Lecture notes in Botany

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BIOLOGY BOTANY HIGHER SECONDARY - FIRST YEAR REVISED BASED ON THE RECOMMENDATIONS OF THE TEXT BOOK DEVELOPMENT COMMITTEE A Publication Under Government of Tamilnadu Distribution of Free Textbook Programme (NOT FOR SALE) Untouchability is a sin Untouchability is a crime Untouchability is inhuman TAMILNADU TEXTBOOK CORPORATION College Road, Chennai - 600 006.© Government of Tamilnadu First Edition - 2005 Revised Edition - 2007 Chairperson Dr. A. JAFFAR HUSSAIN Head of the Department of Botany (Rtd.) Presidency College (Autonomous) Chennai - 600 005. Authors Reviewers Dr. MUJEERA FATHIMA T. R. A. DEVAKUMAR Senior Scale Lecturer in Botany Selection Grade Lecturer in Botany Government Arts College (Men) Government Arts College (Men) Nandanam, Chennai - 600 035. Nandanam, Chennai - 600 035. NALINI P. RAJAGOVINDAN Dr. V. MURGANANDAM Assistant Head Mistress Reader in Botany Corporation Boys' R.M. Vivekananda College, Higher Secondary School Chennai - 600 004. Saidapet, Chennai - 600 015. N. SHANTHA P.G. Teacher Government Higher Secondary School Kodambakkam, Chennai - 600 024. Price : Rs. 33-00 This book has been prepared by the Directorate of School Education on behalf of the Government of Tamil Nadu. This Book has been printed on 60 GSM PaperPREFACE We are passing through an "Era of Biology". Words like "Biotechnology', 'Bioremediation", "Biochips", "Biomineralization", "Bioinformatics" etc. have become familiar even with "common man". Certainly there is a new unusual never-before- tried approach to address and solve many problems associated with modern life and to enhance the quality and standard of living by application of modern tools of Biology; particularly the Genetically Modified Foods (GM Foods) and other GM Products have revolutionized our Life. Application and exploitation of biological principles has become possible because of extensive knowledge and study of descriptive and functional aspects of living organisms over the years commonly studied under "Biology" which broadly comprises Botany and Zoology. Infact, Botany/Zoology is the 'mother science' of Molecular Genetics, Biochemistry, Microbiology, Molecular Cell Biology, Biochemical Engineering and ultimately Biotechnology, These recent applied fields are natrual outcome of a sound knowledge and study of basic Science, Botany (and of course Zoology). Without the study of structural and functional aspects of Green plants, Fungi, Bacteria, Viruses and their interrelationships, the"modern biology" is not possible. In fact, applied sciences, however 'modern', cannot replace basic sciences. The notion of 'Boring Botany' with its tasks of memorising 'technical terms', drawing diagrams can be dispensed with if only it is realized that application of the discipline BOTANY has unlimited potentialities in our complicated modern life through what is called Modern Biology. In this Book, BIOLOGY (Botany), a sincere attempt is made by my colleagues, on the basis of syllabus placed, to provide a simple and lucid account of Botany at the XI Standard which hopefully could be further extended to XII Standard. Each chapter is discussed with simplicity and clarity with Self-Evaluation at the end of each lesson. While preparing for the examination, students should not restrict themselves to the question/problems given in the Self-Evaluvation, they must be prepared to answer the questions and problems from the entire text. Infact, they are advised to refer to 'Reference Books' listed at the end to further their knowledge. DR. A. JAFFAR HUSSAIN Chairperson Text-Book Writing Committee (XI-Bio-Botany) iiiSYLLABUS : HIGHER SECONDARY - FIRST YEAR BIOLOGY : PART-A BOTANY Unit 1 : Biodiversity (12 Hours) Systematics : Two Kingdom and Five Kingdom systems - Salient features of various Plant Groups : Algae, Fungi, Bryophytes, Pteridophytes and Gymnosperms - Viruses - Bacteria. Unit 2 : Cell Biology (8 Hours) Cell as the basic unit of life - Cell Theory - Prokaryotic and Eukaryotic Cell (Plant Cell) - Light Microscope and Electron Microscope (TEM & SEM) - Ultra Structure of Prokaryotic