Biology as a natural science: The study of Life in all its Forms 1774695014, 9781774695012

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Biology as a Natural Science: The Study of Life in all its Forms

BIOLOGY AS A NATURAL SCIENCE: THE STUDY OF LIFE IN ALL ITS FORMS

Saket Kushwaha

www.delvepublishing.com

Biology as a Natural Science: The Study of Life in all its Forms Saket Kushwaha Delve Publishing 224 Shoreacres Road Burlington, ON L7L 2H2 Canada www.delvepublishing.com Email: [email protected] e-book Edition 2023 ISBN: 978-1-77469-566-1 (e-book)

This book contains information obtained from highly regarded resources. Reprinted material sources are indicated and copyright remains with the original owners. Copyright for images and other graphics remains with the original owners as indicated. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data. Authors or Editors or Publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The authors or editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify.

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ABOUT THE AUTHOR

Prof. Saket Kushwaha, currently the Vice Chancellor of Rajiv Gandhi University is specialized in resource management and sustainable agriculture development. After his higher studies from Banaras Hindu University, he joined Abubakar Tafawa Balewa University (ATBU), Bauchi, Nigeria in 1993 and taught various courses on Agriculture Economics and Management at undergraduate, post graduate and Ph.D. level. Prof. Kushwaha rose to the rank of Professor at ATBU in the year 1999 and in 2006 he joined Bananas Hindu University (BHU), India as professor in agriculture economics and became the Vice Chancellor of Lalit Narayan Mithila University for one term 20142017. He has more than 100 publications in national and international journals of repute, supervised 24 Ph.D. students and authored 17 books / book chapters. Prof Kushwaha is life member of 10 Professional Bodies and sits in the panel of editorial boards. Worked extensively in the field of Zero Emission Research Initiatives (ZERI) propagating the mission of sustainable development under the aegis of “Waste is Wealth” concept. He is also the recipient of 17 national and international awards which includes award from Sulabh International Gold Medal in 2016 for sanitation management. 27 years of teaching, research, extension and community service experience with 23 years in administration. Handled over 10 national and International projects majorly funded by World Banks. Coordinated USAID project on Cowpea Research Support Programme (CRSP) in Nigeria for 10 years from 1996 to 2006. Working with NGOs and mega agriculture farms for Green Farm Planning.

TABLE OF CONTENTS



List of Figures.................................................................................................xi



List of Abbreviations.....................................................................................xiii

Glossary........................................................................................................xv Preface..................................................................................................... ....xxi Chapter 1

Introduction to Biology.............................................................................. 1 1.1. Introduction......................................................................................... 2 1.2. Properties of Life.................................................................................. 2 1.3. Levels of Organization of Living Things................................................ 5 1.4. Five Core Themes of Biology................................................................ 9 1.5. Evolutionary Theory/ Theory of Evolution........................................... 11 1.6. The Diversity of Life........................................................................... 20 1.7. Branches of Biological Study.............................................................. 21 1.8. The Process of Science....................................................................... 22 1.9. Natural Sciences................................................................................ 23 1.10. Basic and Applied Science............................................................... 27 1.11. Conclusion...................................................................................... 29 References................................................................................................ 30

Chapter 2

The Science of Life and its Chemical Basis............................................... 31 2.1. Introduction....................................................................................... 32 2.2. What Kind of Molecules Characterize Living Things?......................... 37 2.3. What are the Chemical Structures and Functions of Proteins?............ 39 2.4. What are the Chemical Structures and Functions of Carbohydrates?.. 40 2.5. What are the Chemical Structures and Functions of Lipids?............... 44 2.6. What are the Chemical Structures and Functions of Nucleic Acids?... 48 2.7. How Did the First Cells Originate?..................................................... 51 2.8. What Features Make Cells the Fundamental Units of Life?................. 52

2.9. What is the Structure of a Biological Membrane?............................... 56 2.10. What Are Signals, and How Do Cells Respond to Them?................. 57 2.11. How Do Cells Communicate Directly?............................................ 60 2.12. Conclusion...................................................................................... 63 References................................................................................................ 64 Chapter 3

Cells, Genes and Heredity........................................................................ 65 3.1. Introduction....................................................................................... 66 3.2. The Evolution of Cells........................................................................ 71 3.3. Heredity and Evolution...................................................................... 73 3.4. Chemical Structure of Genes............................................................. 74 3.5. Gene Transcription and Translation.................................................... 74 3.6. How Do Prokaryotic and Eukaryotic Cells Divide?............................. 76 3.7. Prokaryotes Divide By Binary Fission................................................. 76 3.8. How Is Eukaryotic Cell Division Controlled?...................................... 78 3.9. Role Of Cell Division in a Sexual Life Cycle...................................... 79 3.10. In A Living Organism, How Do Cells Die?....................................... 83 3.11. Unregulated Cell Division Lead to Cancer?...................................... 84 3.12. Mendelian Law of Inheritance......................................................... 87 3.13. How Do Genes Interact ?................................................................. 88 3.14. The Environment Affects Gene Action.............................................. 89 3.15. Relationship Between Genes and Chromosomes............................. 91 3.16. Genes on Sex Chromosomes are Inherited in Special Ways............. 92 3.17. How Do Prokaryotes Transmit Genes?............................................. 93 3.18. Conclusion...................................................................................... 94 References................................................................................................ 95

Chapter 4

Flowering Plants: Form and Functions...................................................... 97 4.1. Introduction....................................................................................... 98 4.2. Basic Body Plan of Plants................................................................... 99 4.3. Cell Wall Support Plant Growth and Form....................................... 103 4.4. How Do Plant Tissues and Organs Originate?.................................. 106 4.5. The Plant Body is Constructed From Three Tissue Systems................ 108 4.6. Cells of the Xylem Transport Water and Dissolved Minerals............. 110 4.7. Cells of the Phloem Transport the Products of Photosynthesis........... 111 4.8. How Do Meristems Build a Continuously Growing Plant?............... 111 viii

4.9. Leaves are Determinate Organs Produced by Shoot Apical Meristems................................................................. 114 4.10. How Has Domestication Altered Plant Form?................................ 114 4.11. How Do Plants Acquire Nutrients?................................................. 115 4.12. What Mineral Nutrients Do Plants Require?................................... 116 4.13. Do Carnivorous and Parasitic Plants Obtain a Balanced Diet?........ 117 4.14. Flowering Plants Have Microscopic Gametophytes........................ 119 4.15. The Flowering Stimulus Originates in a Leaf................................... 122 4.16. Flowering Plants Use Animals or Wind to Transfer Pollen Between Flowers................................................................ 122 4.17. Self-Pollination.............................................................................. 125 4.18. Conclusion.................................................................................... 126 References.............................................................................................. 127 Chapter 5

Animals: Forms and Functions................................................................ 129 5.1. Introduction..................................................................................... 130 5.2. Types of Animals With their Class, Category, and Group.................. 132 5.3. How Do Multicellular Animals Supply the Needs of Their Cells?..... 138 5.4. How Do Animals Alter Their Heat Exchange With the Environment?................................................................................ 140 5.5. Animal Hormones........................................................................... 142 5.6. Major Defense System of Animals.................................................... 143 5.7. How Do Animals Make So Many Different Antibodies?................... 144 5.8. Animals Reproduce Without Sex...................................................... 145 5.9. How Do Animals Reproduce Sexually?............................................ 147 5.10. An Individual Animal Can Function as Both Male and Female....... 148 5.11. The Evolution of Vertebrate Reproductive Systems Parallels the Move to Land.......................................................................... 149 5.12. Animal Development..................................................................... 150 5.13. Gas Exchange in Animals............................................................... 153 5.14. Why Do Animals Need Circulatory System?................................... 157 5.15. Conclusion.................................................................................... 159 References.............................................................................................. 160

Chapter 6

The Diversity of Life............................................................................... 161 6.1. Introduction..................................................................................... 162 6.2. History of Life.................................................................................. 166 ix

6.3. Estimates of Current Diversity.......................................................... 169 6.4. Living World Begin to Diversify....................................................... 172 6.5. Where Do Viruses Fit Into the Tree of Life?....................................... 173 6.6. Many RNA Viruses Probably Represent Escaped Genomic Components.................................................................. 174 6.7. How Do Eukaryotic Cells Arise?....................................................... 176 6.8. The Evolution of Seed Plants............................................................ 178 6.9. How Do Plants Support Our World?................................................ 181 6.10. Facts Form the Basis of Our Understanding of Evolution?............... 182 6.11. Mechanisms of Evolutionary Change............................................. 183 6.12. Natural Selection Result In Evolution............................................. 186 6.13. Natural Selection Can Change or Stabilize Populations.................. 186 6.14. Constraints on Evolution................................................................ 189 6.15. Conclusion.................................................................................... 191 References.............................................................................................. 192 Chapter 7

Nervous System, Sensory System and Immune System........................... 193 7.1. Introduction..................................................................................... 194 7.2. The Nervous System Consists of Neurons and Supporting Cells........ 198 7.3. Nerve Impulses are Produced on the Axon Membrane..................... 200 7.4. The Central Nervous System Consists of the Brain............................ 202 7.5. Language and Other Functions........................................................ 206 7.6. The Autonomic Nervous System....................................................... 208 7.7. Animals Employ a Wide Variety of Sensory Receptors...................... 208 7.8. Sensing Muscle Contraction and Blood Pressure.............................. 210 7.9. Sensing Taste, Smell, and Body Position........................................... 212 7.10. Evolution of the Immune System.................................................... 215 7.11. Many of the Body’S Most Effective Defenses are Nonspecific......... 216 7.12. Cells of the Specific Immune System.............................................. 221 7.13. Initiating the Immune Response..................................................... 222 7.14. The Immune System Can Be Defeated............................................ 223 7.15. Conclusion.................................................................................... 224 References.............................................................................................. 226

Index...................................................................................................... 227

LIST OF FIGURES Figure 1.1. The characteristics of living things Figure 1.2. Phylogenetic tree of life Figure 1.3. Image showing evolutionary changes Figure 1.4. Image showing natural selection & coevolution Figure 1.5. Image showing the National Institute of Natural Sciences, Japan Figure 2.1. Image showing cells information Figure 2.2. Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes and prokaryotes Figure 2.3. Structures of proteins Figure 2.4. Image showing polysaccharides Figure 2.5. Image showing phospholipids bilayer Figure 2.6. The cell membrane Figure 3.1. Animal Cell and Components Figure 3.2. Arabidopsis thaliana plant Figure 3.3. An RNA molecule Figure 3.4. Gene Transcription and Translation Figure 3.5. DNA Replication Figure 3.6. A picture of Cholla Cactus Figure 3.7. Production of Gametes Figure 3.8. Life cycles involve Meiosis and Fertilization Figure 3.9. Human karyotype Figure 3.10. Diagram showing cancer cells spreading into the bloodstream Figure 4.1. A picture of Secale cereale flowering Figure 4.2. An example of axillary buds located in the axil of the leaf Figure 4.3. Plasmodesma. (A) Schematic representation of plant cells connected by cell wall-piercing plasmodesmata. (B) Transverse and longitudinal sections through the plasmodesmata of the two forms. Figure 4.4. Herbaceous Dicot Stem: Collenchyma, Sclerenchyma and Parenchyma in Cucurbita

Figure 4.5. Carbon uptake and photosynthesis in a seagrass meadow Figure 4.6. A picture of Venus Fly Trap (Dionaea muscipula) Figure 4.7. Conifer male gametophyte Figure 4.8. Angiosperm embryo sac diagram Figure 5.1. Sturgeon (Acipenser) at the Oregon Zoo Figure 5.2. Bee hummingbird (Mellisuga helenae) Figure 5.3. Mammal Diversity Figure 5.4. A picture of Mollusks Figure 5.5. A Picture of a Sponge Figure 6.1. A typical animal cell with labeled organelles Figure 6.2. Potential evolutionary outcomes of hybridization Figure 7.1. Human Nervous System diagram Figure 7.2. The NK cell releases perforin and granzyme, leading to cancer cell apoptosis. Figure 7.3. Na+ influx via voltage-gated ion channels during depolarization Figure 7.4. Four lobes of the Human Forebrain Figure 7.5. Sketch of the lateral line sensory organ, used by fish to sense pressure differences in the water.

LIST OF ABBREVIATIONS A Adenine AIDS

Acquired Immune Deficiency Syndrome

ATP

Adenosine Triphosphate

AZT

Azidothymidine

C Cytosine cAMP

Cyclic Adenosine Monophosphate

CAT

Computed Axial Tomography

cDNA

Complementary DNA

CNS

Central Nervous System

DNA

Deoxyribonucleic Acid

ECF

Extracellular fluid

EEG

Electroencephalogram

EPSP

Excitatory Postsynaptic Potential

ER

Endoplasmic Reticula

G

Guanine

GTP

Guanosine Triphosphate

HGP

Human Genome Project

HIV

Human Immunodeficiency Virus

HLA

Human Leukocyte Antigens

HPV

Human Papillomaviruses

MHC

Major Histocompatibility Complex

mRNA

Messenger RNA

NK-cells

Natural Killer Cells

PNS

Peripheral Nervous System

REM

Rapid Eye Movement

RNA

Ribonucleic Acid

T Thymine TMV

Tobacco Mosaic Virus

TTX

Tetrodotoxin

U Uracil

GLOSSARY

A Adaptation - In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Adaptation - In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Allopatric Speciation - Allopatric speciation – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow. Apoptosis - Apoptosis is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay B Biodiversity - Biodiversity is all the different kinds of life you’ll find in one area—the variety of animals, plants, fungi, and even microorganisms like bacteria that make up our natural world. Each of these species and organisms work together in ecosystems, like an intricate web, to maintain balance and support life. Biosphere - The biosphere is made up of the parts of Earth where life exists—all ecosystems. The biosphere extends from the deepest root systems of trees, to the dark environments of ocean trenches, to lush rain forests, high mountaintops, and transition zones like this one, where ocean and terrestrial ecosystems meet. C Cytosol - The cytosol, also known as cytoplasmic matrix or ground plasm, is one of the liquids found inside cells. It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into many compartments. D Disease - A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that are associated with specific signs and symptoms.

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E Ecological Barriers - Ecological barrier deterioration driven by human activities. The ecological barrier blocks the three main viral transmission routes (wild intermediate hosts, breeding animal hosts, and environmental media) from natural hosts to human society. Ecosystem - An ecosystem is a geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life. Ecosystems contain biotic or living, parts, as well as abiotic factors, or nonliving parts. Biotic factors include plants, animals, and other organisms. Ecosystem - An ecosystem is a geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life. Ecosystems contain biotic or living, parts, as well as abiotic factors, or nonliving parts. Biotic factors include plants, animals, and other organisms. Ectotherms - An ectotherm is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature. Such organisms rely on environmental heat sources, which permit them to operate at very economical metabolic rates. Environment - The surroundings or conditions in which a person, animal, or plant lives or operates, or the natural world, as a whole or in a particular geographical area, especially as affected by human activity. Eudicots - The eudicots, Eudicotidae, or eudicotyledons are a clade of flowering plants mainly characterized by having two seed leaves upon germination. The term derives from Dicotyledons. Traditionally they were called tricolpates or non-magnoliid dicots by previous authors. F Flaps - (of a bird’s wings) to wave up and down while flying, or (of objects that cannot fly) to move quickly from side to side or up and down G Gametes - Gametes are an organism’s reproductive cells. They are also referred to as sex cells. Female gametes are called ova or egg cells, and male gametes are called sperm. Gametes are haploid cells, and each cell carries only one copy of each chromosome. Genetics - Genetics is the scientific study of genes and heredity—of how certain qualities or traits are passed from parents to offspring as a result of changes in DNA sequence. A gene is a segment of DNA that contains instructions for building one or more molecules that help the body work. H Habitat - In ecology, the term habitat summarizes the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species’ habitat can be seen as the physical manifestation of its ecological niche.

Homeostasis - Homeostasis is any self-regulating process by which an organism tends to maintain stability while adjusting to conditions that are best for its survival. If homeostasis is successful, life continues; if it’s unsuccessful, it results in a disaster or death of the organism. I Interoceptors - A sensory nerve terminal located in and transmitting impulses from the viscera K Kinocilium - A kinocilium is a special type of cilium on the apex of hair cells located in the sensory epithelium of the vertebrate inner ear. L Living things - An individual form of life, such as a bacterium, protist, fungus, plant or animal consisting of a singular cell or a complex of cells in which cell organelles or organs work together to carry out the various processes of life. Lobes - The lobes of the brain are the major identifiable zones of the cerebral cortex, and they comprise the surface of each hemisphere of the cerebrum. Lysosomes - Lysosomes are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids. M Meadow - A meadow is an open habitat, or field, vegetated by grasses, herbs, and other non-woody plants. Trees or shrubs may sparsely populate meadows, as long as these areas maintain an open character. Meadows may be naturally occurring or artificially created from cleared shrub or woodland. Mechanoreception - Ability of an animal to detect and respond to certain kinds of stimuli—notably touch, sound, and changes in pressure or posture—in its environment. Sensitivity to mechanical stimuli is a common endowment among animals. Mitosis - In cell biology, mitosis is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintained. Therefore, mitosis is also known as equational division. Monocots - Monocotyledons, commonly referred to as monocots, are grass and grasslike flowering plants, the seeds of which typically contain only one embryonic leaf, or cotyledon.  Mycota - an alternative taxonomic name for the kingdom Fungi, A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms.

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P Pathogens – A pathogen in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s. Phenotypes - Phenotype is the set of observable characteristics or traits of an organism. The term covers the organism’s morphology or physical form and structure, its developmental processes, its biochemical and physiological properties, its behavior, and the products of behavior.  Photoreception – It refers to mechanisms of light detection that lead to vision and depends on specialized light-sensitive cells called photoreceptors, which are located in the eye. Phyllotaxy - In botany, phyllotaxis or phyllotaxy is the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature. Plasmodesmata - Plasmodesmata are microscopic channels that traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plethora - a bodily condition characterized by an excess of blood and marked by turgescence and a florid complexion. Pollination - Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilization and the production of seeds, most often by an animal or by wind.  Prokaryotes - A prokaryote is a single-celled organism that lacks a nucleus, and other membrane-bound organelles. The word prokaryote comes from the Greek πρό and κάρυον. In the two-Kingdom system arising from the work of Édouard Chatton, prokaryotes were classified within the Kingdom Prokaryota. P Spontaneous Generation - Spontaneous generation is a body of thought on the ordinary formation of living organisms without descent from similar organisms. The theory of spontaneous generation held that living organisms could arise from nonliving matter and that such processes were commonplace and regular. Squamata - an order of reptiles comprising the snakes and lizards and sometimes the extinct Pythonomorpha Sterile - Free from bacteria or other living microorganisms; totally clean. Stimulus - In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism’s internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be made, is called sensitivity. Sturgeon - any of a family (Acipenseridae) of usually large elongate anadromous or freshwater bony fishes which are widely distributed in the north temperate zone and whose roe is made into caviar

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T Taxonomy - Taxonomy is the science of naming, describing and classifying organisms and includes all plants, animals and microorganisms of the world. Toad - Toad is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands. Tropical Rainforest - Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as lowland equatorial evergreen rainforest. The Amazon rainforest is the world’s largest tropical rainforest. It is home to around 40,000 plant species, nearly 1,300 bird species, 3,000 types of fish, 427 species of mammals, and 2.5 million different insects. Tumors - A tumor is an abnormal growth of body tissue. Tumors can be cancerous (malignant) or noncancerous (benign).

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PREFACE

This book takes the readers through several aspects of Biology. This book gives an introduction to biology, the science of life and its chemical basis, cells, genes, heredity, flowering plants, animals, diversity of life and nervous system, sensory system and immune system. The first chapter stresses the basic introduction to the biology, so that the readers are clear about the philosophies behind that form the utmost basics in the field. This chapter will also emphasize the characteristics, levels and five core themes of life. The chapter explains the evolutionary theory, branches of biology, natural sciences and basic and applied science. The second chapter takes the readers through the concepts on the science of life and its chemical basis. This chapter will provide highlights on the various key aspects of molecules characterizing living cells, chemical structures and functions of carbohydrates, proteins, lipids and nucleic acids. The chapter also explains the features, structure and functions of a biological membrane, how cells communicate, genes interact Then, the third chapter explains cells, genes and heredity. It also explains the evolution of cells, chemical structure of genes, transcription, translation, prokaryotic and eukaryotic cell division, unregulated cell division and gene interaction. It also explains chromosomes, their relationship with genes and prokaryotes transmitting genes. The fourth chapter introduces the readers to flowering plants; forms and functions. This chapter also explains the basic body plan of plants, cell wall support, plant tissues and organs, plant vascular system, plant mineral nutrition and carnivorous and parasitic plants. The chapter also sheds light on microscopic gametophytes, flowering stimulus in plants and pollination. The fifth chapter throws light on animals; forms and functions. This chapter contains the definition of animal Kingdom, types of animals with their class, category and group, multicellular animals and their cell functioning, animal hormones, defense system in animals, sexual and asexual reproduction in animals, animal development, gas exchange and circulatory system in animals. The sixth chapter takes the readers through the concept of diversity of life. The readers are then told about the history, estimate of current diversity, living world diversity, viruses’ position in the tree of life and RNA Viruses. The chapter also explains the evolution of eukaryotic cells and plant seeds, the mechanism of evolutionary change, natural selection and constraints on evolution.

The last chapter of this book sheds light on the nervous system, nerve impulses, CNS, Autonomic nervous system, etc. This chapter also mentions various aspects of sensory receptors, sensing muscle contraction and blood pressure, sensing of taste, smell and body position. The evolution of immune system, cells of the immune system, immune response and how the immune system can be defeated are explained well in this chapter. This book has been designed to suit the knowledge and pursuit of the researcher and scholars and to empower them with various aspects of biology, so that they are updated with the information. I hope that the readers find the book explanatory and insightful and that this book is referred by scholars across various fields.

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1

CHAPTER

INTRODUCTION TO BIOLOGY

CONTENTS 1.1. Introduction......................................................................................... 2 1.2. Properties of Life.................................................................................. 2 1.3. Levels of Organization of Living Things................................................ 5 1.4. Five Core Themes of Biology................................................................ 9 1.5. Evolutionary Theory/ Theory of Evolution........................................... 11 1.6. The Diversity of Life........................................................................... 20 1.7. Branches of Biological Study.............................................................. 21 1.8. The Process of Science....................................................................... 22 1.9. Natural Sciences................................................................................ 23 1.10. Basic and Applied Science............................................................... 27 1.11. Conclusion...................................................................................... 29 References................................................................................................ 30

2

Biology as a Natural Science: The Study of Life in all its Forms

Biology, in its widest meaning, is the study of living things. From an objective point of view, it may also be referred to as the science of life. This branch of science studies all live things or living beings. It focuses on and studies topics relating to living beings such as life organization, functions, patterns and order of organisms, growth and development of living organisms, etc. Living things come in all sorts of sizes, forms, and purposes, and biologists study life from the tiniest cell to the largest ecosystems. As a result, biology has many branches and divisions, including evolutionary biology, cellular biology, genetics, growth and developmental biology, and so on.

1.1. INTRODUCTION Microorganisms are assumed to have been the original lifeforms on the planet, existing billions of years before animals and plants evolved. The animals, birds, and flowers humans know and love are all very recent, having evolved between 130 and 200 million years ago. Humans have been on this earth for the last 2.5 million years, and only in the last 200,000 years have humans begun to appear as humans do now. Biology is the study of living things. What is the definition of life? This could appear to be a foolish question with an apparent answer, yet defining life is difficult. For example, virology is a discipline of biology that examines viruses, which have some of the properties of living beings but lack others. Viruses, despite their ability to assault live beings, inflict illnesses, and even reproduce, do not fit the requirements that biologists use to define life. Biology has grappled with four questions since its inception: What are the common characteristics that make anything “alive”? How do the various living things work? When confronted with the incredible diversity of life, how can we organize the many types of organismsso that we can better understand them? Finally, scientists want to know how this variety arose and how it is maintained. As new organismsare found day after day, scientists continue to seek answers to these and other issues.

1.2. PROPERTIES OF LIFE Many important traits or activities are shared by all groups of living organisms: order, sensitivity or reaction to stimuli, reproduction, adaptability, growth

Introduction to Biology

3

and development, control, homeostasis, and energy processing. When taken collectively, these eight elements contribute to defining life.

1.2.1. Order Cells are highly structured entities made up of one or more cells. Although single-celled organisms are amazingly sophisticated. Atoms combine to form molecules within each cell. These, in turn, form cell components or organelles. Multicellular creatures, which may have millions of individual cells, have an advantage over single-celled organisms in that their cells can be specialized to fulfill certain roles and even sacrificed in some instances for the sake of the organism as a whole. How these specialized cells join together to produce organs like the heart, lung, or skin in organisms like the toad.

1.2.2. Sensitivity Or Response To Stimuli Organisms react to a variety of signals. Plants, for instance, can grow toward a light source or react to touch. Even microscopic bacteria may migrate toward or away from chemicals or light (a process known as chemotaxis) (phototaxis). The movement toward a stimulus is regarded as positive, whereas movement away from a stimulus is regarded as negative.

1.2.3. Reproduction Single-celled organisms multiply by copying their genetic material, and DNA, and afterwards splitting it evenly as the cell prepares to split to generate two new cells. Most multicellular organisms (those composed of much more than one cell) have specialized reproductive cells from which new organisms are formed. When such an organism reproduces, DNA-carrying genes are handed down to its offspring. These genes are responsible for the child belonging to the same species and having identical traits to the parent, such as fur color and blood type.

1.2.4. Adaptation Most living species have a “fit” to their surroundings. That fit is referred to by biologists as adaptation, and it is the result of natural selection, which acts in every lineage of reproducing organisms. The distinctive heat-resistant protein structures of bacteria that thrive in boiling hot springs to the tongue

4

Biology as a Natural Science: The Study of Life in all its Forms

length of a nectar-feeding moth that fits the size of the blossom from which it eats are instances of adaptations.

Figure 1.1. The characteristics of living things. Source: Image by Wikimedia commons

All adaptations improve an individual’s reproductive potential, particularly one‘s capacity to live and reproduce. Adaptations are not always consistent. Natural selection drives the traits of individuals in a group to reflect environmental factors.

1.2.5. Growth And Development All organisms grow and evolve in accordance with the guidance encoded in one‘s DNA. Those genes encode directions for cellular progress and expansion, guaranteeing that a species’ offspring have many of these features as its parents.

1.2.6. Regulation Even the tiniest organisms are complicated, requiring various regulatory systems to regulate internal activities such as nutrition transport, response to stimuli, and stress tolerance. Organ systems, for example, the digestive or circulatory systems, provide specialized activities such as conveying oxygen throughout the body, eliminating wastes, supplying nutrients to every cell, and cooling the body.

Introduction to Biology

5

1.2.7. Homeostasis Cells need certain parameters to function effectively, such as temperature, pH, and chemical concentrations. These parameters, however, may shift from one moment to the next. Organisms may keep internal conditions within a restricted range practically consistently, despite outside changes, thanks to a mechanism known as homeostasis or “steady state”—the ability of an organism to maintain constant internal circumstances. Many creatures, for example, use thermoregulation to maintain their body temperature. Organisms that dwell in cold areas, such as the polar bear, have physical features that assist them to tolerate cold temperatures and storing body heat. In hot temperatures, organisms have ways (such as sweating in humans or panting in canines) to remove extra body heat.

1.2.8. Energy Processing For their metabolic functions, most living things require a supply of energy. A few species catch solar energy and transform this into chemical energy in food, whereas others utilize chemical energy from molecules they consume. Such qualities are shared by all living species, and they will be used to determine whether or not things are alive. More information is provided below.

1.3. LEVELS OF ORGANIZATION OF LIVING THINGS Living organisms are highly structured and ordered, with a hierarchy ranging from small to huge. The atom is the most basic and smallest unit of matter. It is made up of a nucleus that is surrounded by electrons. Atoms combine to form molecules. A molecule is a chemical structure made up of at least two atoms connected by a chemical bond. Many physiologically essential compounds are macromolecules, which are huge molecules created by combining smaller components termed monomers. Deoxyribonucleic acid (DNA) is an example of a macromolecule since it holds the instructions for the creature that possesses it. Organelles are collections of macromolecules enclosed by membranes seen in some cells. Organelles are tiny structures found within cells that conduct certain tasks. Cells make up all living things; the cell is the smallest essential unit of structure and function in living beings. (This is why viruses aren’t regarded as living: because they don’t have cells.)

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To create new viruses, viruses must first infiltrate and hijack a live cell; only then can they gather the resources required for reproduction. Some organisms are made up of a single cell, whereas others are multicellular. Prokaryotic and eukaryotic cells are the two types of cells. Prokaryotes are single-celled organisms that lack organelles and do not have nuclei surrounded by nuclear membranes. Eukaryotic cells, on the other hand, have membrane-bound organelles and nuclei. Cells in most multicellular organisms join to form tissues, which are clusters of similar cells that perform the same job. Organs are groups of tissues that share a similar purpose. Organs may be found in both animals and plants. Organ systems are higher levels of structure made up of functionally connected organs.

Figure 1.2. Phylogenetic tree of life. Source: Image by Wikimedia commons

Many organ systems exist in vertebrate animals, such as the circulatory system, which distributes blood throughout the body and to and from the lungs; it comprises organs such as the heart and blood arteries. Individual living organisms are known as organisms.

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Each tree in a forest, for example, is an organism. Single-celled prokaryotes and eukaryotes are also organisms and thus are usually referred to as microbes. A population is all the members of a species that live in a certain region. A forest, for example, may contain many white pine trees. This forest’s population of white pine trees is represented by these pine trees. Different populations may coexist in the same geographical location. For example, the pine forest has communities of blooming plants as well as insects and microbiological species. A community is the collection of individuals who live in a specific location. For example, a forest’s community consists of all of its trees, flowers, insects, and other inhabitants. The forest is an ecosystem in and of itself. An ecosystem is made up of all the living organisms in a given region, as well as the abiotic, or non-living, elements of that environment, such as nitrogen in the soil or precipitation. The biosphere, at the highest level of organization, is the collection of all ecosystems and reflects the zones of life on Earth. It consists of land, water, and parts of the atmosphere.

1.3.1. Hierarchical Organization Seems to be the world of living things in disarray because it is not organized? Any living thing appears to have a hierarchical organization, which it does not. Every living thing has a hierarchical rank. The portrayal of levels in order to provide an easier image of the notion of hierarchy is referred to as life hierarchy. Life’s hierarchies can have a number of levels. In general, there are 10 levels, ranging from atoms at the lowest level to ecosystems at the highest. From lowest to highest, the atom is the lowest, followed by cell, tissue, organ, organ system, organism or person, population, community, and ecosystem at the highest. For instance, two or more atoms are brought together just to create a molecule, and certain molecules are formed into cells, and specific cells are constructed into the tissue, and so on. This is referred to as the hierarchical order of life. Each level is regarded as a separate organization.

The Cellular Level: Atoms, the basic components of matter, are linked together and form clusters termed molecules at the cellular level. Complex biological molecules are built into microscopic structures known as organelles, which are components of

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cells and are put together to make cells. Organelles are built on the nucleus and mitochondria. As previously said, the cell is the fundamental unit of life, and many species in the world are made up of single cells. Bacteria, for example, are solitary cells. All animals and plants, as well as the majority of fungi and algae, are multicellular, meaning they have more than one cell.

