various mechanisms whichfacilitate their co-existence. Some of the most fascinating cases ofmutualism in nature are seen in plant-pollinator interactions.EXERCISES1. How is diapause different from hibernation?2. If a marine fish is placed in a fresh water aquarium, will the fish beable to survive? Why or why not?3. Most living organisms cannot survive at temperature above 450C. Howare some microbes able to live in habitats with temperatures exceeding1000C?4. List the attributes that populations possess but not individuals.5. If a population growing exponentially double in size in 3 years, what isthe intrinsic rate of increase (r) of the population?6. Name important defence mechanisms in plants against herbivory.2022-23240BIOLOGY7. An orchid plant is growing on the branch of mango tree. How do youdescribe this interaction between the orchid and the mango tree?8. What is the ecological principle behind the biological control method ofmanaging with pest insects?9. Distinguish between the following:(a) Hibernation and Aestivation(b) Ectotherms and Endotherms10. Write a short note on(a) Adaptations of desert plants and animals(b) Adaptations of plants to water scarcity(c) Behavioural adaptations in animals(d) Importance of light to plants(e) Effect of temperature or water scarcity and the adaptations of animals.11. List the various abiotic environmental factors.12. Give an example for:(a) An endothermic animal(b) An ectothermic animal(c) An organism of benthic zone13. Define population and community.14. Define the following terms and give one example for each:(a) Commensalism(b) Parasitism(c) Camouflage(d) Mutualism(e) Interspecific competition15. With the help of suitable diagram describe the logistic populationgrowth curve.16. Select the statement which explains best parasitism.(a) One organism is benefited.(b) Both the organisms are benefited.(c) One organism is benefited, other is not affected.(d) One organism is benefited, other is affected.17. List any three important characteristics of a population and explain.2022-23An ecosystem can be visualised as a functional unit ofnature, where living organisms interact among themselvesand also with the surrounding physical environment.Ecosystem varies greatly in size from a small pond to alarge forest or a sea. Many ecologists regard the entirebiosphere as a global ecosystem, as a composite of alllocal ecosystems on Earth. Since this system is too muchbig and complex to be studied at one time, it is convenientto divide it into two basic categories, namely theterrestrial and the aquatic. Forest, grassland and desertare some examples of terrestrial ecosystems; pond, lake,wetland, river and estuary are some examples of aquaticecosystems. Crop fields and an aquarium may also beconsidered as man-made ecosystems.We will first look at the structure of the ecosystem, inorder to appreciate the input (productivity), transfer ofenergy (food chain/web, nutrient cycling) and the output(degradation and energy loss). We will also look at therelationships â cycles, chains, webs â that are created asa result of these energy flows within the system and theirinter- relationship.CHAPTER 14ECOSYSTEM14.1 EcosystemâStructureand Function14.2. Productivity14.3 Decomposition14.4 Energy Flow14.5 Ecological Pyramids14.6 Ecological Succession14.7 Nutrient Cycling14.8 Ecosystem Services2022-23242BIOLOGY14.1 ECOSYSTEM â STRUCTURE AND FUNCTIONIn chapter 13, you have looked at the various components of theenvironment- abiotic and biotic. You studied how the individual bioticand abiotic factors affected each other and their surrounding. Let us lookat these components in a more integrated manner and see how the flow ofenergy takes place within these components of the ecosystem.Interaction of biotic and abiotic components result in a physicalstructure that is characteristic for each type of ecosystem. Identificationand enumeration of plant and animal species of an ecosystem gives itsspecies composition. Vertical distribution of different species occupyingdifferent levels is called stratification. For example, trees occupy topvertical strata or layer of a forest, shrubs the second and herbs and grassesoccupy the bottom layers.The components of the ecosystem are seen to function as a unit whenyou consider the following aspects:(i) Productivity;(ii) Decomposition;(iii) Energy flow; and(iv) Nutrient cycling.To understand the ethos of an aquatic ecosystem let us take a smallpond as an example. This is fairly a self-sustainable unit and rather simpleexample that explain even the complex interactions that exist in an aquaticecosystem. A pond is a shallow water body in which all the abovementioned four basic components of an ecosystem are well exhibited.The abiotic component is the water with all the dissolved inorganic andorganic substances and the rich soil deposit at the bottom of the pond.The solar input, the cycle of temperature, day-length and other climaticconditions regulate the rate of function of the entire pond. The autotrophiccomponents include the phytoplankton, some algae and the floating,submerged and marginal plants found at the edges. The consumers arerepresented by the zooplankton, the free swimming and bottom dwellingforms. The decomposers are the fungi, bacteria and flagellates especiallyabundant in the bottom of the pond. This system performs all the functionsof any ecosystem and of the biosphere as a whole, i.e., conversion ofinorganic into organic material with the help of the radiant energy of thesun by the autotrophs; consumption of the autotrophs by heterotrophs;decomposition and mineralisation of the dead matter to release them backfor reuse by the autotrophs, these event are repeated over and over again.There is unidirectional movement of energy towards the higher trophiclevels and its dissipation and loss as heat to the environment.14.2. PRODUCTIVITYA constant input of solar energy is the basic requirement for any ecosystemto function and sustain. Primary production is defined as the amount of2022-23243ECOSYSTEMbiomass or organic matter produced per unit area over a time period byplants during photosynthesis. It is expressed in terms of weight (gmâ2) orenergy (kcal mâ2). The rate of biomass production is called productivity.It is expressed in terms of gmâ2 yrâ1 or (kcal mâ2) yrâ1 to compare theproductivity of different ecosystems. It can be divided into gross primaryproductivity (GPP) and net primary productivity (NPP). Gross primaryproductivity of an ecosystem is the rate of production of organic matterduring photosynthesis. A considerable amount of GPP is utilised by plantsin respiration. Gross primary productivity minus respiration losses (R),is the net primary productivity (NPP).GPP â R = NPPNet primary productivity is the available biomass for the consumptionto heterotrophs (herbiviores and decomposers). Secondary productivityis defined as the rate of formation of new organic matter byconsumers.Primary productivity depends on the plant species inhabiting aparticular area. It also depends on a variety of environmental factors,availability of nutrients and photosynthetic capacity of plants. Therefore,it varies in different types of ecosystems. The annual net primaryproductivity of the whole biosphere is approximately 170 billion tons(dry weight) of organic matter. Of this, despite occupying about 70 percent of the surface, the productivity of the oceans are only 55 billion tons.Rest of course, is on land. Discuss the main reason for the lowproductivity of ocean with your teacher.14.3 DECOMPOSITIONYou may have heard of the earthworm being referred to as the farmerâsâfriendâ. This is so because they help in the breakdown of complex organicmatter as well as in loosening of the soil. Similarly, decomposers breakdown complex organic matter into inorganic substances like carbondioxide, water and nutrients and the process is called decomposition.Dead plant remains such as leaves, bark, flowers and dead remains ofanimals, including fecal matter, constitute detritus, which is the rawmaterial for decomposition. The important steps in the process ofdecomposition are fragmentation, leaching, catabolism, humification andmineralisation.Detritivores (e.g., earthworm) break down detritus into smaller particles.This process is called fragmentation. By the process of leaching, watersolubleinorganic nutrients go down into the soil horizon and get precipitatedas unavailable salts. Bacterial and fungal enzymes degrade detritus intosimpler inorganic substances. This process is called as catabolism.It is important to note that all the above steps in decomposition operatesimultaneously on the detritus (Figure 14.1). Humification andmineralisation occur during decomposition in the soil. Humification leads2022-23244BIOLOGYto accumulation of a dark coloured amorphous substance called humusthat is highly resistant to microbial action and undergoes decompositionat an extremely slow rate. Being colloidal in nature it serves as a reservoirof nutrients. The humus is further degraded by some microbes and releaseof inorganic nutrients occur by the process known as mineralisation.Decomposition is largely an oxygen-requiring process. The rate ofdecomposition is controlled by chemical composition of detritus andclimatic factors. In a particular climatic condition, decomposition rateis slower if detritus is rich in lignin and chitin, and quicker, if detritus isrich in nitrogen and water-soluble substances like sugars. Temperatureand soil moisture are the most important climatic factors that regulatedecomposition through their effects on the activities of soil microbes.Warm and moist environment favour decomposition whereas lowtemperature and anaerobiosis inhibit decomposition resulting in buildup of organic materials.Figure 14.1 Diagrammatic representation of decomposition cycle in a terrestrial ecosystem2022-23245ECOSYSTEM14.4 ENERGY FLOWExcept for the deep sea hydro-thermal ecosystem, sun is the only sourceof energy for all ecosystems on Earth. Of the incident solar radiation lessthan 50 per cent of it is photosynthetically active radiation (PAR). Weknow that plants and photosynthetic bacteria (autotrophs), fix Sunâsradiant energy to make food from simple inorganic materials. Plantscapture only 2-10 per cent of the PAR and this small amount of energysustains the entire living world. So, it is very important to know how thesolar energy captured by plants flows through different organisms of anecosystem. All organisms are dependent for their food on producers, eitherdirectly or indirectly. So you find unidirectional flow of energy from thesun to producers and then to consumers. Is this in keeping with the firstlaw of thermodynamics?Further, ecosystems are not exempt from the Second Law ofthermodynamics. They need a constant supply of energy to synthesisethe molecules they require, to counteract the universal tendency towardincreasing disorderliness.The green plant in the ecosystem are called producers. In a terrestrialecosystem, major producers are herbaceous and woody plants. Likewise,producers in an aquatic ecosystem are various species like phytoplankton,algae and higher plants.You have read about the food chains and webs that exist in nature.Starting from the plants (or producers) food chains or rather webs areformed such that an animal feeds on a plant or on another animal and inturn is food for another. The chain or web is formed because of thisinterdependency. No energy that is trapped into an organism remains init for ever. The energy trapped by the producer, hence, is either passed onto a consumer or the organism dies. Death of organism is the beginningof the detritus food chain/web.All animals depend on plants (directly or indirectly) for their food needs.They are hence called consumers and also heterotrophs. If they feed onthe producers, the plants, they are called primary consumers, and if theanimals eat other animals which in turn eat the plants (or their produce)they are called secondary consumers. Likewise, you could have tertiaryconsumers too. Obviously the primary consumers will be herbivores.Some common herbivores are insects, birds and mammals in terrestrialecosystem and molluscs in aquatic ecosystem.The consumers that feed on these herbivores are carnivores, or morecorrectly primary carnivores (though secondary consumers). Thoseanimals that depend on the primary carnivores for food are labelledsecondary carnivores. A simple grazing food chain (GFC) is depictedbelow:Grass Goat Man(Producer) (Primary Consumer) (Secondary consumer)2022-23246BIOLOGYThe detritus food chain (DFC) begins with dead