and Eukaryotic Cells - Cell Wall - Cell Membrane (Fluid Mosaic Model) Membrane Transport Model - Cell organelles : Nucleus, Mitochondria, Plastids, Ribosomes - Cell Divisions : Amitosis, Mitosis & Meiosis and their significance. Unit 3 : Plant Morphology (8 Hours) Structure and Modification of Root, Stem and Leaf - Structure and Types of Inflorescences - Structure and Types of Flowers, Fruits and Seeds. Unit 4 : Genetics (12 Hours) Concept of Heredity and Variation - Mendel’s Laws of Inheritance - Chromosomal basis of Inheritance - Intermediate Inheritance (Incomplete Dominance) - Epistasis. Unit 5 : Plant Physiology (18 Hours) Cell as a Physiological Unit : Properties of Protoplasm - Water relations - Absorption and movement : Diffusion, Osmosis, Plasmolysis, Imbibition - Permeability, Water Potential - Theories of Water Transport : Root Pressure - Transpiration pull - Factors affecting Rate of Transpiration - Mechanism of Stomatal Opening and Closing (Potassium ion theory) - Factors affecting Stomatal Movement - Mineral Nutrition : Functions of minerals - Essential Major elements and Trace elements - Deficiency symptoms of elements - Theories of Translocation - Translocation of Solutes - Nitrogen Metablosim and Biological Nitrogen Fixation. ivUnit 6 : Reproduction Biology (10 Hours) Modes of Reproduction in Angiosperms : Vegetative propagation (natural and artificial) - Micropropagation - Sexual Reproduction - Pollination: Types - Double Fertilization - Germination of seed : Parts of seed - Types of germination - Abscission, Senescence. Unit 7 : Environmental Biology (10 Hours) Organisms and their environment - Factors : Air, Water, Soil, Temperature, Light and Biota - Hydrophytes, Mesophytes, Xerophytes and their adaptations - Natural Resources - types, uses and misuse Conservation of water (RWH). vCONTENTS I. BIODIVERSITY 1-60 1. Systematics 1 2. Viruses 11 3. Bacteria 22 4. Salient features of Various Groups 31 4.1. Fungi 31 4.2. Algae 40 4.3. Bryophytes 48 4.4. Pteridophytes 52 4.5. Spermatophytes (Gymnosperms) 56 II. CELL BIOLOGY 61-105 1. Cell as the basic unit of life 61 2. Cell Theory 65 3. Prokaryotic and Eukaryotic Cell 67 4. Light Microscope and Electron Microscope 72 5. Cell Wall 75 6. Cell Membrane 80 7. Cell organelles 87 8. Cell Division 97 III. PLANT MORPHOLOGY 106-159 1. Root, Stem and Leaf 106 2. Inflorescence 124 3. Flowers, Fruits and Seeds 134 IV. GENETICS 160-186 1. Heredity and Variation 160 2. Mendel’s Laws of Inheritance 164 3. Chromosomal basis of Inheritance 175 4. Intermediate Inheritance 179 5. Epistasis 181 viV . PLANT PHYSIOLOGY 187-228 1. Cell as a Physiological Unit 187 1.a. Properties of Protoplasm 188 1.b. Water relations 190 1.c. Absorption and movement 190 1.d. Permeability and Water Potential 195 2. Water Transport 198 2.a. Transpiration pull 198 2.b. Factors Affecting Rate of Transpiration - 200 2.c. Mechanism of Stomatal Opening and Closing 202 2.d. Factors Affecting Stomatal Movement 204 3. Mineral Nutrition 207 3.a. Functions of Minerals 207 3.b. Essential Elements 209 3.c. Physiological Role and Deficiency Symptoms of Elements 209 3.d. Theories of Translocation 216 3.e. Translocation of Solutes 219 3.f. Nitrogen Metablosim 221 VI. REPRODUCTION BIOLOGY 229-259 1. Reproduction in Angiosperms 229 1.a. Vegetative propagation 229 1.b. Micropropagation 235 2. Sexual Reproduction 237 2.a. Pollination 237 2.b. Double Fertilization 244 3. Germination of Seeds 248 VII. ENVIRONMENTAL BIOLOGY 260-292 1. Organisms and Their Environment 260 2. Hydrophytes, Mesophytes, Xerophytes 269 3. Natural Resources 281 REFERENCES 292 viiBIO - BOTANY CHAPTERS I. BIODIVERSITY 1-65 II. CELL BIOLOGY 66-112 III. PLANT MORPHOLOGY 113-166 IV. GENETICS 167-199 V. PLANT PHYSIOLOGY 200-244 VI. REPRODUCTION BIOLOGY 245-278 VII. ENVIRONMENTAL BIOLOGY 279-311I. BIODIVERSITY 1. Systematics Diversity in living organisms There is a great diversity among living organisms found on the planet earth. They differ in their structure, habit, habitat, mode of nutrition, and physiology. The Biodiversity of the earth is enormous. Current estimates suggest that the earth may have anywhere from 10 to