The Organismal Level: Cells are categorized into three organizational levels. The most fundamental level is that of tissues, which are clusters of similar cells that work together to form a functional unit, such as muscle tissues and blood tissues. Tissues are further subdivided into organs, which are biological structures made up of numerous distinct tissues gathered together in a structural and functional unit, such as the heart and brain. Your brain is a vital organ that is made up of billions of nerve cells and a variety of connective tissues. Organs are organized into organ systems at the third level of organization. For example, the nervous system is made up of sensory organs, the brain, and the spinal cord.

The Populational Level: Individual organisms within the living world are arranged into various hierarchical levels. The most fundamental of them is the population, which is a collection of animals of the same species living in the same location, such as a human population or a fish population. All populations of a single type of an organism comprise a species, with individuals that look similar and may interbreed. A biological community is a higher degree of biological organization that comprises all the populations of various species living together in one location. A biological community and the physical habitat in which it lives together comprise an ecological system, or ecosystem, at the highest level of biological organization.

1.3.2. Emergent Properties As one progresses from the lower to higher levels of the living system, many new traits arise. Those would be known as emergent qualities. For instance, we can detect features and energy in a cell built from the aggregates of many molecules that are not present in a single molecule. For instance, there are

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qualities in a molecule like protein which does not exist in the atoms that make it up. That is, consider diamond and coal, which are both constituted of the same carbon molecule but have vastly different features and characteristics despite the fact that they are generated differently. The hierarchy of life teaches us this. The emergent attribute of life can tell us about the hierarchical nature of life.

1.4. FIVE CORE THEMES OF BIOLOGY 1.4.1. Organization of Life: The Cell Theory As previously said, all organisms are made up of cells, which are the basic building blocks of life. Despite the vast diversity in size and structure of all living species, from the smallest single-celled animals to the greatest 10 kilometers-squared size fungus, Armillaria ostoyae is made up of cells. This finding is the foundation of what has become known as the cell hypothesis. The cell theory, one of biology’s fundamental concepts, serves as the framework for comprehending all organisms’ reproduction and growth. Two major inferences may be drawn from the cell hypothesis. One, it determined that all living entities are made up of cells, and two, that all cells descend from previous cells.

1.4.2. Continuity of Life: The Molecular Basis of Inheritance Even, the smallest cell, which cannot be seen with the human eye, is extraordinarily complicated—more complex than a computer. Cells have many various forms, bases, and purposes, and what distinguishes one cell from another is encoded inside a nucleus of a cell called a gene. The gene determines the work, forms, functions, and energy of cells. Aside from DNA, genes are created. DNA is a long, cable-like molecule composed of two long chains of building units known as nucleotides that are twisted around each other. The two strands of DNA form a double helix by winding around each other like the rails of a spiral staircase. The faithful copying of a cell’s DNA into daughter cells is required for the continuation of life from one generation to the next (heredity). As a result, DNA is a fundamental molecular unit of heredity characteristics.

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1.4.3. The Correlation of Structure and Function At all levels of biological organization, structure and function are linked. This subject provides an introduction to the anatomy of life at all structural levels, from molecules to organisms. Analyzing a biological structure reveals information about what it does and how it functions. Knowing the purpose of a building, on the other hand, offers information on its construction. We can easily perceive the relationship between the structure and form of bird wings and their functions. The aerodynamically effective design of a bird’s wing, chest muscle for flight efficacy, the breadth of chest bones to build up the muscles, and airbags in the skeleton to reduce body weight all contribute to a bird’s flight.

1.4.4. Diversity of Life: Evolutionary Change The nature of diversity is a defining feature of existence. About 1.5 million species have been recognized and named by biologists, including around 260,000 plants, nearly 50,000 vertebrates (animals with backbones), and over 750,000 insects. Every year, thousands of new species are added to the list. The entire variety of life is estimated to be between 5 million and 100 million species. Biologists classify life’s vast variety into three major groupings known as domains: Bacteria, Archaea, and Eukarya. Prokaryotes (single-celled organisms with little internal structures) make up the domains Bacteria and Archaea, whereas eukaryotes (organisms formed of a complexly structured cell or numerous complex cells) make up the domain Eukarya. Archaea, on the other hand, appears to be more closely connected to Eukarya than Bacteria. The Eukarya is divided into four major categories known as kingdoms. Humans are included in the Kingdom Animalia, while all plants are included in the Kingdom Plantae. Kingdoms are further classified as phylum, the phylum to class, class to order, order to family, family to genus, and genus to species. These are the eight most important classifications of life. When we question ourselves, “How do such various life forms of living organisms form?” All scientists agree that it occurs as a result of evolution.

1.4.5. Unity of Life: Evolutionary Conservation Biologists think that all living things descended from a primitive cellular organism that appeared around 2.5 billion years ago. Some of the first

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organism’s traits have been retained in all living organisms today. DNA, for example, is used by all living organisms to store genetic information. Furthermore, all eukaryotes have a nucleus that contains chromosomes. The persistence of these preserved features in a long line of descent typically suggests a basic role in the biology of the organism, one that is difficult to modify once chosen. These five features are the observable basic themes in biology, and we must analyze all biological hypotheses in relation to these themes.

1.5. EVOLUTIONARY THEORY/ THEORY OF EVOLUTION Biology, generally, is a complex subject with numerous sub-categories; nevertheless, this introduction book is a preparatory textbook, and so it will offer an important overview of biology chapters. That is scientific evolutionary theory, cell and gene theory, development and reproduction theory, and immune system theory. A number of classical Greek philosophers held that life evolved gradually. However, the most influential thinkers in Western civilization, Plato and his disciple Aristotle held views that resisted any idea of evolution in which species are permanent, flawless, and do not develop.

Figure 1.3. Image showing evolutionary changes. Source: Image by Wikimedia commons

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Biology as a Natural Science: The Study of Life in all its Forms

The Old Testament narrative of creation reinforced the belief that species were uniquely formed and permanent in Judeo-Christian society. Natural theology, a philosophy committed to uncovering the Creator’s design by examining nature, dominated biology in Europe and America in the 1700s. Natural theologians interpreted organism adaptations as proof that the Creator had formed each and every species for a specific purpose. One of the primary goals of natural theology was to classify species in order to disclose the stages of life that God had created. Carolus Linnaeus, a Swedish physician and botanist, tried to seek order in the diversity of life in the 18th century “for the greater glory of God.” Linnaeus established taxonomy, the discipline of biology concerned with identifying and categorizing the many forms of life. He devised the twopart, or binomial, system of identifying organisms by genus and species, which is still in use today. Linnaeus also developed a technique for categorizing comparable species into a hierarchy of increasingly generic groups. Comparable species, for example, are placed in the same genus, and similar genera (plural of genus) are grouped in the same family, and so on. Linnaeus believed that grouping similar species together showed no evolution. However, his taxonomy system would become a major point in Darwin’s arguments for evolution a century later. Natural scientists began investigating fossils in the early nineteenth and late nineteenth centuries. Fossils are the petrified remnants or imprints of an ancient creature preserved as a mold or cast in rock. There were several viewpoints and discussions regarding the geological formation and how life developed it based on fossils. Georges Cuvier, a French anatomist, was substantially responsible for the development of paleontology, or the study of fossils. He studied the succession of fossil species in the Paris Basin after realizing that the history of life is recorded in strata holding fossils. He observed that each stratum is distinguished by a distinct set of ancient species and that the deeper (older) the stratum, the more distant the flora (plant life) and fauna (animal life) are from the present life. Cuvier even acknowledged that extinction was a recurrent occurrence throughout the history of life. New species emerge while others vanish from layer to stratum. Nonetheless, Cuvier was a fervent opponent of evolutionists in his day. Instead, he promoted catastrophism, the theory that each border between

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layers corresponded in time to a disaster, such as a flood or a drought, that had wiped out many of the species that lived there at the time. He suggested that these cyclical catastrophes were typically restricted to particular geographical zones and that the ravaged region was repopulated by species moving from other locations, such as mountain sliding to make ground and plane bulging to build mountains. The doctrine of gradualism had been attacked by the foremost geologist of Darwin’s time; a Scot called Charles Lyell. He included Hutton’s gradualism in his uniformitarianism thesis. The word relates to Lyell’s theory that geological processes have not altered throughout time. Thus, the forces that form and destroy mountains, as well as the speeds at which these forces work, are the same now as they were in the past. Darwin was heavily affected by two findings that sprang directly from Hutton and Lyell’s discoveries. First, since geological change is caused by steady, ongoing acts rather than abrupt catastrophes, Earth must be very ancient, considerably older than the 6000 years ascribed by many theologians based on biblical argument. Second, relatively slow and subtle processes that continue over time can produce significant change. Darwin was not the first to apply the gradualism concept to biological evolution. Toward the close of the 18th century, numerous naturalists proposed that life developed in tandem with Earth’s development. However, only one of Darwin’s precursors, Jean Baptiste Lamarck, devised a complete model that sought to describe how life evolves. Lamarck’s theory of evolution was published in 1809, the year Darwin was born. He agreed that living species on Earth developed from a simpler nature to a more complex one, and he gave an explanation for how specific adaptations emerged. It included two prevalent notions during Lamarck’s time. The first was use and disuse, which proposed that those elements of the body that were frequently employed to cope with the environment grew stronger and bigger. Those that are not in use degrade. Lamarck gave instances such as a blacksmith growing a larger bicep in the arm that handles the hammer and a giraffe lengthening its neck to reach leaves on high trees. Lamarck’s second hypothesis was known as the inheritance of acquired qualities. The alterations that an organism develops over its existence can be handed down to its offspring according to this idea of heredity.

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Lamarck reasoned that the giraffe’s long neck evolved gradually as the result of many generations of ancestors rising progressively higher. Yet, there really is no indication of whether learned traits may be inherited. Charles Darwin not just founded the evolutionary theory solidly, and yet also demonstrated precisely how evolution happens, transforming it into a fundamental premise of biology. Charles had a lifelong fascination with nature, even as a child. Darwin was 22 years old when Darwin set off for five years from Great Britain aboard HMS Beagle in December 1831 on his expedition to numerous continents and islands across the world. During this long journey, Darwin had the opportunity to examine and study in detail a broad range of birds, plants, flowers, fossils, and other organisms from various regions of the globe. Throughout this long journey, he discovered data that challenged the core assumptions of evolution. Shortly after his return, Darwin began reevaluating all the knowledge and data and had gathered on the Beagle trip, and eventually arrived at the conclusion that the enormous variety of life on Earth resulted from evolution and natural selection. That is, biological creatures, evolve by natural selection, and natural selection happens through organisms’ adaptation to their environment. It occurred to him that by gradually accumulating adaptations to a different environment, a new species may emerge from an original one. Darwin developed his renowned evolutionary concepts and amassed significant data to publish The Origin of Species in 1859. Following the publishing of the book, it quickly became a biological manifesto.

1.5.1. Descent with Modification Second, there are five points to consider while discussing the general meaning of evolution. These are mutational descent, variation, natural selection, adaptability, and species genesis. Darwin saw life as a unified whole, with all species descended from an unidentified prototype that existed in the distant past.

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Over millions of years, the descendants of that first creature gained numerous alterations, or adaptations, that adapted them to certain ways of living. Descent with modification refers to the transmission of exterior phenotypic features as well as an internal genetic variation to offspring. This is the central concept in Darwin’s theory on the nature of life, which encompassed thoughts on the transmission of genetic ancestral traits to descendants. When questioned if it is feasible to carry on all the qualities of ancestors to offspring, the answer is no, because genetic features cannot be handed on. Color of eyes and skin, blood type, baldness, and so on are all transferrable features in humans. Most features acquired via learning and other conditions in life, like large blacksmith arms and organ deterioration caused by shock, are not transferable. Transferable qualities are passed on from parents to child via DNA, a gene-holding molecule. As a result of changes in the transferable traits caused by causes and situations, new characteristics are passed on to offspring and eventually spread over the population of that organism, and therefore evolution occurs in the population of that organism.

1.5.2. Variation Second, variation relates to population disparities. When humans pay attention to our surroundings, humans can see many different variations among populations, such as differences in skin color, facial structure, body height, and so on in the human population, various character traits in the same species of domesticated animals, and various kinds of color and size in the same species of flowers. The phrase dominant gene refers to visible qualities, while characteristicbased genes are referred to as genetic makeup. One may infer population genetic composition differences based on population dominant gene variations, and population dominant genetic variations come from genetic composition differences. The genetic content of the population changes through time and spreads across the population, and when this causes a change in the character of the population, it is regarded that evolution has occurred in the population. And what if we questioned by what has the variation occurred?

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Biology as a Natural Science: The Study of Life in all its Forms

It arose as a result of changes in genetic makeup, gene modification from mating, gene flow, and gene movement from one group to another as a result of population migration. More information will be provided in the second-year chapters. Variation is a major source of evolution because evolution will not occur if there are no varied variations in the population. But if one asked how evolution happens in a population with such variance, the process is known as natural selection, which is detailed further below.

1.5.3. Natural Selection Third, there are three methods to talk about natural selection: broad meaning, examples of it happening, and kinds. First, consider the definition or meaning. Organisms with characteristics that change in responding effectively to their environment than the rest of the same species have a better chance of survival and reproduction, resulting in more descendants and, as a result, a good chance of passing on the advantageous genetic composition to coming generations. The genes of organisms with favorable features are thereby retained and handed on to successors. This is known as Natural Selection. One wellknown empirical illustration of natural selection is the fact that throughout the industrial revolution, many factories were operating in several locations, emitting smoke from the combustion of coal. The fumes rendered nearby woodlands black. There were two kinds of moths: white and black. Because of the darkening of the trees and branches caused by the black smoke, provided ideal conditions for the survival of the black moth relative to the white moth, allowing it to survive longer and reproduce more effectively. Birds cannot see since moths and tree branches are the same color, making it easier for them to flee the danger. White moths become prey to birds because they are easier to notice on black trees. Gradually, factual evidence of the change of a former population of mixed moths to a black population is among the easiest and most well-known instances of natural selection.

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Figure 1.4. Image showing natural selection & coevolution. Source: Image by Wikimedia commons

Second, we may use an example to demonstrate how natural selection works, including how there really are few conditions for natural selection to occur in a population of beetles. The first criterion is trait variation. For example, beetles have many various physical colors, such as green and brown, while moths have white and black features. The second situation is that of inherited reproductive disparities. Not only may the population not increase indefinitely, but it can also make full use of the potential for hereditary reproduction, resulting in disparities in hereditary reproduction in the population of organisms. Because of the appearance of green insects, in this case, these fell victim to birds, therefore their potential for genetic reproduction is less than that of brown beetles. The third condition is traits are inherited genetically. Since these features have a genetic foundation, brown beetles spawn brown offspring and green insects create green offspring. As a result, organisms with qualities that provide them with an advantage over their rivals are much more likely to be passed on their characteristics to the next generations than those with traits that do not offer an advantage, resulting in population spread.

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The population as a whole becomes the population with such qualities. And if there are differences in qualities within the population, as well as changes in the reproduction of each organism in the population, features that are genetically inherited, then follows that evolution happens via natural selection. Finally, there are several forms of natural selection, such as directional selection, disruptive selection, stabilizing selection, sexual selection, artificial selection, and so on. When we ask about the effects of evolution via natural selection, we find that, while evolution does alter the characteristics and behavior of animals, the most evident results are behavioral adaptation and physical adaptation. As a result, we shall talk a little bit about adaptation.

1.5.4. Adaptation Fourth, there are three perspectives when discussing adaptation: the nature of adaptation, the method of adaptation, and the effects of adaptation. To begin, adaptation is a procedure through which organisms become more fitted to their environment. Second, adaptedness is the condition of being adapted: the extent to whereby an organism can live and reproduce in a particular set of environments. Organisms must compete for adequate circumstances to survive and reproduce, and while doing so, organisms with environmental adaptation features are naturally selected and passed on to the next generation. Third, in terms of the effects of adaptation, adaptation allows or increases the likelihood of that organism surviving and reproducing. When an organism lacks adaptive features, its survival and reproduction suffer. As a result, adaption aids in population growth.

1.5.5. The Origin of Life Sixth, scientists generally believe the universe started around 13.7 billion years ago and Earth formed around 4.5 billion years ago. Scientists are unable to pinpoint the precise time of the genesis of life, nor have scientists discovered the required proof. As per the assertion, when the earth began to form, all of its interior and exterior portions were composed of fire, and the surface temperature began to fall. The Earth is located at an appropriate distance from the Sun, and various conditions, such as vast pools of water, have prepared it for life to start and grow continually. Mercury and Venus are too close to the Sun, while Mars

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and Jupiter are too far away, posing hurdles to the formation of life on these planets. Carbon is an essential atom for all creatures, and it is thought to have influenced the creation of many types of life in the early days of Earth. The vapors produced by volcanic eruptions are one of the principal sources of carbon. Carbon, hydrogen, oxygen, and nitrogen are combined to produce an essential type of chemical known as amino acid, which serves as a core component for protein. Because amino acids are the building blocks of protein, they are also the foundation of early forms of life. Enzymes and catalysts made of proteins have become essential for organisms to carry out their various biological tasks. In general, organisms evolve from their most basic to their most sophisticated form. For example, even though organisms began in extremely modest numbers and sizes, they progressively grew and became more diversified in nature. What is evident however is that an organism that is dependent on other cells cannot be called the source of life. It is due to the fact that one must rely on other organisms in order to survive and reproduce, and hence it cannot be claimed as originating from the condition of an earlier life form. However, viruses and other organisms that are dependent on other cells might be considered to have evolved after the beginning of the development of living things. One must regard something physical as a living entity on this planet when it draws nourishment from others and begins to breathe, and this is to consider the production of amino acids through chemical reactions in the world’s oceans and lakes stretching back 3 billion years. However, other scientists believe that life evolved on other exoplanets rather than on Earth. A few researchers have discovered proof of RNA molecule creation by chemical processes in the initial stages of the planet. According to some experts, life must be evaluated from the standpoint of reproduction ability to a mixture of chemical compounds. At the moment, most researchers undertake various types of experiments in laboratories in order to gain a better knowledge of the origins of life and seek to create comparable chemical compounds or entities.

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1.6. THE DIVERSITY OF LIFE Due to the obviously incredible diversity of life on Earth, biology is a discipline with a very broad reach. Evolution, the process of progressive change in which new species emerge from older ones, is the origin of this variety. Evolutionary biologists examine the evolution of living things from the microscopic to the macrocosmic. Carl Linnaeus, an 18th-century scientist, was the first to suggest arranging known species of organisms into a hierarchical taxonomy. Species most similar to each other are grouped under a genus in this arrangement. Furthermore, within a family, comparable genera (the plural of genus) are grouped together. This process is repeated until all organisms are grouped together at the highest level. From lowest to highest, the present taxonomic system contains eight degrees of hierarchy: species, genus, family, order, class, phylum, kingdom, and domain. Thus, species are categorized inside genera, genera within families, families within orders, and so forth. The highest level, domain, was introduced to the system in the 1990s. Scientists today acknowledge three types of life: Eukarya, Archaea, and Bacteria (also called the Eubacteria). Organisms having nuclei are found in the domain Eukarya. It comprises the kingdoms of fungi, plants, animals, and various protist kingdoms. Archaea are single-celled organismslacking nuclei that thrive in extreme settings such as hot springs. Bacteria are a distinct type of single-celled creature with no nucleus. Archaea and Bacteria are both prokaryotes, which is an informal term for cells that lack nuclei. The discovery in the 1990s that certain “bacteria,” now known as Archaea, were genetically and biochemically distinct from other bacterial cells as they were from eukaryotes prompted the suggestion that life is divided into three domains. This tremendous shift in our understanding of the tree of life shows that categories are not fixed and will shift whenever new evidence comes to light. In addition to the hierarchical taxonomic system, Linnaeus was the first to name organisms using two unique names, now called the binomial naming system. Before Linnaeus, the use of common names to refer to organisms caused confusion because there were regional differences in these common names. Binomial names consist of the genus name (which is capitalized) and the species name (all lower-case).

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Both names are set in italics when they are printed. Every species is given a unique binomial that is recognized the world over so that a scientist in any location can know which organism is being referred to. For example, the North American blue jay is known uniquely as Cyanocitta cristata. Our own species is Homo sapiens.

1.7. BRANCHES OF BIOLOGICAL STUDY Because biology has such a large reach, it has several branches and subdisciplines. Biologists can specialize in one of these sub-disciplines and work in a narrower field. For example, molecular biology investigates biological processes at the molecular level, such as interactions between molecules like DNA, RNA, and proteins, as well as how they are controlled. The study of microorganisms, or microbiology, is the study of the structure and function of single-celled organisms. It is a vast field, including microbial physiologists, ecologists, and geneticists, among others, depending on the area of research. Neurobiology is another discipline of biological research that explores the biology of the nervous system. Although it is considered a part of biology, Neuroscience is also acknowledged as an interdisciplinary branch of research. Because of its multidisciplinary character, this subfield investigates many nervous system functions utilizing molecular, cellular, developmental, medicinal, and computational methodologies. Paleontology, another area of biology, studies the evolution of life via the use of fossils. The study of animals and plants is known as zoology and botany, accordingly. Biologists can also pursue careers as biotechnologists, ecologists, or physiologists, to mention a few. Biotechnologists employ their biological expertise to make beneficial goods. Ecologists investigate how organisms interact with their surroundings. Physiologists investigate the functions of cells, tissues, and organs. This is only a small selection of the various careers available to biologists. Biology discoveries may have a direct and significant impact on us, from our own bodies to the environment individuals live in. People rely on these findings for our health, food supplies, and the advantages that our environment provides. As a result, understanding biology may help us make decisions in our daily life. The development of technology in the twentieth century, notably the ability to define and control genetic material, DNA, has altered biology.

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This shift will help biologists to better understand the history of life, how the human body functions, our human beginnings, and how humans may exist as a species on this planet despite the challenges created by our growing population. Biologists continue to solve vast riddles regarding life, implying that we have just scratched the surface of our understanding of life on Earth, its history, and our relationship with it. For that and other reasons, the biology information obtained from this textbook and other paper and digital sources should be useful in whichever career you choose.

1.8. THE PROCESS OF SCIENCE Biology, like geology, physics, and chemistry, is a science that collects aspects of the natural world. Biology, in particular, is the study of life. Biology is discovered by a community of scientists that work independently and collaboratively through agreed-upon procedures. In this sense, biology, like other disciplines, is a social endeavor in the same way that politics or the arts are. Science procedures involve diligent inspection, maintaining records, logical and mathematical reasoning, experimentation, and presenting results to peer review. Science necessitates a great deal of imagination and ingenuity; a well-designed experiment is sometimes regarded as elegant or beautiful. Science, like politics, has significant functional ramifications, and some science is dedicated to practical applications, such as illness prevention. Other science is driven mostly by curiosity. Whatever its purpose, there is little question that science, particularly biology, has and will continue to revolutionize human existence.

1.8.1. The Nature of Science Biology is a science, but what is science in general? What are the similarities and differences between biology as well as other scientific disciplines? Science (from the Latin Scientia, which means “knowledge”) is the study of the natural world. Science is a highly specialized approach to learning or understanding the universe. The last 500 years have also shown that science is a highly strong means of understanding the world; it is mainly responsible for the technological revolutions that have occurred throughout this period. Nevertheless, there

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are a few realms of information and human experience where scientific approaches cannot be used. This includes resolving solely moral issues, artistic concerns, and religious issues in general. Science has nothing to offer in these areas since they are outside the sphere of material phenomena, the phenomena of matter and energy, and cannot be viewed or quantified. A science-based methodology is a research approach that includes experimentation and thorough analysis. The phases of the scientific process would be discussed in depth afterwards, however testing hypotheses is among the most significant components of this process. A hypothesis is an explanation for an occurrence that can be tested. In most cases, hypotheses, or preliminary explanations, are developed within the context of a scientific theory. A scientific theory is an explanation for a collection of facts or events that is widely accepted, carefully examined, and proven. The cornerstone of scientific understanding is a scientific theory. Furthermore, there are scientific laws, frequently stated in mathematical formulae, that govern how components of nature will act under particular precise situations in many scientific disciplines (less so in biology). There is no progression from hypotheses to theories to laws, as though each indicated an increase in certainty about the world. Hypotheses are the material that scientists deal with on a daily basis, and they are generated within the context of theories. Laws are brief explanations of aspects of the world that may be described using formulas or mathematics.

1.9. NATURAL SCIENCES What would one anticipate finding in a natural sciences museum? Frogs? Plants? Dinosaur bones? Brain-related displays? What’s a planetarium? What about gemstones and minerals? Or might it be all of the above? Astronomy, biology, computer sciences, geology, logic, physics, chemistry, and mathematics are all branches of science. Natural sciences, on the other hand, are those branches of study concerned with the physical world and its events and processes. As a result, any of the things described above may be found in a natural science museum.

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Biology as a Natural Science: The Study of Life in all its Forms

Figure 1.5. Image showing the National Institute of Natural Sciences, Japan. Source: Image by Wikimedia commons

Whenever it comes to determining whatever the natural sciences are, there is no universal consensus. According to some experts, the natural sciences include astronomy, biology, chemistry, earth science, and physics. Others split natural sciences into life sciences, which investigate living organismsand include biology, and physical sciences, which investigate the nonliving matter and include astronomy, physics, and chemistry. Some interdisciplinary studies, such as biophysics and biochemistry, are composed of two sciences.

1.9.1. Scientific Inquiry Another element most types of science have in common is the ultimate objective of “knowing.” Curiosity and investigation are the driving elements behind scientific progress. Scientists aim to comprehend the world and how it works. There are two types of logical reasoning: inductive reasoning and deductive reasoning. Inductive reasoning is a type of logical reasoning in which related observations are used to reach a general conclusion. In descriptive science, this type of reasoning is common. A biologist, for example, makes observations and records them. These data might be qualitative (descriptive) or quantitative (numerical), and they can be augmented with drawings, photographs, photos, or videos.

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Supported by evidence, the researcher can make conclusions (inductions) from a large number of observations. Deductive approach reasoning is the process of constructing assumptions based on attentive observation and data analysis. This is how many brain studies operate. While individuals are performing a task, several brains are observed. The area of the brain that lights up, showing activity, is then shown to be the part of the brain that controls the reaction to that task. In hypothesisbased science, deductive reasoning or deduction is applied. When compared to inductive reasoning, the pattern of thought in deductive reasoning moves in the other way. Deductive reasoning is a type of logical reasoning in which a general principle or law is used to foresee specific conclusions. A scientist can extrapolate and anticipate particular consequences from those broad principles as long as the general principles are correct. For example, as the temperature in a region warms, the distribution of plants and animals should alter. Comparisons of distributions in the past and present have been done, and the numerous changes discovered are compatible with a warming environment. The discovery of the shift in dispersion provides proof that perhaps the global warming conclusion is correct. Both kinds of logical reasoning are associated with the two major scientific research paths: descriptive science and hypothesis-based science. Descriptive (or discovery) science is concerned with observing, exploring, and discovering, whereas hypothesis-based science begins with a particular question or issue and a probable response or solution which can be verified. Since most scientific activities involve both methodologies, the line between these two types of research is frequently blurry. Findings raise some questions, and inquiries result in the formation of a hypothesis as a possible solution to those concerns, which is subsequently tested. As a result, descriptive science and hypothesis-based science are constantly conversing.

1.9.2. Hypothesis Testing Biologists investigate the living world by asking questions and finding scientific answers. Such technique is used in many different fields and therefore is usually referred to as the natural sciences. The scientific method was employed in olden history, but it was first published by Sir Francis Bacon (1561–1626) of England, who established inductive procedures for scientific research.

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The scientific method is not limited to biologists; it may be used in practically any logical problem-solving situation. Usually, the scientific method begins with an observation (often an issue to be addressed) which proceeds to a question. Consider a basic problem that begins with an observation and is solved using the scientific process. A student comes to class on a Monday morning and immediately learns that the classroom is far too warm. That observation also highlights a problem: the classroom is overheated. The youngster then inquires, “Why is the classroom so warm?” Remember that a hypothesis is an explanation that can be tested. Several theories may be suggested to address an issue. One theory may be, “The classroom is hot because no one switched on the air conditioner.” However, various replies to the question may exist, and hence other hypotheses may be suggested. A second possibility is that “the classroom is heated because there is a power outage and the air conditioner is not working.” A prediction can be made once a hypothesis has been chosen. A prediction is similar to a hypothesis, although it usually follows the formula “If... then... The prediction for the first hypothesis, for example, maybe, “If the student switches on the air conditioning, the classroom will no longer be overly warm.” To be valid, a theory must be tested. A hypothesis that is dependent on what a bear thinks, for example, is unverifiable since it is impossible to determine what a bear thinks. It must also be falsifiable, which implies that it could be proven false by experimental evidence. “Botticelli’s Birth of Venus is lovely,” for example, is an example of an unfalsifiable hypothesis. There is no research that could prove this assertion wrong. To test a hypothesis, a scientist will undertake one or more experiments aimed to rule out one or more possibilities. This is critical. A theory can be confirmed or proven false, but it cannot be proved. Science, unlike mathematics, doesn’t quite deal with evidence. If such a project fails to refute a hypothesis, then seek support for that reason; nevertheless, this does not mean that a better clarification or a more precisely constructed experiment will not be discovered later to invalidate the hypothesis. There will be one or more factors and one or more controls in each experiment. A variable is any aspect of the experiment that can alter or vary during the course of the experiment. A control is a constant component of

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the experiment. Look identify the variables and controls in the following example. As an example, an experiment may be carried out to test the theory that phosphorus reduces algal development in freshwater ponds. A series of artificial ponds are filled with water, and half of them are treated each week by adding phosphate, while the other half is treated by introducing a salt that algae are known not to consume. The variable here is phosphate (or lack thereof), the experimental or treatment instances are the ponds with additional phosphate, and the control ponds are those with something inert added, such as salt. Simply adding stuff is also a control against the potential that adding more matter to the pond has an impact. If the treated ponds exhibit decreased algal growth, we have discovered evidence for our idea. If they do not, we reject our hypothesis. Be mindful that rejecting one hypothesis does not indicate whether or not the other hypotheses may be accepted; it merely removes one hypothesis that is invalid. Hypotheses that are incongruous with experimental findings are rejected using the scientific process. In actuality, the scientific process is not as rigorous and regimented as it appears on the surface. Sometimes an experiment yields results that support a different method; other times, an experiment raises totally new scientific problems. Science does not always proceed in a straight method; instead, scientists constantly draw conclusions and generalizations, discovering patterns as their study progresses. Scientific thinking is more complicated than the scientific process implies.

1.10. BASIC AND APPLIED SCIENCE For decades, the scientific community has debated the importance of various sorts of science. Is it worthwhile to pursue science just for the sake of knowledge, or does scientific information only have value if we can apply it to addressing a specific problem or improving our lives? This question focuses on the distinctions between fundamental science and applied science. Basic science, sometimes known as “pure” science, attempts to broaden knowledge independent of its immediate application. It is not concerned with creating a product or service with immediate public or economic value. The immediate purpose of fundamental research is knowledge for the sake

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of knowledge, but this does not rule out the possibility of an application in the future. In contrast, applied science or “technology” tries to apply research to address real-world issues, such as increasing crop productivity, discovering a cure for a specific disease, or saving animals endangered by a natural disaster. This issue is commonly described by scientists in applied science. Some people may see applied science as “useful,” but basic science as “useless.” “What for?” these folks could ask a scientist who advocates for knowledge gain. A close examination of the history of science, on the other hand, indicates that basic knowledge has resulted in several surprising applications of tremendous value. Many scientists believe that a fundamental grasp of science is required before developing an application; hence, applied science is dependent on basic research outcomes. Others believe that it is time to go beyond fundamental research and focus on finding answers to real-world challenges. Both techniques are correct. Certainly, there are situations that require immediate attention; nonetheless, few remedies would be discovered without the assistance of basic scientific understanding. After the discovery of DNA structure led to a knowledge of the molecular mechanisms driving DNA replication, one example of how fundamental and applied research might collaborate to tackle practical issues happened. Strands of DNA, which are unique to each individual, are present in our cells, where they offer the instructions for life. New copies of DNA are created soon before a cell splits to make new cells during DNA replication. Knowing the processes of DNA replication allowed researchers to develop laboratory procedures that are currently used to diagnose genetic illnesses, identify persons who were present at a scene of the crime, and establish paternity. It seems doubtful that applied science would exist in the absence of basic science. Another example of the relationship between fundamental and practical research is the Human Genome Project, which studied and mapped each human chromosome to identify the specific sequence of DNA subunits and the precise position of each gene. (The gene is the fundamental unit of inheritance; an individual’s whole collection of genes is referred to as his or her genome.)