organic matter. It ismade up of decomposers which are heterotrophic organisms, mainlyfungi and bacteria. They meet their energy and nutrient requirements bydegrading dead organic matter or detritus. These are also known assaprotrophs (sapro: to decompose). Decomposers secrete digestiveenzymes that breakdown dead and waste materials into simple, inorganicmaterials, which are subsequently absorbed by them.In an aquatic ecosystem, GFC is the major conduit for energy flow.As against this, in a terrestrial ecosystem, a much larger fraction of energyflows through the detritus food chain than through the GFC. Detritusfood chain may be connected with the grazing food chain at some levels:some of the organisms of DFC are prey to the GFC animals, and in a naturalecosystem, some animals like cockroaches, crows, etc., are omnivores.These natural interconnection of food chains make it a food web. Howwould you classify human beings!Organisms occupy a place in the natural surroundings or in acommunity according to their feeding relationship with other organisms.Based on the source of their nutrition or food, organisms occupy a specificplace in the food chain that is known as their trophic level. Producersbelong to the first trophic level, herbivores (primary consumer) to thesecond and carnivores (secondary consumer) to the third (Figure 14.2).Figure 14.2 Diagrammatic representation of trophic levels in an ecosystem2022-23ECOSYSTEMFigure 14.3 Energy flow through different trophic levels247The important point to note is that the amount of energy decreases atsuccessive trophic levels. When any organism dies it is converted todetritus or dead biomass that serves as an energy source for decomposers.Organisms at each trophic level depend on those at the lower trophic levelfor their energy demands.Each trophic level has a certain mass of living material at a particulartime called as the standing crop. The standing crop is measured as themass of living organisms (biomass) or the number in a unit area. Thebiomass of a species is expressed in terms of fresh or dry weight.Measurement of biomass in terms of dry weight is more accurate. Why?The number of trophic levels in the grazing food chain is restricted asthe transfer of energy follows 10 per cent law â only 10 per cent of theenergy is transferred to each trophic level from the lower trophic level. Innature, it is possible to have so many levels â producer, herbivore, primarycarnivore, secondary carnivore in the grazing food chain (Figure 14.3) .Do you think there is any such limitation in a detritus food chain?14.5 ECOLOGICAL PYRAMIDSYou must be familiar with the shape of a pyramid. The base of a pyramidis broad and it narrows towards the apex. One gets a similar shape,whether you express the food or energy relationship between organisms2022-23248BIOLOGYat different trophic levels. This, relationship is expressed in terms ofnumber, biomass or energy. The base of each pyramid represents theproducers or the first trophic level while the apex represents tertiary ortop level consumer. The three types of ecological pyramids that are usuallystudied are (a) pyramid of number; (b) pyramid of biomass and (c) pyramidof energy. For detail (see Figure 14.4 a, b, c and d).Figure 14.4 (a) Pyramid of numbers in a grassland ecosystem. Only three top-carnivores aresupported in an ecosystem based on production of nearly 6 millions plantsFigure 14.4 (b) Pyramid of biomass shows a sharp decrease in biomass at higher trophic levelsFigure 14.4 (c) Inverted pyramid of biomass-small standing crop of phytoplankton supports largestanding crop of zooplankton2022-23249ECOSYSTEMFigure 14.4 (d) An ideal pyramid of energy. Observe that primary producers convert only 1% ofthe energy in the sunlight available to them into NPPAny calculations of energy content, biomass or numbers, has to includeall organisms at that trophic level. No generalisations we make will betrue if we take only a few individuals at any trophic level into account.Also a given organism may occupy more than one trophic levelsimultaneously. One must remember that the trophic level represents afunctional level, not a species as such. A given species may occupy morethan one trophic level in the same ecosystem at the same time; for example,a sparrow is a primary consumer when it eats seeds, fruits, peas, and asecondary consumer when it eats insects and worms. Can you work outhow many trophic levels human beings function at in a food chain?In most ecosystems, all the pyramids, of number, of energy andbiomass are upright, i.e., producers are more in number and biomassthan the herbivores, and herbivores are more in number and biomassthan the carnivores. Also energy at a lower trophic level is always morethan at a higher level.There are exceptions to this generalisation: If you were to count thenumber of insects feeding on a big tree what kind of pyramid would youget? Now add an estimate of the number of small birds depending on theinsects, as also the number of larger birds eating the smaller. Draw theshape you would get.The pyramid of biomass in sea is generally inverted because thebiomass of fishes far exceeds that of phytoplankton. Isnât that a paradox?How would you explain this?Pyramid of energy is always upright, can never be inverted, becausewhen energy flows from a particular trophic level to the next trophic level,some energy is always lost as heat at each step. Each bar in the energypyramid indicates the amount of energy present at each trophic level in agiven time or annually per unit area.2022-23250BIOLOGYHowever, there are certain limitations of ecological pyramids such asit does not take into account the same species belonging to two or moretrophic levels. It assumes a simple food chain, something that almostnever exists in nature; it does not accommodate a food web. Moreover,saprophytes are not given any place in ecological pyramids even thoughthey play a vital role in the ecosystem.14.6 ECOLOGICAL SUCCESSIONYou have learnt in Chapter 13, the characteristics of population andcommunity and also their response to environment and how suchresponses vary from an individual response. Let us examine another aspectof community response to environment over time.An important characteristic of all communities is that theircomposition and structure constantly change in response to the changingenvironmental conditions. This change is orderly and sequential, parallelwith the changes in the physical environment. These changes lead finallyto a community that is in near equilibrium with the environment andthat is called a climax community. The gradual and fairly predictablechange in the species composition of a given area is called ecologicalsuccession. During succession some species colonise an area and theirpopulation become more numerous whereas populations of other speciesdecline and even disappear.The entire sequence of communities that successively change in agiven area are called sere(s). The individual transitional communities aretermed seral stages or seral communities. In the successive seral stagesthere is a change in the diversity of species of organisms, increase in thenumber of species and organisms as well as an increase in the total biomass.The present day communities in the world have come to be becauseof succession that has occurred over millions of years since life started onearth. Actually succession and evolution would have been parallelprocesses at that time.Succession is hence a process that starts in an area where no livingorganisms are there â these could be areas where no living organismsever existed, say bare rock; or in areas that somehow, lost all the livingorganisms that existed there. The former is called primary succession,while the latter is termed secondary succession.Examples of areas where primary succession occurs are newly cooledlava, bare rock, newly created pond or reservoir. The establishment of anew biotic community is generally slow. Before a biotic community ofdiverse organisms can become established, there must be soil. Dependingmostly on the climate, it takes natural processes several hundred to severalthousand years to produce fertile soil on bare rock.2022-23251ECOSYSTEMSecondary succession begins in areas where natural bioticcommunities have been destroyed such as in abandoned farm lands,burned or cut forests, lands that have been flooded. Since some soil orsediment is present, succession is faster than primary succession.Description of ecological succession usually focuses on changes invegetation. However, these vegetational changes in turn affect food andshelter for various types of animals. Thus, as succession proceeds, thenumbers and types of animals and decomposers also change.At any time during primary or secondary succession, natural orhuman induced disturbances (fire, deforestation, etc.), can convert aparticular seral stage of succession to an earlier stage. Also suchdisturbances create new conditions that encourage some species anddiscourage or eliminate other species.14.6.1 Succession of PlantsBased on the nature of the habitat â whether it is water (or very wet areas)or it is on very dry areas â succession of plants is called hydrarch orxerarch, respectively. Hydrarch succession takes place in wet areas andthe successional series progress from hydric to the mesic conditions. Asagainst this, xerarch succession takes place in dry areas and the seriesprogress from xeric to mesic conditions. Hence, both hydrarch and xerarchsuccessions lead to medium water conditions (mesic) â neither too dry(xeric) nor too wet (hydric).The species that invade a bare area are called pioneer species. Inprimary succession on rocks these are usually lichens which are able tosecrete acids to dissolve rock, helping in weathering and soil formation.These later pave way to some very small plants like bryophytes, whichare able to take hold in the small amount of soil. They are, with time,succeeded by higher plants, and after several more stages, ultimately astable climax forest community is formed. The climax community remainsstable as long as the environment remains unchanged. With time thexerophytic habitat gets converted into a mesophytic one.In primary succession in water, the pioneers are the smallphytoplanktons, which are replaced with time by rooted-submerged plants,rooted-floating angiosperms followed by free-floating plants, then reedswamp,marsh-meadow, scrub and finally the trees. The climax again wouldbe a forest. With time the water body is converted into land (Figure 14.5).In secondary succession the species that invade depend on thecondition of the soil, availability of water, the environment as also theseeds or other propagules present. Since soil is already there, the rate ofsuccession is much faster and hence, climax is also reached more quickly.What is important to understand is that succession, particularlyprimary succession, is a very slow process, taking maybe thousands of2022-23252BIOLOGYyears for the climax to be reached. Another important fact is to understandthat all succession whether taking place in water or on land, proceeds toa similar climax community â the mesic.Figure 14.5 Diagrammatic representation of primary succession(a) (d)(b) (e)(c)(f)(g)2022-23253ECOSYSTEM14.7 NUTRIENT CYCLINGYou have studied in Class XI that organisms need a constant supply ofnutrients to grow, reproduce and regulate various body functions. Theamount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc.,present in the soil at any given time, is referred to as the standing state. Itvaries in different kinds of ecosystems and also on a seasonal basis.What is important is to appreciate that nutrients which are never lostfrom the ecosystems, rather they are recycled time and again indefinitely.The movement of nutrient elements through the various components ofan ecosystem is called nutrient cycling. Another name of nutrient cyclingis biogeochemical cycles (bio: living organism, geo: rocks, air, water).Nutrient cycles are of two types: (a) gaseous and (b) sedimentary. TheFigure 14.6 Simplified model of carbon cycle in the biospherereservoir for gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle)exists in the atmosphere and for the sedimentary cycle (e.g., sulphur andphosphorus cycle), the reservoir is located in Earthâs crust. Environmentalfactors, e.g., soil, moisture, pH, temperature, etc., regulate the rate ofrelease of nutrients into the atmosphere. The function of the reservoir is2022-23254BIOLOGYto meet with the deficit which occurs due to imbalance in the rate of influxand efflux.You have made a detailed