over 40 million species of organisms, but only about 1.7 million have actually been described including over 7,50,000 insects, about 2,50,000 flowering plants and 47,000 vertebrate animals. We call such a diversity among living organisms as Biodiversity. Even though there is such a variety and diversity among them, the living organisms show a lot of similarities and common features so that they can be arranged into many groups. In order to understand them and study them systematically, these living organisms, mainly the plants and animals are grouped under different categories. The branch of biology dealing with identification, naming and classifying the living organisms is known as Taxonomy. Taxonomy in Greek means rendering of order. The word Systematics means to put together. It was Carolus Linnaeus who used this word first in his book ‘Systema Naturae’. Systematics may be defined as the systematic placing of organisms into groups or taxa on the basis of certain relationships between organisms. Need for Classification It is not possible for any one to study all the organisms. But if they are grouped in some convenient way the study would become easier as the characters of a particular group or a family would apply to all the individuals of that group. Classification allows us to understand diversity better. History of Classification rd th In the 3 and 4 century BC Aristotle and others categorized organisms into plants and animals. They even identified a few thousands or more of living organisms. Hippocrates (460-377 BC), the Father of Medicine listed organisms with medicinal value. Aristotle and his student Theophrastus (370-282 BC) made 1the first attempt to classify organisms without stressing their medicinal value. They tried to classify the plants and animals on the basis of their form and habitat. It was followed by Pliny the Elder (23-79 AD) who introduced the first artificial system of classification in his book ‘Historia Naturalis’. John Ray an English naturalist introduced the term species for the first time for any kind of living th things. It was then Carolus Linnaeus the Swedish naturalist of 18 century now known as Father of Taxonomy developed the Binomial System of nomenclature which is the current scientific system of naming the species. In his famous book ‘Species Plantarum’(1753) he described 5,900 species of plants and in “systema Naturae’(1758) he described 4200 species of animals. Taxonomy and Phylogeny Taxonomy is the branch of biology that deals with identification and nomenclature (naming) of living organisms and their classification on the basis of their similarities and differences. It was the Swiss-French botanist Augustin- Pyramus de Candolle(1778-1841) who coined the word Taxonomy, the science of naming and classifying of organisms. Species Species is the basic unit of Classification. It is defined as the group of individuals which resemble in their morphological and reproductive characters and interbreed among themselves and produce fertile offsprings. Species are then grouped into more inclusive taxa, which are grouped into larger taxa so that the classification is a hierarchy of a system of units that increase in inclusiveness from each level to the next higher level. The seven main categories used in any plan of classification are given below. 1. Kingdom 2.Phylum or Division 3.Class 4.Order 5.Family 6.Genus 7.Species Phylogeny The evolutionary history of a particular taxon like species is called phylogeny. The classification based on the basis of evolution is called phylogenetic classification. Phylogenetic classification is not always possible since there are 2several gaps in the fossil records which form the basis of phylogenetic studies and also evolution is never unidirectional. Classification not explicitly based on evolutionary relationships is called artificial, for example, organisms are grouped according to usefulness (economic plants) size (herbs, shrubs) colour (flowers) ecological role (ground cover) and so-forth. Nevertheless many biologists make use of this non-systematic classification. Two Kingdom System of Classification Carolus Linnaeus(1758) divided all the living organisms into two kingdoms. 