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As part of this study, other organisms have been examined in order to acquire a better knowledge of human chromosomes. The Human Genome Project(HGP) was founded on fundamental research conducted on nonhuman species and, later, on the human genome. One significant end objective was to use the data for applied research to find remedies for genetically connected disorders. Since scientific research in both fundamental and applied science is normally properly planned, it is crucial to highlight that certain findings are produced by serendipity, that is, by a fortunate incident or a happy surprise. When researcher Alexander Fleming mistakenly left an uncovered petri dish of Staphylococcus germs, Fleming developed penicillin. An unwelcome mold developed, destroying the germs. The mold proved to be Penicillium, and a new antibiotic was developed. Though in the highly organized field of science, luck, if paired with an alert, inquiring mind, may lead to unwanted discoveries.

1.11. CONCLUSION How do the various living things work? Living things come in all sorts of sizes, forms, and purposes, and biologists study life from the tiniest cell to the largest ecosystems. This branch of science studies all live things or living beings. Biology, Biology, in its widest meaning, is the study of living things. For example, virology is a discipline of biology that examines viruses, which have some of the properties of living beings but lack others. Biology is the study of living things. It focuses on and studies topics relating to living beings such as life organization, functions, patterns and order of organisms, growth and development of living organisms, etc. As a result, biology has many branches and divisions, including evolutionary biology, cellular biology, genetics, growth and developmental biology, and so on. From an objective point of view, it may also be referred to as the science of life. Microorganisms are assumed to have been the original lifeforms on the Planet, existing billions of years before animals and plants evolved.

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REFERENCES 1.

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Rms.rsccd.edu. n.d. Concepts of Biology. [online] Available at: [Accessed 2 July 2022]. S3-us-west-2.amazonaws.com. n.d. Introduction to Biology. [online] Available at: [Accessed 2 July 2022]. Tibet.emory.edu. n.d. An Introduction to Biology. [online] Available at: [Accessed 2 July 2022]. Baldwin, G., Bayer, T., Dickinson, R., Ellis, T., Freemont, P., Kitney, R., Polizzi, K. and Stan, G., 2015. Introduction to Biology. Synthetic Biology — A Primer, [online] pp.1-18. Available at: [Accessed 4 July 2022]. Warwick, R., 1974. Introduction to Biology.  BMJ, [online] 1(5900), pp.205-205. Available at: [Accessed 4 July 2022].

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THE SCIENCE OF LIFE AND ITS CHEMICAL BASIS

CONTENTS 2.1. Introduction....................................................................................... 32 2.2. What Kind of Molecules Characterize Living Things?......................... 37 2.3. What are the Chemical Structures and Functions of Proteins?............ 39 2.4. What are the Chemical Structures and Functions of Carbohydrates?.. 40 2.5. What are the Chemical Structures and Functions of Lipids?............... 44 2.6. What are the Chemical Structures and Functions of Nucleic Acids?... 48 2.7. How Did the First Cells Originate?..................................................... 51 2.8. What Features Make Cells the Fundamental Units of Life?................. 52 2.9. What is the Structure of a Biological Membrane?............................... 56 2.10. What Are Signals, and How Do Cells Respond to Them?................. 57 2.11. How Do Cells Communicate Directly?............................................ 60 2.12. Conclusion...................................................................................... 63 References................................................................................................ 64

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In its broadest sense, biology is the study of living things—the science of life. Living things come in an astounding variety of shapes and forms, and biologists study life in many different ways. They live with gorillas, collect fossils, and listen to whales. They isolate viruses, grow mushrooms, and examine the structure of fruit flies. They read the messages encoded in the long molecules of heredity and count how many times a hummingbird’s wings beat each second.

2.1. INTRODUCTION Biology is the scientific study of living things. Biologists define “living things” as all the diverse organisms descended from a single-celled ancestor that evolved almost 4 billion years ago. Because of their common ancestry, living organisms share many characteristics that are not found in the nonliving world. Living organisms: • • • • •

consist of one or more cells contain genetic information use genetic information to reproduce themselves are genetically related and have evolved can convert molecules obtained from their environment into new biological molecules • can extract energy from the environment and use it to do biological work • can regulate their internal environment This simple list, however, belies the incredible complexity and diversity of life. Some forms of life may not display all of these characteristics all of the time. For example, the seed of a desert plant may go for many years without extracting energy from the environment, converting molecules, regulating its internal environment, or reproducing; yet the seed is alive.

2.1.1. Cells are the Basic Units of Life The discovery of cells was made possible by the invention of the microscope in the 1590s by the Dutch spectacle makers Hans and Zacharias Janssen (father and son). In the mid to late 1600s, Antony van Leeuwenhoek of Holland and Robert Hooke of England both made improvements to the Janssens’ technology and used it to study living organisms.

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Van Leeuwenhoek discovered that drops of pond water teemed with single-celled organisms, and he made many other discoveries as he progressively improved his microscopes over a long lifetime of research. Hooke put pieces of plants under his microscope and observed that they were made up of repeated units he called cells. In 1676, Hooke wrote that van Leeuwenhoek had observed: “a vast number of small animals in his Excrements which were most abounding when he was troubled with a Looseness and very few or none when he was well.” This simple observation represents the discovery of bacteria—and makes one wonder why scientists do some of the things they do.

Figure 2.1. Image showing cells information. Source: Image by Flickr

More than a hundred years passed before studies of cells advanced significantly. As they were dining together one evening in 1838, Matthias Schleiden, a German biologist, and Theodor Schwann, from Belgium, discussed their work on plant and animal tissues, respectively. They were struck by the similarities in their observations and concluded that the basic structural elements of plants and animals were essentially the same. They formulated their conclusion as the cell theory, which states that:

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•

Cells are the basic structural and physiological units of all living organisms. • Cells are both distinct entities and building blocks of more complex organisms. Schleiden and Schwann mistakenly believed that cells emerged by the self-assembly of nonliving materials, similar to crystals that form in a solution of salt. This conclusion was aligned with the prevailing view of the day that stated, that life can arise from non-life by spontaneous generation— mice from dirty clothes, maggots from dead meat, or insects from pond water. The debate continued until 1859 when the French Academy of Sciences sponsored a contest for the most convincing experiment to prove or disprove spontaneous generation. The prize was won by the celebrated French scientist Louis Pasteur, who demonstrated that sterile broth directly exposed to the dirt and dust in air developed a culture of microorganisms, but a similar container of broth not directly exposed to air remained sterile. Even though Pasteur’s experiment could not prove that microorganisms in the air caused the broth to become dangerously infected, it did uphold the conclusion that life must be present to generate new life. Today scientists accept the fact that all cells come from pre-existing cells and that the functional properties of organisms derive from the properties of their cells. Since cells of all kinds share both essential mechanisms and a common ancestry that goes back billions of years, modern cell theory has additional elements: • • •

All cells come from pre-existing cells. All cells are similar in chemical composition. Most of the chemical reactions of life occur in an aqueous solution within cells. • Complete sets of genetic information are replicated and passed on during cell division. • Viruses lack cellular structure but remain dependent on cellular organisms. During the same period when Schleiden and Schwann were building the foundation for the cell theory, Charles Darwin had begun his work to understand how organisms underwent evolutionary changes.

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2.1.2. Cells Use Nutrients to Supply Energy and Build New Structures The environment provides nutrients to living organisms. Organisms need nutrients to perform biochemical reactions, which provide them with energy. Each cell is responsible for thousands of biochemical reactions. The reactions that break down nutrient molecules into smaller units are thought to liberate some of the energy from the chemical bonds of the nutrients, which are then used for different types of cellular work. Cells perform mechanical functions, including moving molecules from one location to another, moving entire cells or tissues, or even moving the organism itself, as muscles do. The most basic cellular work is the building, or synthesis, of new complex molecules and structures from smaller chemical units. For example, we are all familiar with the fact that carbohydrates eaten today may be deposited in the body as fat tomorrow. There is one more kind of work that is the essence of nervous system information processing: electrical work. The metabolic rate of an organism is the sum of all the chemical shifts and other activities that occur in every cell.

The reactions that occur within each cell are intrinsically linked in that the raw materials for each one are the products of the previous one. Disease results when these complex networks of reactions are not controlled precisely and integrated.

2.1.3. Living Organisms Regulate Their Internal Environment The internal environment of multicellular organisms is not cellular. They do this by bathing their cells in extracellular fluids, from where they receive nutrients and into which they expel metabolic waste. The cells of multicellular organisms are specialized or differentiated, to contribute in some way to the maintenance of the internal environment. With the evolution of specialization, differentiated cells lost many of the functions carried out by single-celled organisms, and must depend on the internal environment for essential services. Individual cells are organized into tissues so that they can perform their specialized tasks. As an example, a single muscle cell cannot produce much force, but when multiple cells work together, a great deal of force and movement can be generated. An organ is made up of different tissues that perform specific

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functions. Hearts, brains, and stomachs, for instance, are all constructed of a variety of cells. The digestive system consists of the stomach, intestines, and esophagus, among other organs whose functions are interconnected. A multicellular organism consists of cells, tissues, organs, and organ systems. 

2.1.4. Living Organisms Interact With One Another The internal hierarchy of the individual organism is matched by the external hierarchy of the biological world. Organisms do not live in isolation. A group of individuals of the same species that interact with one another is a population, and populations of all the species that live and interact in the same area are called a community. Communities together with their abiotic environment constitute an ecosystem. Individuals in a population interact in many different ways. Animals eat plants and other animals (usual members of another species) and compete with other species for food and other resources. Some animals will prevent other individuals of their species from exploiting a resource, whether it be food, nesting sites, or mates. Animals may also cooperate with members of their species, forming social units such as a termite colony or a flock of birds. Such interactions have resulted in the evolution of social behaviors such as communication.

Figure 2.2. Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes and prokaryotes. Source: Image by Wikimedia commons

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Plants also interact with their external environment, which includes other plants, animals, and microorganisms. All terrestrial plants depend on complex partnerships with fungi, bacteria, and animals. Some of these partnerships are necessary to obtain nutrients, some to produce fertile seeds, and still, others to disperse seeds. Plants compete with each other for light and water, and they have ongoing evolutionary interactions with the animals that eat them, evolving anti-predation adaptations or ways to attract the animals that assist in their reproduction. The interactions of populations of different plant and animal species in a community are major evolutionary forces that produce specialized adaptations. Communities interacting over a broad geographic area with distinguishing physical features form ecosystems; examples might include an Arctic tundra, a coral reef, or a tropical rainforest.

2.2. WHAT KIND OF MOLECULES CHARACTERIZE LIVING THINGS? Four kinds of molecules are characteristic of living things: proteins, carbohydrates, lipids, and nucleic acids. Except for the lipids, these biological molecules are polymers (poly, “many”; Mer, “unit”) constructed by the covalent bonding of smaller molecules called monomers. The monomers that make up each kind of biological molecule have similar chemical structures: •

Proteins are formed from different combinations of 20 amino acids, all of which share chemical similarities. • Carbohydrates can form giant molecules by linking together chemically similar sugar monomers (monosaccharides) to form polysaccharides. • Nucleic acids are formed from four kinds of nucleotide monomers linked together in long chains. • Lipids also form large structures from a limited set of smaller molecules, but in this case, noncovalent forces maintain the interactions between the lipid monomers Polymers with molecular weights exceeding 1,000 grams per mole are considered to be macromolecules. The proteins, carbohydrates, and nucleic acids of living systems certainly fall into this category.

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Although large lipid structures are not polymers in the strictest sense, it is convenient to treat them as a special type of macromolecule. How the macromolecules function and interact with other molecules depends on the properties of certain chemical groups in their monomers, the functional groups.

2.2.1. Functional Groups Give Specific Properties to Biological Molecules Many different types of molecules consist of small groups of atoms referred to as functional groups. You will encounter several functional groups repeatedly in your study of biology. Each functional group has specific chemical properties and, when it is attached to a larger molecule, it confers those properties on the larger molecule. One of these properties is polarity. The consistent chemical behaviors of functional groups help us understand the properties of the molecules that possess them. Because macromolecules are so large, they contain many different functional groups. A single large protein may contain hydrophobic, polar, and charged functional groups, each of which gives unique specific properties to local sites on the macromolecule. As we will see, sometimes these different groups interact on the same macromolecule. These diverse groups and their properties help determine the shapes of macromolecules as well as how they interact with other macromolecules and with smaller molecules.

2.2.2. The Structures of Macromolecules Reflect Their Functions The four sorts of biological macromolecules are available to generally similar extents in every single living life form. Moreover, a protein that has a specific capacity in an apple tree most likely has a comparable capacity in a person because its science is the equivalent to any place it is found. Such biochemical solidarity mirrors the development of all life from a common ancestor, by descent with modification. A significant benefit of

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biochemical unity is that a few life forms can procure required unrefined substances by eating different creatures. At the point when you eat an apple, the molecules you take in incorporate carbohydrates, lipids, and proteins that can be separated and modified into the assortments of those molecules required by living beings. Each sort of macromolecule plays out a combination of functions, like energy storage, structural support, protection, catalysis (accelerating a chemical reaction), transport, defense, regulation, movement, and information storage. These functions are not restrictive; for instance, carbohydrates and proteins can assume underlying parts, supporting and safeguarding tissues and organs. However, only nucleic acids specialize in information storage and transmission. These macromolecules work as inherited material, conveying the qualities of the two species and people from one age to another. The elements of macromolecules are straightforwardly connected with their three-dimensional shapes and to the arrangements and synthetic properties of their monomers. A few macromolecules fold into reduced circular structures with surface properties that make them water-dissolvable and fit for bonding with different molecules. A few proteins and starches structure long, thread-like frameworks (like those in hair) that give strength and unbending nature to cells and tissues. The long, thin assemblies of proteins such as those in muscles can contract, resulting in movement.

2.3. WHAT ARE THE CHEMICAL STRUCTURES AND FUNCTIONS OF PROTEINS? While all the kinds of large molecules are essential to the function of organisms, few have such diverse roles as proteins. All proteins are polymers made up of different proportions and sequences of 20 amino acids. Proteins range in size from small ones such as insulin, which has a molecular weight of 5,733 Daltons and 51 amino acids, to huge molecules such as the muscle protein titin, with a molecular weight of 2,993,451 Daltons and 26,926 amino acids. All proteins consist of one or more polypeptide chains—unbranched (linear) polymers of covalently linked amino acids.

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Figure 2.3. Structures of proteins. Source: Image by Wikimedia commons

The composition of a protein alludes to the relative amount of the different amino acids present in its polypeptide chains. Variety in the grouping of the amino acids in polypeptide chains is the source of the variation in protein structure and function because each chain folds into a definite three-layered shape that is characterized by the exact arrangement of the amino acids present in the chain. Numerous proteins are comprised of more than one polypeptide chain. For instance, the oxygen-conveying protein hemoglobin has four chains that are collapsed independently and met up to make up the functional protein. Proteins can likewise connect, shaping multi-protein structures that do many-sided roles like DNA bonding.

2.4. WHAT ARE THE CHEMICAL STRUCTURES AND FUNCTIONS OF CARBOHYDRATES? Carbohydrates are a large group of molecules that all have more similar atomic compositions but differ greatly in size, chemical properties, and biological functions. Carbohydrates have the general formula Cn (H2O) n,

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which makes them appear as hydrates of carbon (association between water molecules and carbon in the ratio C1H2O1), hence their name.

When their molecular structures are examined, the linked carbon atoms are seen to be bonded with hydrogen atoms (—H) and hydroxyl groups (— OH), the components of water. Carbohydrates have three major biochemical roles: •

They are a source of stored energy that can be released in a form usable by organisms. • They are used to transport stored energy within complex organisms. • They serve as carbon skeletons that can be rearranged to form new molecules. Some carbohydrates are relatively small, with molecular weights of less than 100 Da. Others are true macromolecules, with molecular weights in the hundreds of thousands. There are four categories of biologically important carbohydrates: •

•

• •

Monosaccharides (mono, “one”; saccharide, “sugar”), such as glucose, ribose, and fructose, are simple sugars. They are the monomers from which the larger carbohydrates are constructed. Disaccharides (di, “two”) consist of two monosaccharides linked together by covalent bonds. The most familiar is sucrose, which is made up of covalently bonded glucose and fructose molecules. Oligosaccharides (oligo, “several”) are made up of several (3–20) monosaccharides. Polysaccharides (poly, “many”), such as starch, glycogen, and cellulose, are polymers made up of hundreds or thousands of monosaccharides.

2.4.1. Monosaccharides Are Simple Sugars All living cells contain the monosaccharide glucose; it is the recognizable “glucose,” used to transport energy in people. Cells use glucose as an energy source, separating it through a progression of responses that deliver put away energy and produce water and carbon dioxide; this is a cell type of the burning response. Glucose exists in straight chains and in-ring forms. The ring forms predominate in virtually all biological circumstances because they are more stable under physiological conditions. There are two versions of glucose

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ring, called α- and β-glucose, which differ only in the orientation of the —H and —OH attached to carbon 1. The α and β forms interconvert and exist in equilibrium when dissolved in water. Different monosaccharides contain different numbers of carbons. Some monosaccharides are structural isomers, with the same kinds and numbers of atoms, but in different arrangements. Such seemingly small structural changes can significantly alter properties. Most of the monosaccharides in living systems belong to the D (righthanded) series of isomers. Pentoses (pence, “five”) are five-carbon sugars. Two pentoses are of particular biological importance: the backbones of the nucleic acids RNA and DNA contain ribose and deoxyribose, respectively. These two pentoses are not isomers of each other; rather, one oxygen atom is missing from carbon 2 in deoxyribose (de-, “absent”). The absence of this oxygen atom is an important distinction between RNA and DNA. The hexoses (hex, “six”), a group of structural isomers, all have the formula C6H12O6. Included among the hexoses are glucose, fructose (so named because it was first found in fruits), mannose, and galactose.

2.4.2. Glycosidic Linkages Bond Monosaccharides The disaccharides, oligosaccharides, and polysaccharides are all constructed from monosaccharides that are covalently merged by condensation reactions that form glycosidic linkages. A single glycosidic association between two monosaccharides forms a disaccharide. For example, sucrose—common table sugar in the human diet and a chief disaccharide in plants—is a disaccharide molded from glucose and a fructose molecule. The disaccharides maltose and cellobiose are made from two glucose molecules. Maltose and cellobiose are structural isomers, both having the formula C12H22O11.

However, they have different chemical properties and are recognized by different enzymes in biological tissues. For example, maltose can be hydrolyzed into its monosaccharides in the human body, whereas cellobiose cannot. Oligosaccharides contain several monosaccharides bound by glycosidic linkages at various sites. Many oligosaccharides have additional functional groups, which give them special properties. Oligosaccharides are often covalently bonded to proteins and lipids on the outer cell surface, where they serve as recognition signals. The different human blood groups (for example, the ABO blood types) get their specificity from oligosaccharide chains.

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2.4.3. Polysaccharides Store Energy and Provide Structural Materials Polysaccharides are large (occasionally gigantic) polymers of monosaccharides linked by glycosidic linkages. In contrast to proteins, polysaccharides are not essentially linear chains of monomers. Each monomer unit has several sites that may be capable of forming glycosidic linkages, and thus branched molecules are conceivable.

Starch Starches comprise a family of giant molecules of broadly similar structure. While all starches are polysaccharides of glucose with α-glycosidic linkages (α−1,4 and α−1,6 glycosidic bonds), the different starches can be distinguished by the amount of branching that occurs at carbons 1 and 6. Starch is the principal energy storage compound of plants. Some plant starches, such as amylose, are unbranched; others are moderately branched (amylopectin, for example). Starch readily binds water. When that water is removed, however, hydrogen bonds tend to form between the unbranched polysaccharide chains, which then aggregate, as in the large starch grains observed in the storage material of plant seeds.

Figure 2.4. Image showing polysaccharides. Source: Image by Wikimedia commons

Glycogen Glycogen is a water-insoluble, highly branched polymer of glucose. It stores glucose in the liver and muscle, serving as an energy storage compound for animals similar to what starch does for plants. Both glycogen and starch are readily hydrolyzed into glucose monomers, which further can be broken down to liberate their stored energy.

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However, if it is glucose that is needed for fuel, why is it stored in the form of glycogen? The reason is that 1,000 glucose molecules would exert 1,000 times the osmotic pressure of a single glycogen molecule, causing water to enter the cells. If it were not for polysaccharides, many organisms would expend a lot of energy expelling excess water from their cells.

Cellulose Cellulose is a predominant component of plant cell walls. It is also the most abundant organic compound on Earth. Similar to starch and glycogen, cellulose is a polysaccharide of glucose. However, its monosaccharides are connected by β- rather than by α-glycosidic linkages. Starch is easily degraded by the actions of chemicals or enzymes. Cellulose, however, is chemically more stable because of its β-glycosidic linkages. Thus, whereas starch is easily broken down to supply glucose for energy-producing reactions, cellulose is an excellent structural material that can withstand harsh environmental conditions without substantial change.

2.5. WHAT ARE THE CHEMICAL STRUCTURES AND FUNCTIONS OF LIPIDS? Lipids—colloquially called fats—are hydrocarbons that are insoluble in water because of their many nonpolar covalent bonds. Nonpolar hydrocarbon molecules are hydrophobic and preferentially aggregate among themselves, away from water (which is polar). When nonpolar hydrocarbons are sufficiently close together, weak but additive van der Waals forces hold them together. The huge macromolecular aggregations that can form are not polymers in a strict chemical sense, because the individual lipid molecules are not covalently bonded. With this understanding, it is still useful to consider aggregations of individual lipids as a different sort of polymer. There are several different types of lipids, and they play several roles in living organisms: • • • • •

Fats and oils store energy. Phospholipids play important structural roles in cell membranes. Carotenoids and chlorophylls help plants capture light energy. Steroids and modified fatty acids play regulatory roles as hormones and vitamins. Fat in animal bodies serves as thermal insulation.

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A lipid coating around nerves provides electrical insulation. Oil or wax on the surfaces of skin, fur, and feathers repels water.

2.5.1. Fats and Oils Are Hydrophobic Chemically, fats and oils are triglycerides, also known as simple lipids. Triglycerides that are solid at room temperature (around 20°C) are called fats; those that are liquid at room temperature are called oils. Triglycerides are composed of two types of building blocks: fatty acids and glycerol. Glycerol is a small molecule with three hydroxyl (—OH) groups (thus it is alcohol). A fatty acid is made up of a long nonpolar hydrocarbon chain and a polar carboxyl group (—COOH). These chains are very hydrophobic, with their abundant C—H and C—C bonds, which have low electronegativity and are nonpolar. A triglyceride contains three fatty acid molecules and one molecule of glycerol. The synthesis of a triglyceride involves three condensations (dehydration) reactions. In each reaction, the carboxyl group of fatty acid bonds with a hydroxyl group of glycerol, resulting in a covalent bond called an ester linkage and the release of a water molecule. The three fatty acids in a triglyceride molecule need not all have the same hydrocarbon chain length or structure; some may be saturated fatty acids, while others may be unsaturated: •

In saturated fatty acids, all the bonds between the carbon atoms in the hydrocarbon chain are single—there are no double bonds. That is, all the bonds are saturated with hydrogen atoms. These fatty acid molecules are relatively rigid and straight, and they pack together tightly, like pencils in a box. • In unsaturated fatty acids, the hydrocarbon chain contains one or more double bonds. Linoleic acid is an example of a polyunsaturated fatty acid that has two double bonds near the middle of the hydrocarbon chain, which causes kinks in the molecule. Such kinks prevent the unsaturated fat molecules from packing together tightly. The crimps in fatty acids are significant in deciding the fluidity and melting point of a lipid. The triglycerides of animal fats tend to have many long-chain saturated fatty acids, packed tightly together; these facts are usually solids at room temperature and have a high melting point.

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The triglycerides of plants, such as corn oil, tend to have short or unsaturated fatty acids. Because of their kinks, these fatty acids pack together poorly and have a low melting point, and these triglycerides are usually liquids at room temperature. Fats are phenomenal storage facilities for substance energy. At the point when the C — H bond is broken, it discharges huge energy that an organism can use for its motivations, like development or developing complex molecules. For each weight considered, broken down fats yield over two times as much energy as degraded carbohydrates.

2.5.2. Phospholipids Form Biological Membranes As mentioned previously, the hydrophobic nature of the many C—C and C—H bonds in fatty acids. However, what about the carboxyl functional group at the end of the molecule? When it ionizes and forms COO–, it is strongly hydrophilic. Further, a fatty acid is a molecule with a hydrophilic end and a long hydrophobic tail. It has two opposing chemical properties; the technical term for this is amphipathic. This explains what happens when oil (fatty acid) and water mix: the fatty acids orient themselves so that their polar ends face outward (i.e., toward the water) and their nonpolar tails face inward (away from water). Although no covalent bonds link individual lipids in large aggregations, such stable aggregations form readily in aqueous conditions. So, these large lipid structures can be considered a different kind of macromolecule. Like triglycerides, phospholipids contain fatty acids bound to glycerol by ester linkages.

Figure 2.5. Image showing phospholipids bilayer. Source: Image by Wikimedia commons

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In phospholipids, however, any one of several phosphate-containing compounds replaces one of the fatty acids, giving these molecules amphipathic properties—that is properties of both water-soluble and waterinsoluble molecules. The phosphate functional group has a negative electric charge, so this portion of the molecule is hydrophilic, attracting polar water molecules. Nonetheless, the two fatty acids are hydrophobic, so they tend to avoid water and aggregate together or with other hydrophobic substances. In an aqueous environment, phospholipids line up in such a way that the nonpolar, hydrophobic “tails” pack tightly together and the phosphatecontaining “heads” face outward, where they interact with water. The phospholipids thus form a bilayer: a sheet two molecules thick, with water excluded from the core.

2.5.3. Lipids Have Roles in Energy Conversion, Regulation and Protection Carotenoids The carotenoids are a family of light-absorbing pigments found in plants and animals. Beta-carotene (β-carotene) is one of the pigments that traps light energy in leaves during photosynthesis. In humans, a molecule of β-carotene can be broken down into two vitamin A molecules, from which we make the pigment cis-retinal, which is required for vision. Carotenoids are responsible for the colors of carrots, tomatoes, pumpkins, egg yolks, and butter.

Steroids Steroids are a family of organic compounds whose multiple rings share carbons. The steroid cholesterol is an important constituent of membranes. Other steroids function as hormones, chemical signals that carry messages from one part of the body to another. Cholesterol is synthesized in the liver and is the starting material for making testosterone and other steroid hormones, such as estrogen.

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Vitamins Vitamins are small molecules that are not synthesized by the human body and must be acquired from the diet. For example, vitamin A is formed from the β-carotene found in green and yellow vegetables. In humans, a deficiency of vitamin A leads to dry skin, eyes, and internal body surfaces, retarded growth and development, and night blindness, which is a diagnostic symptom of the deficiency. Vitamins D, E, and K are also lipids.

Waxes The sheen on human hair is more than cosmetic. Glands in the skin secrete a waxy coating that repels water and keeps the hair pliable. Birds that live near water have a similar waxy coating on their feathers. The shiny leaves of plants such as holly, familiar during winter holidays, also have a waxy coating. Finally, bees make their honeycombs out of wax. All waxes have the same basic structure: they are formed by an ester linkage between a saturated, long-chain fatty acid and saturated, long-chain alcohol. The result is a very long molecule, with 40–60 CH2 groups.

2.6. WHAT ARE THE CHEMICAL STRUCTURES AND FUNCTIONS OF NUCLEIC ACIDS? From medication to development, from agribusiness to criminology, the properties of nucleic acids influence our lives consistently. It is with nucleic acids that the idea of “data” entered the natural jargon. Nucleic acids are particularly fit for coding for and communicating organic data. The nucleic acids are polymers particular for the capacity, transmission among ages, and utilization of hereditary data. There are two sorts of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA is a macromolecule that encodes inherited data and passes it from one age to another. Through an RNA intermediate, the data encoded in DNA is utilized to determine the amino acid arrangements of proteins. Information is transfered from one DNA molecule to another during propagation. In the non-regenerative exercises of the cell, data is transfered from DNA to RNA to proteins. The proteins eventually do life’s functions.

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2.6.1. Nucleotides Are the Building Blocks of Nucleic Acids Nucleic acids are composed of monomers called nucleotides, each of which consists of a pentose sugar, a phosphate group, and a nitrogen-containing base. (Molecules consisting of a pentose sugar and a nitrogenous base—but no phosphate group— are called nucleosides.) The bases of the nucleic acids take one of two chemical forms: a sixmembered single-ring structure called a pyrimidine, or a fused double-ring structure called a purine. In DNA, the pentose sugar is deoxyribose, which differs from the ribose found in RNA by the absence of one oxygen atom. In both RNA and DNA, the backbone of the macromolecule consists of a chain of alternating pentose sugars and phosphate groups (sugar-phosphate– sugar–phosphate). The bases are attached to the sugars and project from the polynucleotide chain. The nucleotides are joined by phosphodiester linkages between the sugar of one nucleotide and the phosphate of the next (diester refers to the two covalent bonds formed by —OH groups reacting with acidic phosphate groups). The phosphate groups link carbon 3’ in one pentose sugar to carbon 5’ in the adjacent sugar. Most RNA molecules consist of only one polynucleotide chain. DNA, however, is usually double-stranded; its two polynucleotide chains are held together by hydrogen bonding between their nitrogenous bases. The two strands of DNA run in opposite directions. You can see what this means by drawing an arrow through a phosphate group from carbon 5’ to carbon 3’ in the next ribose. This antiparallel orientation allows the strands to fit together in three-dimensional space.

2.6.2. Base Pairing Occurs In both DNA and RNA Only four nitrogenous bases—and thus only four nucleotides— are found in DNA. The DNA bases and their abbreviations are adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine and guanine are purines; thymine and cytosine are pyrimidines. RNA is also made up of four different monomers, but its nucleotides differ from those of DNA. In RNA, the nucleotides are termed ribonucleotides (the ones in DNA are deoxyribonucleotides). They contain ribose rather than deoxyribose, and instead of the base thymine, RNA uses the base uracil (U). The other three bases are the same in RNA and DNA.