study of nitrogen cycle in class XI. Here wediscuss carbon and phosphorus cycles.14.7.1 Ecosystem â Carbon CycleWhen you study the composition of living organisms, carbon constitutes49 per cent of dry weight of organisms and is next only to water. If welook at the total quantity of global carbon, we find that 71 per cent carbonis found dissolved in oceans. This oceanic reservoir regulates the amountof carbon dioxide in the atmosphere (Figure 14.6). Do you know that theatmosphere only contains about 1per cent of total global carbon?Fossil fuel also represent a reservoir of carbon. Carbon cycling occursthrough atmosphere, ocean and through living and dead organisms.According to one estimate 4 à 1013 kg of carbon is fixed annually in thebiosphere through photosynthesis. A considerable amount of carbonreturns to the atmosphere as CO2 through respiratory activities of theproducers and consumers. Decomposers also contribute substantiallyto CO2 pool by their processing of waste materials and dead organic matterof land or oceans. Some amount of the fixed carbon is lost to sedimentsand removed from circulation. Burning of wood, forest fire and combustionof organic matter, fossil fuel, volcanic activity are additional sources forreleasing CO2 in the atmosphere.Human activities have significantly influenced the carbon cycle. Rapiddeforestation and massive burning of fossil fuel for energy and transporthave significantly increased the rate of release of carbon dioxide into theatmosphere (see greenhouse effect in Chapter 16).14.7.2 Ecosystem â Phosphorus CyclePhosphorus is a major constituent of biological membranes, nucleic acidsand cellular energy transfer systems. Many animals also need largequantities of this element to make shells, bones and teeth. The naturalreservoir of phosphorus is rock, which contains phosphorus in the formof phosphates. When rocks are weathered, minute amounts of thesephosphates dissolve in soil solution and are absorbed by the roots of theplants (Figure 14.7). Herbivores and other animals obtain this elementfrom plants. The waste products and the dead organisms are decomposedby phosphate-solubilising bacteria releasing phosphorus. Unlike carboncycle, there is no respiratory release of phosphorus into atmosphere. Canyou differentiate between the carbon and the phosphorus cycle?The other two major and important differences between carbon andphosphorus cycle are firstly, atmospheric inputs of phosphorus throughrainfall are much smaller than carbon inputs, and, secondly, gaseous2022-23255ECOSYSTEMexchanges of phosphorus between organism and environment arenegligible.14.8 ECOSYSTEM SERVICESHealthy ecosystems are the base for a wide range of economic,environmental and aesthetic goods and services. The products ofecosystem processes are named as ecosystem services, for example,healthy forest ecosystems purify air and water, mitigate droughts andfloods, cycle nutrients, generate fertile soils, provide wildlife habitat,maintain biodiversity, pollinate crops, provide storage site for carbonand also provide aesthetic, cultural and spiritual values. Though valueof such services of biodiversity is difficult to determine, it seemsreasonable to think that biodiversity should carry a hefty price tag.Robert Constanza and his colleagues have very recently tried toput price tags on natureâs life-support services. Researchers have putan average price tag of US $ 33 trillion a year on these fundamentalecosystems services, which are largely taken for granted because theyare free. This is nearly twice the value of the global gross nationalproduct GNP which is (US $ 18 trillion).Out of the total cost of various ecosystem services, the soilformation accounts for about 50 per cent, and contributions of otherservices like recreation and nutrient cycling, are less than 10 percent each. The cost of climate regulation and habitat for wildlife areabout 6 per cent each.Figure 14.7 A simplified model of phosphorus cycling in a terrestrialecosystem2022-23256BIOLOGYSUMMARYAn ecosystem is a structural and functional unit of nature and itcomprises abiotic and biotic components. Abiotic components areinorganic materials- air, water and soil, whereas biotic componentsare producers, consumers and decomposers. Each ecosystem hascharacteristic physical structure resulting from interaction amongstabiotic and biotic components. Species composition and stratificationare the two main structural features of an ecosystem. Based on sourceof nutrition every organism occupies a place in an ecosystem.Productivity, decomposition, energy flow, and nutrient cycling arethe four important components of an ecosystem. Primary productivityis the rate of capture of solar energy or biomass production of theproducers. It is divided into two types: gross primary productivity (GPP)and net primary productivity (NPP). Rate of capture of solar energy ortotal production of organic matter is called as GPP. NPP is the remainingbiomass or the energy left after utilisation of producers. Secondaryproductivity is the rate of assimilation of food energy by the consumers.In decomposition, complex organic compounds of detritus are convertedto carbon dioxide, water and inorganic nutrients by the decomposers.Decomposition involves three processes, namely fragmentation ofdetritus, leaching and catabolism.Energy flow is unidirectional. First, plants capture solar energyand then, food is transferred from the producers to decomposers.Organisms of different trophic levels in nature are connected to eachother for food or energy relationship forming a food chain. The storageand movement of nutrient elements through the various componentsof the ecosystem is called nutrient cycling; nutrients are repeatedlyused through this process. Nutrient cycling is of two typesâgaseousand sedimentary. Atmosphere or hydrosphere is the reservoir for thegaseous type of cycle (carbon), whereas Earthâs crust is the reservoirfor sedimentary type (phosphorus). Products of ecosystem processesare named as ecosystem services, e.g., purification of air and water byforests.The biotic community is dynamic and undergoes changes with thepassage of time. These changes are sequentially ordered and constituteecological succession. Succession begins with invasion of a bare lifelessarea by pioneers which later pave way for successors and ultimately astable climax community is formed. The climax community remainsstable as long as the environment remains unchanged.EXERCISES1. Fill in the blanks.(a) Plants are called as_________because they fix carbon dioxide.(b) In an ecosystem dominated by trees, the pyramid (of numbers)is_________type.(c) In aquatic ecosystems, the limiting factor for the productivityis_________.2022-23257ECOSYSTEM(d) Common detritivores in our ecosystem are_________.(e) The major reservoir of carbon on earth is_________.2. Which one of the following has the largest population in a food chain?(a) Producers(b) Primary consumers(c) Secondary consumers(d) Decomposers3. The second trophic level in a lake is(a) Phytoplankton(b) Zooplankton(c) Benthos(d) Fishes4. Secondary producers are(a) Herbivores(b) Producers(c) Carnivores(d) None of the above5. What is the percentage of photosynthetically active radiation (PAR) inthe incident solar radiation?(a) 100%(b) 50 %(c) 1-5%(d) 2-10%6. Distinguish between(a) Grazing food chain and detritus food chain(b) Production and decomposition(c) Upright and inverted pyramid(d) Food chain and Food web(e) Litter and detritus(f) Primary and secondary productivity7. Describe the components of an ecosystem.8. Define ecological pyramids and describe with examples, pyramids ofnumber and biomass.9. What is primary productivity? Give brief description of factors that affectprimary productivity.10. Define decomposition and describe the processes and products ofdecomposition.11. Give an account of energy flow in an ecosystem.12. Write important features of a sedimentary cycle in an ecosystem.13. Outline salient features of carbon cycling in an ecosystem.2022-23If an alien from a distant galaxy were to visit our planetEarth, the first thing that would amaze and baffle himwould most probably be the enormous diversity of lifethat he would encounter. Even for humans, the rich varietyof living organisms with which they share this planet neverceases to astonish and fascinate us. The common manwould find it hard to believe that there are more than20,000 species of ants, 3,00,000 species of beetles, 28,000species of fishes and nearly 20,000 species of orchids.Ecologists and evolutionary biologists have been tryingto understand the significance of such diversity by askingimportant questionsâ Why are there so many species?Did such great diversity exist throughout earthâs history?How did this diversification come about? How and whyis this diversity important to the biosphere? Would itfunction any differently if the diversity was much less?How do humans benefit from the diversity of life?15.1 BIODIVERSITYIn our biosphere immense diversity (or heterogeneity)exists not only at the species level but at all levels ofbiological organisation ranging from macromoleculeswithin cells to biomes. Biodiversity is the term popularisedby the sociobiologist Edward Wilson to describe theCHAPTER 15BIODIVERSITY ANDCONSERVATION15.1 Biodiversity15.2 Biodiversity Conservation2022-23259BIODIVERSITY AND CONSERVATIONcombined diversity at all the levels of biological organisation.The most important of them areâ(i) Genetic diversity: A single species might show high diversity atthe genetic level over its distributional range. The genetic variationshown by the medicinal plant Rauwolfia vomitoria growing indifferent Himalayan ranges might be in terms of the potency andconcentration of the active chemical (reserpine) that the plantproduces. India has more than 50,000 genetically different strainsof rice, and 1,000 varieties of mango.(ii) Species diversity: The diversity at the species level, for example,the Western Ghats have a greater amphibian species diversity thanthe Eastern Ghats.(iii) Ecological diversity: At the ecosystem level, India, for instance,with its deserts, rain forests, mangroves, coral reefs, wetlands,estuaries, and alpine meadows has a greater ecosystem diversitythan a Scandinavian country like Norway.It has taken millions of years of evolution, to accumulate this richdiversity in nature, but we could lose all that wealth in less than twocenturies if the present rates of species losses continue. Biodiversity andits conservation are now vital environmental issues of international concernas more and more people around the world begin to realise the criticalimportance of biodiversity for our survival and well- being on this planet.15.1.1 How Many Species are there on Earth and HowMany in India?Since there are published records of all the species discovered and named,we know how many species in all have been recorded so far, but it is noteasy to answer the question of how many species there are on earth.According to the International Union for Conservation of Nature andNatural Resources (IUCN) (2004), the total number of plant and animalspecies described so far is slightly more than 1.5 million, but we have noclear idea of how many species are yet to be discovered and described.Estimates vary widely and many of them are only educated guesses. Formany taxonomic groups, species inventories are more complete intemperate than in tropical countries. Considering that an overwhelminglylarge proportion of the species waiting to be discovered are in the tropics,biologists make a statistical comparison of the temperate-tropical speciesrichness of an exhaustively studied group of insects and extrapolate thisratio to other groups of animals and plants to come up with a grossestimate of the total number of species on earth. Some extreme estimatesrange from 20 to 50 million, but a more conservative and scientificallysound estimate made by Robert May places the global species diversityat about 7 million.2022-23260BIOLOGYLet us look at some interesting aspects about earthâs biodiversity basedon the currently available species inventories. More than 70 per cent ofall the species recorded are animals, while plants (including algae, fungi,bryophytes, gymnosperms and angiosperms) comprise no more than 22per cent of the total. Among animals, insects are the most species-richtaxonomic group, making up more than 70 per cent of the total. Thatmeans, out of every 10 animals on this planet, 7 are insects. Again, howdo we explain this enormous diversification of insects? The number offungi species in the world is more