1. Kingdom Plantae 2. Kingdom Animalia 1. Kingdom Plantae: This kingdom includes bacteria(Prokaryotes), photosynthetic plants and non - photosynthetic fungi. The characteristic features of this kingdom are: 1. Plants have branches, asymmetrical body with green leaves. 2. Plants are non motile and fixed in a place. 3. During the day time plants more actively involve in photosynthesis than in respiration and hence take more of CO and liberate O & during night O is 2 2 2 taken in and CO is liberated. 2 4. They are autotrophic in their mode of nutrition since they synthesize their own food. 5. Plants have growing points which have unlimited growth. 6. Excretory system and nervous system are absent. 7. Reserve food material is starch. 8. Cells have a cell wall. Cells have a lager vacuole. Plant cells lack centrosome and they may have inorganic crystals. 9. Reproduction takes place with help of agents such as air, water and insects. Asexual and vegetative method of reproduction is also not uncommon. 2. Kingdom Animalia This kingdom includes unicellular protozoans and multi-cellular animals or metazoans. They are characterized by 1. Definite shape of the body and absence of branches. 2. Ability to move from place to place. 3Pteridophtes Algae Spermatophytes Bacteria Fungi Bryophytes j j 3. During day and night take in O and release CO i.e only respiration takes 2 2 place and there is no photosynthesis. 4. Holozoic mode of nutrition since no chlorophylls present and hence they are heterotrophs. 5. Growth is limited in animals. Growth stops after attaining a particular size and age. 6. Excretory system and nervous system are well developed. 7. Reserve food material is glycogen. 8. Lacks cell wall. They have small vacuoles. Centrosomes are present. Cells do not have inorganic crystals. 9. Animals do not depend on any external agents for sexual reproduction. Regeneration of body parts and asexual reproduction is found only in lower organisms. Limitations of Two Kingdom System of Classification The two kingdom system Animalia of Classification proposed by Plantae Linnaeus has been in use for a long time. But later it proved to be inadequate and unsatisfactory in view of new information and discoveries about the lower forms of organisms. The following are the shortcomings of the two kingdom system of classification. 1. Certain organisms share the characteristics of both plants and animals. eg. Euglena Fig: 1.1. Diagrammatic representation of and Sponges. In Two Kingdom System Euglena, some species have chlorophyll and are autotrophic like plants. However like animals they are dependent on an external supply of vitamins B, and B which they 12 cannot synthesize themselves. A few species of Euglena lack chloroplasts and are therefore colourless and non-photosynthetic (heterotrophic). They have a saprotrophic mode of nutrition, carrying out extra-cellular digestion. 4 j j j j j j j j j Chordates InvertebratesOther colorless forms ingest small food particles and carryout intracellular digestion (holozoic nutrition). If green species of Euglena are kept in darkness they lose their chloroplasts and become colourless and survive saprotrophically. Chloroplasts return when the organisms are returned to light. Euglena is also characterized by the presence of an animal pigment astaxanthin in the eye spot. 2. Fungi are a group of organisms which have features of their own. They lack chlorophyll. They are heterotrophic like animals. They are placed along with green plants. 3. Many primitive organisms such as bacteria did not fit into either category and organisms like slime moulds are amoeboid but form fruiting bodies similar to fungi. 4. The status of virus whether they are living or non living is a point of debate even to -day. For all these reasons the two hundred and fifty years old Linnaeus system of classifying organisms into two rigid groups animals and plants is considered highly arbitrary and artificial. The Five Kingdom System of Classification In order to suggest a better system of classification of living organisms, R.H. Whittaker (1969) an American Taxonomist divided all the organisms into 5 kingdoms based on their phylogenetic relationships. This classification takes into account the following important criteria. 