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The key to understanding the structure and function of nucleic acids is the principle of complementary base pairing. In double-stranded DNA, adenine and thymine always pair (A-T), and cytosine and guanine always pair (C-G). Three factors make base-pairing complementary: • •

The sites for hydrogen bonding on each base The geometry of the sugar-phosphate backbone, which brings complementary bases near each other • The molecular sizes of the paired bases; the pairing of a larger purine with a smaller pyrimidine ensures stability and uniformity in the double-stranded molecule of DNA Although RNA is generally single-stranded, complementary hydrogen bonding between ribonucleotides plays important role in determining the three-dimensional shapes of some types of RNA molecules, since portions of the single-stranded RNA can fold back and pair with each other. Complementary base pairing can also take place between ribonucleotides and deoxyribonucleotides. In RNA, guanine and cytosine pair (G-C), as in DNA, but adenine pairs with uracil (A-U). Adenine is an RNA strand that can pair either with uracil (in another RNA strand) or with thymine (in a DNA strand). The three-layered actual appearance of DNA is strikingly uniform. The varieties in DNA — the various arrangements of bases — are inner. Through hydrogen holding, the two reciprocal polynucleotide strands pair and twist to form a twofold helix. When contrasted with the complicated and differed tertiary structures of proteins, this consistency is astonishing. Yet, this underlying difference appears to be legit regarding the elements of these two classes of macromolecules. The unique states of proteins license these macromolecules to perceive specific “target” atoms. The region on the outer layer of a protein that connects with the target atom should match the state of to some degree part of the target molecule. As such, underlying variety in the objective atoms requires comparing variety in the designs of the actual proteins. Primary variety is important in DNA too. Nonetheless, the variety of DNA is found in its base succession as opposed to in the actual state of the molecule. Different DNA base arrangements encode specific data.

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2.6.3. Nucleotides Have Other Important Roles Nucleotides are more than just the building blocks of nucleic acids, there are several nucleotides with other functions: •

ATP (adenosine triphosphate) acts as an energy transducer in many biochemical reactions. • GTP (guanosine triphosphate) serves as an energy source, especially in protein synthesis. It also plays a role in the transfer of information from the environment to cells. • cAMP (cyclic adenosine monophosphate) is a special nucleotide in which an additional bond forms between the sugar and phosphate group. It is essential in many processes, including the actions of hormones and the transmission of information by the nervous system. We have seen that the nucleic acids RNA and DNA carry the blueprint of life and that the inheritance of these macromolecules reaches back to the beginning of evolutionary time.

2.7. HOW DID THE FIRST CELLS ORIGINATE? As seen from a significant number of the speculations for the beginning of life, the development of biochemistry happened under limited conditions. That is, the chemical reactions of metabolism, polymerization, and replication could not occur in a dilute aqueous environment. There must be a compartment or the like that united and focused the mixtures engaged with these occasions. Biologists have recommended that at first, this compartment might have been a minuscule bead of water on the outer layer of a stone. Yet, one more significant occasion at the beginning of life was fundamental. Life as far as we might be concerned is isolated from the climate inside fundamentally characterized units called cells. The inward items in a cell are isolated from the nonbiological environment by a unique boundary — a membrane. The membrane isn’t simply an obstruction; it directs what goes into and out of the cell. This job of the surface layer is vital because it allows the inside of the cell to keep a compound structure that is not the same as its outside climate. How did the first cells with membranes come into existence?

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2.7.1. Experiments Describe the Origin of Cells Jack Szostak and his colleagues at Harvard University built a laboratory model that gives insights into the origin of cells. To do this, they first put fatty acids (which can be made in prebiotic experiments) into water. Fatty acids are amphipathic: they have a hydrophilic polar end and a long, nonpolar tail that is hydrophobic. When placed in water, fatty acids will arrange themselves in a round “huddle” much like a football team: the hydrophilic ends point outward to interact with the aqueous environment and the fatty acid tails point inward, away from the water molecules. What if some water becomes trapped in the interior of this “huddle”? Now the layer of hydrophobic fatty acid tails is in water, which is an unstable situation. To stabilize this, the second layer of fatty acids forms. This lipid bilayer has the polar ends of the fatty acids facing both outward and inward because they are attracted to the polar water molecules present on each side of the double layer. The nonpolar tails form the interior of the bilayer. These prebiotic, waterfilled structures, defined by a lipid bilayer membrane, very much resemble living cells. Scientists refer to these compartments as protocells. Examining their properties revealed that •

•

Large molecules such as DNA or RNA could not pass through the bilayer to enter the protocells, but small molecules such as sugars and individual nucleotides could. Nucleic acids inside the protocells could replicate using the nucleotides from outside. When the investigators placed a short nucleic acid strand capable of self-replication inside protocells and added nucleotides to the watery environment outside, the nucleotides crossed the barrier, entered the protocells, and became incorporated into new polynucleotide chains. This may have been the first step toward cell reproduction, and it took place without protein catalysis.

2.8. WHAT FEATURES MAKE CELLS THE FUNDAMENTAL UNITS OF LIFE? Cells contain water and other small and large molecules. Each cell contains at least 10,000 different types of molecules, most of them present in many copies. Cells use these molecules to transform matter and energy, respond

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to their environments, and reproduce themselves. The cell theory has three important implications: •

Studying cell biology is in some sense the same as studying life. The principles that underlie the functions of the single cell of a bacterium are similar to those governing the approximately 60 trillion cells of your body. • Life is continuous. All those cells in your body came from a single cell, a fertilized egg. That egg came from the fusion of two cells, a sperm and an egg, from your parents. The cells of your parents’ bodies were all derived from their parents, and so on back through generations and evolution to the first living cell. • The origin of life on Earth was marked by the origin of the first cells. Even the largest organisms on Earth are composed of cells, but the cells themselves are usually too small for the naked eye to see.

2.8.1. Microscopes Reveal the Features of Cells Microscopes do two unique things to permit cells and subtleties inside them to be seen by the human eye. To start with, they increment the clear size of the item: this is called Magnification. However, simply expanding the magnification doesn’t be guaranteed to imply that the item will be seen plainly. As well as being bigger, an amplified object should be sharp, or clear. This is a property called resolution. Officially characterized, the resolution is the base distance two articles can be separated and nevertheless be viewed as two items. The resolution for the natural eye is around 0.2 mm (200 μm). Most cells are a lot more modest than 200 μm, and subsequently are undetectable to the natural eye. Magnifying lenses amplify and increment resolution so cells and their inner designs should be visible plainly. There are two fundamental kinds of magnifying instruments —light microscopes and electron microscopes—that utilize various types of radiation. While the resolution is better in electron microscopy, one ought to underscore that since cells are ready in a vacuum, just dead, dried-out cells are visualized. Consequently, the arrangement of cells for electron microscopy might change them, and this should be thought about when deciphering the pictures

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delivered. Then again, light microscopes can be utilized to envision living cells (for instance, by phase-contrast microscopy). Before considering the subtleties of cell structure, taking into account the many purposes of microscopy is valuable. A whole part of medicine, pathology, utilizes various techniques for microscopy to help with the investigation of cells and the diagnosis of sicknesses. For instance, a surgeon might remove from a body some tissue suspected of being cancerous. The pathologist might: •

•

•

•

examine the tissue quickly by phase-contrast microscopy or interference-contrast microscopy to determine the size, shape, and spread of the cells stain the tissue with a general dye and examine it by bright field microscopy to bring out features such as the shape of the nucleus, or cell division characteristics stain the tissue with a fluorescent dye and examine it by fluorescence microscopy or confocal microscopy for the presence of specific proteins that are diagnostic of a particular cancer examine the tissue under the electron microscope to observe its most minute internal structures, such as the shapes of the mitochondria and the chromatin.

2.8.2. The Plasma Membrane Forms the Outer Surface of Every Cell While the structural diversity of cells can often be observed using light microscopy, the plasma membrane is best observed with an electron microscope. This very thin structure forms the outer surface of every cell, and it has more or less the same thickness and molecular structure in all cells. Biochemical methods have shown that membranes have great functional diversity. These methods have revealed that the thin, almost invisible plasma membrane is actively involved in many cellular functions—it is not a static structure. The plasma membrane separates the interior of the cell from its outside environment, creating a segregated (but not isolated) compartment. The presence of this outer limiting membrane is a feature of all cells. What are the composition and molecular architecture of this amazing structure? The plasma membrane is composed of a phospholipid bilayer (or simply lipid bilayer), with the hydrophilic “heads” of the lipids facing the

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cell’s aqueous interior on one side of the membrane and the extracellular environment on the other.

Figure 2.6. The cell membrane. Source: Image by Wikimedia commons

Proteins and other molecules are embedded in the lipids. The membrane is not a rigid, static structure. Rather, it is an oily fluid, in which the proteins and lipids are in constant motion. This allows the membrane to move and change the shape of the cell. Here is a summary: •

•

The plasma membrane acts as a selectively permeable barrier, preventing some substances from crossing it while permitting other substances to enter and leave the cell. For example, macromolecules such as DNA and proteins cannot normally cross the plasma membrane, but some smaller molecules such as oxygen can. In addition to size, other factors such as polarity determine a molecule’s ability to cross the plasma membrane: because the membrane is composed mostly of hydrophobic fatty acids, nonpolar molecules cross it more easily than polar or charged molecules. The plasma membrane allows the cell to maintain a more or less constant internal environment. A self-maintaining,

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•

•

constant internal environment (known as homeostasis) is a key characteristic of life. One way that the membrane does this is by actively regulating the transport of substances across it. This dynamic process is distinct from the more passive process of diffusion, which is dependent on the size of a molecule. As the cell’s boundary with the outside environment, the plasma membrane is important in communicating with adjacent cells and receiving signals from the environment. The plasma membrane often has proteins protruding from it that are responsible for binding and adhering to adjacent cells. Thus, the plasma membrane plays an important structural role and contributes to cell shape.

2.9. WHAT IS THE STRUCTURE OF A BIOLOGICAL MEMBRANE? The physical organization and functioning of all biological membranes depend on their constituents: lipids, proteins, and carbohydrates. The lipids establish the physical integrity of the membrane and create an effective barrier to the rapid passage of hydrophilic materials such as water and ions. In addition, the phospholipid bilayer serves as a lipid “lake” in which a variety of proteins “float”. This general design is known as the fluid mosaic model. In the fluid mosaic model for biological membranes, the proteins are noncovalently embedded in the phospholipid bilayer by their hydrophobic regions (or domains), but their hydrophilic domains are exposed to the watery conditions on either side of the bilayer. These membrane proteins have several functions, including moving materials through the membrane and receiving chemical signals from the cell’s external environment. Each membrane has a set of proteins suitable for the specialized functions of the cell or organelle it surrounds. The carbohydrates associated with membranes are attached either to the lipids or to protein molecules. In plasma membranes, carbohydrates are located on the outside of the cell, where they may interact with substances in the external environment. Like some of the membrane proteins, carbohydrates are crucial in recognizing specific molecules, such as those on the surfaces of adjacent cells. Although the fluid mosaic model is largely valid for membrane structure, it does not say much about membrane composition.

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As you read about the different molecules in membranes in the next sections, keep in mind that some membranes have more protein than lipids, others are lipid-rich, others have significant amounts of cholesterol or other sterols, and still others are rich in carbohydrates.

2.10. WHAT ARE SIGNALS, AND HOW DO CELLS RESPOND TO THEM? Both prokaryotic and eukaryotic cells process information from their environments. This information can be in the form of a physical stimulus, such as the light reaching your eyes as you read this book or chemicals that bathe a cell, such as the lactose in a bacterial growth medium. It may come from outside the organism, such as the scent of a female moth seeking a mate in the dark, or from a neighboring cell within the organism, such as in the heart, where thousands of muscle cells contract in unison by transmitting signals to one another. Of course, the mere presence of a signal does not mean that a cell will respond to it, just as you do not pay close attention to every image in your environment as you study. To respond to a signal, the cell must have a specific receptor that can detect it. This section provides examples of some types of cellular signals and one model of signal transduction. A signal transduction pathway is a sequence of molecular events and chemical reactions that lead to a cell’s response to a signal.

2.10.1. Cells Receive Signals from the Physical Environment and Other Cells The physical environment is full of signals. Our sense organs allow us to respond to light, odors and tastes (chemical signals), temperature, touch, and sound. Bacteria and protists can respond to minute chemical changes in their environments. Plants respond to light as a signal as well as an energy source. The amount and wavelengths of light reaching a plant’s surface differ from day to night and in direct sunlight versus shade. These variations act as signals that affect plant growth and reproduction. Some plants also respond to temperature: when the weather gets cold, they may respond either by becoming tolerant to cold or by accelerating flowering.

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A cell deep inside a large multicellular organism is far away from the exterior environment. Such a cell’s environment consists of other cells and extracellular fluids. Cells receive their nutrients from and pass their wastes into, extracellular fluids. Cells also receive signals—mostly chemical signals—from their extracellular fluid environment. Most of these chemical signals come from other cells, but they can also come from the environment via the digestive and respiratory systems. And cells can respond to changes in the extracellular concentrations of certain chemicals, such as CO2 and H+, which are affected by the metabolic activities of other cells. Inside a large multicellular organism, chemical signals made by the body itself reach a target cell by local diffusion or by circulation within the blood. These signals are usually in tiny concentrations (as low as 10–10 M). Autocrine signals diffuse to and affect the cells that make them; for example, part of the reason many tumor cells reproduce uncontrollably is that they self-stimulate cell division by making their division signals. Paracrine signals diffuse to and affect nearby cells; an example is a neurotransmitter made by one nerve cell that diffuses to an adjacent cell and stimulates it. Signals to distant cells called hormones travel through the circulatory system.

2.10.2. A Signal Transduction Pathway Involves a Signal, a Receptor, and Responses For the information from a signal to be transmitted to a cell, the target cell must be able to receive or sense the signal and respond to it, and the response must have some effect on the function of the cell. In a multicellular organism, all cells may receive chemical signals that are circulated in the blood, such as the peptides oxytocin and vasopressin that are released following a mating in voles (see the opening of this chapter), but most body cells are not capable of responding to the signals. Only the cells with the necessary receptors can respond. The sorts of reactions fluctuate significantly contingent upon the sign and the objective cell. Only a couple of models are: a skin cell starting cell division to recuperate an injury; a cell moving to another area in the embryoto form a tissue; a cell delivering compounds to process food; a plant cell loosening molecules that keep its cell wall polymers intact so it can grow; and a cell in the eye sending messages to the mind about the book you are perusing.

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A sign transduction pathway includes a sign, a receptor, and a reaction. We should look at an illustration of such a pathway in the bacterium Escherichia coli (E. coli). Follow the highlights of this pathway overall and specifically.

Signal Like a prokaryotic cell, a bacterium is very sensitive to changes in its environment. One thing that can change is the total solute concentration in the environment surrounding the cell. In the mammalian intestine where E. coli lives, the solute concentration around the bacterium often rises far above the solute concentration inside the cell. A fundamental characteristic of all living cells is that they maintain a constant internal environment or homeostasis. To do this, the bacterium must perceive and quickly respond to this environmental signal. The cell does this by a signal transduction pathway involving two major components: a receptor and a responder.

Receptor The E. coli. receptor protein for changes in solute concentration is called EnvZ. EnvZ is a transmembrane protein that extends across the bacterium’s plasma membrane into the space between the plasma membrane and the highly porous outer membrane, which forms a complex with the cell wall. When the solute concentration of the extracellular environment rises, so does the solute concentration in the space between the two membranes. This change in the aqueous solution causes the part of the receptor protein that sticks out into the intermembrane space to change conformation (its threedimensional shape). The conformational change in the intermembrane domain (a domain is a sequence of amino acids folded into a particular shape) causes a conformational change in the domain that lies in the cytoplasm and initiates the events of signal transduction. The cytoplasmic domain of EnvZ can act as an enzyme. The conformational change in EnvZ exposes an active site that was previously buried within the protein so that EnvZ becomes a protein kinase—an enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule. EnvZ transfers the phosphate group to one of its histidine amino acids. In other words, EnvZ phosphorylates itself.

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Responder A responder is the second component of a signal transduction pathway. The charged phosphate group added to the histidine of the EnvZ protein causes its cytoplasmic domain to change its shape again. It now binds to a second protein, OmpR, and transfers the phosphate to it. In turn, this phosphorylation changes the shape of OmpR. The change in the responder is a key event in signaling, for three reasons: •

The signal on the outside of the cell has now been transduced to a protein that lies totally within the cell’s cytoplasm. • The altered responder can do something. In the case of the phosphorylated OmpR, that “something” is to bind to DNA to alter the expression of many genes; in particular, it increases the expression of the protein OmpC. This binding begins the final phase of the signaling pathway: the effect of the signal, which is an alteration in cell function. • The signal has been amplified. Because a single enzyme can catalyze the conversion of many substrate molecules, one EnvZ molecule alters the structure of many OmpR molecules. Phosphorylated OmpR has the correct three-dimensional structure to bind to the ompC DNA, increasing the transcription of that gene. This results in the production of OmpC protein, which enables the cell to respond to the increase in osmotic concentration in its environment. The OmpC protein is inserted into the outer membrane of the cell, where it blocks pores and prevents solutes from entering the intermembrane space. As a result, the solute concentration in the intermembrane space is lowered, and homeostasis is restored. Thus, the EnvZ-OmpR signal transduction pathway allows the E. coli cell to function just as if the external environment had a normal solute concentration.

2.11. HOW DO CELLS COMMUNICATE DIRECTLY? Most cells are in touch with their neighbors. There are different manners by which cells stick to each other, for example, utilizing receptor proteins that distend from the cell surface, or through close junctions and desmosomes. However, as we probably are aware from our involvement with our neighbors, simply being in the vicinity doesn’t be guaranteed intend that there is utilitarian correspondence.

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Neither tight junctions nor desmosomes are specific for intercellular signaling. Nonetheless, numerous multicellular life forms have particular cell junctions that permit their cells to connect. In animals, these designs are gap junctions; in plants, they are plasmodesmata.

2.11.1. Animal Cells Communicate By Gap Junctions Gap junctions are channels between adjacent cells that occur in many animals, occupying up to 25 percent of the area of the plasma membrane. Gap junctions traverse the narrow space between the plasma membranes of two cells (the “gap”) by means of channel structures called connexons. The walls of a connexon are composed of six subunits of the integral membrane protein connexin. In adjacent cells, two connexons come together to form a gap junction that links the cytoplasm of the two cells. There may be hundreds of these channels between a cell and its neighbors. The channel pores are about 1.5 nm in diameter—far too narrow for the passage of large molecules such as proteins. But they are wide enough to allow small molecules to pass between the cells. Experiments in which labeled signal molecules or ions are injected into one cell show that they can readily pass into adjacent cells if the cells are connected by gap junctions. Why is it necessary to have these linkages between the cytoplasm of adjacent cells? Gap junctions permit metabolic cooperation between the linked cells. Such cooperation ensures the sharing between cells of important small molecules such as ATP, metabolic intermediates, amino acids, and coenzymes. In some tissues, metabolic cooperation is needed so that signals and metabolic products can be passed from cells at the edges of tissues to cells in the interior and vice versa. It is not clear how important this function is in many tissues, but it is known to be vital in some. For example, in the lens of the mammalian eye, only the cells at the periphery are close enough to the blood supply to allow the diffusion of nutrients and wastes. But because lens cells are connected by large numbers of gap junctions, the material can diffuse between them rapidly and efficiently. As mentioned above, there is evidence that signal molecules such as hormones and second messengers such as cAMP can move through gap junctions. If this is true, then only a few cells would need receptors for a signal for the signal to be transduced throughout the tissue. In this way, a tissue can have a coordinated response to the signal.

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2.11.2. Plant Cells Communicate By Plasmodesmata Instead of gap junctions, plants have plasmodesmata (singular plasmodesma), which are membrane-lined tunnels that traverse the thick cell walls separating plant cells from one another. A typical plant cell has several thousand plasmodesmata. Plasmodesmata differ from gap junctions in one fundamental way: unlike gap junctions, in which the wall of the channel is made of integral proteins from the adjacent plasma membranes, plasmodesmata are lined by the fused plasma membranes themselves. Plant biologists are so familiar with the notion of tissue as cells interconnected in this way that they refer to this continuous cytoplasm as a symplast. The diameter of a plasmodesma is about 6 nm, far larger than a gap junction channel. But the actual space available for diffusion is about the same—1.5 nm. Examination of the interior of the plasmodesma by transmission electron microscopy reveals that a tubule called the desmotubule, apparently derived from the endoplasmic reticulum, fills up most of the opening of the plasmodesma. Typically, only small metabolites and ions can move between plant cells. This fact is important in plant physiology because the bulk transport system in plants, the vascular system, lacks the tiny circulatory vessels (capillaries) that many animals have for bringing gases and nutrients to every cell. Diffusion from cell to cell across plasma membranes is probably inadequate to account for the movement of a plant hormone from the site of production to the site of action. Instead, plants rely on more rapid diffusion through plasmodesmata to ensure that all cells of tissue respond to a signal at the same time. There are cases in which larger molecules or particles can pass between cells via plasmodesmata. For example, some viruses can move through plasmodesmata by using “movement proteins” to assist their passage.

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2.12. CONCLUSION This chapter discussed the various kinds of molecules that characterize living things. It also discussed the chemical structures and different functions of proteins, lipids, nucleic acids, and carbohydrates. Towards the end of the chapter, it also discussed the origins of the first cell. In this chapter, various features that make cells the fundamental unit of life have also been explained. It also discussed the structure of biological membranes.

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David Sadava, H. Craig Heller, David M. Hillis and May Berenbaum, 2009. Life: The Science of Biology. 9th ed. [ebook] p.1393. Available at: [Accessed 3 July 2022]. Alberts, B., 2017. Science for life. Science, [online] 355(6332), pp.1353-1353. Available at: [Accessed 4 July 2022]. Banavar, J. and Maritan, A., 2002. The science of life. Computer Physics Communications, [online] 146(1), pp.129-130. Available at:

[Accessed 4 July 2022]. Jordan, P., 2007. Science for life?. Nature, [online] 446(7138), pp.946946. Available at: [Accessed 4 July 2022]. W.J.T, 1975. The chemical basis of life. Journal of Molecular Structure, [online] 25(2), pp.448-449. Available at: [Accessed 4 July 2022].

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CELLS, GENES AND HEREDITY

CONTENTS 3.1. Introduction....................................................................................... 66 3.2. The Evolution of Cells........................................................................ 71 3.3. Heredity and Evolution...................................................................... 73 3.4. Chemical Structure of Genes............................................................. 74 3.5. Gene Transcription and Translation.................................................... 74 3.6. How Do Prokaryotic and Eukaryotic Cells Divide?............................. 76 3.7. Prokaryotes Divide By Binary Fission................................................. 76 3.8. How Is Eukaryotic Cell Division Controlled?...................................... 78 3.9. Role Of Cell Division in a Sexual Life Cycle...................................... 79 3.10. In A Living Organism, How Do Cells Die?....................................... 83 3.11. Unregulated Cell Division Lead to Cancer?...................................... 84 3.12. Mendelian Law of Inheritance......................................................... 87 3.13. How Do Genes Interact ?................................................................. 88 3.14. The Environment Affects Gene Action.............................................. 89 3.15. Relationship Between Genes and Chromosomes............................. 91 3.16. Genes on Sex Chromosomes are Inherited in Special Ways............. 92 3.17. How Do Prokaryotes Transmit Genes?............................................. 93 3.18. Conclusion...................................................................................... 94 References................................................................................................ 95

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The basic membrane-bound unit is the cell, in biology, that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in a bacterium or yeast. As they mature, other cells acquire specialized functions. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms such as humans and other animals. Atoms are still very small, although cells are much larger than atoms.

3.1. INTRODUCTION 3.1.1. Cells Mycoplasmas known as the smallest known cells are a group of tiny bacteria; as small as 0.2 μm in diameter (1μm = about 0.000039 inches) some of these single-celled organisms are spheres, with a total mass of 10−14 gram—equal to that of 8,000,000,000 hydrogen atoms. Larger than the mass of a single mycoplasma bacterium cells of humans typically has a mass of 400,000 times, but even human cells are only about 20 μm across. It would require a sheet of about 10,000 human cells to cover the head of a pin and each human organism is composed of more than 30,000,000,000,000 cells. A selective barrier that allows nutrients to enter and waste products to leave is formed when a cell is enclosed by a plasma membrane. The interior of the cell is organized into many specialized compartments or organelles, each surrounded by a separate membrane. One major organelle, the nucleus, contains the genetic information necessary for cell growth and reproduction. Other types of organelles are present in multiple copies in the cellular contents, or cytoplasm whereas each cell contains only one nucleus. Organelles include mitochondria are responsible for the energy transactions are necessary for cell survival; lysosomes, within the cell which digest unwanted materials; and the endoplasmic reticulum and the Golgi apparatus, which play important roles by synthesizing selected molecules and then processing, sorting, and directing them to their proper locations in the internal organization of the cell. In addition, plant cells contain chloroplasts, which are responsible for photosynthesis, whereby the energy of sunlight is used to convert molecules

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of carbon dioxide (CO2) and water (H2O) into carbohydrates. Between all these organelles is the space in the cytoplasm called the cytosol. An organized framework of fibrous molecules in the cytosol constitutes the cytoskeleton, which enables organelles to move within the cell, gives a cell its shape, and provides a mechanism by which the cell itself can move. The cytosol also contains more than 10,000 different kinds of molecules that are involved in cellular biosynthesis, the process of making large biological molecules from small ones.

Figure 3.1. Animal Cell and Components. Source: Image by Wikimedia Commons

3.1.2. Gene On a chromosome, gene is a unit of hereditary information that occupies a fixed position (locus). By directing the synthesis of proteins genes achieve their effects. Within the cell nucleus in eukaryotes (such as animals, plants, and fungi), genes are contained. Distinct from the genes found in the nucleus, the mitochondria (in animals) and the chloroplasts (in plants) also contain small subsets of genes.

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Genes are contained in a single chromosome that is free-floating in the cell cytoplasm, in prokaryotes (organisms lacking a distinct nucleus, such as bacteria). Many bacteria also contain plasmids, extrachromosomal genetic elements — with a small number of genes. Between species, the number of genes in an organism’s genome (the entire set of chromosomes) varies significantly. As an example, an estimated 20,000 to 25,000 genes are contained in the human genome, while the genome of the bacterium Escherichia coli O157:H7 houses precisely 5,416 genes. The bacterium Mycoplasma genitalium has the fewest number of genes, just 517, Arabidopsis thaliana—the first plant for which a complete genomic sequence was recovered—has roughly 25,500 genes; its genome is one of the smallest known to plants.

Figure 3.2. Arabidopsis thaliana plant. Source: Image by Wikimedia Commons

Except in some viruses, genes are composed of deoxyribonucleic acid (DNA), which have genes consisting of a closely related compound called ribonucleic acid (RNA). Two chains of nucleotides are make a DNA molecule that wind about each other to resemble a twisted ladder. The sides of the ladder are made up of sugars and phosphates, and the rungs are formed by bonded pairs of nitrogenous bases.

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These bases are thymine (T), adenine (A), guanine (G), cytosine (C), and. A C on one chain bonds to a G similarly an A on one chain bonds to a T on the other (thus forming an A–T ladder rung). The two chains unwind, if the bonds between the bases are broken, and free nucleotides within the cell attach themselves to the exposed bases of the now-separated chains. According to the base-pairing rule, the free nucleotides line up along each chain—A bonds to T, C bonds to G. In the creation of two identical DNA molecules resulted from this process from one original and is the method by which hereditary information is passed from one generation of cells to the next.

3.1.3. Heredity The sum of all biological processes by which characteristics are transmitted from parents to their offspring is known as heredity. Two seemingly paradoxical observations about organisms encompasses by the concept of heredity: the variation among individuals within a species and the constancy of a species from generation to generation as becomes clear in the study of genetics that constancy and variation are two sides of the same coin. The functional units of heritable material that are found within all living cells both aspects of heredity can be explained by genes. Specific to that species, every member of a species has a set of genes. The constancy of the species provided by it is this set of genes. However, variations can occur in the form each gene takes, providing the genetic basis for the fact that no two individuals (except identical twins) have the same traits among individuals within a species. A combination of the genetic material of each, the set of genes that an offspring inherits from both parents is called the organism’s genotype. The genotype is contrasted to the phenotype, which is the developmental outcome of its genes and the organism’s outward appearance.

An organism’s bodily structures, physiological processes, and behaviors are included by the phenotype. Although the broad limits of the features an organism can develop as determined by the genotype, the features that develop, i.e., the phenotype, depend on complex interactions between genes and their environment. However, because the organism’s internal and external environments change continuously, so does its phenotype, however, the genotype remains constant throughout an organism’s lifetime. It is crucial to discover the

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degree to which the observable trait is attributable to the pattern of genes in the cells and to what extent it arises from environmental influence, in conducting genetic studies. Genetics also can be defined as the study of genes because genes are integral to the explanation of hereditary observations. Genes are important determinants of all aspects of an organism’s makeup shown by the discoveries into the nature of genes. For this reason, the study of genetics has a position of central importance in biology and most areas of biological research now have a genetic component. With genes that are built on the same chemical principle and that function according to similar mechanisms, genetic research also has demonstrated that virtually all organisms on this planet have similar genetic systems. Many similar genes are found across a wide range of species, although species differ in the sets of genes they contain. For example, in humans, a large proportion of genes of baker’s yeast are also present. By the evolutionary relatedness of virtually all life-forms on Earth, this similarity in genetic makeup between organisms that have such disparate phenotypes can be explained. Between humans and all other organisms, this genetic unity has radically reshaped the understanding of the relationship. Genetics also has had a profound impact on human functioning. By subjecting plants, animals, and microbes to the ancient techniques of selective breeding and to the modern methods of recombinant DNA technology throughout history, humans have created or improved many different medicines, foods, and textiles. In recent years medical researchers have begun to discover the role that genes play in disease. As the structure and function of more and more human genes are characterized, the significance of genetics only promises to become greater. One of the most puzzling and mysterious phenomena of nature for a long time has been heredity. This was so because the bridges across which heredity must pass between the generations are formed by the sex cells, and are usually invisible to the naked eye. In the early 17th century only after the invention of the microscope and the subsequent discovery of the sex cells could the essentials of heredity be grasped. Ancient Greek philosopher and scientist Aristotle (4th century BC) speculated that the relative contributions of the female and the male parents

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were very unequal before that time; the female was thought to supply what he called the “matter” and the male the “motion.” Composed in India between 100 and 300 AD, The texts of Manu, consider the role of the female like that of the field and of the male like that of the seed; new bodies are formed “by the united operation of the seed and the field.” On average, children resemble their mothers as much as they do their fathers and, both parents transmit the heredity pattern equally. Nevertheless, the mass of an egg cell is sometimes millions of times greater than that of a spermatozoon; the female and male sex cells may be very different in size and structure.

3.2. THE EVOLUTION OF CELLS Until a collection of catalysts appeared that could promote the synthesis of more catalysts of the same kind, life on Earth could not exist. In the evolutionary pathway of cells, early stages presumably centered on RNA molecules, which not only present specific catalytic surfaces but for their own duplication through the formation of a complementary RNA molecule also contain the potential. It is assumed that a small RNA molecule eventually appeared that was able to catalyze its own duplication,. Imperfections in primitive RNA replication likely gave rise to many variant autocatalytic RNA molecules. Molecules of RNA that acquired variations that increased the speed, or the fidelity of self-replication would have out multiplied other, less-competent RNA molecules. In addition, for their ability, other small RNA molecules that existed in symbiosis with autocatalytic RNA molecules underwent natural selection to catalyze useful secondary reactions such as the production of better precursor molecules. Since cooperation between different molecules produced a system that was much more effective at self-replication than a collection of individual RNA catalysts, in this way, sophisticated families of RNA catalysts could have evolved together. In the evolution of the cell another major step would have been the development of a primitive mechanism of protein synthesis in one family of self-replicating RNA. For the synthesis of other protein molecules like themselves, protein molecules cannot provide the information. From a

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nucleic acid sequence, this information must ultimately be derived. Before a group of powerful RNA catalysts evolved, protein synthesis is much more complex than RNA synthesis.