than the combined total of the speciesof fishes, amphibians, reptiles and mammals. In Figure 15.1, biodiversityis depicted showing species number of major taxa.Figure 15.1 Representing global biodiversity: proportionate number ofspecies of major taxa of plants, invertebrates and vertebratesIt should be noted that these estimates do not give any figures forprokaryotes. Biologists are not sure about how many prokaryotic speciesthere might be. The problem is that conventional taxonomic methods arenot suitable for identifying microbial species and many species are simplynot culturable under laboratory conditions. If we accept biochemical ormolecular criteria for delineating species for this group, then their diversityalone might run into millions.2022-23261BIODIVERSITY AND CONSERVATIONAlthough India has only 2.4 per cent of the worldâs land area, its shareof the global species diversity is an impressive 8.1 per cent. That is whatmakes our country one of the 12 mega diversity countries of the world.Nearly 45,000 species of plants and twice as many of animals have beenrecorded from India. How many living species are actually there waitingto be discovered and named? If we accept Mayâs global estimates, only22 per cent of the total species have been recorded so far. Applying thisproportion to Indiaâs diversity figures, we estimate that there are probablymore than 1,00,000 plant species and more than 3,00,000 animal speciesyet to be discovered and described. Would we ever be able to completethe inventory of the biological wealth of our country? Consider the immensetrained manpower (taxonomists) and the time required to complete thejob. The situation appears more hopeless when we realise that a largefraction of these species faces the threat of becoming extinct even beforewe discover them. Natureâs biological library is burning even before wecatalogued the titles of all the books stocked there.15.1.2 Patterns of Biodiversity(i) Latitudinal gradients: The diversity of plants and animals isnot uniform throughout the world but shows a rather unevendistribution. For many group of animals or plants, there areinteresting patterns in diversity, the most well- known being thelatitudinal gradient in diversity. In general, species diversitydecreases as we move away from the equator towards the poles.With very few exceptions, tropics (latitudinal range of 23.5° N to23.5° S) harbour more species than temperate or polar areas.Colombia located near the equator has nearly 1,400 species of birdswhile New York at 41° N has 105 species and Greenland at 71° Nonly 56 species. India, with much of its land area in the tropicallatitudes, has more than 1,200 species of birds. A forest in a tropicalregion like Equador has up to 10 times as many species of vascularplants as a forest of equal area in a temperate region like the Midwestof the USA. The largely tropical Amazonian rain forest in SouthAmerica has the greatest biodiversity on earth- it is home to morethan 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427of mammals, 427 of amphibians, 378 of reptiles and of more than1,25,000 invertebrates. Scientists estimate that in these rain foreststhere might be at least two million insect species waiting to bediscovered and named.What is so special about tropics that might account for their greaterbiological diversity? Ecologists and evolutionary biologists haveproposed various hypotheses; some important ones are (a) Speciationis generally a function of time, unlike temperate regions subjectedto frequent glaciations in the past, tropical latitudes have remainedrelatively undisturbed for millions of years and thus, had a long2022-23262BIOLOGYevolutionary time for species diversification, (b) Tropical environments,unlike temperate ones, are less seasonal, relatively more constantand predictable. Such constant environments promote nichespecialisation and lead to a greater species diversity and (c) Thereis more solar energy available in the tropics, which contributes tohigher productivity; this in turn might contribute indirectly to greaterdiversity.(ii) Species-Area relationships: During his pioneering and extensiveexplorations in the wilderness of South American jungles, the greatGerman naturalist and geographer Alexander von Humboldtobserved that within a region speciesrichness increased with increasingexplored area, but only up to a limit. Infact, the relation between species richnessand area for a wide variety of taxa(angiosperm plants, birds, bats,freshwater fishes) turns out to be arectangular hyperbola (Figure15.2). Ona logarithmic scale, the relationship is astraight line described by the equationlog S = log C + Z log AwhereS= Species richness A= AreaZ = slope of the line (regressioncoefficient)C = Y-interceptEcologists have discovered that thevalue of Z lies in the range of 0.1 to 0.2,regardless of the taxonomic group or theregion (whether it is the plants in Britain,birds in California or molluscs in New York state, the slopes of the regressionline are amazingly similar). But, if you analyse the species-arearelationships among very large areas like the entire continents, you willfind that the slope of the line to be much steeper (Z values in the rangeof 0.6 to 1.2). For example, for frugivorous (fruit-eating) birds andmammals in the tropical forests of different continents, the slope is foundto be 1.15. What do steeper slopes mean in this context?15.1.3 The importance of Species Diversity to the EcosystemDoes the number of species in a community really matter to the functioningof the ecosystem?This is a question for which ecologists have not beenable to give a definitive answer. For many decades, ecologists believedthat communities with more species, generally, tend to be more stablethan those with less species. What exactly is stability for a biologicalFigure 15.2 Showing species area relationship.Note that on log scale the relationshipbecomes linear2022-23263BIODIVERSITY AND CONSERVATIONcommunity? A stable community should not show too much variationin productivity from year to year; it must be either resistant or resilient tooccasional disturbances (natural or man-made), and it must also beresistant to invasions by alien species. We donât know how these attributesare linked to species richness in a community, but David Tilmanâslong-term ecosystem experiments using outdoor plots provide sometentative answers. Tilman found that plots with more species showedless year-to-year variation in total biomass. He also showed that in hisexperiments, increased diversity contributed to higher productivity.Although, we may not understand completely how species richnesscontributes to the well-being of an ecosystem, we know enough to realisethat rich biodiversity is not only essential for ecosystem health butimperative for the very survival of the human race on this planet. At atime when we are losing species at an alarming pace, one might askâDoes it really matter to us if a few species become extinct? Would WesternGhats ecosystems be less functional if one of its tree frog species is lostforever? How is our quality of life affected if, say, instead of 20,000 wehave only 15,000 species of ants on earth?There are no direct answers to such näive questions but we can developa proper perspective through an analogy (the ârivet popper hypothesisâ)used by Stanford ecologist Paul Ehrlich. In an airplane (ecosystem) allparts are joined together using thousands of rivets (species). If everypassenger travelling in it starts popping a rivet to take home (causing aspecies to become extinct), it may not affect flight safety (proper functioningof the ecosystem) initially, but as more and more rivets are removed, theplane becomes dangerously weak over a period of time. Furthermore,which rivet is removed may also be critical. Loss of rivets on the wings(key species that drive major ecosystem functions) is obviously a moreserious threat to flight safety than loss of a few rivets on the seats orwindows inside the plane.15.1.4 Loss of BiodiversityWhile it is doubtful if any new species are being added (through speciation)into the earthâs treasury of species, there is no doubt about their continuinglosses. The biological wealth of our planet has been declining rapidlyand the accusing finger is clearly pointing to human activities. Thecolonisation of tropical Pacific Islands by humans is said to have led tothe extinction of more than 2,000 species of native birds. The IUCN RedList (2004) documents the extinction of 784 species (including 338vertebrates, 359 invertebrates and 87 plants) in the last 500 years. Someexamples of recent extinctions include the dodo (Mauritius), quagga(Africa), thylacine (Australia), Stellerâs Sea Cow (Russia) and threesubspecies (Bali, Javan, Caspian) of tiger. The last twenty years alonehave witnessed the disappearance of 27 species. Careful analysis of records2022-23264BIOLOGYshows that extinctions across taxa are not random; some groups likeamphibians appear to be more vulnerable to extinction. Adding to thegrim scenario of extinctions is the fact that more than 15,500 speciesworld-wide are facing the threat of extinction. Presently, 12 per cent ofall bird species, 23 per cent of all mammal species, 32 per cent of allamphibian species and 31per cent of all gymnosperm species in the worldface the threat of extinction.From a study of the history of life on earth through fossil records, welearn that large-scale loss of species like the one we are currentlywitnessing have also happened earlier, even before humans appeared onthe scene. During the long period (> 3 billion years) since the origin anddiversification of life on earth there were five episodes of mass extinctionof species. How is the âSixth Extinctionâ presently in progress differentfrom the previous episodes? The difference is in the rates; the currentspecies extinction rates are estimated to be 100 to 1,000 times fasterthan in the pre-human times and our activities are responsible for thefaster rates. Ecologists warn that if the present trends continue,nearly half of all the species on earth might be wiped out within the next100 years.In general, loss of biodiversity in a region may lead to (a) decline inplant production, (b) lowered resistance to environmental perturbationssuch as drought and (c) increased variability in certain ecosystem processessuch as plant productivity, water use, and pest and disease cycles.Causes of biodiversity losses: The accelerated rates of speciesextinctions that the world is facing now are largely due to humanactivities. There are four major causes (â The Evil Quartet â is the sobriquetused to describe them).(i) Habitat loss and fragmentation: This is the most importantcause driving animals and plants to extinction. The most dramaticexamples of habitat loss come from tropical rain forests. Oncecovering more than 14 per cent of the earthâs land surface, theserain forests now cover no more than 6 per cent. They are beingdestroyed fast. By the time you finish reading this chapter, 1000more hectares of rain forest would have been lost. The Amazonrain forest (it is so huge that it is called the âlungs of the planetâ)harbouring probably millions of species is being cut and clearedfor cultivating soya beans or for conversion to grasslands for raisingbeef cattle. Besides total loss, the degradation of many habitats bypollution also threatens the survival of many species. When largehabitats are broken up into small fragments due to various humanactivities, mammals and birds requiring large territories and certainanimals with migratory habits are badly affected, leading topopulation declines.(ii) Over-exploitation: Humans have always depended on nature forfood and shelter, but when âneedâ turns to âgreedâ, it leads to2022-23265BIODIVERSITY AND CONSERVATIONover-exploitation of natural resources. Many species extinctionsin the last 500 years (Stellerâs sea cow, passenger pigeon) were dueto overexploitation by humans. Presently many marine fishpopulations around the world are over harvested, endangering thecontinued existence of some commercially important species.(iii) Alien species invasions: When alien species are introducedunintentionally or deliberately for whatever purpose, some of themturn invasive, and cause decline or extinction of indigenous species.The Nile perch introduced into Lake Victoria in east Africa ledeventually to the extinction of an ecologically unique assemblage ofmore than 200 species of cichlid fish in the lake. You must befamiliar with the environmental damage caused and threat posedto our native species by invasive weed species like carrot grass(Parthenium), Lantana and water hyacinth (Eicchornia). The recentillegal introduction of the African catfish Clarias gariepinus foraquaculture purposes is posing a threat to the indigenous catfishesin our rivers.