1. Complexity of Cell structure – prokaryote to Eukaryote 2. Mode of nutrition – autotrophs and heterotrophs 3. Body organization -unicellular or multi-cellular 4. Phylogenetic or evolutionary relationship The Five kingdoms are Monera, Protista, Fungi, Plantae and Animalia. 1. Monera The Kingdom of Prokaryotes This kingdom includes all prokaryotic organisms i.e. mycoplasma, bacteria, actinomycetes(filamentous bacteria) and cyanobacteria (blue green Algae). They show the following characters. 1. They are microscopic. They do not possess a true nucleus. They lack membrane bound organelles. 52. Their mode of nutrition is autotrophic or heterotrophic. Some bacteria are autotrophic and are photosynthetic. i.e. they can synthesize their organic food in the presence of sunlight eg. Spirillum. Some bacteria are chemosynthetic i.e. they can synthesize their organic food by deriving energy from some chemical reactions. eg. Nitrosomonas and Nitrobacter. 3. Many other bacteria like Rhizobium, Azotobacter and Clostridium can fix atmospheric nitrogen into ammonia. This phenomenon is called Biological Nitrogen Fixation . 4. Some bacteria are parasites and others live as symbionts. 5. Some monerans like Archaebacteria can live in extreme environmental conditions like absence of oxygen (anaerobic), high salt condition, high 0 temperature like 80 c or above and highly acidic soils. 2. Kingdom Protista This kingdom includes eukaryotic unicellular mostly aquatic cells. They show the following characters. 1. They have a typical Eukaryotic cell organization. 2. They often bear cilia or flagella for locomotion. Most of them are photosynthetic autotrophs. They form the chief producers of food in oceans and in fresh water. All unicellular plants are collectively called as phytoplanktons and unicellular animals as zooplanktons. Phytoplanktons are photosynthetically active and have cell wall. 3. Zooplanktons are mostly predatory. They lack cell wall and show holozoic mode of nutrition as in Amoeba. 4. Some protists are parasitic. Some are symbionts while others are decomposers. Euglena, a protozoan has two modes of nutrition. In the presence of sunlight it is autotrophic and in the absence of sunlight it is heterotrophic. This mode of nutrition is known as myxotrophic and hence they form a border line between plants and animals and can be classified in both. 3. Kingdom Fungi This kingdom includes moulds, mushrooms, toad stools, puffballs and bracket fungi. They have eukaryotic cell organization. They show the following characteristics. 1. They are either unicellular or multi-cellular organisms. 62. Their mode of nutrition is heterotrophic since they lack the green pigment chlorophyll. Some fungi like Puccinia are parasites while others like Rhizopus are saprotrophic and feed on dead organic matter. 3. Their body is made up of numerous filamentous structures called hyphae. 4. Their cell wall is made up of chitin. 4. Kingdom Plantae It includes all multi-cellular plants of land and water. Major groups of Algae, Bryophytes, Pteridophytes, Gymnosperms and Angiosperms belong to this kingdom. It shows the following characteristics. 1. The cells have a rigid cell wall made up of cellulose. 2. They show various modes of nutrition. Most of them are autotrophs since they have chlorophyll. Some plants are heterotrophs. For eg. Cuscuta is a parasite. Nepenthes and Drosera are insectivorous plants. 5. Kingdom Animalia This kingdom includes all multi-cellular eukaryotic organisms. They are also referred to as metazoans. They show the following characteristic features. 1. All animals show heterotrophic mode of nutrition. They form the consumers of an ecosystem. 2. They have contractibility of the muscle cells. 3. They can transmit impulses due to the presence of nerve cells. 4. Some groups of animals are parasites eg. tapeworms and roundworms. Merits of the Five Kindom Classification 1. It shows the phylogenetic relationships among the organisms. 