Figure 3.3. An RNA molecule. Source: Image by Wikimedia Commons

Among the RNA molecules that function in the current cell each of these catalysts presumably has its counterpart: (1) Much like messenger RNA (mRNA), whose nucleotide sequence was read to create an amino acid sequence there, was an information RNA molecule; (2) Much like transfer RNA (tRNA) that could bind to both mRNA, there was a group of adaptor RNA molecules, and a specific activated amino acid; and (3) Finally, there was an RNA catalyst, much like ribosomal RNA (rRNA), that facilitated the joining together of the amino acids aligned on the mRNA by the adaptor RNA. The first cell was formed, at some point in the evolution of biological catalysts. The partitioning of the primitive biological catalysts into individual units is required by this, each surrounded by a membrane. Since many amphiphilic molecules—half hydrophobic (waterrepelling) and half hydrophilic (water-loving)— membrane formation might

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have occurred quite simply, aggregate to form bilayer sheets in which the hydrophobic portions of the molecules line up in rows to form the interior of the sheet and leave the hydrophilic portions to face the water. To form the walls of small, spherical vesicles, as can the phospholipid bilayer membranes of present-day cells such bilayer sheets can spontaneously close. The units would have begun to compete with one another for the same resources as soon as the biological catalysts became compartmentalized into small individual units or cells. For the development of more efficient cells, the active competition that ensued must have greatly accelerated evolutionary change, serving as a powerful force. In this way, cells enabling them to use simpler more abundant precursor molecules for their growth, eventually arose that contained new catalysts,. These cells were able to spread far beyond the limited environments where the first primitive cells arose because they were no longer dependent on preformed ingredients for their survival. Only after the development of a primitive form of protein synthesis, it is often assumed that the first cells appeared. However, it has been suggested that the first cells contained only RNA catalysts and it is by no means certain that cells cannot exist without proteins. Protein molecules, with their chemically varied side chains, are more powerful catalysts than RNA molecules, in either case; therefore, cells arose in which RNA served primarily as genetic material, as time passed, in each generation and inherited by all progeny cells in order to specify proteins being directly replicated. A need would have arisen for a stabler form of genetic information storage than that provided by RNA, as cells became more complex. In the evolutionary history of cells, DNA, related to RNA yet chemically stabler, probably appeared rather late. The genetic information in RNA sequences was transferred to DNA sequences over a period of time, and to replicate directly the ability of RNA molecules was lost. The central process of biology—the synthesis, one after the other, of DNA, RNA, and protein—appeared it was only at this point.

3.3. HEREDITY AND EVOLUTION As proposed by Charles Darwin and Alfred Russell Wallace at the center of the theory of evolution were the concepts of variation and natural selection.

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Hereditary variants were either selected for or against by the contemporary environmental conditions and then thought to arise naturally in populations. Specific variant types of subsequent generations either became enriched or impoverished, in this way. The accumulation of such changes in populations could lead to the formation of new species and higher taxonomic categories, over the long term. However, in the 19th-century world of Darwin and Wallace, although hereditary change was basic to the theory, the fundamental unit of heredity— the gene—was unknown. In the 20th century after the discovery of Mendel’s laws, the birth and proliferation of the science of genetics, made it possible to consider the process of evolution by natural selection in terms of known genetic processes.

3.4. CHEMICAL STRUCTURE OF GENES Except in some viruses, genes are composed of deoxyribonucleic acid (DNA), which have genes consisting of a closely related compound called ribonucleic acid (RNA). To resemble a twisted ladder, a DNA molecule is composed of two chains of nucleotides that wind about each other. The rungs are formed by bonded pairs of nitrogenous bases and the sides of the ladder are made up of sugars and phosphates. These bases are thymine (T)2, adenine (A), guanine (G), and cytosine (C). A C on one chain bonds to a G; similarly, an A on one chain bonds to a T on the other (thus forming an A–T ladder rung). The two chains unwind, if the bonds between the bases are broken, and free nucleotides within the cell attach themselves to the exposed bases of the now-separated chains. According to the base-pairing rule—A bonds to T, C bonds to G the free nucleotides line up along each chain. This process is the method by which hereditary information is passed from one generation of cells to the next and results in the creation of two identical DNA molecules from one original.

3.5. GENE TRANSCRIPTION AND TRANSLATION Along a strand of DNA, the sequence of bases determines the genetic code. The portion of the DNA molecule that contains that gene will split when the product of a particular gene is needed.

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A strand of RNA with bases complementary to those of the gene is created from the free nucleotides in the cell through the process of transcription. (A and U form base pairs during RNA synthesis because RNA has the base uracil [U] instead of thymine.) The process of translation, or protein synthesis, takes place when this single chain of RNA, called messenger RNA (mRNA), then passes to the organelles called ribosomes. A second type of RNA, transfer RNA (tRNA), matches up the nucleotides on mRNA during translation, with specific amino acids. Three nucleotides code for one amino acid.

Figure 3.4. Gene Transcription and Translation. Source: Image by Wikimedia Commons

According to the sequence of nucleotides forms a polypeptide chain the series of amino acids built; all proteins are made from one or more linked polypeptide chains. Experiments conducted in the 1940s indicated one gene as being responsible for the assembly of one enzyme, or one polypeptide chain. This is known as the one gene–one enzyme hypothesis. However, it has been realized that not all genes encode an enzyme since this discovery, and by two or more genes that some enzymes are made up of several short polypeptides encoded.

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3.6. HOW DO PROKARYOTIC AND EUKARYOTIC CELLS DIVIDE? The life cycle of an organism is intimately linked to cell division from birth to death. Cell division plays important roles in the growth, repair of tissues and in the reproduction of all organisms as well as in multicellular organisms. Four events must occur, for any cell to divide: •

There must be a reproductive signal. This signal may originate from either inside or outside the cell and initiates cell division. • Each of the two new cells will have identical genes and complete cell functions when replication of DNA (the genetic material) must occur. • To each of the two new cells, the cell must distribute the replicated DNA. This process is called segregation. • In order to separate the two new cells by a process called cytokinesis, in addition to synthesizing needed enzymes and organelles, new material must be added to the plasma membrane (and the cell wall, in organisms that have one). In prokaryotes and eukaryotes, these four events proceed somewhat differently.

3.7. PROKARYOTES DIVIDE BY BINARY FISSION Cell division results in the reproduction of the entire single-celled organism in prokaryotes. The cell replicates its DNA, grows, and by a process called binary fission, then separates the cytoplasm and DNA into two new cells.

3.7.1. Reproductive Signals In the environment, the reproductive rates of many prokaryotes respond to conditions. A species commonly used in genetic studies, the bacterium Escherichia coli, is a “cell division machine”; if abundant sources of carbohydrates and mineral nutrients are available, it can divide as often as every 20 minutes. Another bacterium, Bacillus subtilis, does not just slow its growth when nutrients are low but stops dividing and then resumes dividing when conditions improve. Clearly, for the initiation of cell division in prokaryotes external factors such as environmental conditions and nutrient concentrations are signals.

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3.7.2. Replication of DNA A chromosome can be defined in molecular terms as a DNA molecule containing genetic information. All its chromosomes must be replicated, and one copy of each chromosome must find its way into one of the two new cells when a cell divides. A single long DNA molecule with proteins bound to it, most prokaryotes have only one chromosome. To create a circular chromosome in E. coli, the ends of the DNA molecule are joined. Some viruses are also found in the chloroplasts and mitochondria of eukaryotic cells as well as circular chromosomes are characteristic of most prokaryotes. It would be about 500 μm in diameter if the E. coli DNA was spread out into an actual circle. The bacterium itself is only about 2 μm long and 1μm in diameter. Thus, it would form a circle over 200 times larger than the cell if the bacterial DNA were fully extended.

Figure 3.5. DNA Replication. Source: Image by Wikimedia Commons

Bacterial DNA must be compacted to fit into the cell. The DNA folds in on itself, and positively charged (basic) proteins bound to the negatively charged (acidic) DNA contribute to this folding. Near the center of the cell, chromosome replication takes place as the DNA is threaded through a “replication complex” of proteins. Replication begins at the ORI site and moves toward the TER site. Anabolic metabolism is active, and the cell grows while the DNA replicates.

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At opposite ends of the cell when replication is complete, the two daughter DNA molecules separate and segregate from one another. DNA replication occupies the entire time between cell divisions in rapidly dividing prokaryotes.

3.7.3. Segregation of DNA molecules Replication proceeds the ORI regions move toward opposite ends of the cell because it begins near the center of the cell. For this segregation, DNA sequences adjacent to the ORI region bind proteins that are essential. Since the binding proteins hydrolyze ATP, this is an active process. Either actively moving the DNA along, or passively acting as a “railroad track” along which DNA moves, the prokaryotic cytoskeleton may be involved in DNA segregation.

3.7.4. Cytokinesis After chromosome replication is finished in rapidly growing cells, the actual division of a single cell and its contents into two cells is called cytokinesis and begins immediately. To form a ring of fibers similar to a purse string the first event of cytokinesis is a pinching in of the plasma membrane. A protein that is related to eukaryotic tubulin (which makes up microtubules) is the major component of these fibers. New cell wall materials are deposited, which finally separate the two cells as the membrane pinches in.

3.8. HOW IS EUKARYOTIC CELL DIVISION CONTROLLED? Stem cells in bone marrow, some cells, such as those in an early embryo, or cells in the growing tip of a plant root, divide rapidly and continuously. Cells like neurons in the brain or phloem cells in a plant stem, do not divide at all. Clearly, for cells to divide, they are highly controlled by the signaling pathways. The period between cell divisions is referred to as the cell cycle. The cell cycle is divided into Mitosis/cytokinesis and interphase. Including in DNA replication during interphase, the cell nucleus is visible and typical cell functions occur. When cytokinesis is completed and ends when mitosis begins this phase of the cell cycle begins. Especially those that trigger mitosis the events of interphase. Cells spend most of their time in interphase even when rapidly dividing.

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So, most of the cells will be in interphase if we take a snapshot through the microscope of a cell population; at any given moment only, a small percentage will be in mitosis or cytokinesis.

3.9. ROLE OF CELL DIVISION IN A SEXUAL LIFE CYCLE A single cell can give rise to a vast number of cells with identical nuclear DNA- the mitotic cell cycle repeats itself and by this process. On the other hand, meiosis produces just four daughter cells. In reproduction mitosis and meiosis both are involved, but they have different roles: sexual reproduction involves both mitosis and meiosis while asexual reproduction involves only mitosis.

3.9.1. Asexual Reproduction by Mitosis Results in Genetic Constancy Asexual reproduction is based on the mitotic division of the nucleus sometimes called vegetative reproduction. An organism reproducing itself with each cell cycle, that reproduces asexually may be single-celled like yeast, or it may be multicellular like the cholla cactus, to produce a new multicellular organism that breaks off a piece.

Figure 3.6. A picture of Cholla Cactus. Source: Image by Flickr

Asexual reproduction is common in nature, and it is a rapid and effective means of making new individuals. The offspring are clones of the parent

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organism in asexual reproduction; that is, the offspring are genetically identical to the parent. A mutation is known as any genetic variation among the offspring that is most likely due to small environmentally caused changes in the DNA. In sexually reproducing organisms, this small amount of variation contrasts with the extensive variation possible.

3.9.2. Sexual Reproduction by Meiosis Results in Genetic Diversity Sexual reproduction results in an organism that is not identical to its parents, unlike asexual reproduction. Two parents each contribute one gamete to each of their offspring; sexual reproduction requires gametes created by meiosis. Gametes produced by meiosis—and thus offspring—differ genetically from each other and from the parents. Because of this genetic variation, some offspring may be better adapted than others to survive and reproduce in a particular environment. The genetic diversity generated by the meiosis is the raw material for natural selection and evolution.

Figure 3.7. Production of Gametes. Source: Image by Flickr

The body cells that are not specialized for reproduction called somatic cells, in most multicellular organisms, each contain two sets of chromosomes, which are found in pairs. From each of the organism’s two parents, one

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chromosome of each pair is received; for example, in humans with 46 chromosomes, 23 come from the father and 23 from the mother. Except for the sex chromosomes found in some species the members of such a homologous pair are similar in size and appearance. In a homologous pair (called homologs) the two chromosomes bear corresponding, though often not identical, genetic information. As an example, in a plant, a homologous pair of chromosomes may carry different versions of a gene that controls seed shape. For wrinkled seeds, one homolog may carry this wrinkled form while the other may carry the type for smooth seeds. A single set of chromosomes contained by gametes on the other hand— that is, one homolog from each pair. In a gamete, the number of chromosomes is denoted by n, and the cell is said to be haploid. To form a zygote, two haploid gametes fuse in a process called fertilization.

Figure 3.8. Life cycles involve Meiosis and Fertilization. Source: Image by Wikimedia Commons

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Just as somatic cells do, the zygote has two sets of chromosomes. The zygote is said to be diploid, and its chromosome number is denoted by 2n. The zygote may divide by either meiosis or mitosis, depending on the organism. A new mature organism that is capable of sexual reproduction develops either way. To produce gametes or cells that are haploid all sexual life cycles involve meiosis. Eventually, beginning the diploid stage of the lifecycle, the haploid cells or gametes fuse to produce a zygote. Evolution has generated many different versions of the sexual life cycle since the origin of sexual reproduction.

3.9.3. The Number, Shapes, and Sizes of the Metaphase Chromosomes Constitute the Karyotype It is often possible to count and characterize their individual chromosomes when cells are in metaphase of mitosis, The images of the individual chromosomes can be manipulated if a photomicrograph of the entire set of chromosomes is made, pairing and placing them in an orderly arrangement. Which together constitute its karyotype, such a rearranged photomicrograph reveals the number, shapes, and sizes of the chromosomes in a cell. In humans, karyotypes can aid in the diagnosis of certain diseases and this has led to an entire branch of medicine called cytogenetics. Chromosome analysis with the microscope is being replaced by direct analysis of DNA,. When the chromosomes are stained and observed at high magnification individual chromosomes can be recognized by their lengths, the positions of their centromeres, and characteristic banding patterns that are visible. The karyotype consists of homologous pairs of chromosomes in diploid cells—for instance, a total of 46 chromosomes in humans and there are 23 pairs. Between the size of an organism and its chromosome number, there is no simple relationship. A housefly has 5 chromosome pairs, and a horse has 32, but the smaller carp (a fish) has 52 pairs. Probably the highest number of chromosomes in any organism which has 1,260 (630 pairs) is in the fern Ophioglossum reticulatum.

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Figure 3.9. Human karyotype. Source: Image by Wikimedia Commons

3.10. IN A LIVING ORGANISM, HOW DO CELLS DIE? In one of two ways cells die. When cells are damaged by mechanical means or toxins or are starved of oxygen or nutrients the first type of cell death, necrosis, occurs. Releasing their contents into the extracellular environment, these cells usually swell up and burst. This process often results in inflammation. Cell death is more typically due to apoptosis (Greek, “falling apart”). Cell death resulting from apoptosis is a programmed series of events. Why would a cell initiate apoptosis, which is essentially cell suicide? There are two possible reasons, in animals: •

•

By the organism, the cell is no longer needed. For example, a human fetus has weblike hands, before birth, with connective tissue between the fingers. This unneeded tissue disappears as its cells undergo apoptosis in response to specific signals, as development proceeds. The the longer cells live the more prone they are to genetic damage that could lead to cancer. It is exposed to radiation or

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toxic substances; this is especially true of epithelial cells on the surface of an organism. After only days or weeks and are replaced by new cells, such cells normally die. In many organisms, the outward events of apoptosis are similar. From its neighbors, the cell becomes detached, cuts up its chromatin into nucleosomesized pieces, and forms membranous lobes, or “blebs,” that break up into cell fragments. The surrounding living cells usually ingest the remains of the dead cell by phagocytosis in a remarkable example of the economy of nature. The digested components are recycled and neighboring cells digest the apoptotic cell contents in their lysosomes. In an important defense mechanism called the hypersensitive response, apoptosis is also used by plant cells. By undergoing apoptosis at the site of infection by a fungus or bacterium, plants can protect themselves from disease. The invading organism is not able to spread to other parts of the plant, with no living tissue to grow in. Plant cells do not form blebs the way that animal cells do, because of their rigid cell wall. Instead, in the vacuole, they digest their own cell contents and into the vascular system then release the digested components. They share many of the signal transduction pathways that lead to apoptosis, despite these differences between plant and animal cells. Programmed cell death is controlled by signals, like the cell division cycle, which may come from inside or outside the cell. Internal signals may be linked to the absence of mitosis or the recognition of damaged DNA,. In the plasma membrane external signals (or a lack of them) can cause a receptor protein to change its shape, and in turn, activate a signal transduction pathway. Both internal and external signals can lead is the activation of a class of enzymes known as caspases. In a cascade of events, these enzymes are proteases that hydrolyze target molecules. The cell dies as the caspases hydrolyze proteins of the nuclear envelope, nucleosomes, and plasma membrane, as a result.

3.11. UNREGULATED CELL DIVISION LEAD TO CANCER? Most people realize that cancer involves an inappropriate increase in cell numbers, perhaps no malady affecting people in the industrialized world

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instills more fear than cancer. One in four will die of it, one in three Americans will have some form of cancer in their lifetimes. Cancer ranks second only to heart disease as a killer, with 1.5 million new cases and half a million deaths in the United States annually.

3.11.1. Cancer Cells Differ From Normal Cells From the normal cells, cancer cells differ, the latter originate from the former in two ways: • Over cell division cancer cells lose control. • To other locations in the body cancer, cells can migrate. Only if they are exposed to extracellular signals such as growth factors do most cells in the body divide. Ultimately forming tumors (large masses of cells) cancer cells do not respond to these controls, and instead, divide continuously. A physician can feel a tumor or see one on an X-ray film or CAT scan but by the time, it already contains millions of cells. Tumors can be benign or malignant.

Figure 3.10. Diagram showing cancer cells spreading into the bloodstream. Source: Image by Wikipedia Commons

Benign tumors remain localized where they develop and resemble the tissue they came from, and grow slowly. For instance, a benign tumor of fat cells is a lipoma that may arise in the armpit and remain there. Benign tumors must be removed if they impinge on an organ, obstructing its function but they are not cancers.

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Malignant tumors do not look like their parent tissue at all. In the lung wall a flat, specialized epithelial cell may turn into a relatively featureless, round, malignant lung cancer cell. Malignant cells often have irregular structures such as variable nucleus sizes and shapes. to invade surrounding tissues and spread to other parts of the body by traveling through the bloodstream or lymphatic ducts. The second and most fearsome characteristic of cancer cells is their ability In distant parts of the body where the malignant cells become lodged, they go on dividing and growing, establishing a tumor at that new site. This spreading is known as metastasis that results in organ failures and makes the cancer very hard to treat.

3.11.2. Cancer Cells Lose Control over the Cell Cycle and Apoptosis Earlier in this chapter, you learned about proteins that regulate the progress of a eukaryotic cell through the cell cycle: •

Positive regulators such as growth factors stimulate the cell cycle: they are like “gas pedals.” • Negative regulators such as RB inhibit the cell cycle: they are like “brakes.” A cell will go through a division cycle only if the positive regulators are active and the negative regulators are inactive just as driving a car requires stepping on the gas pedal and releasing the brakes. The two regulatory systems ensure that cells divide only when needed, in most cells. These two processes are abnormal, in cancer cells. •

In cancer cells, oncogene proteins are positive regulators. They stimulate the cancer cells to divide more often, and they are derived from normal positive regulators that have become mutated to be overly active or that are present in excess. In the signal transduction pathway, oncogene products could be growth factors, their receptors, or other components. The growth factor receptor in a breast cancer cell is an example of an oncogene protein. Low numbers of the growth factor receptor HER2 are relatively contained by the normal breast cells. So, it does not find many breast cell receptors with which to bind and initiate cell division when this growth factor is made.

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A DNA change results in the increased production of the HER2 receptor, in about 25 percent of breast cancers. This results in a rapid proliferation of cells with the altered DNA and positive stimulation of the cell cycle. •

In both cancer and normal cells, tumor suppressors are negative regulators, but in cancer cells they are inactive. An example is the RB protein that acts at R (the restriction point) in G1. When RB is is active, the cell cycle does not proceed, but in cancer cells it allows the cell cycle to occur. Some viral proteins can inactivate tumor suppressors. HPV infects cells of the cervix and produces a protein called E7. E7 binds to the RB protein and it prevents it from inhibiting the cell cycle.

3.12. MENDELIAN LAW OF INHERITANCE Much of the early study of biological inheritance was done on plants and animals of economic importance. As early as 5,000 years ago, records show that people were deliberately crossbreeding date palm trees and horses. Especially for ornamental flowers such as tulips, by the early nineteenthcentury plant breeding was widespread. Under two key assumptions about how inheritance worked plant breeders of that time were operating. By experimental evidence, only one of those assumptions turned out to be supported. •

To offspring each parent contributes equally (supported by experiments). The German botanist Josef Gottlieb Kölreuter studied the offspring of reciprocal crosses in the 1770s, in which plants were crossed (mated with each other) in both directions. For instance, to pollinate related plants with red flowers in one cross, plants with white flowers were used as males. In the reciprocal crosses, the red-flowered plants were used as males in crosses with the white-flowered plants. Such reciprocal crosses always gave identical results, in Kölreuter’s studies, showing that both parents contributed equally to the offspring. •

Hereditary determinants blend in offspring (not supported by experiments). In the egg and sperm cells, Kölreuter and others proposed that there were hereditary determinants. In a single cell after mating when these determinants came together, they were believed to blend. The offspring would have a blended combination of the two parents’ characteristics (purple flowers) if a plant with one form of a character (say,

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red flowers) was crossed with a plant with a different form of that character (blue flowers). Once heritable elements were combined, they could not be separated again according to the blending theory (like inks of different colors mixed). The red and blue hereditary determinants were thought to be forever blended into the new purple one. Gregor Mendel confirmed the first of these two assumptions but refuted the second, in his experiments in the 1860s.

3.13. HOW DO GENES INTERACT ? To produce a phenotype, two alleles of the same gene can interact. It will realize that they are influenced by the products of many genes, considering most complex phenotypes, such as human height. It is moved to the genetics of such gene interactions. When the phenotypic expression of one gene is affected by another gene epistasis occurs. As an example, two genes (B and E) determine coat color in Labrador retrievers: • •

Allele B (black pigment) is dominant to b (brown) Allele E (pigment deposition in hair) is dominant to e (no deposition, so hair is yellow) So, an EE or Ee dog with BB or Bb is black; one with bb is brown; and one with ee is yellow regardless of the Bb alleles present. Clearly, the expression of Bb is determined by the gene E.

3.13.1. Hybrid Vigor Results From New Gene Combinations and Interactions Charles Darwin reported that when he crossed two different true breeding, in 1876, the offspring were 25 percent taller than either of the parent strains, homozygous genetic strains of corn. For the next 30 years, Darwin’s observation was largely ignored. In 1908, George Shull “rediscovered” this idea, reporting that not just plant height but the weight of the corn grain produced was dramatically higher in the offspring. In the field of applied genetics, agricultural scientists took note, and Shull’s paper had a lasting impact. Mattings among close relatives (known as inbreeding) can result in offspring of lower quality than mattings between unrelated individuals

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known by farmers for centuries. In breeding depression agricultural scientists call this. Because close relatives tend to have the same recessive alleles the problems with inbreeding arise, some of which may be harmful. Heterosis is known as the “hybrid vigor” after crossing inbred lines (short for heterozygosis). Quadrupling grain production, the cultivation of hybrid corn spread rapidly in the United States and all over the world. This scientific advance was not universally adopted, unfortunately, as in regions such as the Russian empire that fell far behind in corn production. The practice of hybridization has spread to many other crops and animals used in agriculture. For example, beef cattle that are crossbred are larger and live longer than cattle bred within their own genetic strain. The mechanism by which heterosis works is not known. A widely accepted hypothesis is overdominance, in which the heterozygous condition is superior to the homozygous condition in either or both genes in certain important genes whose products interact. Another hypothesis is that the homozygotes have alleles that inhibit growth, and these are less active or absent in the heterozygote.

3.14. THE ENVIRONMENT AFFECTS GENE ACTION The phenotype of an individual does not result from its genotype alone. To determine the phenotype of an organism genotype and environment interact. To remember in the era of genome sequencing this is especially important. It was hailed as the “book of life,” when the sequence of the human genome was completed in 2003, and public expectations of the benefits gained from this knowledge were (and are) high. But this kind of “genetic determinism” is wrong. Environmental variables such as light, temperature, and nutrition can affect the phenotypic expression of a genotype informed by the common knowledge. A familiar example of this phenomenon involves “point restriction” coat patterns found in Siamese cats and certain rabbit breeds.These animals carry a mutant allele of a gene that controls the growth of black fur all over the body. The enzyme encoded by the gene is inactive at temperatures above a certain point (usually around 35°C) as a result of this mutation. Their fur is mostly light because the animals maintain a body temperature above this point.

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The fur on these regions is dark, however, the extremities—feet, ears, nose, and tail—are cooler, about 25°C. Because the extremities were kept warm in the mother’s womb, these animals are all white when they are born. The dark fur is temperature dependent by a simple experiment it is shown. The fur that grows back will be dark, if a patch of white fur on a point-restricted rabbit’s back is removed and an ice pack is placed on the skin where the patch was. The environment inhibited the activity of the mutant enzyme this indicates that although the gene for dark fur was expressed all along. The effects of genes and environment on the phenotype are described by the two parameters: •

In a group, penetrance is the proportion of individuals with a given genotype that show the expected phenotype. • In an individual, expressivity is the degree to which a genotype is expressed. Penetrance affects, for instance, in humans the incidence of Huntington’s disease. The disease results from the presence of a dominant allele, but 5 percent of people do not express the disease with the allele. So, this allele is said to be 95 percent penetrant. Consider for an example of environmental effects on expressivityhow Siamese cats kept indoors or outdoors in different climates might look .

3.14.1. Most Complex Phenotypes Are Determined By Multiple Genes and the Environment The differences between individual organisms in simple characters such as those that Mendel studied in pea plants, are distinct and qualitative. For example, the individuals are either short or tall in a population of pea plants. However, the phenotype varies continuously over a range, such as height in humans. Many are in between the two extremes, some people are short, others are tall. Such variation is called quantitative, or continuous, variation within a population. This variation is largely genetic sometimes. For example, the result of several genes controlling the synthesis and distribution of dark melanin pigment is human eye color. Brown eyes less, and green, gray, and blue eyes even less, but dark eyes have a lot of it. The distribution of other pigments in the eye is what determines light reflection and color, in the latter cases.

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However, quantitative variation is due to both genes and the environment in most cases. In humans, height certainly falls into this category. It is often seen that parent and their offspring all tend to be tall or short, if looked at families. American 18-year-olds today are about 20 percent taller than their greatgrandparents were at the same age, however, nutrition also plays a role in height. For mutations that would exert such a dramatic effect to occur three generations are not enough time, so the height difference must not be due to genetics. Geneticists call the genes that together determine such complex character’s quantitative trait as loci. Identifying these loci is an important one and is a major challenge. For example, by many interacting genetic factors, the amount of grain that a variety of rice produces in a growing season is determined. Crop plant breeders have worked hard to decipher these factors in order to breed higher-yielding rice strains. Human characteristics such as disease susceptibility and behavior are caused in part in a similar way by quantitative trait loci. Recently, one of the many genes involved with human height was identified. The gene, HMGA2, has an allele that apparently has the potential to add 4 mm to human height.

3.15. RELATIONSHIP BETWEEN GENES AND CHROMOSOMES There are far more genes than chromosomes. The same chromosome reveal inheritance patterns that are not Mendelian. Not only in detecting the linkage of genes have such these patterns have been useful, but also in determining how far apart they are from one another on the chromosome The fruit fly or Drosophila melanogaster is the organism that revealed genetic linkage. This animal is an attractive experimental subject because of its small size, the ease with which it can be bred, and its short generation time. Thomas Hunt Morgan and his students at Columbia University pioneered the study of Drosophila, beginning in 1909, and in studies of genetics, it remains a very important organism.

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3.16. GENES ON SEX CHROMOSOMES ARE INHERITED IN SPECIAL WAYS The Mendelian patterns of inheritance is not shown by genes on sex chromosomes. The X chromosome carries a substantial number of genes that affect a great variety of characters, but the Y chromosome carries few known genes, in Drosophila and in humans. These genes are present in only one copy in males but in two copies in females. Therefore, on the X chromosome males are always hemizygous for genes—they have only one copy of each, and it is expressed. For genes are carried on the sex chromosomes, so reciprocal crosses do not give identical results for characters, and these characters do not show the usual Mendelian inheritance ratios. Eye color in Drosophila is a good example of inheritance of a character that is governed by a locus on a sex chromosome (sex-linked inheritance). Red is the color of the wild-type eye flies. Morgan discovered a mutation that causes white eyes in 1910. He crossed flies of the wild-type and mutant phenotypes and demonstrated that the eye color locus is on the X chromosome. The presence of the alleles on the X chromosome is designated by XR and X ,if we abbreviate the eye color alleles as R (red eyes) and r (white eyes). All the male progeny and female progeny had red eyes, when a homozygous, red-eyed female (XRXR) was crossed with a (hemizygous) white-eyed male (Xr Y), all the progenies had inherited a wild-type X chromosome (XR) from their mothers and because red (R) is dominant over white (r). r

All the male progeny were white-eyed and all the female progeny were red-eyed, in the reciprocal cross, in which a white-eyed female (Xr Xr ) was mated with a red-eyed male (XRY). The male progeny from the reciprocal cross inherited the Y chromosome they inherited from their father and did not carry the eye color locus and their only X chromosome was from their white-eyed mother. On the other hand, an X chromosome bearing the red allele from their father and the female progeny got an X chromosome bearing the white allele from their mother; therefore, they were red-eyed heterozygotes. Half of their male progeny had white eyes, when heterozygous females were mated with red-eyed males, but all their female progeny had red eyes. Together, these three results showed that eye color was carried on the X chromosome and not on the Y,.

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3.17. HOW DO PROKARYOTES TRANSMIT GENES? Prokaryotic cells lack a nucleus but have their genetic material as mostly a single chromosome in a central region of the cell. A process that gives rise to virtually genetically identical products, bacteria reproduce asexually by cell division. That is, in bacteria, the offspring of cell reproduction constitute a clone. However, the resulting new alleles increase genetic diversity; mutations occur in bacteria just as they do in eukaryotes. Therefore, it might be expected that there is no way for individuals of these species to exchange genes, as in sexual reproduction. Prokaryotes do have a sexual process, it turns out, though.

3.17.1. Bacteria Exchange Genes by Conjugation Like the bacterial cell, the bacterial chromosome is considerably smaller than its eukaryotic counterpart. In humans, in a haploid set, each of the 23 chromosome set may have thousands of linked genes and be a highly compacted linear strand several centimeters in length. E. coli has a single, circular chromosome that carries a few thousand genes and is only about 1 μm in circumference in contrast. After a chromosome is transferred from one cell to another genetic recombination in bacteria occurs, which brings the chromosomes of two cells into close proximity within a single cell. Bacterial conjugation is the process of transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This process usually takes place through a pilus. It is a parasexual mode of reproduction in bacteria. Also, it is a mechanism of horizontal gene transfer as are transformation and transduction although these two other mechanisms do not involve cell-to-cell contact. Process• • • •

Donor cell produces pilus. Pilus attaches to recipient cell and brings the two cells together. The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donor for the F-factor.