(iv) Co-extinctions: When a species becomes extinct, the plant andanimal species associated with it in an obligatory way also becomeextinct. When a host fish species becomes extinct, its uniqueassemblage of parasites also meets the same fate. Another exampleis the case of a coevolved plant-pollinator mutualism whereextinction of one invariably leads to the extinction of the other.15.2 BIODIVERSITY CONSERVATION15.2.1 Why Should We Conserve Biodiversity?There are many reasons, some obvious and others not so obvious, but allequally important. They can be grouped into three categories: narrowlyutilitarian, broadly utilitarian, and ethical.The narrowly utilitarian arguments for conserving biodiversity areobvious; humans derive countless direct economic benefits from naturefood(cereals, pulses, fruits), firewood, fibre, construction material,industrial products (tannins, lubricants, dyes, resins, perfumes ) andproducts of medicinal importance. More than 25 per cent of the drugscurrently sold in the market worldwide are derived from plants and 25,000species of plants contribute to the traditional medicines used by nativepeoples around the world. Nobody knows how many more medicinallyuseful plants there are in tropical rain forests waiting to be explored.With increasing resources put into âbioprospectingâ (exploring molecular,genetic and species-level diversity for products of economic importance),nations endowed with rich biodiversity can expect to reap enormousbenefits.The broadly utilitarian argument says that biodiversity plays amajor role in many ecosystem services that nature provides. The fast-2022-23266BIOLOGYdwindling Amazon forest is estimated to produce, throughphotosynthesis, 20 per cent of the total oxygen in the earthâs atmosphere.Can we put an economic value on this service by nature? You can getsome idea by finding out how much your neighborhood hospital spendson a cylinder of oxygen. Pollination (without which plants cannot giveus fruits or seeds) is another service, ecosystems provide throughpollinators layer â bees, bumblebees, birds and bats. What will be thecosts of accomplishing pollination without help from naturalpollinators? There are other intangible benefits â that we derive fromnatureâthe aesthetic pleasures of walking through thick woods, watchingspring flowers in full bloom or waking up to a bulbulâs song in themorning. Can we put a price tag on such things?The ethical argument for conserving biodiversity relates to what weowe to millions of plant, animal and microbe species with whom we sharethis planet. Philosophically or spiritually, we need to realise that everyspecies has an intrinsic value, even if it may not be of current or anyeconomic value to us. We have a moral duty to care for their well-beingand pass on our biological legacy in good order to future generations.15.2.2 How do we conserve Biodiversity?When we conserve and protect the whole ecosystem, its biodiversity at alllevels is protected - we save the entire forest to save the tiger. This approachis called in situ (on site) conservation. However, when there are situationswhere an animal or plant is endangered or threatened (organisms facinga very high risk of extinction in the wild in the near future) and needsurgent measures to save it from extinction, ex situ (off site) conservationis the desirable approach.In situ conservationâ Faced with the conflict between development andconservation, many nations find it unrealistic and economically not feasibleto conserve all their biological wealth. Invariably, the number of specieswaiting to be saved from extinction far exceeds the conservation resourcesavailable. On a global basis, this problem has been addressed by eminentconservationists. They identified for maximum protection certainâbiodiversity hotspotsâ regions with very high levels of species richnessand high degree of endemism (that is, species confined to that regionand not found anywhere else). Initially 25 biodiversity hotspots wereidentified but subsequently nine more have been added to the list,bringing the total number of biodiversity hotspots in the world to 34.These hotspots are also regions of accelerated habitat loss. Three ofthese hotspots â Western Ghats and Sri Lanka, Indo-Burma andHimalaya â cover our countryâs exceptionally high biodiversity regions.Although all the biodiversity hotspots put together cover less than2 per cent of the earthâs land area, the number of species they collectively2022-23267BIODIVERSITY AND CONSERVATIONharbour is extremely high and strict protection of these hotspots couldreduce the ongoing mass extinctions by almost 30 per cent.In India, ecologically unique and biodiversity-rich regions are legallyprotected as biosphere reserves, national parks and sanctuaries. Indianow has 14 biosphere reserves, 90 national parks and 448 wildlifesanctuaries. India has also a history of religious and cultural traditionsthat emphasised protection of nature. In many cultures, tracts of forestwere set aside, and all the trees and wildlife within were venerated andgiven total protection. Such sacred groves are found in Khasi and JaintiaHills in Meghalaya, Aravalli Hills of Rajasthan, Western Ghat regions ofKarnataka and Maharashtra and the Sarguja, Chanda and Bastar areasof Madhya Pradesh. In Meghalaya, the sacred groves are the last refugesfor a large number of rare and threatened plants.Ex situ Conservationâ In this approach, threatened animals and plantsare taken out from their natural habitat and placed in special settingwhere they can be protected and given special care. Zoological parks,botanical gardens and wildlife safari parks serve this purpose. There aremany animals that have become extinct in the wild but continue to bemaintained in zoological parks. In recent years ex situ conservation hasadvanced beyond keeping threatened species in enclosures. Now gametesof threatened species can be preserved in viable and fertile condition forlong periods using cryopreservation techniques, eggs can be fertilised invitro, and plants can be propagated using tissue culture methods. Seedsof different genetic strains of commercially important plants can be keptfor long periods in seed banks.Biodiversity knows no political boundaries and its conservation istherefore a collective responsibility of all nations. The historic Conventionon Biological Diversity (âThe Earth Summitâ) held in Rio de Janeiro in1992, called upon all nations to take appropriate measures forconservation of biodiversity and sustainable utilisation of its benefits. Ina follow-up, the World Summit on Sustainable Development held in 2002in Johannesburg, South Africa, 190 countries pledged their commitmentto achieve by 2010, a significant reduction in the current rate of biodiversityloss at global, regional and local levels.SUMMARYSince life originated on earth nearly 3.8 billion years ago, there hadbeen enormous diversification of life forms on earth. Biodiversity refersto the sum total of diversity that exists at all levels of biologicalorganisation. Of particular importance is the diversity at genetic, speciesand ecosystem levels and conservation efforts are aimed at protectingdiversity at all these levels.More than 1.5 million species have been recorded in the world, butthere might still be nearly 6 million species on earth waiting to be2022-23268BIOLOGYdiscovered and named. Of the named species, > 70 per cent are animals,of which 70 per cent are insects. The group Fungi has more speciesthan all the vertebrate species combined. India, with about 45,000species of plants and twice as many species of animals, is one of the 12mega diversity countries of the world.Species diversity on earth is not uniformly distributed but showsinteresting patterns. It is generally highest in the tropics and decreasestowards the poles. Important explanations for the species richness ofthe tropics are: Tropics had more evolutionary time; they provide arelatively constant environment and, they receive more solar energywhich contributes to greater productivity. Species richness is alsofunction of the area of a region; the species-area relationship is generallya rectangular hyperbolic function.It is believed that communities with high diversity tend to be lessvariable, more productive and more resistant to biological invasions.Earthâs fossil history reveals incidence of mass extinctions in the past,but the present rates of extinction, largely attributed to human activities,are 100 to 1000 times higher. Nearly 700 species have become extinctin recent times and more than 15,500 species (of which > 650 are fromIndia) currently face the threat of extinction. The causes of highextinction rates at present include habitat (particularly forests) lossand fragmentation, over -exploitation, biological invasions andco-extinctions.Earthâs rich biodiversity is vital for the very survival of mankind.The reasons for conserving biodiversity are narrowly utilitarian, broadlyutilitarian and ethical. Besides the direct benefits (food, fibre, firewood,pharmaceuticals, etc.), there are many indirect benefits we receivethrough ecosystem services such as pollination, pest control, climatemoderation and flood control. We also have a moral responsibility totake good care of earthâs biodiversity and pass it on in good order to ournext generation.Biodiversity conservation may be in situ as well as ex situ. In in situconservation, the endangered species are protected in their naturalhabitat so that the entire ecosystem is protected. Recently, 34âbiodiversity hotspotsâ in the world have been proposed for intensiveconservation efforts. Of these, three (Western Ghats-Sri Lanka,Himalaya and Indo-Burma) cover Indiaâs rich biodiversity regions. Ourcountryâs in situ conservation efforts are reflected in its 14 biospherereserves, 90 national parks, > 450 wildlife sanctuaries and many sacredgroves. Ex situ conservation methods include protective maintenanceof threatened species in zoological parks and botanical gardens, in vitrofertilisation, tissue culture propagation and cryopreservation ofgametes.EXERCISES1. Name the three important components of biodiversity.2. How do ecologists estimate the total number of species present in theworld?2022-23269BIODIVERSITY AND CONSERVATION3. Give three hypotheses for explaining why tropics show greatest levelsof species richness.4. What is the significance of the slope of regression in a species â arearelationship?5. What are the major causes of species losses in a geographical region?6. How is biodiversity important for ecosystem functioning?7. What are sacred groves? What is their role in conservation?8. Among the ecosystem services are control of floods and soil erosion.How is this achieved by the biotic components of the ecosystem?9. The species diversity of plants (22 per cent) is much less than that ofanimals (72 per cent). What could be the explanations to how animalsachieved greater diversification?10. Can you think of a situation where we deliberately want to make aspecies extinct? How would you justify it?2022-23Human population size has grown enormously over thelast hundred years. This means increase in demand forfood, water, home, electricity, roads, automobiles andnumerous other commodities. These demands are exertingtremendous pressure on our natural resources, and arealso contributing to pollution of air, water and soil. Theneed of the hour is to check the degradation and depletionof our precious natural resources and pollution withouthalting the process of development.Pollution is any undesirable change in physical,chemical or biological characteristics of air, land, water orsoil. Agents that bring about such an undesirable changeare called as pollutants. In order to control environmentalpollution, the Government of India has passed theEnvironment (Protection) Act, 1986 to protectand improve the quality of our environment (air, waterand soil).16.1 AIR POLLUTION AND ITS CONTROLWe are dependent on air for our respiratory needs. Airpollutants cause injury to all living organisms. Theyreduce growth and yield of crops and cause prematuredeath of plants. Air pollutants also deleteriously affect therespiratory system of humans and of animals. HarmfulCHAPTER 16ENVIRONMENTAL ISSUES16.1 Air Pollution and ItsControl16.2 Water Pollution and ItsControl16.3 Solid Wastes16.4 Agro-chemicals andtheir Effects16.5 Radioactive Wastes16.6 