2. It is based on the complexity of the cell structure from prokaryotic to eukaryotic cell organization. 3. It is based on the complexity of body organization from unicellular to multi- cellular. 4. It is based on the modes of nutrition: autotrophic or heterotrophic mode of nutrition. Demerits of Five Kingdom Classification 1. Chlamydomonas and Chlorella are included under the kingdom Plantae. They should have been included under kingdom Protista since they are unicellular. 72. Animal protozoans are not included along with animals. 3. Animal protozoans are included under the kingdom Protista which include unicellular plants. They show different modes of nutrition. 4. Yeasts, though unicellular eukaryotes, are not placed in the kingdom Protista. 8 Table : 1.1 Major differences among five kingdoms in the Five Kingdom System of Classification: Property Monera Protista Fungi Plantae Animalia Cell type Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic Cell Mostly unicellular Mostly Multicellular Mostly Mostly organization unicellular and unicellular Multicellular Multicellular Cell wall Present in most Present in Present Present absent some: absent in others Nutritional Phototrophic,heterotrophic Heterotrophic Heterotrophic phototrophic Heterotrophic class or chemoautotrophic and phototrophic Mode of Absorptive Absorptive or Absorptive Mostly Mostly nutrition ingestive Absorptive ingestive Motility Motile or non Motile orNonmotile Mostly Mostly Motile motile nonmotile nonmotileDifficulties in classification Since living organisms exhibit great variety and diversity and also they have evolved through millions of years and there are many missing links between groups, it is very difficult to have a clear cut and well defined classification. Biological classification reflects the state of our knowledge. It changes as we acquire new information. By the 1970s molecular biologists realized that prokaryotes consist of two different and unrelated groups. To accommodate this new information three microbiologists, C.Woese, O.Kandler, and M.L Wheelis introduced a new classification scheme in 1990. They proposed that all organisms be divided into three major groups called domains: the Eucarya (containing all eukaryotes), the Bacteria (containing most familiar prokaryotes), and the Archaea (originally called archaebacteria and containing prokaryotes that live mostly in extreme environments.) This scheme is currently accepted by most biologists. Classification will undoubtedly continue to change. SELF EVALUATION One Mark Choose the correct answer 1. The basic unit of classification is a. genus b.species c.family d.taxon 2. Unicellular plants found floating in oceans and freshwater are called a. algae b.zooplanktons c.phytoplanktons d.epiphytes 3. Carolus Linnaeus proposed the following system of classification a. Phylogenetic b. Two kingdoms c. Five Kingdoms d. Natural Fill in the blanks 1. “Systema Naturae” is written by_____________ 2. Father of Ayurveda is_______________ 3. __________ introduced the term species for the first time. 4. The author of “Species Plantarum” is ___________ 5. __________ coined the word Taxonomy. 9Match the following Fossil records - Five kingdom System Whittaker - Species Carolus Linnaeus - Taxonomy John Ray - Phylogenetic studies Augustin de Candolle - Species Plantarum Two Marks 1. Define biodiversity. 2. What are the aims of classification? 3. Define Taxonomy. 4. Define species. 5. Write the hierarchy of the units of classification. 6. Define phylogeny. 7. Give any two reasons why phylogenetic classification is not always possible? 8. What is meant by phylogenetic classification? 9. What is meant by artificial system of classification? Give example. 10. What are Archaebacteria? 11. Name the three domains according to the modern classification proposed by C.Woese, O.Kandler and M.C.Wheelis. 12. Define systematics. Five Marks 1. List the differences between plants and animals. 2. How do you justify a separate kingdom status for fungi. 3. What are the difficulties encountered in classifying Euglena? Ten Marks 1. Discuss the Five kingdom system of classification. List it’s merits and demerits. 