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3.18. CONCLUSION In this chapter, the evolution of the cells, and heredity have been discussed. In this chapter, chemical structures of the genes, gene transcription and translation have also been discussed. It also discussed the binary fission with reproductive signals and the replication of DNA. Towards the end of the chapter, the role of cell division in a sexual life cycle has been discussed. It also discussed that the unregulated cell division leads to cancer. In this chapter, the Mendelian law of inheritance, environmental effects of gene action, and the relationship between genes and chromosomes have also been discussed.

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REFERENCES 1.

2.

3.

4.

5.

Sadava, D., Craig Heller, H., Hillis, D. and Berenbaum, M., 2009. Life: The Science of Biology. 9th ed. [ebook] WHFreeman,2009. Available at: [Accessed 3 July 2022]. Encyclopedia Britannica. n.d. chromosome | Structure & Function. [online] Available at: [Accessed 3 July 2022]. Griffiths, A., 1999. heredity | Definition & Facts. [online] Encyclopedia Britannica. Available at: [Accessed 3 July 2022]. Hege, K., 1996. T-cell gene therapy. Current Opinion in Biotechnology, [online] 7(6), pp.629-634. Available at: [Accessed 3 July 2022]. Zahn, L., 2021. Bacterial cell gene expression. Science, [online] 371(6531), pp.793.13-795. Available at: [Accessed 3 July 2022].

4

CHAPTER

FLOWERING PLANTS: FORM AND FUNCTIONS

CONTENTS 4.1. Introduction....................................................................................... 98 4.2. Basic Body Plan of Plants................................................................... 99 4.3. Cell Wall Support Plant Growth and Form....................................... 103 4.4. How Do Plant Tissues and Organs Originate?.................................. 106 4.5. The Plant Body is Constructed From Three Tissue Systems................ 108 4.6. Cells of the Xylem Transport Water and Dissolved Minerals............. 110 4.7. Cells of the Phloem Transport the Products of Photosynthesis........... 111 4.8. How Do Meristems Build a Continuously Growing Plant?............... 111 4.9. Leaves are Determinate Organs Produced by Shoot Apical Meristems................................................................. 114 4.10. How Has Domestication Altered Plant Form?................................ 114 4.11. How Do Plants Acquire Nutrients?................................................. 115 4.12. What Mineral Nutrients Do Plants Require?................................... 116 4.13. Do Carnivorous and Parasitic Plants Obtain a Balanced Diet?........ 117 4.14. Flowering Plants Have Microscopic Gametophytes........................ 119 4.15. The Flowering Stimulus Originates in a Leaf................................... 122 4.16. Flowering Plants Use Animals or Wind to Transfer Pollen Between Flowers................................................................ 122 4.17. Self-Pollination.............................................................................. 125 4.18. Conclusion.................................................................................... 126 References.............................................................................................. 127

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This chapter deals with the various characteristics of plants along with the functions and processes in plant biology. Beginning with the reproductive process and the role of flowers, the same the chapter later talks about the function of the stem, which is to support and uphold the plant. Further, the feature and importance of leaves are elaborated upon, as they are the sites for photosynthesis. Root system is the last macroscopic feature that is discussed. Elaboration on the cellular organization of the plants is highlighted that mainly includes the multiple functions of the cell wall which is only present in plant cells. The Tissue system is later on touched upon after discussing the cell wall. In the end, the importance of the meristem is discussed keeping in, mind the various differences between plants and animals and how it has proved to be beneficial to plants.

4.1. INTRODUCTION The plants which produce flowers as their reproductive structures are known as flowering plants. They are found in a great variety of shapes and sizes including grasses, sedges, rushes, many trees, as well as familiar plants like daffodils and daisies. Although not always but, often, flowers have brightly colored petals. Within the flowers, the female ovules are concealed that are further fertilized by pollen grains. The pollen grains are the flower’s male parts. Fertilized ovules mature into seeds from which new plants grow. The fact that ovules are protected within flowers has greatly contributed to the success of flowering plants. They can be found in a variety of habitats, including deserts, rivers, and even the sea. There are approximately 250,000 known species. The currently known, described and accepted number of plant species are ca 374,000, out of which approximately 308,312 are vascular plants, with 295,383 flowering plants (angiosperms; monocots: 74,273; eudicots: 210,008). Global numbers of smaller plant groups are as follows: algae ca 44,000, liverworts ca 9,000, hornworts ca 225, mosses 12,700, lycopods 1,290, ferns 10,560 and gymnosperms 1,079. Phytotaxa is currently contributing more than a quarter of the ca 2000 species that are described every year, showing that it has become a major contributor to the dissemination of new species discovery. Wind pollinates some flowering plants. This means they rely on wind to transport pollen grains from one flower to the next. Wind pollinated plants,

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such as grasses and many trees, such as oak and birch, typically have small, inconspicuous flowers. Insects pollinate some flowering plants. These plants frequently have brightly colored, highly scented flowers that produce sugar-filled nectar. These characteristics attract insects, which carry pollen from one flower to the next as they search for hidden nectar. Flowering plants (phylum Anthophyta) exhibit a virtually limitless variety of shapes, sizes, and textures. They range in size from the massive Tasmanian Eucalyptus trees, which have nearly the same mass as the giant redwoods, to the smallest duckweeds, which are less than 1 millimeter long. Aside from the common flattened green leaves, flowering plant leaves can be succulent, floating, submerged, cup-shaped, spine-like, scale-like, feathery, papery, hairy, or insect trapping, and almost any color. Some are so small that they require a microscope to examine them, while others, such as the Seychelles Island palm, can grow to be up to 6 meters long. Their flowers range from simple buttercup flowers to the extraordinarily complex flowers of some orchids, which may entice pollinators with molecules, force pollen bags onto their bodies, or coat them in the fluid they secrete. Flowers can weigh less than one gram and last only a few minutes, or they can weigh up to nine kilograms and last for months. Plants from many families are parasitic or partially parasitic on other plants (for example, dodder, or mistletoe), or mycotrophic (deriving their nutrients from fungi that form a mutualism with plant roots). Numerous orchids, for example, are epiphytic (attached to other plants, with no roots in the ground, and not in any way parasitic).

4.2. BASIC BODY PLAN OF PLANTS Plants survive by absorbing sunlight energy and extracting water and mineral nutrients from the soil. These resources, however, are extremely scarce in the environment, so plants must collect them from vast areas both above and below ground. Another difficulty that plants face is their inability to move. Plants cannot move from, say, a dry, shady location to a wet, sunny location. Due to the virtue of a plant’s body plan, they are enabled to respond to the said challenges. Plants are allowed by stems, leaves, and roots to be anchored to a certain spot and capture scanty resources effectively, both above and below the ground.

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More importantly, plants can grow throughout their lives to compensate for their inability to move. While plants cannot move to a new water source or a sunny clearing, they can respond to environmental cues by redirecting their growth to capitalize on opportunities that arise in their immediate surroundings.

4.2.1. The Root System Anchors the Plant and Takes up Water and Dissolved Minerals Most plants get their water and minerals from the soil via the root system. But since light does not penetrate the soil, roots typically lack photosynthesis capacity. Despite being hidden from view, the root system is frequently larger than the visible shoot system. The root system of a 4-month-old winter rye plant (Secale cereale) was discovered to be 130 times larger than the shoot system, with nearly 13 million branches totaling over 500 km! The root system of angiosperms develops from an embryonic root known as the radicle. The root systems of monocots and eudicots develop differently from this common starting point.

Figure 4.1. A picture of Secale cereale flowering. Source: Image by Wikimedia Commons

Following seed germination, the radicle of most eudicots develops as a primary root (called the taproot), which extends downward through tip

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growth and outward through lateral root formation. The taproot and lateral roots combine to form a taproot system, which can take various forms. The taproot, for example, is frequently used as a nutrient storage organ in carrots (Daucus carota), sugar beets (Beta vulgaris), and sweet potatoes (Ipomoea batatas). Monocot (and some eudicot) primary roots, on the contrary, are shortlived. The roots of a typical monocot are called adventitious (“arriving from outside”) roots because they emerge from the stem at ground level or below, and they form a fibrous root system composed of numerous thin roots that are all roughly equal in diameter. Several more fibrous root systems have a large surface area for water and mineral absorption. A fibrous root system adheres strongly to the soil. The fibrous root systems of grasses, for example, may protect steep hillside erosion caused by rain runoff. Adventitious roots serve as props to help support the shoot in certain monocots, such as corn, banyan trees, and some palms. Prop roots are essential for these plants, which, unlike most eudicot tree species, cannot support aboveground growth through stem thickening.

4.2.2. The Stem Supports Leaves and Flowers To elevate and support the photosynthetic organs (leaves) as well as the reproductive organs (flowers) is the fundamental function of the stems. Unlike roots, stems produce a variety of buds. A bud is a developing shoot that may or may not produce additional branches or leaves. Shoots are made up of repeating modules known as phytomers. A phytomer consists of one or more leaves attached to the stem at a node; an internode (the distance between two nodes on the stem); and one or more axillary buds that form at the angle (axil) where each leaf meets the stem. Axillary buds are distinguished from terminal buds, which grow at the end of a stem or branch. If an axillary bud becomes active, it can develop into a new branch or extension of the shoot system. The arrangement of leaves along the stem (called the phyllotaxy) is often typical of the plant species. Nature exhibits a variety of stem modifications. A potato tuber, for example—the part of the plant eaten by humans—is not a root, but rather an underground stem. A potato’s “eyes” are depressions that contain axillary buds; in other words, a sprouting potato is just a branching stem.

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Many desert plants have water-retaining stems that have grown in size. Strawberry runners are horizontal stems from which roots grow at regular intervals. If the connections between the rooted portions are broken, independent plants can form on each side of the break—a type of vegetative (asexual) reproduction.

Figure 4.2. An example of axillary buds located in the axil of the leaf. Source: Image by Wikimedia Commons

4.2.3. Leaves Are the Primary Sites of Photosynthesis The leaves of gymnosperms and most flowering plants are responsible for the majority of photosynthesis. Leaves are brilliantly adapted for light gathering. A leaf blade is typically a thin, flat structure attached to the stem by a stalk called a petiole. The petiole of many plants holds the leaf blade at an almost perpendicular angle to the sun’s rays. With the leaf surface facing the sun, this orientation maximizes the amount of light available for photosynthesis. Some leaves follow the sun throughout the day, shifting their position so that they are always facing it. The leaves of certain plant species have been heavily modified to perform specific functions. Some modified leaves, such as those seen in onion bulbs, act as storage depots for energy-rich compounds. The leaves of other species, such as succulents, hold water.

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Cacti have modified leaves as defensive spines. Plants with modified parts of leaves, such as peas, have tendrils that support the plant by wrapping around other structures or plants.

4.3. CELL WALL SUPPORT PLANT GROWTH AND FORM Plant cells have the same fundamental organelles as all other eukaryotes, but they have a few extra structures and organelles that set them apart from many other eukaryotic cells: • Plastids (chloroplasts and other plastids) • A vacuole in the center • Cell walls that are rigid and contain cellulose The need to acquire energy for photosynthesis, which takes place in the chloroplasts, dictates plant structure in part. The importance of vacuoles and cell walls in defining plant shape is less evident.

4.3.1. Cell Walls and Vacuoles Help Determine Plant Form A single central vacuole is found in most mature plant cells, accounting for up to 90% of the cell’s volume. The vacuole is a watery sac that contains a high concentration of solutes, such as enzymes, amino acids, and photosynthesisproduced carbohydrates. Many of these solutes are transported into the vacuole by transporter proteins found in the vacuolar membrane, the tonoplast. The osmotic force for water uptake into the vacuole is provided by this active buildup of solutes. The cell wall is subjected to turgor pressure when the vacuole expands. Turgor pressure is necessary for plant growth as well as for keeping plants erect.

4.3.2. The Structure of Cell Walls Allows Plants to Grow Cell walls are a feature of bacteria, fungi, algae, and plants. They are responsible for regulating cell volume, determining cell shape, and protecting the contents of the cell. The chemical makeup of plant cell walls gives them distinct characteristics. Furthermore, the chemicals that make up plant cell walls sequester the majority of carbon in terrestrial ecosystems. As a result, it’s worthwhile

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to examine their genesis and structure more closely. When a plant cell’s cytokinesis is completed, a cell plate separates the two daughter cells. The middle lamella is a gluelike substance that originates within the cell plate and survives as a thin layer between the walls of the two daughter cells. To create the main cell wall, each daughter cell secretes three types of polysaccharides. •

Cellulose is made up of thousands of glucose molecules that are arranged into microfibril bundles that form a lattice within the cell wall. • Hemicelluloses are polysaccharide chains with a lot of branches that cross-link the cellulose microfibrils. • Pectins are polysaccharides that are diverse and more soluble than the other components. (The gel characteristics of fruit jams and jellies are due to pectin.) As the cell develops to its final size, the production and deposition of polysaccharides continue. Plants grow in a variety of ways, one of which is cell growth. Some cells can expand by 100,000 to 1,000,000 times their original size! When a plant cell is enclosed by a stiff cell wall, how can it expand? Remember that osmotic pressure causes the central vacuole to expand, putting turgor pressure on the cell wall. The protoplast is made up of the living contents of the plant cell, such as the plasma membrane and everything contained within it. The cell wall loosens the connections between cellulose microfibrils in response to the protoplast’s growing size. Expansins, a type of cell wall protein, are hypothesized to help loosen the cell wall by breaking the noncovalent bonds that connect the hemicelluloses and pectins to the cellulose microfibrils. New cell wall components are generated and incorporated to prevent the cell wall from becoming too thin (and so blowing out like an overinflated balloon). As the cell expansion stops, certain types of plant cells deposit one or more additional cellulosic layers in order to form a thick secondary cell wall internal to the primary cell wall. Secondary cell walls provide mechanical support for some plants, allowing them to grow huge stems.

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The secondary wall, like the primary wall, is made up of layers of organized cellulose microfibrils. Rather than being embedded in pectins, the microfibrils are embedded in lignin, a fascinating material to scientists. The primary wall and even the central lamella become lignified when subsidiary walls become lignified. Lignin is a polymer made up of carbon atoms that forms a hydrophobic matrix that is robust, waterproof, and resistant to animal digestion. Lignin is the second most prevalent biological polymer on Earth, behind cellulose, accounting for 20–35 percent of the dry weight of wood.

Plant cell walls are still being assessed by scientists due to their complexity and dynamics. Their core components—celluloses, hemicelluloses, pectins, and lignins—are molecular classes that can be constructed from a range of components and modified in a number of ways. As a result, the composition of plant cell walls differs depending on the type of plant cell. Furthermore, the composition of a single plant cell’s wall may not be homogeneous. It’s possible, for example, that directional growth reflects the deposition of cell wall components that are easier to release at one end of the cell than at the other. The discovery that the genome of the microscopic plant Arabidopsis thaliana has more than a thousand genes related to cell wall production, the functions of only a small fraction of which are currently known, is one measure of how much remains unknown. Cooperation between groups of cells is required to build a plant body. Despite their isolated appearance through the cell walls, plant cells interact in two ways in order to build as well as maintain a complex organism. First, the cell wall is permeable to water and mineral ions in most places, allowing tiny molecules to pass through to the plasma membrane. Second, adjacent cells’ endoplasmic reticula (ER) are linked by plasmodesmata, which are cytoplasm-filled canals that pierce through the main wall and allow direct contact between plant cells.

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Figure 4.3. Plasmodesma. (A) Schematic representation of plant cells connected by cell wall-piercing plasmodesmata. (B) Transverse and longitudinal sections through the plasmodesmata of the two forms. Source: Image by Wikimedia Commons

A single plant cell can have up to a thousand plasmodesmata connecting it to its neighbors, allowing proteins and even RNA to pass from one cell to the next. Some of these plasmodesmata occur when the cell plate is deposited during cytokinesis,. Some plant viruses have evolved a cunning strategy to take advantage of this intercellular highway. Tobacco mosaic virus (TMV), for example, encodes a protein which is called the movement protein, or MP. It helps the virus spread throughout the plant. In the absence of MP, the RNA genome of TMV cannot move from cell to cell. However, in some unknown way, the MP–RNA complex is able to move easily from cell to cell via plasmodesmata.

4.4. HOW DO PLANT TISSUES AND ORGANS ORIGINATE? How can a single plant cell (a zygote) split and grow into an organism such as a redwood tree, which may grow to a height of over 100 meters in over a thousand years? A plant’s basic body design for its mature shape is established while it is still in the seed.

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The embryo establishes two patterns that contribute to the plant body plan: •

The basal–apical axis: the arrangement of cells and tissues from root to shoot along the main axis. • The radial axis: the tissue systems are arranged in a concentric pattern. At the terminals of the embryonic root and shoot, two clusters of undifferentiated cells arise. Meristems (from the Greek merizein, “to divide”) are the organ-forming clusters that control all postembryonic development and allow the plant to generate organs throughout its lifecycle. Axes and meristems are best understood in terms of development. We will concentrate on embryogenesis (embryo formation) in the model eudicot Arabidopsis thaliana, which has been the subject of most research. The mitotic division of the zygote, which produces two daughter cells, is the first step in the creation of a plant embryo. The destinies of these two cells are very different. One daughter cell produces the embryo proper, while the other daughter cell produces the suspensor, due to an asymmetrical (uneven) distribution of cytoplasm within the zygote. The zygote’s asymmetrical division establishes polarity as well as the future plant’s basal–apical axis. After only four mitotic divisions, a long, thin suspensor can be distinguished from a more spherical or globular embryo. The suspensor quickly stops elongating. In the case of eudicots, the initially globular embryo develops into the characteristic heart stage as the cotyledons (“seed leaves”) begin to grow. The torpedo stage is characterized by further extension of the cotyledons and the embryo’s main axis, during which some internal tissues begin to develop. The shoot apical meristem is located between the cotyledons, while the root apical meristem is located at the other end of the axis. Each of these areas contains undifferentiated cells that will continue to divide in order to give rise to the organs that will develop throughout the plant’s existence. The plant’s radial axis is also established at the end of embryogenesis. Three tissue systems, grouped concentrically in the embryonic plant, will give rise to the tissues of the adult plant body.

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4.5. THE PLANT BODY IS CONSTRUCTED FROM THREE TISSUE SYSTEMS A tissue is an organized collection of cells that share characteristics and operate as a structural and functional unit. Tissues, in turn, are organized into tissue systems in plants. Regardless of anatomical differences, all vascular plants are made up of three tissue systems: dermal, vascular, and ground. These three tissue systems emerge during embryogenesis and eventually expand in a concentric pattern throughout the plant body. Each tissue system serves a specific purpose and is made up of several cell types.

4.5.1. Dermal Tissue System The epidermis, or outer covering of a plant is formed by the dermal tissue system, which normally consists of a single cell layer. The periderm is a type of dermal tissue found on the stems and roots of woody plants. The epidermis must stretch to cover the developing plant body throughout development. The epidermal cells are tiny and spherical, with a small central vacuole or no vacuole at all. When cell division in an organ’s epidermis stops, the epidermal cells enlarge. Some epidermal cells develop into one of three types of structures: •

Stomatal guard cells, which create stomata (pores) in leaves that allow gas exchange. • Trichomes, or leaf hairs, which give protection against insects and damaging sun rays • Root hairs, which considerably increase root surface area, thereby providing a greater surface for the uptake of water and mineral nutrients Cutin (a polymer formed of long chains of fatty acids), a complex mixture of waxes, and cell wall polysaccharides are secreted by aboveground epidermal cells to form a protective extracellular cuticle. The cuticle prevents water loss, reflects potentially harmful solar radiation, and acts as a pathogen barrier.

4.5.2. Ground Tissue System The ground tissue system encompasses almost all of the tissue in both shoots and roots that lies between dermal and vascular tissue, accounting for the majority of the plant’s body. Storage, support, and photosynthesis are the primary roles of ground tissue.

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Ground tissues contain three cell types that are categorized according to their cell wall construction to perform these various functions: parenchyma, collenchyma, and schlerenchyma. The parenchyma cell is the most abundant cell type in plants. Large vacuoles and thin walls make up parenchyma cells, which only have the main wall and a common middle lamella. They are involved in photosynthesis (in leaves) as well as the storage of protein and carbohydrates (in fruits and in roots). Many cells retain the ability to divide and hence can produce new cells, such as when a wound causes cell proliferation. Collenchyma cells resemble modified parenchyma cells that provide flexible support. Their major walls are thick at the corners of the cells, which is a distinguishing feature. Collenchyma cells are elongated in general. The primary wall thickens in these cells in part due to pectin deposition, but no secondary wall forms. Support to leaf petioles, nonwoody stems, and growing organs through collenchyma. Tissue made of collenchyma cells is flexible, permitting stems and petioles to sway in the wind without snapping. Celery’s “strings” are made up mostly of collenchyma cells. Sclerenchyma cells have enlarged secondary walls that help them maintain themselves. After lignifying their cell walls, several sclerenchyma cells undergo programmed cell death and so perform their supportive function while dead.

Figure 4.4. Herbaceous Dicot Stem: Collenchyma, Sclerenchyma and Parenchyma in Cucurbita. Source: Image by Flickr

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Elongated fibers and variably shaped sclereids are two forms of sclerenchyma cells. Fibers, which are commonly grouped into bundles, give relatively rigid support to wood and other elements of the plant. Long fibers provide a lot of the mechanical strength in tree bark. Sclereids can be densely packed, as in the shell of a nut or some seed coverings. Stone cells, which are isolated clumps of sclereids, give pears and other fruits their characteristic gritty texture.

4.5.3. Vascular Tissue System The vascular tissue system is the plant’s plumbing or transport system, and it is the distinctive trait in vascular plants. The xylem and phloem, two of its constituent tissues, transport materials throughout the plant. The xylem transports water and mineral ions from the roots to all of the stem and leaf cells. Phloem can perform a number of roles due to its cellular complexity, including transport, support, and storage. The plant body’s live cells require a source of energy and chemical building components. The phloem satisfies these requirements by transferring carbohydrates from sources (mainly leaves) to places of usage or storage (called sinks, such as growing tissue, storage tubers, and developing flowers). Let’s take a closer look at the structure of the different cell types which form these vascular tissues.

4.6. CELLS OF THE XYLEM TRANSPORT WATER AND DISSOLVED MINERALS Conducting cells called tracheary elements are present in the xylem. Before they can perform their duty of carrying water and dissolved minerals, these elements must go through apoptosis. Tracheids and vessel elements are the two types of tracheary elements. Tracheids are spindle-shaped cells present in gymnosperms and other vascular plants, and they are older than vessel elements in terms of evolution. Water and minerals can travel with little resistance from one tracheid to its neighbors via pits, breaks in the secondary wall that leave the primary wall unobstructed when the protoplast disintegrates during cell death. Flowering plants developed a water-conducting system made up of endto-end vessels made up of individual cells called vessel elements. Vessel elements, like tracheids, have pits in their cell walls, but they are often

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bigger in diameter. Vessel elements secrete lignin into their secondary walls before undergoing apoptosis and partially break down their end walls. The result is a continuous hollow tube made up of multiple vessel parts that serves as an open water conduit. The end walls of vessel components have grown less obliquely orientated and less blocked as angiosperm evolution has progressed, probably boosting the efficiency of water flow through them. Many angiosperms have tracheids and vessel components in their xylem.

4.7. CELLS OF THE PHLOEM TRANSPORT THE PRODUCTS OF PHOTOSYNTHESIS The phloem’s transport cells, unlike those of the adult xylem, are alive. The distinctive cells of the phloem in blooming plants are sieve tube components. These cells connect end-to-end, just like vessel elements. They create lengthy sieve tubes that convey carbohydrates and a variety of other substances from their origins to tissues that absorb or store them. Photosynthesis products, for example, migrate from leaves to root tissues in mature plants. Unlike vessel elements, which have end walls that break down, sieve tube elements have end walls with plasmodesmata that grow to form pores. Sieve plates, or end walls that resemble sieves, are the outcome. Even though the sieve tube elements remain alive, some protoplast components degrade. They are intimately linked to companion cells, which retain all of their organelles and serve as the sieve tube elements’ “life support system.” The plant embryo is encased in a seed towards the culmination of embryogenesis and is ready to germinate. For the time being, let’s look at the beginning of life as it pertains to plants, and see how the cells and tissues we just discussed enable the embryo to develop into an adult plant body.

4.8. HOW DO MERISTEMS BUILD A CONTINUOUSLY GROWING PLANT? As mentioned at the beginning of this chapter, the functioning and development of plants and animals are different. Plants are sessile and must collect restricted resources from above and below ground by growing, whereas animals use their mobility to seek food. Through the growth of

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shoots and roots, plants grow in two directions: toward the sunlight and toward water and dissolved minerals in the soil. As an animal develops from embryo to adult, all parts of the animal body increase, but in most animals, this growth is determined, meaning that the individual and all of its components stop growing once the adult state is attained. Some plant organs, such as leaves, flowers, and fruits, have a determined growth pattern. Shoot and root development, on the other hand, is a lifelong process. Indeterminate growth is a phrase used to describe open-ended growth.

4.8.1. Plants Increase in Size through Primary and Secondary Growth Plants expand their surface region above and below by developing. All plants experience essential development, which is described by the stretching of roots and shoots and by the multiplication of new roots and shoots through expansion. Also, numerous gymnosperms and eudicots, particularly trees, experience optional development, by which they expand in size. Essential and auxiliary development lead to particular attributes in the plant body. Essential development creates what is known as the essential plant body, while auxiliary development fosters the optional plant body. All seed plants have an essential plant body, which comprises the relative multitude of nonwoody pieces of the plant. Numerous herbaceous plants — monocots specifically — comprise completely of an essential plant body. Woody plants, like trees and bushes, have, notwithstanding the essential plant body, an optional plant body comprising of wood and bark. As the tissues of the optional plant body are set out, the stems and roots thicken. The optional plant body proceeds to develop and thicken over the lifetime of the plant. The essential plant body additionally keeps on developing, protracting and spreading the shoot and underground roots and shaping new leaves.

4.8.2. A Hierarchy of Meristems Generates the Plant Body Meristems, as seen, are confined locales of undifferentiated cells that are the sources of all new organs in the grown-up plant. Indeed, even before seed germination, the plant embryo has two meristems: a shoot apical meristem toward the tip of the undeveloped shoot, and a root apical meristem close to the furthest limit of the early-stage root.

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Meristematic cells are little and firmly pressed, with tiny focal vacuoles and an extremely delicate basic cell wall. Meristematic cells are undifferentiated and perpetually active, holding the capacity to endlessly deliver new cells. The cells that maintain the meristems, called progenitor cells, are equivalent to animal germ cells. At the point when a cell divides partitions, one little daughter cell forms into one more meristem cell the size of its parent, while the other daughter girl cell separates into a more specific cell. While the plant embryo undergoes essential development through the functions of the root and shoot apical meristems, the development of the grown-up plant mirrors the movement of extra meristem types. The process of postembryonic plant development starts with a more intensive look at how the grown-up plant develops all through its lifetime and the basic functions of meristems in that development.

4.8.3. The Products of The Stem’s Primary Meristems Become Stem Tissues Remember that shoots are made out of repeating modules called phytomers, each comprising of a node with its joined leaf or leaves, the internode among nodes, and axillary buds in the point between each leaf and the stem. Shoots develop by adding new phytomers. Those new phytomers begin from cells in shoot apical meristems, which are shaped at the tips of stems and in axillary buds. The shoot apical meristem, similar to the root apical meristem, structures three essential meristems: protoderm, ground meristem, and procambium. These essential meristems bring about the three shoot tissue frameworks. The shoot apical meristem dully sets out the starting points of leaves and axillary buds. Leaves emerge from points called leaf primordia, which form as cells partition on the sides of the shoot apical meristem. Bud primordia form at the roots of the leaf early stage and where they might turn out to be new apical meristems and start new shoots. The developing stem has no defensive design practically equivalent to the root cap, yet the leaf primordia can go about as a defensive covering for the shoot apical meristem.

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The channels of stems contrast from that of roots. In a root, the vascular tissue lies somewhere down on the inside, with the xylem at or close to the middle. The vascular tissue of a young stem, notwithstanding, is separated into distinct vascular bundles. Each vascular bundle contains both xylem and phloem. In eudicots, the vascular bundles for the most part structure a chamber, however in monocots, they are apparently dissipated all through the stem.

4.9. LEAVES ARE DETERMINATE ORGANS PRODUCED BY SHOOT APICAL MERISTEMS For the greater part of its life, a plant produces leaves from apical meristems. Apical meristems that produce leaves are called vegetative meristems. Leaves start from the edges of the apical meristem as beginning cells that separate into leaf primordia. An exceptionally improved method for thinking about the development of the leaf from the leaf primordia is to envision leaves as leveled stems. Be that as it may, there are two significant contrasts. In the first place, dissimilar to the uncertain development of the stem, the development of a leaf is determinate. Second, while the tissues of the stem are organized in a spiral example, the leaf, as a level organ, has an unmistakable top side and base side.

4.10. HOW HAS DOMESTICATION ALTERED PLANT FORM? Plant body plan — with roots, stems, leaves, meristems, and moderately hardly any tissue and cell types — underlies the surprising variety of the flowering plants that cover our planet. Notwithstanding, while a distinction in plant structure between individuals from various species is normal, individuals from similar species can be surprisingly different in structure too. According to a hereditary viewpoint, this perception proposes that minor distinctions in genes or gene control can underlie huge contrasts in plant structure. (By and by, various plant species in all actuality do contrast extraordinarily in some cases in genes and genome association.)

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Large numbers of these seed banks focus on seeds from a specific harvest animal variety, for example, the Maize Stock Center at the University of Illinois (corn) or the Genetic Resources Center of the International Rice Research Institute in the Philippines (rice). As well as containing huge assortments of developed assortments, these seed banks additionally contain seeds from populations of their wild family members. Why keep up with assortments of seed from both developed crops and their wild family members? Notwithstanding some of the time, tremendous morphological contrasts, crops and their wild family members are still individuals from similar species. Accordingly, when they are crossbred, they can create feasible offspring. These descendants will convey new blends of their folks’ characteristics. It is difficult to accept that advanced corn was trained from the wild grass teosinte, which actually fills in the slopes of Mexico. Quite possibly the most prominent distinction is that teosinte, as other wild grasses, is exceptionally stretched, with many shoots, while trained corn has a solitary shoot. This morphological contrast is to a great extent because of the action of a single gene called teosinte branches 1 (tb1). The protein product of tb1 controls the development of axillary buds. The allele of tb1 in domesticated corn quells branching, while the allele in teosinte allows expanding.

4.11. HOW DO PLANTS ACQUIRE NUTRIENTS? Each living thing — and plants are no special case — should get pure elements from their current circumstance surroundings These nutrients are the significant elements of macromolecules: carbon, hydrogen, oxygen, and nitrogen. Plants are autotrophs, and acquire carbon from atmospheric carbon dioxide through the carbon-fixing responses of photosynthesis. Hydrogen and oxygen come mostly from water, so these components are ample with a satisfactory water supply. Nitrogen, enters essentially through the activities of microorganisms.

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Figure 4.5. Carbon uptake and photosynthesis in a seagrass meadow. Source: Image by Wikimedia Commons

Living life forms need other mineral nutrients also, which most plants get from the soil. For instance, proteins contain sulfur (S), nucleic acids contain phosphorus (P), chlorophyll contains magnesium (Mg), cytochromes contain iron (Fe), and cell signaling can include calcium (Ca). Inside the soil, these and different minerals break down in water as particles, forming a solution — called the soil system — that contains the roots of plants.