Greenhouse Effect andGlobal Warming16.7 Ozone Depletion in theStratosphere16.8 Degradation by ImproperResource Utilisation andMaintenance16.9 Deforestation2022-23271ENVIRONMENTAL ISSUESeffects depend on the concentration of pollutants, duration of exposureand the organism.Smokestacks of thermal power plants, smelters and other industriesrelease particulate and gaseous air pollutants together with harmlessgases, such as nitrogen, oxygen, etc. These pollutants must be separated/filtered out before releasing the harmless gases into the atmosphere.Figure 16.1 Electrostatic precipitatorThere are several ways of removing particulate matter; the most widelyused of which is the electrostatic precipitator (Figure 16.1), which canremove over 99 per cent particulate matter present in the exhaust from athermal power plant. It has electrode wires that are maintained at severalthousand volts, which produce a corona that releases electrons. Theseelectrons attach to dust particles giving them a net negative charge. Thecollecting plates are grounded and attract the charged dust particles.The velocity of air between the plates must be low enough to allow thedust to fall. A scrubber (Figure 16.1) can remove gases like sulphurdioxide. In a scrubber, the exhaust is passed through a spray of water orlime. Recently we have realised the dangers of particulate matter that arevery very small and are not removed by these precipitators. According toCentral Pollution Control Board (CPCB), particulate size 2.5 micrometersor less in diameter (PM 2.5) are responsible for causing the greatest harmto human health. These fine particulates can be inhaled deep into thelungs and can cause breathing and respiratory symptoms, irritation,inflammations and damage to the lungs and premature deaths.2022-23272BIOLOGYAutomobiles are a major cause for atmospheric pollution atleast inthe metro cities. As the number of vehicles increase on the streets, thisproblem is now shifting to the other cities too. Proper maintenance ofautomobiles along with use of lead-free petrol or diesel can reduce thepollutants they emit. Catalytic converters, having expensive metals namelyplatinum-palladium and rhodium as the catalysts, are fitted intoautomobiles for reducing emission of poisonous gases. As the exhaustpasses through the catalytic converter, unburnt hydrocarbons areconverted into carbon dioxide and water, and carbon monoxide and nitricoxide are changed to carbon dioxide and nitrogen gas, respectively. Motorvehicles equipped with catalytic converter should use unleaded petrolbecause lead in the petrol inactivates the catalyst.In India, the Air (Prevention and Control of Pollution) Act cameinto force in 1981, but was amended in 1987 to include noise as an airpollutant. Noise is undesired high level of sound. We have got used toassociating loud sounds with pleasure and entertainment not realisingthat noise causes psychological and physiological disorders in humans.The bigger the city, the bigger the function, the greater the noise!! Abrief exposure to extremely high sound level, 150 dB or more generatedby take off of a jet plane or rocket, may damage ear drums thuspermanently impairing hearing ability. Even chronic exposure to arelatively lower noise level of cities may permanently damage hearingabilities of humans. Noise also causes sleeplessness, increased heartbeat, altered breathing pattern, thus considerably stressing humans.Considering the many dangerous effects of noise pollution can youidentify the unnecessary sources of noise pollution around you whichcan be reduced immediately without any financial loss to anybody?Reduction of noise in our industries can be affected by use of soundabsorbentmaterials or by muffling noise. Stringent following of laws laiddown in relation to noise like delimitation of horn-free zones aroundhospitals and schools, permissible sound-levels of crackers and of loudspeakers,timings after which loudspeakers cannot be played, etc., needto be enforced to protect ourselves from noise pollution.16.1.1 Controlling Vehicular Air Pollution: A CaseStudy of DelhiWith its very large population of vehicular traffic, Delhi leads the countryin its levels of air-pollution â it has more cars than the states ofGujarat and West Bengal put together. In the 1990s, Delhi rankedfourth among the 41 most polluted cities of the world. Air pollutionproblems in Delhi became so serious that a public interest litigation(PIL) was filed in the Supreme Court of India. After being censured verystrongly by the Supreme Court, under its directives, the governmentwas asked to take, within a specified time period, appropriate measures,including switching over the entire fleet of public transport, i.e.,buses, from diesel to compressed natural gas (CNG). All the buses ofDelhi were converted to run on CNG by the end of 2002. You may askthe question as to why CNG is better than diesel. The answer is that2022-23273ENVIRONMENTAL ISSUESCNG burns most efficiently, unlike petrol or diesel, in the automobilesand very little of it is left unburnt. Moreover, CNG is cheaper than petrolor diesel, cannot be siphoned off by thieves and adulterated like petrolor diesel. The main problem with switching over to CNG is the difficultyof laying down pipelines to deliver CNG through distribution points/pumps and ensuring uninterrupted supply. Simultaneously parallelsteps taken in Delhi for reducing vehicular pollution include phasingout of old vehicles, use of unleaded petrol, use of low-sulphur petroland diesel, use of catalytic converters in vehicles, application of stringentpollution-level norms for vehicles, etc.The Government of India through a new auto fuel policy has laidout a roadmap to cut down vehicular pollution in Indian cities. Morestringent norms for fuels means steadily reducing the sulphur andaromatic content in petrol and diesel fuels. Euro III norms, for example,stipulate that sulphur be controlled at 350 parts-per-million (ppm) indiesel and 150 ppm in petrol. Aromatic hydrocarbons are to be containedat 42 per cent of the concerned fuel. The goal, according to the roadmap,is to reduce sulphur to 50 ppm in petrol and diesel and bring down thelevel to 35 per cent. Corresponding to the fuel, vehicle engines will alsoneed to be upgraded.Mass Emission Standards (Bharat Stage II which is equivalent toEuro-II norms) are no more applicable in any of the cities of India.Details of the latest Mass Emission Standards in India are providedbelow (Table 16.1)Type of Vehicles Norms Cities of Implementation4 Wheelers Bharat Stage IV Throughout the countrysince April 20173 Wheelers Bharat Stage IV Throughout the countrysince 1st April 20172 Wheelers Bharat Stage IV Throughout the countrysince April 2017Table 16.1: Table Showing the Mass Emission Standards in IndiaThanks to the efforts made, the air quality of Delhi has significantlyimproved. According to an estimate, a substantial fall in CO2 and SO2level has been found in Delhi between 1997 and 2005.16.2 WATER POLLUTION AND ITS CONTROLHuman beings have been abusing the water-bodies around the world bydisposing into them all kinds of waste. We tend to believe that water canwash away everything not taking cognizance of the fact that the waterbodies are our lifeline as well as that of all other living organisms. Canyou list what all we tend to try and wash away through our rivers anddrains? Due to such activities of human kind, the ponds, lakes, stream,2022-23274BIOLOGYrivers, estuaries and oceans are becoming polluted in several parts of theworld. Realising the importance of maintaining the cleanliness of the waterbodies, the Government of India has passed the Water (Prevention andControl of Pollution) Act, 1974 to safeguard our water resources.16.2.1 Domestic Sewage and Industrial EffluentsAs we work with water in our homes in the cities and towns, we washeverything into drains. Have youever wondered where the sewagethat comes out of our houses go?What happens in villages? Is thesewage treated before beingtransported to the nearest riverand mixed with it? A mere 0.1per cent impurities makedomestic sewage unfit for humanuse (Figure 16.2). You have readabout sewage treatmentplants in Chapter 10. Solids arerelatively easy to remove, whatis most difficult to remove areFigure 16.2 Composition of waste waterFigure 16.3 Effect of sewage discharge on some important characteristics of a river2022-23275ENVIRONMENTAL ISSUESdissolved salts such as nitrates, phosphates, and other nutrients, andtoxic metal ions and organic compounds. Domestic sewage primarilycontains biodegradable organic matter, which readily decomposes âthanks to bacteria and other micro-organisms, which can multiply usingthese organic substances as substrates and hence utilise some of thecomponents of sewage. It is possible to estimate the amount ofbiodegradable organic matter in sewage water by measuring BiochemicalOxygen Demand (BOD). Can you explain how? In the chapter on microorganismsyou have read about the relation between BOD, microorganismsand the amount of biodegradable matter.Figure 16.3 shows some of the changes that one may notice followingdischarge of sewage into a river. Micro-organisms involved inbiodegradation of organic matter in the receiving water body consume alot of oxygen, and as a result there is a sharp decline in dissolved oxygendownstream from the point of sewage discharge. This causes mortality offish and other aquatic creatures.Presence of large amounts of nutrients in waters also causes excessivegrowth of planktonic (free-floating) algae, called an algal bloom(Figure 16.4) which imparts a distinct colour to the water bodies. Algalblooms cause deterioration of the water quality and fish mortality. Somebloom-forming algae are extremely toxic to human beings and animals.You may have seen the beautiful mauve-colored flowers found onvery appealingly-shaped floating plants in water bodies. These plantswhich were introduced into India for their lovely flowers have caused havocby their excessive growth by causing blocks in our waterways. They growfaster than our ability to remove them. These are plants of water hyacinth(Eichhornia crassipes), the worldâs most problematic aquatic weed, alsoFigure 16.4 Pictorial view of an algal bloom2022-23276BIOLOGYcalled â Terror of Bengalâ. They grow abundantly ineutrophic water bodies, and lead to an imbalance in theecosystem dynamics of the water body.Sewage from our homes as well as from hospitals arelikely to contain many undesirable pathogenic microorganisms,and its disposal into a water without propertreatment may cause outbreak of serious diseases, suchas, dysentery, typhoid, jaundice, cholera, etc.Unlike domestic sewage, waste water from industrieslike petroleum, paper manufacturing, metal extraction andprocessing, chemical manufacturing, etc., often containtoxic substances, notably, heavy metals (defined aselements with density > 5 g/cm3 such as mercury,cadmium, copper, lead, etc.) and a variety of organiccompounds.A few toxic substances, often present in industrialwaste waters, can undergo biological magnification(Biomagnification) in the aquatic food chain.Biomagnification refers to increase in concentration ofthe toxicant at successive trophic levels. This happensbecause a toxic substance accumulated by an organismcannot be metabolised or excreted, and is thus passed onto the next higher trophic level. This phenomenon is wellknownfor mercury and DDT. Figure 16.5 showsbiomagnification of DDT in an aquatic food chain. In thismanner, the concentration of DDT is increased atsuccessive trophic levels; say if it starts at 0.003 ppb(ppb = parts per billion) in water, it can ultimately reach25 ppm (ppm = parts per million) in fish-eating birds,through biomagnification. High concentrations of DDTdisturb calcium metabolism in birds, which causesthinning of eggshell and their premature breaking,eventually causing decline in bird populations.Eutrophication is the natural aging of a lake bynutrient enrichment of its water. In a young lake the water is cold andclear, supporting little life. With time, streams draining into the lakeintroduce nutrients such as nitrogen and phosphorus, which encouragethe growth of aquatic organisms. As the lakeâs fertility increases, plantand animal life burgeons, and organic remains begin to be deposited onthe lake bottom. Over the centuries, as silt and organic debris pile up, thelake grows shallower and warmer, with warm-water organismssupplanting those that thrive in a cold environment. Marsh plants takeroot in the shallows and begin to fill in the original lake basin. Eventually,the lake