2. Discuss the Two kingdom system of classification. List it’s merits and demerits. 102. Viruses Introduction Viruses are still biologists’ puzzle because they show both living and non- living characters. Hence viruses are regarded as a separate entity. It is not taken into account in Whittaker’s five kingdom classification. Viruses are now defined as ultramicroscopic, disease causing intra cellular obligate parasites. Brief history of discovery Viruses were not known to biologists for a long time due to their ultramicroscopic structure though their presence was apparent by infectious diseases which were proved not due to bacteria. It attracted the attention of investigators th only in the 19 century when a virus called tobacco mosaic virus (TMV) caused severe damage to commercially important tobacco crop. Table : 1.2. Enigma of Viruses Living characteristics of virus Non-living characteristics of virus 1. Ability to multiply inside a host Inability to multiply extra cellularly plant or animal cell 2. Ability to cause diseases Absence of any metabolic activity 3. Possession of nucleic acid, Absence of protoplasm protein, enzyme, etc. 4. Ability to undergo mutation Can be crystallized. Mayer demonstrated that the disease could be transmitted just by applying the sap of infected leaf to the leaf of healthy plant. He thought that the disease was due to a bacterium. It was then the Russian biologist Iwanowsky (1892) who demonstrated that the sap of infected leaves even after passing through bacterial filter remained infective, ruling out the bacterium as the causative agent. Dutch microbiologist Beijerinck (1898) confirmed the findings of Iwanowsky and called the fluid “contagium vivum fluidum” which means contagious living fluid. This was later on called virion (poison) and the disease causing agent as virus. W.M. Stanley (1935), the American biochemist, isolated virus in crystalline form and demonstrated that even in that state it maintained the infectivity. This marked the beginning of a new branch of science called virology. General characteristics Viruses are ultramicroscopic and can cause diseases in plants and animals. They are very simple in their structure. They are composed of nucleic acid 11surrounded by a protein coat. Nucleic acid can be either RNA or DNA, but never both. They have no cellular organization and have no machinery for any metabolic activity. They are obligate intracellular parasites and they multiply within their host cells. Once outside the host cell they are completely inactive. Size and Shape Viruses are very minute particles that they can be seen only under electron microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). (1micron – 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size. Very small size and ability to pass through bacterial filters are classic attributes of viruses. The following methods are used to determine the size of the viruses. 1. Direct observation by using electron microscope: 2. Filtration through membranes of graded porosity: In this method viruses are made to pass through a series of membranes of known pore size, the approximate size of any virus can be measured by determining which membrane allows the virus to pass through and which membrane holds it back. 3. Sedimentation by ultra centrifugation : The relationship between the size and shape of a particle and its rate of sedimentation permits determination of particle size. 4. Comparative measurements: The following data is used for reference. a. Staphylococcus has a diameter of 1000 mm. Cubical (A den o V ir us) b. Bacteriophage varies in size from 10-100 nm. Helical M atrix Broadly speaking viruses occur in three main shapes: N ucleo capsid Com plex (P ro tein) 1.Cubic symmetry: (B acteriophage) Envelope polyhedral or spherical – eg. (L ip id ) Head DNA Adeno virus, HIV Spikes Neck (G ly co 2. Helical symmetry:eg. (Capsid) Collar protein) Tail Tobacco Mosaic virus Core tube (TMV), Influenza virus. Tail Fibres 3. Complex or atypical eg. End P late Spikes Bacteriophage, Pox virus. Fig : 1.2 Different shapes of Viruses 12

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