4.12. WHAT MINERAL NUTRIENTS DO PLANTS REQUIRE? Plants require numerous mineral nutrients. With the exception of nitrogen, all mineral nutrients are from rock and are typically taken up from the soil. A nutrient is called essential in the event that its absence causes serious disturbance of ordinary plant development and generation. An essential nutrient can’t be supplanted by another component. Essential nutrients fall generally into two classes — macronutrients and micronutrients — in view of the levels needed by plants. • •

A plant needs macronutrients in levels of no less than 1 gram for every kilogram of the plant’s dry matter. A plant needs micronutrients in levels of under 100 milligrams for each kilogram of the plant’s dry matter.

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4.12.1. Hydroponic Experiments Identified Essential Elements A nutrient is viewed as essential for plants assuming a plant doesn’t finish its life cycle or develops abnormally when that component is missing or inadequate. The elements for plants were distinguished by developing plants hydroponically — that is, with their roots suspended in nutrient systems rather than soil. An element missing any of these mixtures like calcium nitrate, magnesium sulfate, and potassium phosphate couldn’t uphold typical development. Tests with different mixtures that included different mixes of these components before long identified six macronutrients — calcium, nitrogen, magnesium, sulfur, potassium, and phosphorus — as essential elements.

4.13. DO CARNIVOROUS AND PARASITIC PLANTS OBTAIN A BALANCED DIET? Most plants acquire their mineral nutrients from the soil (with the assistance of microbes), however some utilize different sources. Predatory and parasitic plants are instances of such plants.

4.13.1. Carnivorous Plants Supplement Their Mineral Nutrition A few plants increase their nitrogen supply by catching and processing flies and different insects. There are around 500 of these meat-eating plant species, the most popular of which are Venus flytraps, sundews, and pitcher plants (Sarracenia). Carnivorous plants are commonly found in boggy environments that are acidic and nutrient lacking. To acquire additional nitrogen, these plants catch insects, digest their proteins, and retain the amino acids. Pitcher plants have pitcher-molded leaves that gather enough quantities of water. Insects and, surprisingly, little rodents are tricked into the pitchers by splendid colors or appealing aromas and are kept from leaving by solid, descending pointing hairs. The animals ultimately die and are processed by a blend of plant enzymes and microscopic organisms in the water. Sundews have with hairs that emit a reasonable, tacky, sweet fluid. Bugs become adhered to these hairs, and more hairs bend over to additionally capture them. Enzymes from the plant digest the bugs.

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Venus flytraps have specific leaves with two parts that overlap together. At the point when a bug contacts trigger hairs on a leaf, its two parts immediately meet up, their barbed edges interlocking and detaining the bug before it can get away. The leaf then, at that point, secretes enzymes that digest its prey.

Figure 4.6. A picture of Venus Fly Trap (Dionaea muscipula). Source: Image by Wikimedia Commons

The Venus flytrap’s leaf is perhaps the quickest movement in the plant world, requiring just 0.1 sec. To figure out how this occurs, Dr. Lakshminarayanan Mahadevan and associates painted fluorescent spots on the outer layer of the flytrap’s leaf surface and utilized high-velocity cameras to record the snare snapping shut when its trigger hairs were contacted. They then, at that point, utilized PC examination of the recorded spot developments to create a numerical model to assist with making sense of the development. The specialists observed that the initial step is the stretching of cells on the external surface of the leaf. The extension of only one side of the leaf makes it snap from a curved into a sunken shape, similar to a contact focal point flipping back to front. These plants don’t have to benefit from bugs, however, doing so assists them with nutrition in their normal living spaces. They utilize the extra nitrogen from the insects to make more proteins, chlorophyll, and other nitrogencontaining compounds.

4.13.2. Parasitic Plants Take Advantage of Other Plants Around 1% of flowering plant species infer some or the entirety of their water, nutrients, and some of the time even photosynthate from different

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plants. In these parasitic plants, absorptive organs called haustoria have advanced that attack the host and tap into the vascular tissues in the root or stem. Parasitic plants are divided into two wide classes in view of their nourishing connections with their hosts. Hemiparasites can in any case photosynthesize, yet determine water and mineral nutrients from the living assortments of different plants. Maybe the most recognizable hemiparasites are the few genera of mistletoes. Mistletoes are green and continue some photosynthesis, yet they parasitize different plants for water and mineral nutrients and may get photosynthetic items from them also. Bantam mistletoe (Arceuthobium americanum) is a serious parasite in woodlands of the western United States, obliterating multiple billion board feet of timber each year. Holoparasites are totally parasitic and don’t perform photosynthesis. They are systematically and morphologically different. Some, like individuals from the dodder family, are plantlike apparently, with little leaf leftovers and flowers. Some holoparasites don’t have leaves or stems since they go through the greater part of their time on earth underground and just come to the surface to bloom. A few parasitic plant animal types need a large number of the genes typically present in the chloroplast genome (which thusly is just a remainder of the genome in the first endosymbiont from which the chloroplast developed). These genes, which are required for photosynthesis, have been lost since there is no developmental stress to keep them.

4.14. FLOWERING PLANTS HAVE MICROSCOPIC GAMETOPHYTES The haploid gametophytes — the gamete creating structures — create from haploid spores in the flower: • •

Female gametophytes (megagametophytes), which are called embryosacs, foster in megasporangia. Male gametophytes (microgametophytes), which are called dust grains, foster in microsporangia.

4.14.1. Female Gametophyte Situated on the ovule in the blossom. Inside the ovule, a megasporocyte — a cell inside the megasporangium — partitions meiotically to create four

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haploid megaspores. In most blooming plants, everything except one of these megaspores then, at that point, go through apoptosis. The megaspore typically goes through three mitotic divisions without cytokinesis, delivering eight haploid nuclei, all at first held inside a solitary cell — three cores toward one side, three at the other, and two in the center. The resulting cell wall development prompts a circular, seven-celled megagametophyte with a sum of eight nuclei.

4.14.2. Male Gametophyte The pollen grain (microgametophyte) comprises of less cells and nuclei than the embryosac. The development of a pollen grain starts when a microsporocyte inside the anther isolates meiotically. Each subsequent haploid microspore fosters a spore wall, inside which it regularly goes through one mitotic division before the anthers open and delivery these twocelled pollen grains. The two cells are the tube cell and the generative cell. Further development of the pollen grain, which will develop when the pollen arrives at a stigma (the open piece of the carpel). In angiosperms, the exchange of pollen from the anther to the stigma is alluded to as fertilization.

Figure 4.7. Conifer male gametophyte. Source: Image by Wikimedia Commons

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4.14.3. Flowering Plants Prevent Inbreeding A few plants can replicate physically by both cross-pollination and selffertilization. Self-fertilization expands the possibilities of effective fertilization, yet prompts homozygosity, which diminishes hereditary variety. Since variation is the unrefined substance of development by regular choice, homozygosity can be specifically disadvantageous. Most plants have advanced components that forestall self-preparation. The two essential means to forestall self-treatment are: (1) Actual detachment of male and female gametophytes, and (2) Genetic Self-Incompatibility

4.14.4. Separation of Male and Female Gametophytes Self-fertilization is forestalled in dioecious species, which bear just male or female flowers on a specific plant. Fertilization in dioecious species is achieved just when one plant pollinates another. In monoecious plants, which bear both male and female flowers on the similar plant, the actual division of the male and female flowers is frequently adequate to forestall self-fertilization. A few monoecious plant groups forestall self-treatment by preventing the development of male and female flowers so they don’t germinate simultaneously, making these species practically dioecious.

4.14.5. Genetic Self-Incompatibility A pollen grain that terrains on the stigma of a similar plant will treat the female gamete provided that the plant is self-viable, meaning equipped for self-fertilization. To forestall self- fertilization, many plants are selfincompatible, which relies on the capacity of a plant to decide if pollen is geneticaly compatible or hereditarily unique in relation to “self.” Rejection of “same-as-self” pollen forestalls self-fertlization. How can it happen? Self- Incompatibility in plants is constrained by genes called the S locus (for self-contrariness). The S locus encodes proteins in the pollen and style that associate during the development cycle. A self- incompatible plant variety ordinarily has numerous alleles of the S locus, and when the pollen conveys an allele that matches one of the alleles of the pistil, the pollen is rejected. Contingent upon the kind of selfincompatibility system, the rejected pollen either neglects to germinate

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or is kept from developing through the style; one way or the other, selffertilization is prevented.

4.15. THE FLOWERING STIMULUS ORIGINATES IN A LEAF Early experiments specified that reception of the photoperiodic stimulus takes place within the leaf. For an instance, in spinach, an LDP, flowering would take place if the leaves were exposed to long-day periods of light while the bud meristem was masked to simulate short days. Flowering could not occur when its leaves were masked to simulate short days while the bud was exposed to long-day periods of light. These “masking” experiments were extended to SDP plants as well. As the receptor of the stimulus (in the leaf) is physically separated from the tissue on which the stimulus acts (the bud meristem), the inference can be drawn that a systemic signal travels from the leaf through the plant’s tissues to the bud meristem.

4.16. FLOWERING PLANTS USE ANIMALS OR WIND TO TRANSFER POLLEN BETWEEN FLOWERS 4.16.1. Formation of Angiosperm Gametes Reproductive success relies upon joining the gametes (egg and sperm) found in the embryo sacs and pollen grains of flowers. As referenced beforehand, plant sexual life cycles are described by an alternation of generations, in which a diploid sporophyte age leads to a haploid gametophyte age. In angiosperms, the gametophyte age is tiny and is totally encased inside the tissues of the parent sporophyte. The male gametophytes, or microgametophytes, are pollen grains. The female gametophyte, or megagametophyte, is the embryo sac. Pollen grains and the embryo sac both are created in isolated, specific parts of the angiosperm flower. Like animals, angiosperms have separate parts for forming male and female gametes , however, the regenerative organs of angiosperms are not the same as those of organisms in two ways. In the first place, in angiosperms, both male and female parts occur together in a similar individual flower. Second, angiosperm reproductive parts are not extremely durable adult. Angiosperm flowers and reproductive organs grow occasionally, now and

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again of the year generally suitable for fertilization. At times, reproductive organs are formed just a single time and the parent plant dies.

Pollen Formation Pollen grains form in the two pollen sacs situated in the anther. Every pollen sac contains specific chambers in which the microspore mother cells are encased and safeguarded. The microspore mother cells go through meiosis to form four haploid microspores. In this manner, mitotic division’s form four pollen grains. Inside every pollen grain is a generative cell; this cell will later separation to create two sperm cells. Pollen grain shapes are particular for definite bloom species. Pollen grain grows a tube that enters the style until it encounters the ovary. Most pollen grains have a wrinkle from which this pollen tube arises; a few grains have three wrinkles.

Embryo Sac Formation Eggs develop in the ovules of the angiosperm bloom. Inside every ovule is a megaspore mother cell. Every megaspore mother cell goes through meiosis to create four haploid megaspores. In many plants, only one of these megaspores, nonetheless, develops; the rest are consumed by the ovule. The single megaspore goes through repeated mitotic divisions to create eight haploid nuclei that are encased inside a seven-celled embryo sac. Inside the embryo sac, the eight nuclei are organized in exact positions.

Figure 4.8. Angiosperm embryo sac diagram. Source: Image by Flickr

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One nucleus is situated close to the start of the embryo sac in the egg cell. Two are situated in a solitary cell in the embryosac and are called polar nuclei; two nuclei are contained in cells called synergids that flank the egg cell; and the other three nuclei are with cells called the antipodals, situated toward the end of the sac, opposite the egg cell. The most vital phase in joining the sperm in the pollen grain with the egg and polar nuclei is to get pollen germinating on the stigma of the carpel and developing toward the embryo sac.

4.16.2. Pollination Pollination is the interaction by which pollen is put on the stigma. Pollen might be conveyed to the flower by wind or by animals, or it might start inside the singular flower itself. At the point when pollen from the anther pollinates a similar flower’s stigma, the cycle is called self-fertilization.

Pollination in Early Seed Plants Early seed plants were pollinated inactively, by the activity of the wind. As in present-day conifers, extraordinary amounts of pollen were shed and blown about, sometimes arriving at the area of the ovules of similar species. Individual plants of some random species should become generally near each other for such a system to proficiently work. If not, the opportunity that any pollen will show up at the suitable targettt is tiny. By far most the windblown pollen travels under 100 meters. This brief distance is critical contrasted and the significant distances pollen is regularly conveyed by specific insects, birds, and different creatures.

Pollination by Animals The spreading of pollen from one plant to another by pollinators visiting flowers of definite angiosperm species plays a significant impact. It currently appears to be evident that the earliest angiosperms, and maybe their ancestors additionally, were bug pollinated, and the coevolution of insects and plants has been significant for the two groups for more than 100 million years. Such cooperation has additionally been significant in achieving expanded flower specialization. As flowers become progressively specific, so do their associations with specific groups of insects and different animals.

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Wind-Pollinated Angiosperms Numerous angiosperms, are wind-pollinated — a trait of early seed plants. Among them are such natural plants as oaks, birches, cottonwoods, grasses, sedges, and brambles. The flowers of these plants are little, greenish, and unscented; their corollas are decreased or missing. Such flowers frequently are gathered in genuinely huge numbers and may hang down in tufts that wave about in the breeze and shed pollen unreservedly. Numerous wind-pollinated plants have stamen-and carpelcontaining flowers. In the event that the pollen delivering and ovule-bearing flowers are separate, it is sure that pollen delivered to the wind will arrive at a flower other than the one that sheds it, a system that extraordinarily promotes outcrossing. Some wind-pollinated plants, particularly trees and bushes, flower in the spring, before the development of their leaves can slow down the wind-borne pollen.

4.17. SELF-POLLINATION Each of the methods of fertilization that have been thought about up to this point will quite often prompt cross-pollination, which is as exceptionally favorable for plants for what it’s worth for eukaryotic organisms for the most part. All things considered, self-fertilization additionally happens among angiosperms, especially in temperate locations. The majority of self-pollinating plants have little, generally subtle flowers that shed pollen straightforwardly onto the stigma, once in a while even before the bud opens. The question arises why there are numerous selfpollinated plant species on the off chance that outcrossing is similarly as significant hereditarily for plants for what it’s worth for animals. There are two essential purposes behind the regular event of selfpollinated angiosperms: •

Self-fertilization clearly is naturally worthwhile under particular conditions in light of the fact that self-pollinators needn’t bother with being visited by animals to deliver pollen. Accordingly, self-pollinated plants spend less energy in the creation of pollinator attractants and can fill in regions where the sorts of insects or different organisms that could visit them are missing or exceptionally scant — as in the Arctic or at high heights.

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•

In hereditary terms, self-fertilization produces descendants that are more uniform than those that come about because of crosspollination. Recollect that since meiosis is involved, there is still recombination and the offspring won’t be indistinguishable from the parent. In any case, such descendants might contain descendants well adapted to specific living spaces. Self-fertilization in regularly cross-pollination species will in general deliver enormous quantities of diseased individuals since it unites harmful latent genes; however, a portion of these mixes might be exceptionally favorable specifically in natural surroundings. In such environments, it very well might be worthwhile for the plant to self-fertilize endlessly. This is the primary explanation numerous selfpollinating plant species are weeds — besides the fact that people have made natural surroundings uniform, they have likewise spread the weeds from one side of the planet to the other.

4.18. CONCLUSION All plants experience a fundamental turn of events, which is depicted by the extending of roots and shoots. Additionally, various gymnosperms and eudicots, especially trees, experience many events, by which they grow in size. Development makes what is known as the fundamental plant body, while auxillary development cultivates the plant body. Meristematic cells are undifferentiated and always active, holding the ability to form new cells unendingly. Right when a basic parts, one daughter forms into another meristem cell the size of its parent. Leaves rise out of grows called leaf primordia, which form as cells divide on the sides of the shoot apical meristem. The forming stem has no protective plan basically comparable to the root cap, yet it can go probably as a covering for the developing cell. The majority of self-pollinating plants have small, often delicate flowers that shed pollen directly onto the ground, perhaps even before the bud opens.

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REFERENCES 1.

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Channarayappa, C. and Biradar, D., 2018. Plant Forms and Functions. Soil Basics, Management, and Rhizosphere Engineering for Sustainable Agriculture, [online] pp.497-521. Available at: [Accessed 3 July 2022]. Johnson, R., n.d. [online] Biology.org.ua. Available at: [Accessed 3 July 2022]. LADD, P., 1994. Pollen presenters in the flowering plants-form and function.  Botanical Journal of the Linnean Society, [online] 115(3), pp.165-195. Available at: [Accessed 3 July 2022]. Sattler, R. and Bell, A., 1991. Plant Form: An Illustrated Guide to Flowering Plant Morphology. Taxon, [online] 40(3), p.534. Available at: [Accessed 3 July 2022]. Sadava, D., Craig Heller, H., Hillis, D. and Berenbaum, M., 2009. Life: The Science of Biology. 9th ed. [ebook] WHFreeman,2009. Available at: [Accessed 3 July 2022].

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ANIMALS: FORMS AND FUNCTIONS

CONTENTS 5.1. Introduction..................................................................................... 130 5.2. Types of Animals With their Class, Category, and Group.................. 132 5.3. How Do Multicellular Animals Supply the Needs of Their Cells?..... 138 5.4. How Do Animals Alter Their Heat Exchange With the Environment?................................................................................ 140 5.5. Animal Hormones........................................................................... 142 5.6. Major Defense System of Animals.................................................... 143 5.7. How Do Animals Make So Many Different Antibodies?................... 144 5.8. Animals Reproduce Without Sex...................................................... 145 5.9. How Do Animals Reproduce Sexually?............................................ 147 5.10. An Individual Animal Can Function as Both Male and Female....... 148 5.11. The Evolution of Vertebrate Reproductive Systems Parallels the Move to Land.......................................................................... 149 5.12. Animal Development..................................................................... 150 5.13. Gas Exchange in Animals............................................................... 153 5.14. Why Do Animals Need Circulatory System?................................... 157 5.15. Conclusion.................................................................................... 159 References.............................................................................................. 160

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Animal, any of a class of multicellular eukaryotic organisms( Kingdom Animalia) (i.e., as distinct from bacteria, their deoxyribonucleic acid, or DNA, is contained in a membrane-bound nucleus). It is assumed that they developed separately from unicellular eukaryotes. Animals vary fundamentally from members of the two other kingdoms of multicellular eukaryotes, plants (Plantae) and fungus (Mycota), in form and physiology. This is primarily due to the development of muscles and hence motion in animals, which has promoted the continued development of tissues and organ systems.

5.1. INTRODUCTION Animals dominate human perceptions of life on Earth not just because of their size, quantity, and overwhelming diversity, but because of their mobility, which people share. So important is the movement to the idea of animals that sponges, which lack muscular structures, were long thought to be plants. Only when their little motions were detected in 1765 was the animal character of sponges gradually recognized. Plants outgrow animals in size on land, where they may frequently hide within the greenery. In contrast, photosynthetic algae that feed the open oceans are typically too tiny to see, yet marine organisms can grow to be the size of whales. In contrast to size, diversity of form has a smaller impact on our perception of life and hence receives less attention. Nonetheless, animals account for three-quarters or more of all species on the planet, a variety that reflects the flexibility in food, defense, and reproduction that mobility provides. Animals follow practically every known method of living outlined for Earth’s animals. Animals move in search of food, mates, or refuge from predators, and this movement draws attention and intrigue, especially when it becomes clear that certain species’ behavior is not so unlike human behavior. Aside from pure curiosity, people study animals to learn about themselves, which is a relatively new result of animal development. Muscles and the movement animals provide are characteristics of members of the animal kingdom. Mobility has a big impact on how an organism gets nutrients for growth and reproduction. Animals normally travel to feed on other living species; however, some absorb dead and decaying debris and even photosynthesize by harboring symbiotic algae.

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In separating animals from the other two multicellular kingdoms, the form of feeding is not as important as the sort of movement. In contrast to the myofilament-based mobility found in mammals, certain plants and fungi prey on animals by exploiting movements based on fluctuating turgor pressure in critical cells. Mobility necessitates the development of far more complex senses and internal communication systems than are present in plants or fungi. It also necessitates a different form of growth: mammals develop by growing all portions of their bodies, whereas plants and fungi grow by extending their terminal edges. The muscles that separate animals from plants and fungi are variations on the actin and myosin microfilaments that are found in all eukaryotic cells. In other aspects, ancestral sponges are not much more complicated than aggregations of protozoans that feed in a similar manner. Although the sensory and neurological systems of animals are also formed of modified cells that plants and fungi lack, the underlying process of communication is just a specialization of a chemical system common in protists, plants, and fungi. The boundaries of an evolutionary continuum are rarely distinct.

5.1.1. The Animal Kingdom Unicellular eukaryotes gave rise to animals. In eukaryotes, the presence of a nuclear membrane allows the two steps of protein synthesis to be separated: transcription (copying) of deoxyribonucleic acid (DNA) in the nucleus and translation (decoding) of the message into protein in the cytoplasm. In comparison to the bacterial cell structure, this allows for better control over which proteins are generated. Such control enables cell specialization, with each cell having identical DNA but the capacity to finely select which genes effectively deliver copies into the cytoplasm. As a result, tissues and organs can develop. Animals lack the semirigid cell walls seen in plants and fungi, which restrict the form and hence the range of conceivable cell types. It would be impossible to move an animal if nerve and muscle cells were not present.

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5.2. TYPES OF ANIMALS WITH THEIR CLASS, CATEGORY, AND GROUP Now let us look more closely at the six major animal groups that occupy this world. Fish, birds, mammals, reptiles, and amphibians are the five bestknown groups of vertebrates (animals having backbones). Invertebrates (animals without a backbone) are the sixth class.

5.2.1. Fish Fish are aquatic organisms that are classified as vertebrates in the phylum Chordata. Fish differ from other animals in that fish have scales, fins, and gills. Fish, as cold-blooded creatures, must swim in the water at the appropriate temperature to regulate their body temperature. The fish category includes most organismsthat live in seas, oceans, lakes, and rivers. Unlike amphibians, who might survive on both land and water, fish must spend their entire lives in water. Whereas fish can breathe air, fish usually breathe through their gills and obtain oxygen from the water. There are about 33,600 fish species believed to reside in water bodies ranging from tiny lakes to the deep reaches of the sea. Per some estimation, fish have the most species of any vertebrate group on the globe. The biggest forms of fish are cartilaginous fish. Whale sharks, basking sharks, and other forms of sharks are among these massive swimming creatures. The beluga (sturgeon) is one of the biggest bony fish species found in the seas. Some of the tiniest fish are only a few millimeters long.

Figure 5.1. Sturgeon (Acipenser) at the Oregon Zoo. Source: Image by Wikimedia Commons

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It’s important to keep in mind that just because an animal lives in water doesn’t guarantee it belongs to the fish world. Whales, for example, are classified as mammals since they nourish their young with milk. Whales, unlike fish, have lungs and must rise to the water’s surface to breathe. Bony fish are by far the most commonly consumed. These are fish species like salmon, pollock, cod, mackerel, and tuna.

5.2.2. Birds Birds are the animal group with the largest number of flying vertebrates. The presence of feathers distinguishes birds from other animals. Birds are the only type of animal that has feathers. Although all birds hatch from eggs and also most birds fly, different kinds of animals share these qualities. Birds are classified as Aves and belong to the phylum Chordata. This group of flying avian species is classified as warm-blooded organisms. Most birds may be identified by their beak, wings, feathers, and the fact that they hatch from an egg. There are about 18,000 bird species, according to estimates. The huge flightless ostrich is over 9 ft. (2.75 m) tall, whereas the charming small hummingbird stands barely 2” tall (5 cm). Birds are also among the most colorful and attractive of all creatures. Parrots, birds of paradise, ducks, wrens, finches, and peacocks, for example, can have stunning yellow, blue, red, green, and orange hues. There are many bird species that you would not think of as birds. One such species is the Spheniscidae family of birds, which includes the penguins. This species’ genera are found in the Southern Hemisphere and South America. Despite the fact that they are unable to fly and spend a significant amount of time swimming, they are in the bird classification.

Figure 5.2. Bee hummingbird (Mellisuga helenae). Source: Image by Wikimedia Commons

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Whenever it concerns bird taxonomy, few individuals mistakenly believe birds are animals. There are significant distinctions between mammals and birds, despite the fact as both have backbones and are warm-blooded. One of the distinctions is that birds have a beak but no teeth. Birds are not mammals although they do not give birth to or feed the young. Human beings rely heavily on birds as a source of food. Eggs, for example, are high in protein, and several species of poultry have less fat than red meats like beef and hog.

5.2.3. Mammals Humans are best familiar with the group of organismsknown as mammals. Dogs, cats, rabbits, and even pigs are popular pets among humans. Surprisingly, the animal class known as Mammalia is not the biggest. There are just roughly 4,000 different varieties of mammals, whereas there are above 900,000 different types of insects. Mammal species may not be the most populous in the animal world, but they are the most diversified. Mammals may be distinguished from other animals by at least three distinguishing characteristics. These are mammals having three middle ear bones, fur or hair, and female animals producing milk.

Figure 5.3. Mammal Diversity. Source: Image by Wikimedia Commons

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Mammals, as warm-blooded creatures, can adapt to a broad variety of temperatures. Polar bears, arctic wolves, and muskox, for example, can all live in frigid temperatures. Mammals like camels, lions, tigers, wildcats, and coyotes, on the other hand, can survive in extremely high temperatures. There are also some animals that appear to be fish but are actually mammals. Dolphins, whales, porpoises and other marine animals are examples of marine mammals. Aquatic mammals (mammals that inhabit water) feed the young the same way as terrestrial mammals do. The duckbill platypus and the spiny anteater are two of the most fascinating creatures. These spiky and fuzzy animals lay eggs yet are classified as mammals since the young are fed milk.

5.2.4. Reptiles Reptiles are cold-blooded animals with scales on their bodies. These are the two distinguishing characteristics of the animal subclassification Chordata class Reptilia. Snakes and lizards (Squamata), turtles and tortoises (Testudines), crocodiles and alligators (Crocodilia), and tuataras are the four major categories of reptiles (from New Zealand). It is estimated that there are about 10,000 species of reptiles in the taxonomic group Sauropsida. Their body temperature, like that of other cold-blooded species, is determined by their surroundings. This leads reptiles to exhibit different behavior, such as sitting and waiting for hours. This is a strategy for conserving energy owing to a slow metabolism. Reptiles such as turtles, geckos, snakes, lizards, and crocodiles will remain in the sun for hours to warm themselves. If they need to cool off, they will go to a shaded area or into the water. Reptiles’ scaly skin is their most noticeable feature. These scales might be thin and lustrous, like those on a snake, or rough and lumpy, like those on a crocodile or caiman. Turtles, for example, have horny scales that cover their hard-outer shells.

The fact that reptiles lay eggs is another distinguishing aspect of the animal kingdom. Reptiles hatch from eggs and are not born, which is not unique to this class. Even though they may superficially resemble, the Reptilia category must not be mistaken for amphibians. Despite the fact that both groups are cold-blooded, only a few reptile species like swimming. Amphibians have

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gills and lungs and may survive on land as well as in water. And although reptiles like snakes appear slimy, their skin is remarkably dry. Several individuals keep unusual organismsas pets, such as reptiles. A well-heated terrarium is ideal for snakes, aquatic dragons, geckos, and chameleons. Reptiles eat insects, crickets, fruit, and plants in general.

5.2.5. Amphibians Animals in the class Amphibia are organisms that have gills (similar to fish) and lungs (like reptiles). Frogs and toads (Salientia), salamanders and newts (Caudata), and blindworms (caecilians) are the three categories of amphibians. The bulk of water-loving animal are amphibians, specifically frogs. There seem to be around 4,000 amphibian species. The term “amphibian” originates from the Greek word for “two forms of life.” This references amphibious organisms’ capacity to existing in both water and on land. Although certain animals, such as seals, may live on both land and water, these are not amphibians. Fish, despite the fact having gills, aren’t just an amphibian. Amphibians, like reptiles, are cold-blooded creatures that require adequate environmental conditions to control their temperature. Frogs and toads deposit their eggs in water rather than on dry land, which distinguishes them from reptiles. Amphibians emit poisons as a defensive strategy since they lack scaly skin to defend themselves. Unlike reptiles, which do have dry skin, amphibians can have slimy or sticky skin with no scales.

5.2.6. Invertebrates Invertebrates are organismsthat do not have a backbone. Arthropods like insects, and also mollusks, worms, jellyfish, snails, and squid, may be found in this varied group of creatures. This animal species group is so big that some say it encompasses 97 percent of all animals on the planet. Arthropods are one of the biggest phyla (plural of phylum) of all invertebrates. These are cold-blooded organisms that can be found everywhere you live: mosquitoes, spiders, butterflies, and caterpillars. Even though one might not think of insects as animals, an exoskeleton is another distinguishing characteristic of many arthropods.

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Because arthropods lack a backbone, some develop hard shell-like structures to defend themselves. Crabs, lobsters, and grasshoppers are examples of organisms that have an exoskeleton. Many of the tiniest invertebrates are insects, which account for the vast bulk of arthropods. Unlike arthropods, which typically have a hard-outer shell, many mollusks have a soft body. Some mollusks, such as snails and oysters, have a hard shell that protects them. Other marine mollusks with soft bodies and no spines include jellyfish, squid, and octopuses. Indeed, marine invertebrates are among the biggest and most abundant aquatic organisms in the invertebrate class. Lac insects, mites, and ticks are all anthropoid invertebrates. Though invertebrates like wasps, mites, and insects sometimes are regarded as pests, these are one of the most fascinating of all animals. Several invertebrates are also extremely helpful creatures. Honeybees, for example, create one of the cleanest meals — honey. Lobsters, crabs, and squid may be the centerpiece of a delectable supper. Colorful caterpillars develop into lovely butterflies that pollinate the flowers in your yard. Worms, insects, and other ‘bugs’ are necessary for healthy plant growth in excellent soil.

Figure 5.4. A picture of Mollusks. Source: Image by PxHere

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5.3. HOW DO MULTICELLULAR ANIMALS SUPPLY THE NEEDS OF THEIR CELLS? All animal cells require nutrients and oxygen from their surroundings, and they must expel carbon dioxide and other metabolic waste products into the environment. The cells of extremely tiny or very thin aquatic organisms satisfy these demands through direct interactions with the external environment. No cell in such organisms is far from direct touch with the water in which they exist; the water contains nutrients, absorbs waste, and offers a reasonably stable physical environment. Most cells in bigger animals have no direct interaction with the external world, thus essential demands must be met by an environment that is entirely personal to the animal.

5.3.1. An Internal Environment Makes Complex Multicellular Animals Possible Multicellular animals’ cells dwell inside an extracellular fluid-filled interior environment (ECF). A person, for example, is composed of around 60% water. Two-thirds of the water is held within cells, with the remaining onethird constituting the ECF or our internal environment. The blood plasma that circulates in our blood arteries accounts for around 20% of the extracellular fluid or 3 liters. The remaining, around 11 liters, is interstitial fluid, which bathes every cell in the body. Individual cells obtain nutrients from this interstitial fluid and excrete waste into it. These cells are shielded from changes or severe circumstances in the external environment as long as conditions in this internal environment are kept within prescribed parameters. A steady internal environment allows an animal to live in environments that would destroy its cells if cells were directly exposed to them. How is the interior environment maintained?? Cells were specialized for sustaining particular components of the interior environment as multicellular organisms emerged. Because each cell did not have to cater to all of its own needs, the internal environment facilitated various specializations. Some cells developed to serve as the interface between the internal and exterior surroundings, providing the essential transport activities to bring nutrients in and waste out. Other cells become specialized to perform internal duties such as extracellular fluid circulation, energy storage, motility, and information processing.