gives way to large masses of floating plants (bog), finally convertinginto land. Depending on climate, size of the lake and other factors, theFigure 16.5 Biomagnification ofDDT in an aquaticfood chain2022-23277ENVIRONMENTAL ISSUESnatural aging of a lake may span thousands of years. However, pollutantsfrom manâs activities like effluents from the industries and homes canradically accelerate the aging process. This phenomenon has been calledCultural or Accelerated Eutrophication. During the past century, lakesin many parts of the earth have been severely eutrophied by sewage andagricultural and industrial wastes. The prime contaminants are nitratesand phosphates, which act as plant nutrients. They overstimulate thegrowth of algae, causing unsightly scum and unpleasant odours, androbbing the water of dissolved oxygen vital to other aquatic life. At thesame time, other pollutants flowing into a lake may poison wholepopulations of fish, whose decomposing remains further deplete thewaterâs dissolved oxygen content. In such fashion, a lake can literallychoke to death.Heated (thermal) wastewaters flowing out of electricity-generating units,e.g., thermal power plants, constitute another important category ofpollutants. Thermal wastewater eliminates or reduces the number oforganisms sensitive to high temperature, and may enhance the growth ofplants and fish in extremely cold areas but, only after causing damage tothe indigenous flora and fauna.16.2.2 A Case Study of Integrated Waste Water TreatmentWastewater including sewage can be treated in an integrated manner, byutilising a mix of artificial and natural processes. An example of such aninitiative is the town of Arcata, situated along the northern coast ofCalifornia. Collaborating with biologists from the HumboldtState University, the townspeople created an integrated waste watertreatment process within a natural system. The cleaning occurs in twostages â (a) the conventional sedimentation, filtering and chlorinetreatments are given. After this stage, lots of dangerous pollutants likedissolved heavy metals still remain. To combat this, an innovativeapproach was taken and (b) the biologists developed a series of sixconnected marshes over 60 hectares of marshland. Appropriate plants,algae, fungi and bacteria were seeded into this area, which neutralise,absorb and assimilate the pollutants. Hence, as the water flows throughthe marshes, it gets purified naturally.The marshes also constitute a sanctuary, with a high level ofbiodiversity in the form of fishes, animals and birds that now reside there.A citizens group called Friends of the Arcata Marsh (FOAM) are responsiblefor the upkeep and safeguarding of this wonderful project. .All this time, we have assumed that removal of wastes requires water,i.e., the creation of sewage. But what if water is not necessary to disposeoff human waste, like excreta? Can you imagine the amount of water thatone can save if one didnât have to flush the toilet? Well, this is already areality. Ecological sanitation is a sustainable system for handling human2022-23278BIOLOGYexcreta, using dry composting toilets. This is a practical, hygienic, efficientand cost-effective solution to human waste disposal. The key point tonote here is that with this composting method, human excreta can berecycled into a resource (as natural fertiliser), which reduces the need forchemical fertilisers. There are working âEcoSanâ toilets in many areas ofKerala and Sri Lanka.16.3 SOLID WASTESSolid wastes refer to everything that goes out in trash. Municipal solidwastes are wastes from homes, offices, stores, schools, hospitals, etc.,that are collected and disposed by the municipality. The municipal solidwastes generally comprise paper, food wastes, plastics, glass, metals,rubber, leather, textile, etc. Burning reduces the volume of the wastes,although it is generally not burnt to completion and open dumps oftenserve as the breeding ground for rats and flies. Sanitary landfills wereadopted as the substitute for open-burning dumps. In a sanitary landfill,wastes are dumped in a depression or trench after compaction, andcovered with dirt everyday. If you live in a town or city, do you knowwhere the nearest landfill site is? Landfills are also not really much of asolution since the amount of garbage generation specially in the metroshas increased so much that these sites are getting filled too. Also thereis danger of seepage of chemicals, etc., from these landfills polluting theunderground water resources.A solution to all this can only be in human beings becoming moresensitive to these environment issues. All waste that we generate canbe categorised into three types â (a) bio-degradable, (b) recyclable and(c) the non-biodegradable. It is important that all garbage generated issorted. What can be reused or recycled should be separated out; ourkabadiwallahs and rag-pickers do a great job of separation of materialsfor recycling. The biodegradable materials can be put into deep pits inthe ground and be left for natural breakdown. That leaves only the nonbiodegradableto be disposed off . The need to reduce our garbagegeneration should be a prime goal, instead, we are increasing the use ofnon-biodegradable products. Just pick any readymade packet of anyâgood qualityâ eatable, say a biscuit packet, and study the packaging âdo you see the number of protective layers used? Note that atleast onelayer is of plastic. We have started packaging even our daily use productslike milk and water in polybags!! In cities, fruits and vegetables can bebought packed in beautiful polysterene and plastic packaging â we payso much and what do we do? Contribute heavily to environmentalpollution. State Governments across the country are trying to push forreduction in use of plastics and use of eco-friendly packaging. We can doour bit by carrying cloth or other natural fibre carry-bags when we goshopping and by refusing polythene bags.2022-23279ENVIRONMENTAL ISSUESHospitals generate hazardous wastes that contain disinfectants andother harmful chemicals, and also pathogenic micro-organisms. Suchwastes also require careful treatment and disposal. The use of incineratorsis crucial to disposal of hospital waste.Irreparable computers and other electronic goods are known aselectronic wastes (e-wastes). E-wastes are burried in landfills orincinerated. Over half of the e-wastes generated in the developed worldare exported to developing countries, mainly to China, India and Pakistan,where metals like copper, iron, silicon, nickel and gold are recoveredduring recycling process. Unlike developed countries, which havespecifically built facilities for recycling of e-wastes, recycling in developingcountries often involves manual participation thus exposing workers totoxic substances present in e-wastes. Recycling is the only solution forthe treatment of e-waste, provided it is carried out in an environmentfriendlymanner.16.3.1 Case Study of Remedy for Plastic WasteA plastic sack manufacturer in Bangalore has managed to find the idealsolution to the ever-increasing problem of accumulating plastic waste.Ahmed Khan, aged 57 years old, has been producing plastic sacks for20 years. About 8 years ago, he realised that plastic waste was a realproblem. Polyblend, a fine powder of recycled modified plastic, wasdeveloped then by his company. This mixture is mixed with the bitumenthat is used to lay roads. In collaboration with R.V.College of Engineeringand the Bangalore City Corporation, Ahmed Khan proved that blends ofPolyblend and bitumen, when used to lay roads, enhanced the bitumenâswater repellant properties, and helped to increase road life by a factor ofthree. The raw material for creating Polyblend is any plastic film waste.So, against the price of Rs. 0.40 per kg that rag pickers had been gettingfor plastic waste, Khan now offers Rs.6. Using Khanâs technique, by theyear 2002, more than 40 kms of road in Bangalore has already beenlaid. At this rate, Khan will soon be running short of plastic waste inBangalore, to produce Polyblend. Thanks to innovations like Polyblend,we might still avoid being smothered by plastic waste.16.4 AGRO-CHEMICALS AND THEIR EFFECTSIn the wake of green revolution, use of inorganic fertilisers and pesticideshas increased manifold for enhancing crop production. Pesticides,herbicides, fungicides, etc., are being increasingly used. These incidentally,are also toxic to non-target organisms,that are important components ofthe soil ecosystem. Do you think these can be biomagnified in the terrestrialecosystems? We know what the addition of increasing amounts ofchemical fertilisers can do to aquatic ecosystems vis-à -vis eutrophication.The current problems in agriculture are, therefore, extremely grave.2022-23280BIOLOGY16.4.1 Case Study of Organic FarmingIntegrated organic farming is a cyclical, zero-waste procedure, where wasteproducts from one process are cycled in as nutrients for other processes.This allows the maximum utilisation of resource and increases theefficiency of production. Ramesh Chandra Dagar, a farmer in Sonipat,Haryana, is doing just this. He includes bee-keeping, dairy management,water harvesting, composting and agriculture in a chain of processes,which support each other and allow an extremely economical andsustainable venture. There is no need to use chemical fertilisers for crops,as cattle excreta (dung) are used as manure. Crop waste is used to createcompost, which can be used as a natural fertiliser or can be used togenerate natural gas for satisfying the energy needs of the farm.Enthusiastic about spreading information and help on the practice ofintegrated organic farming, Dagar has created the Haryana Kisan WelfareClub, with a current membership of 5000 farmers.16.5 RADIOACTIVE WASTESInitially, nuclear energy was hailed as a non-polluting way for generatingelectricity. Later on, it was realised that the use of nuclear energy has twovery serious inherent problems. The first is accidental leakage, as occurredin the Three Mile Island and Chernobyl incidents and the second is safedisposal of radioactive wastes.Radiation, that is given off by nuclear waste is extremely damaging toorganisms, because it causes mutations at a very high rate. At high doses,nuclear radiation is lethal but at lower doses, it creates various disorders,the most frequent of all being cancer. Therefore, nuclear waste is anextremely potent pollutant and has to be dealt with utmost caution.It has been recommended that storage of nuclear waste, aftersufficient pre-treatment, should be done in suitably shieldedcontainers buried within the rocks, about 500 m deep below theearthâs surface. However, this method of disposal is meeting stiffopposition from the public. Why do you think this method ofdisposal is not agreeable to many people?16.6 GREENHOUSE EFFECT AND GLOBAL WARMINGThe term âGreenhouse effectâ has been derived from a phenomenon thatoccurs in a greenhouse. Have you ever seen a greenhouse? It looks like asmall glass house and is used for growing plants especially during winter.In a greenhouse the glass panel lets the light in, but does not allow heatto escape. Therefore, the greenhouse warms up, very much like inside acar that has been parked in the sun for a few hours.The greenhouse effect is a naturally occurring phenomenon that isresponsible for heating of Earthâs surface and atmosphere. You would be2022-23281ENVIRONMENTAL ISSUESsurprised to know that without greenhouse effect the average temperatureat surface of Earth would have been a chilly â18oC rather than the presentaverage of 15oC. In order to understand thegreenhouse effect, it is necessary to know thefate of the energy of sunlight that reaches theoutermost atmosphere (Figure16.6). Cloudsand gases reflect about one-fourth of theincoming solar radiation, and absorb some ofit but almost half of incoming solar radiationfalls on Earthâs surface heating it, while a smallproportion is reflected back. Earthâs surfacere-emits heat in the form of infrared radiationbut part of this does not escape into space asatmospheric gases (e.g., carbon dioxide,methane, etc.) absorb a major fraction of it. Themolecules of these gases radiate heat energy,and a major part of which again comes toEarthâs surface, thus heating it up once again.This cycle is repeated many a times. Theabove-mentioned gases â carbon dioxide and methane â are commonlyknown as greenhouse gases (Figure 16.7) because they are responsiblefor the greenhouse effect.Increase in the level of greenhouse gases has led to considerable heatingof Earth leading to global warming. During the past century, thetemperature of Earth has increased by 0.6 oC, most of it during the lastFigure 16.7 Relative contribution of variousgreenhouse gases to total globalwarmingFigure 16.6 Sunlight energy at the outermost atmosphere2022-23282BIOLOGYFigure 16.8 Ozone hole is the area aboveAntarctica, shown in purplecolour, where the ozone layeris the thinnest. Ozonethickness is given in Dobsonunit (see carefully the scaleshown in colour violet to red).The ozone hole over Antarcticadevelops each year betweenlate August and earlyOctober. Courtesy: NASAthree decades. Scientists believe that this rise in temperature is leadingto deleterious changes in the environment and resulting in odd climaticchanges (e.g. El Nino effect) , thus leading to increased melting of polarice caps as well as of other places like the Himalayan snow caps. Overmany years, this will result in a rise in sea level that can submerge manycoastal areas. The total spectrum of changes that global warming canbring about is a subject that is still under active research.How can we control global warming? The measures include cuttingdown use of fossil fuel, improving efficiency of energy usage, reducingdeforestation, planting trees and slowing down the growth of humanpopulation. International initiatives are also being taken to reduce theemission of greenhouse gases into the atmosphere.16.7 OZONE DEPLETION IN THE STRATOSPHEREYou have earlier studied in the Chemistrytextbook of Class XI about âbadâ ozone, formedin the lower atmosphere (troposphere) that harmsplants and animals. There is âgoodâ ozone also;this ozone is found in the upper part of theatmosphere called the stratosphere, and it actsas a shield absorbing ultraviolet radiation fromthe sun. UV rays are highly injurious to livingorganisms since DNA and proteins of livingorganisms preferentially absorb UV rays, and itshigh energy breaks the chemical bonds withinthese molecules. The thickness of the ozone in acolumn of air from the ground to the top of theatmosphere is measured in terms of Dobsonunits (DU).Ozone gas is continuously formed by theaction of UV rays on molecular oxygen, and alsodegraded into molecular oxygen in thestratosphere. There should be a balance betweenproduction and degradation of ozone in thestratosphere. Of late, the balance has beendisrupted due to enhancement of ozonedegradation by chlorofluorocarbons (CFCs).CFCs find wide use as refrigerants. CFCs discharged in the lower part ofatmosphere move upward and reach stratosphere. In stratosphere, UVrays act on them releasing Cl atoms. Cl degrades ozone releasingmolecular oxygen, with these atoms acting merely as catalysts; Cl atomsare not consumed in the reaction. Hence, whatever CFCs are added tothe stratosphere, they have permanent and continuing effects on Ozone2022-23283ENVIRONMENTAL ISSUESlevels. Although ozone depletion is occurring widely in the stratosphere,the depletion is particularly marked over the Antarctic region. This hasresulted in formation of a large area of thinned ozone layer, commonlycalled as the ozone hole (Figure 16.8).UV radiation of wavelengths shorter than UV-B, are almost completelyabsorbed by Earthâs atmosphere, given that the ozone layer is intact. But,UV-B damages DNA and mutation may occur. It causes aging of skin,damage to skin cells and various types of skin cancers. In human eye,cornea absorbs UV-B radiation, and a high dose of UV-B causesinflammation of cornea, called snow-blindness, cataract, etc. Suchexposure may permanently damage the cornea.Recognising the deleterious affects of ozone depletion, an internationaltreaty, known as the Montreal Protocol, was signed at Montreal (Canada)in 1987 (effective in 1989) to control the emission of ozone depletingsubstances. Subsequently many more efforts have been made andprotocols have laid down definite roadmaps, separately for developed anddeveloping countries, for reducing the emission of CFCs and other ozonedepleting chemicals.16.8 DEGRADATION BY IMPROPER RESOURCE UTILISATIONAND MAINTENANCEThe degradation of natural resources can occur, not just by the action ofpollutants but also by improper resource utilisation practices.Soil erosion and desertification: The development of the fertile top-soiltakes centuries. But, it can be removed very easily due to human activitieslike over-cultivation, unrestricted grazing, deforestation and poorirrigation practices, resulting in arid patches of land. When large barrenpatches extend and meet over time, a desert is created. Internationally, ithas been recognised that desertification is a major problem nowadays,particularly due to increased urbanisation.Waterlogging and soil salinity: Irrigation without proper drainage ofwater leads to waterlogging in the soil. Besides affecting the crops,waterlogging draws salt to the surface of the soil. The salt then is depositedas a thin crust on the land surface or starts collecting at the roots of theplants. This increased salt content is inimical to the growth of crops andis extremely damaging to agriculture. Waterlogging and soil salinity aresome of the problems that have come in the wake of the Green Revolution.16.9 DEFORESTATIONDeforestation is the conversion of forested areas to non-forested ones.According to an estimate, almost 40 per cent forests have been lost in thetropics, compared to only 1 per cent in the temperate region. The presentscenario of deforestation is particularly grim in India. At the beginning of2022-23284BIOLOGYthe twentieth century, forests covered about 30 per cent of the land ofIndia. By the end of the century, it shrunk to 21.54 per cent, whereas theNational Forest Policy (1988) of India has recommended 33 per cent forestcover for the plains and 67 per cent for the hills.How does deforestation occur? A number of human activitiescontribute to it. One of the major reasons is the conversion of forest toagricultural land so as to feed the growing human population. Trees areaxed for timber, firewood, cattle ranching and for several other purposes.Slash and burn agriculture, commonly called as Jhum cultivation inthe north-eastern states of India, has also contributed to deforestation.In slash and burn agriculture, the farmers cut down the trees of the forestand burn the plant remains. The ash is used as a fertiliser and the land isthen used for farming or cattle grazing. After cultivation, the area is leftfor several years so as to allow its recovery. The farmers then move on toother areas and repeat this process. In earlier days, when Jhum cultivationwas in prevalence, enough time-gap was given so that the land recoveredfrom the effect of cultivation. With increasing population, and repeatedcultivation, this recovery phase is done away with, resulting indeforestation.What are the consequences of deforestation? One of the major effectsis enhanced carbon dioxide concentration in the atmosphere becausetrees that could hold a lot of carbon in their biomass are lost withdeforestation. Deforestation also causes loss of biodiversity due to habitatdestruction, disturbs hydrologic cycle, causes soil erosion, and may leadto desertification in extreme cases.Reforestation is the process of restoring a forest that once existedbut was removed at some point of time in the past. Reforestation mayoccur naturally in a deforested area. However, we can speed it up byplanting trees with due consideration to biodiversity that earlier existedin that area.16.9.1 Case Study of Peopleâs Participation inConservation of ForestsPeopleâs participation has a long history in India. In 1731, the king ofJodhpur in Rajasthan asked one of his ministers to arrange wood forconstructing a new palace. The minister and workers went to a forestnear a village, inhabited by Bishnois, to cut down trees. The Bishnoicommunity is known for its peaceful co-existence with nature. The effortto cut down trees by the kings was thwarted by the Bishnois. A Bishnoiwoman Amrita Devi showed exemplary courage by hugging a tree anddaring kingâs men to cut her first before cutting the tree. The tree matteredmuch more to her than her own life. Sadly, the kingâs men did not heed toher pleas, and cut down the tree along with Amrita Devi. Her threedaughters and hundreds of other Bishnois followed her, and thus losttheir lives saving trees. Nowhere in history do we find a commitment of2022-23285ENVIRONMENTAL ISSUESSUMMARYMajor issues relating to environmental pollution and depletion ofvaluable natural resources vary in dimension from local, regional toglobal levels. Air pollution primarily results from burning of fossil fuel,e.g., coal and petroleum, in industries and in automobiles. They areharmful to humans, animals and plants, and therefore must be removedto keep our air clean. Domestic sewage, the most common source ofpollution of water bodies, reduces dissolved oxygen but increasesbiochemical oxygen demand of receiving water. Domestic sewage is richin nutrients, especially, nitrogen and phosphorus, which causeeutrophication and nuisance creating algal blooms. Industrial wastewaters are often rich in toxic chemicals, especially heavy metals andorganic compounds. Industrial waste waters harm living organisms.Municipal solid wastes also create problems and must be disposed offin landfills. Disposal of hazardous wastes like defunct ships, radioactivewastes and e-wastes requires additional efforts. Soil pollution primarilyresults from agricultural chemicals (e.g., pesticides) and leachates fromsolid wastes deposited over it.Two major environmental issues of global nature are increasinggreenhouse effect, which is warming Earth, and depletion of ozone inthe stratosphere. Enhanced greenhouse effect is mainly due toincreased emission of carbon dioxide, methane, nitrous oxide and CFCs.,and also due to deforestation. It may drastically change rainfall pattern,global temperature, besides deleteriously affecting living organisms.Ozone in the stratosphere, which protects us from harmful effects ofultraviolet radiation, is depleting fast due to emission of CFCs thusincreasing the risks of skin cancer, mutation and other disorders.this magnitude when human beings sacrificed their lives for the cause ofthe environment. The Government of India has recently instituted theAmrita Devi Bishnoi Wildlife Protection Award for individuals orcommunities from rural areas that have shown extraordinary courageand dedication in protecting wildlife.You may have heard of the Chipko Movement of Garhwal Himalayas.In 1974, local women showed enormous bravery in protecting trees fromthe axe of contractors by hugging them. People all over the world haveacclaimed the Chipko movement.Realising the significance of participation by local communities,the Government of India in 1980s has introduced the concept ofJoint Forest Management (JFM) so as to work closely with the localcommunities for protecting and managing forests. In return for theirservices to the forest, the communities get benefit of various forest products(e.g., fruits, gum, rubber, medicine, etc.), and thus the forest can beconserved in a sustainable manner.2022-23286BIOLOGYEXERCISES1. What are the various constituents of domestic sewage? Discuss theeffects of sewage discharge on a river.2. List all the wastes that you generate, at home, school or during yourtrips to other places. Could you very easily reduce the generation ofthese wastes? Which would be difficult or rather impossible to reduce?3. Discuss the causes and effects of global warming. What measures needto be taken to control global warming?4. Match the items given in column A and B:Column A Column B(a) Catalytic converter (i) Particulate matter(b) Electrostatic precipitator (ii) Carbon monoxide and nitrogen oxides(c) Earmuffs (iii) High noise level(d) Landfills (iv) Solid wastes5. Write critical notes on the following:(a) Eutrophication(b) Biological magnification(c) Groundwater depletion and ways for its replenishment6. Why does ozone hole form over Antarctica? How will enhanced ultravioletradiation affect us?7. Discuss the role of women and communities in protection andconservation of forests.8. What measures, as an individual, would you take to reduceenvironmental pollution?9. Discuss briefly the following:(a) Radioactive wastes(b) Defunct ships and e-wastes(c) Municipal solid wastes10. What initiatives were taken for reducing vehicular air pollution in Delhi?Has air quality improved in Delhi?11. Discuss briefly the following:(a) Greenhouse gases(b) Catalytic converter(c) Ultraviolet B2022-23My Youtube channel https://m.youtube.com/channel/UCiQ29FVhzssHzJK7K2eRNQQ
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