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The evolution of physiological mechanisms to regulate the internal environment allowed multicellular organisms to become bigger, thicker, and more complex, as well as occupy a wide range of environments. The external environment and the metabolic functions of the body’s cells continuously challenge the composition of the interior environment. Homeostasis refers to the preservation of steady circumstances (within a restricted range) in the internal environment. Homeostasis is disturbed when a physiological system fails to work properly, and cells are harmed and can die. To prevent homeostasis loss, physiological systems must be managed and regulated in response to changes in both the external and internal environments.

5.3.2. Physiological Systems Maintain Homeostasis The neurological and endocrine systems regulate the activity of all physiological systems by speeding up or slowing them down. However, information is essential to govern the internal environment. Consider driving a car as an example. To control the speed of your automobile, you must first understand how fast you are going and how fast you want to travel. The target speed is a fixed point or reference point, while the speedometer measurement is feedback information. An error signal is any mismatch between both the set point and the feedback information. Error signals indicate remedial measures, such as pressing the accelerator or braking. Certain physiological system components are referred to be effectors since they produce changes in the internal environment. Effectors are regulated systems since their actions are directed by directives from regulatory systems. Regulatory systems, on the other hand, gather, process, and integrate data before issuing directives to regulated systems. Sensors, which offer feedback to be matched with the internal set point, are crucial parts of any regulatory regime. How is sensor data used?? The most prevalent use of sensory information in regulatory systems is negative feedback. Negative feedback data is used to mitigate the effect that caused an error signal. Whatever is causing the system to deviate from its fixed point must be “negated.” In our automotive analogy, realizing you’re traveling too fast is negative feedback that encourages you to let off the gas and use the brake.

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Positive feedback is observed in physiological systems, albeit it is not as prevalent as negative responses. Positive comments enhance a reaction instead of restoring a system to a predetermined position (i.e., it increases the deviation from the set point). Good feedback regulatory systems include reactions that empty bodily cavities, such as urine and excrement. Another example is sexual activity, wherein a small number of simulations results in greater behavior, which results in even more excitation, and so on. Positive feedback replies tend to reach a plateau and then fade away. A nice example is the birthing process. Labor pains of a uterus stretch the delivery canal, causing more and stronger contractions to occur until the baby is born, during which point contractions cease.

5.4. HOW DO ANIMALS ALTER THEIR HEAT EXCHANGE WITH THE ENVIRONMENT? Many others grew up thinking of animals as either “cold-blooded” or “warmblooded,” implying a comparison with their own body temperature and distinguishing mammals and birds from other species. When we consider that mammals that hibernate become cold, and that many reptiles and insects may be fairly warm when active, this easy categorization falls apart. Physiologists frequently divide animals into two groups based on whether they have a constant body temperature (homeotherms) or a fluctuating body temperature (ectotherms) (poikilotherms). A deep-sea fish, on the other hand, maintains a steady body temperature. A thermal categorization method that prevents such nonsensical outcomes is one that is established on the source of heat that mostly affects the animal’s temperature. Ectotherms are animals whose body temperatures are mostly regulated by external heat sources. Endotherms regulate their body temperatures by either creating heat metabolically or by employing active heat loss mechanisms. Most mammals and birds are endotherms, whereas most other organisms are ectotherms. The endotherm/ectotherm system, like the homeotherm/ poikilotherm scheme, is not ideal. As a result, we have a third category: heterotherms, which are animals that behave as endotherms at times and ectotherms at other times. For example, a hibernating animal is a perfect endotherm throughout the summer, but during the winter, its internal heat generation decreases and it

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behaves more like an ectotherm. Some ectotherms can generate internal heat and behave like endotherms at times.

5.4.1. Some Fishes Elevate Body Temperature by Conserving Metabolic Heat Active fishes may generate a lot of metabolic heat, although they have a hard time maintaining any of it. Blood is pumped straight from the heart to the gills, where it comes into close contact with the surrounding water to exchange breathing gases. As a result, any heat picked up by the blood from metabolically active muscles is lost to the surrounding water as it travels past the gills. As a result, it is astonishing that certain huge, fast-moving fishes, such as bluefin tuna and great white sharks, can sustain temperature gradients of 10° to 15° C between their bodies and the surrounding water. The heat originates from their strong swimming muscles, and their ability to store that heat is due to the extraordinary arrangement of their blood vessels. The oxygenated blood from the gills is collected in a big dorsal artery, the aorta, which passes through the middle of the fish, supplying blood to all organs and muscles. The core dorsal aorta of “warm” fishes is small, as well as the majority of their oxygenated blood is delivered through vast capillaries immediately beneath the skin. The cold blood from the gills is therefore maintained near the fish’s surface. Smaller arteries that bring cold blood into the muscle mass run parallel to veins that transfer warm blood from the muscle mass back to the heart. Because the capillaries delivering cold blood into the muscle are in close proximity to those transporting warm blood out, Heat is transferred from warm to cold blood via conduction and is so maintained in muscle mass. This adaption is known as a countercurrent heat exchanger because heat is transferred between blood veins flowing blood in opposing directions. It retains heat inside the muscle mass, allowing these fish to have an internal body temperature that is significantly greater than that of the temperature of the water. Why should it be beneficial for the fish to be warm? Each 10°C increased muscle temperature nearly triples the fish’s sustained power production, allowing it to forage faster.

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5.5. ANIMAL HORMONES Physiological control and regulation of multicellular organisms require information and cell-to-cell interaction. The majority of intercellular communication occurs via chemical signals that bind to receptors. Hormones are chemical signals produced by specific types of cells that impact the activity of other cells at a distance. Hormone levels and other kinds of chemical signals, including growth factors, morphogens, cytokines, and neurotransmitters. These generic designations are derived from the context in which the chemical signal operates—endocrine system, growth and development, immune system, or neurological system—but the fundamental principles of their action are the same: A chemical signal is released by one cell and travels to and binds to a receptor, resulting in a cellular reaction. The DNA, the endocrine system, the immune system, and the neurological system are the four basic sources of information that animals use to develop, grow, and operate. Data is contained in the selectivity of chemical signals and their receptors in each of these systems. Readers learned a lot about genetic data in earlier chapters. The endocrine, immunological, and neurological systems are covered in this and the next chapters. To avoid the misconception that all signaling is chemical, take into consideration that the nervous system contains receptors that encode physical sources of information such as temperature, pressure, and light. And the nervous system transmits information throughout the body via electrical impulses known as action potentials. Regardless of the method, data processing is dependent on whether cells contain signal receptors and how certain cells react and connect with the other cells.

5.5.1. Chemical Signals Can Act Locally or At a Distance Endocrine cells release chemical signals, and target cells contain receptors for those signals. Chemicals released into extracellular fluid disperse locally and may enter the circulation. Hormones are endocrine signals that reach the bloodstream and can affect target cells distant from the place of release. Testosterone hormone is an example of one. These are either inactivated so quickly by enzymes or absorbed so effectively by local cells which never diffuse into the circulation de sufficient quantities to operate on distant cells.

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These signals are known as paracrine since they only influence target cells near the place of release. Histamine, one of the inflammatory mediators, is an example of a paracrine signal. When an endocrine signal attaches to receptors on or in the same cell that released it, it has the most local effect. It has autocrine activity when a chemical signal affects the cell that produced it. Hormones and paracrine can have autocrine activities to provide negative feedback to limit their secretion rates. Some endocrine cells can be found as solitary cells inside tissues. Gastrointestinal hormones, for instance, are released by distinct endocrine cells in the stomach and small intestine walls. Most hormones, on the other hand, are released by endocrine cell aggregations that form secretory structures known as endocrine glands. The term “endocrine” refers to the fact that these glands discharge their products directly into the extracellular fluid, which is then absorbed by the blood. Exocrine glands, on the other hand, such as sweat glands or salivary glands, have ducts that convey their secretions to the skin’s surface or into a bodily cavity such as the stomach. Multiple hormones can be secreted by a single endocrine gland.

5.5.2. Hormonal Communication Has a Long Evolutionary History Chemical communication between cells was essential for the emergence of multicellularity. Dictyostelium, a slimy mold, employs a chemical signal (cAMP) to organize the aggregation of single cells into a multicellular fruiting structure. The sponges, the most basic multicellular creatures, lack nervous systems but do have intercellular chemical communication. The study of hormone signaling evolution offers an intriguing generalization: the signal molecules themselves are remarkably preserved. The very same chemical molecules are found in large groups of species, yet their roles differ. As animals developed to inhabit diverse habitats and lead varied lifestyles, hormone–receptor systems evolved to fulfill various tasks, such as the hormone prolactin.

5.6. MAJOR DEFENSE SYSTEM OF ANIMALS Animals have quite a variety of defense mechanisms against pathogens, which are hazardous organisms and pathogenic diseases. These defensive

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mechanisms are built on the differentiation between self (the animal’s own molecules) and nonself molecules. The defensive reaction is divided into three stages: • Recognition phase: The organism must be capable of distinguishing between self and no self molecules. • Activation phase: The identification event causes cells and molecules to mobilize in order to resist the intruder. • Effector phase: The intruder is destroyed by the mobilized cells and molecules.: • Nonspecific defenses: Nonspecific defenses, often known as innate defenses, are the body’s initial line of protection against infections. They often act quickly and include barriers like skin, chemicals that are harmful to intruders, and phagocytic cells (phagocytes like macrophages) that eat invaders. This system identifies large groups of organisms or chemicals and responds quickly, within hours or minutes. The majority of animals have general defenses. • Specific defenses: Specific defenses are adaptive mechanisms that are directed against specific diseases. A specialized defensive system, for example, can produce an antibody protein that will detect, bind to, and help in the killing of a certain virus if it ever reaches the circulation. Such systems, which identify certain atomic configurations in a molecule, are often slow to form and long-lasting. Vertebrate animals have specific defensive systems. The aim of this section is on mammals, which have both types of defensive mechanisms. Nonspecific and specific processes work together as a coordinated defensive system in mammals and other animals. But since specific defenses can take days or even weeks towards becoming efficient, nonspecific defenses are the body’s first line of defense.

5.7. HOW DO ANIMALS MAKE SO MANY DIFFERENT ANTIBODIES? Every mature B cell produces one but only one particular antibody with a unique amino acid sequence that is specific to a single antigen. As previously stated, there are millions of potential epitopes to which a human is or could be exposed.

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With millions of potential amino acid compositions in immunoglobulins, the molecular and genetic explanation is that there are millions of genes, each of which codes for one antibody molecule.

5.7.1. Antibody Diversity Results From DNA Rearrangement and Other Mutations Every gene that encodes an immunoglobulin chain is really a “supergene” that was created by genetic recombination from numerous clusters of smaller genes distributed throughout a chromosome. Every cell in the body contains hundreds of immunoglobulin genes organized into distinct clusters that have the capacity to participate in the synthesis of both variable and constant portions of immunoglobulin chains. These genes stay intact and distinct in the majority of body cells and tissues. These genes, however, are taken out, altered, and connected together in DNA recombination processes throughout B cell formation. One gene from each cluster is randomly selected for joining, while the rest are removed. A unique immunoglobulin supergene is therefore built from randomly picked “pieces. “Every B cell precursor gathers two supergenes, one for a specific heavy chain and one for a specific light chain, which is formed separately. From the same starting DNA, this extraordinary example of irreversible cell differentiation creates a huge variety of immunoglobulins. This is a significant exception to the rule that all somatic cells generated from fertilized eggs contain identical DNA. The gene clusters encoding immunoglobulin heavy chains are located on one chromosome pair both in mice and humans, while those encoding light chains are located on two additional pairs. Two gene families encode the light chain’s variable region, while three families encode the heavy chain’s variable area.

5.8. ANIMALS REPRODUCE WITHOUT SEX Even though many species may reproduce asexually and some solely reproduce asexually, sexual reproduction is a fairly universal characteristic in animals. Asexual offspring are genetically identical to one another and to their parents. Because no time or energy is wasted on mating, asexual reproduction is efficient because any member of the community may turn resources into progeny.

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Asexual reproduction, on the other hand, does not create genetic variation in the same way that sexual reproduction does, and this diversity is the raw material that allows natural selection to form adaptations in response to environmental change. When such changes occur, a species’ lack of genetic variation might be detrimental. Asexual reproduction occurs in a wide range of creatures, the majority of which are invertebrates. They are often animals that are tied to their substrate and cannot look for mates, or species that exist in small numbers and encounter few possible mates. Asexually reproducing animals are more likely to be found in very stable settings with low genetic variation. In reality, as long as the habitat doesn’t really vary, asexual reproduction is an excellent approach to conserving a genotype that is effective in that environment. Budding, regeneration, and parthenogenesis are three prevalent ways of asexual reproduction.

5.8.1. Budding and Regeneration Produce New Individuals by Mitosis By budding, most basic multicellular organisms create progeny. New individuals emerge from the body of older animals as outgrowths or buds. A bud develops by mitotic cell division, and the cells differentiate before the bud separates from its parent. The bud is genetically similar to the parent, and it may reach the same size before becoming independent. Regeneration is commonly assumed to be the repair of injured tissues or missing limbs, but in certain situations, parts of an organism may regenerate entire individuals. Echinoderms, for example, have exceptional regeneration powers. If sea stars (starfishes) are split into pieces, each piece including an arm and a bit of the central disc can develop into a new species. Oyster fishermen in Narragansett Bay attempted to eradicate the sea stars that preyed on their oysters in the early 1900s. When they came upon a sea star, they cut it up with knives and dumped it back into the ocean. As a result, the number of sea stars exploded. When an animal is shattered by an outside force, such as wave movement in the intertidal zone, it can regenerate. Breakage can occur in the absence of external pressures in some instances. Some segmented sea worms grow segments with rudimentary heads that include sensory organs. The segments then separate, each becoming a new worm.

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5.8.2. Parthenogenesis Is the Development of Unfertilized Eggs To develop, not all eggs must be fertilized. The formation of progeny from unfertilized eggs is a typical way of asexual reproduction in arthropods. Parthenogenesis is a phenomenon that happens in several fish, amphibians, and reptile species. Most parthenogenetically reproducing organisms also participate in sexual reproduction or at the minimum sexual activity at other times. Parthenogenesis is part of the mechanism that determines sex in some organisms. Male honey bees (as well as most ants and wasps) originate from unfertilized eggs and are haploid. Females are diploid and develop from fertilized eggs. Parthenogenetic reproduction necessitates sexual activity in certain animals, even if sperm is not transported to the female reproductive system and eggs are not fertilized. Parthenogenetic reproduction is a type of whiptail lizard that has been extensively studied by David Crews and his students at the University of Texas. Although there are no males in this species, females may behave like males by engaging in all parts of courting display and mating, even if no sperm is generated or transferred. The cyclical hormonal states determine whether a given female performs as a female or as a man. Whenever estrogen levels are high in females, they behave like a girl. Whenever female progesterone levels are high, they behave like a man. The stimulation caused by sexual intercourse causes the release of eggs from the ovaries. Asexual reproduction is a resource-efficient method of reproduction. However, because most species reproduce sexually, the fact that sexual reproduction promotes genetic variation must be a huge benefit.

5.9. HOW DO ANIMALS REPRODUCE SEXUALLY? Knowing how effective asexual reproduction is at maintaining an organism’s DNA, the frequency of sexual reproduction is somewhat unexpected. Even the development of meiosis, a considerably more intricate process than mitosis, has sparked significant inquiry and controversy among evolutionary scientists. Mating behaviors, of course, have costs and hazards. Time and energy spent locating, attracting, and competing for a mate, as well as the potential

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costs of detracting from other tasks like eating and caring for existing progeny, are all expenses. Risks include greater predation and the possibility of bodily harm. Given these drawbacks, the majority of eukaryotic organisms reproduce sexually. As a result, it appears that the development of genetic variation is an evolutionary advantage that outweighs “the cost of sex.”. •

The merging of two haploid sex cells to produce a diploid person is required for sexual reproduction. Gametogenesis, a process involving meiotic cell divisions, produces these haploid cells, or gametes. Crossing over between homologous chromosomes and independent chromosomal assortment are two phenomena in meiosis that contribute to genetic variation. Sexual reproduction adds to genetic diversity as well. The genetic variance among a single individual’s gametes, as well as the genetic variation between any two parents, creates a huge potential for genetic variation between any two children of a sexually reproducing couple of humans. Animal sexual reproduction consists of three basic steps: • Gametogenesis: the production of gametes • Spawning or mating: the act of bringing gametes together. • Fertilization: combining gametes The gametogenesis process is fairly identical among sexually reproducing animal species. Fertilization processes are also quite similar throughout a large range of species. As a result, while our discussion of gametogenesis will focus on mammals in general and the discussion of fertilization will concentrate on sea creatures, the findings will not be vastly different if we considered many other animal species. In contrast, spawning and mating adaptations exhibit enormous anatomical, physiological, and behavioral variation between species.

5.10. AN INDIVIDUAL ANIMAL CAN FUNCTION AS BOTH MALE AND FEMALE Gametes are generated by either male or female individuals in most animals. Dioecious species (from the Greek meaning “two dwellings”) have distinct male and female members. In other animals, though, a single person may

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generate both sperm and eggs. These species are known as monoecious (“one home”) or hermaphroditic. Hermaphroditic species may be found in almost all invertebrate groups. Earthworms are examples of simultaneous hermaphrodites, which are both male and female at the same time. When two earthworms couple, they exchange sperm, which fertilizes the eggs of both. Some vertebrates are sequential hermaphrodites, which means they can be male or female at various stages throughout their lives. The anemone fish, sometimes known as the clown fish, is a kind of fish that lives in tiny groups among enormous sea anemones. Male anemone fish are born all the time. In a group, the biggest individual becomes a functional female. When the fish is taken from the group, the next largest male transforms into a female. Furthermore, it is the second-largest anemone fish and is the lone male in breeding conditions. What evolutionary benefit does hermaphroditism provide? Some concurrent hermaphrodites, such as parasitic tapeworms, have a minimal chance of encountering a possible mate—it could be the sole tapeworm inside the host. Though tapeworms may fertilize their own eggs, many simultaneous hermaphrodites must mate with some other individual; nonetheless, because each member of the population is both male and female, the likelihood of meeting a potential partner is double that of a strictly monoecious species. All siblings in some consecutive hermaphrodites are either male or female at the same time, which reduces the likelihood of inbreeding.

5.11. THE EVOLUTION OF VERTEBRATE REPRODUCTIVE SYSTEMS PARALLELS THE MOVE TO LAND In watery habitats, the first animals emerged. Modern fishes are the closest surviving relatives of the first animals. They are still only aquatic creatures, and the majority of them perform external fertilization. Lampreys and hagfishes are the most basic fishes, just releasing their gametes into the environment. However, in most fishes, mating rituals bring females and males into close contact during gamete discharge. Fins have developed in sharks and

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rays into claspers that keep the male and female together and allow sperm to be delivered directly into the female reproductive system. The earliest organisms to dwell in terrestrial habitats were amphibians. They overcame the barrier of a dry climate by returning to the water to breed, much as most amphibians do nowadays. Reptiles were the first vertebrate species to address the difficulty of terrestrial reproduction. The amniote egg, their answer, is shared with the birds. An amniote egg is similar to a chicken egg. It provides nourishment (yolk) and water to the growing embryo. A strong shell shields the embryo and prevents water loss while letting oxygen into the egg and carbon dioxide out. The eggshell obviously interferes with fertilization. Because sperm cannot pass through the shell, they must reach the egg before the shell hardens. As a result, internal fertilization, as well as the formation of accessory sex organs, were required for the evolution of the amniote egg. Male snakes and lizards have paired hemipenes that can swell with blood and protrude from the male’s body. Just one hemipenis is put at a moment into the female’s reproductive tract. To create a firm grip as sperm are transmitted along a groove on its surface, it is frequently rough or spiky towards the end. When mating is complete, the hemipenis is pulled back into the male’s body by the retractor muscles. Some archaic bird species have erectile penises that route sperm into the female’s reproductive system via a groove. Birds with more recent evolutionary origins, furthermore, lack erectile penises and instead pull the genital openings closely together just to transmit sperm. It typically involves the male standing on the female’s back.

5.12. ANIMAL DEVELOPMENT 5.12.1. How Does Fertilization Activate Development? The uniting of sperm and egg is known as fertilization. As a result, you may consider it the event that kicks off development. But keep two things in mind. •

First, development occurs without fertilization in organisms that reproduce asexually.

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•

Second, if fertilization does occur in animals, it is followed by important processes in the growing egg that determine later development. Thus, when we research fertilization, we truly want to know how it initiates or restarts multicellular development in sexually reproducing organisms. Fertilization accomplishes significantly more than simply restoring the entire diploid complement of maternal and paternal DNA. The fusing of the plasma membranes of sperm and egg achieves various things: • • • •

It creates barriers to the admission of more sperm. It increases the flow of ions through the egg membrane. It alters the pH of the egg. It boosts egg metabolism and protein synthesis, and it kickstarts the fast succession of dividing cells that results in a multicellular embryo.

5.12.2. The Sperm and the Egg Make Different Contributions to the Zygote Eggs are substantially bigger than sperm in most animals. The cytoplasm of an egg is densely packed with organelles, nutrients, and a wide range of chemicals, including transcription factors and mRNAs. The sperm is only a vehicle for DNA transmission. The mother provides nearly everything the embryo needs throughout its early stages of development. In addition to its haploid nucleus, sperm contributes another vital component to the zygote in most species: a centriole. The centriole transforms into the zygote’s centrosome, which arranges the mitotic spindles for subsequent cell divisions. The centriole is also the source of the primary cilia of cells, which are vital in cell signaling. Cytoplasmic components in the egg serve critical roles in establishing signaling cascades that govern the key developmental stages of determination, division, and morphogenesis.

5.12.3. Rearrangements Of Egg Cytoplasm Set The Stage For Determination. The distinctive qualities of amphibian eggs make them great examples for demonstrating how cytoplasmic rearrangements in the egg set the scene

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for determination. The molecules in the amphibian egg’s cytoplasm are not evenly distributed. The sperm’s entrance into the egg causes cytoplasmic rearrangements that provide extra organization to the egg. This rearrangement determines the zygote’s polarity, and when cell divisions commence, the informative molecules that drive development are not distributed evenly across daughter cells. Due to colors in the cytoplasm, rearrangement of egg cytoplasm after fertilization is easily detected in some frog species. The nutrients in an unhatched frog egg are concentrated by gravity in the lower half of the egg, known as the vegetal hemisphere. The egg’s haploid nucleus is found at the opposite end, in the animal hemisphere. The animal hemisphere’s outermost (cortical) cytoplasm is strongly pigmented, whereas the beneath cytoplasm is more diffusely colored. The vegetal hemisphere lacks pigmentation. Because of these distinctions, it is simple to see how the cytoplasm changes when a frog egg is fertilized. When a frog egg is fertilized, the cytoplasm is altered. Frog eggs are radially symmetrical. All sides are the same if you spin it on its vegetal– animal polar axis. Sperm binding sites are found on the animal hemisphere’s surface, which is where the sperm enters the egg. When a sperm penetrates an egg, an anterior-posterior axis is formed, imposing bilateral symmetry. The cortical cytoplasm rotates toward the sperm entrance point. This rotation puts distinct sections of cytoplasm on opposing sides of the egg into touch with each other, resulting in a band of diffusely colored cytoplasm on the side opposite the location of the sperm entrance. In certain species, the area, known as the grey crescent, represents the site of key developmental stages in several amphibian species.

5.12.4. Early Cell Divisions in Mammals Are Unique Several characteristics of early cell divisions in placental mammals (eutherians) vary so greatly from those observed in other types of animals that some scientists believe the term “cleavage” is incorrect. However, if you name it cleavage or not, it still is the process of initial cell divisions which results in a body of undifferentiated cells that will form the embryo. It is a fairly sluggish process in animals. Cell divisions take 12 to 24 hours, compared to tens of minutes to a few hours in non-mammalian species. Furthermore, mammalian blastomere cell divisions are not synchronous.

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Since the blastomeres need not undergo mitosis simultaneously, the number of cells in the embryo doesn’t really rise in the usual (2, 4, 8, 16, 32, etc.) trend seen in other species. Mammalian cleavage is rotational, with the initial cell division parallel to the animal–vegetal axis and generating two blastomeres. During the second cell division, the two blastomeres split at right angles to one another, with one dividing parallel to and the other perpendicular to the animal–vegetal axis. Another distinguishing aspect of slow, rotating mammalian cleavage is that gene products produced during cleavage serve cleavage-related functions. Gene transcription does not occur in blastomeres in animals such as sea urchins and frogs; therefore, cleavage is driven solely by chemicals present in the egg prior to fertilization. Early cell divisions in a mammalian zygote form a loosely linked ball of cells, as in other animals with full breakage. Nevertheless, beyond the 8-cell stage, the behavior of mammalian blastomeres alters. Cells alter shape to optimize surface contact, develop tight connections, and form a dense mass of cells.

5.13. GAS EXCHANGE IN ANIMALS 5.13.1. What Physical Factors Govern Respiratory Gas Exchange? Gaseous exchange systems consist of gas exchange surfaces as well as the mechanisms that ventilate and perfuse such surfaces. Oxygen (O2) and carbon dioxide (CO2) are the respiratory gases that organisms must exchange. Cells require O2 from their surroundings in order to create an appropriate amount of ATP via cellular respiration. CO2 is a byproduct of cellular respiration that must be eliminated from the body to avoid harmful consequences. The only way for breathing gases to interact between an animal’s internal bodily fluids and the outside medium is through diffusion (air or water). Active transport systems do not exist to carry breathing gases across biological membranes. Because diffusion is a physical process, understanding how physical elements impact diffusion rates aids in understanding the many adaptations of gas exchange systems.

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5.13.2. Diffusion Is Driven By Concentration Differences Diffusion is caused by the random mobility of molecules, and a molecule’s net movement is down its concentration gradient. Concentrations in solutions are simple to understand since they represent the quantity of solute per volume of solution. Gas concentrations are more difficult since the total number of gas molecules in a given volume varies on pressure; a liter of gas under 2 atmospheres of pressure contains twice as many gas molecules as a liter under 1 atmosphere of pressure. The partial pressures of different gases in a mixture are used by biologists to express the concentrations of those gases. Firstly, one must determine the overall pressure, which is measured with a device known as a barometer. There are numerous different types of barometers, but perhaps the most common is a glass tube that is closed at one end and filled with mercury. This is inverted over a pool of mercury, with the open end of the tube beneath the surface. The pressure produced by the atmosphere at sea level maintains, and so equals, a 760 mm high column of mercury in the tube (depending on the weather). As a result, the sea-level barometric pressure (atmospheric pressure) is 760 mm of mercury (mm Hg). Since dry air contains 20.9 percent oxygen (O2), the partial pressure of oxygen (pO2) at sea level is 20.9 percent of 760 mm Hg, or approximately 159 mm Hg. When two gas mixtures are divided by an O2-permeable membrane, O2 will permeate from the mixture with the higher partial pressure to the mixture with the lower partial pressure. The concentration of respiratory gases in a liquid such as water is harder to describe since another component is engaged in the solubility of the gases in the liquid. Therefore, the actual quantity of a gas in a liquid is determined by its partial pressure in the gas phase in contact with the liquid, as well as its solubility in that liquid. But, gas diffusion between both the gaseous and liquid phases is still determined by the partial pressures of the gas in the two phases. The diffusion rate of material relies on its concentration gradient and other elements specified in Fick’s equation of diffusion, whether it be in air or water.

5.13.3. Air Is a Better Respiratory Medium than Water Both air-breathing and water-breathing organisms are affected by the sluggish diffusion of O2 molecules in water. Cellular respiration occurs in

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mitochondria in eukaryotic cells, which are situated in the cytoplasm—an aqueous medium. Extracellular fluid, which is also an aqueous medium, surrounds cells. Furthermore, all respiratory surfaces must be kept moist by a thin coating of fluid through which O2 must pass. Even in animals that breathe air, the slow rate of O2 diffusion in water restricts the efficiency with which O2 is distributed from gas exchange surfaces to sites of cellular respiration. O2 diffusion in water is so sluggish that even animal cells with poor metabolic rates can be only a few millimeters away from a rich supply of ambient O2. As a result, many species of invertebrates that lack internal O2 distribution systems face significant size and form constraints. The majority of these species are rather tiny, but others have expanded in size by developing a flat, thin body with a huge exterior surface area. Others have a very thin body that is formed around a core chamber that allows water to circulate. The emergence of specialized respiratory systems with huge surface surfaces for improving gas exchange has been a major element in permitting bigger, more complex animal bodies.

5.13.4. High Temperatures Create Respiratory Problems for Aquatic Animals As ambient temperatures rise, animals that employ water as their respiratory exchange medium face a dilemma. The majority of water-breathing organisms are ectotherms, which means that their body temperatures are directly related to the water temperature surrounding them. The body temperature and metabolic rate of an ectotherm rise as the temperature of the water rises. As a result, as the water warms, water breathers require more O2. Warm water, on the other hand, carries less dissolved gas than cold water (think of the gases that escape when you open a warm bottle of soda). Furthermore, if an animal works to move water across its gas exchange surfaces (like fish do), it must exert more energy to breathe as the temperature of the water rises. As a result, as the water temperature rises, a water-breathing animal must extract more and more O2 from an increasingly O2 deficient environment, with a reduced percentage of that O2 available to support activities other than breathing.

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5.13.5. Fish Gills Use Counter current Flow to Maximize Gas Exchange Gill arches support the internal gills of fishes, which are located between the mouth cavity and the protective opercular flaps on the sides of the fish directly behind the eyes. Water travels unidirectionally into the fish’s mouth, through the gills, and out through the opercular flaps. The gills are therefore constantly washed with fresh water. This continuous, one-way flow of water over the gills boosts the pO2 on the gill surfaces. The blood circulates on the interior side of the gill membranes reducing the pO2 by sweeping away O2 as quickly as it diffuses across. Since they’re so finely split, gills provide a massive surface area for gas exchange. Hundreds of ribbon-like gill filaments make up each gill. Rows of uniformly spaced folds, or lamellae, cover the upper and bottom flat surfaces of each gill filament. The real gas exchange surfaces are the lamellae. Because they are so thin, the route length (L) for gas diffusion between blood and water is reduced. The surfaces of the lamellae are made up of extremely flattened epithelial cells, so the water and the fish’s red blood cells are separated by only 1 or 2 micrometers. Like the flow of water over the gills, the flow of blood perfusing the inner surfaces of the lamellae is unidirectional. Afferent blood vessels transport deoxygenated blood to the gills, whereas efferent blood vessels transport oxygenated blood away from the gills (. Blood passes through the lamellae in the opposite direction of water flow. This countercurrent flow optimizes the pO2 gradient between water and blood, resulting in more efficient gas exchange than would be the case in a system with the contemporaneous (parallel) flow. Certain fish, such as anchovies, tuna, and some sharks, ventilate their gills by swimming with their mouths open virtually continually. Most fishes, on the other hand, ventilate their gills using a two-pump arrangement. Water flows over the gills when the mouth cavity closes and contracts and water flows over the gills when the opercular cavity expands prior to the opercular flaps opening.

5.13.6. Tidal Ventilation Produces Dead Space That Limits Gas Exchange Efficiency Lungs developed as digestive tract outpocketings in the first “air-gulping” animals. Except for birds, lungs are still dead-end sacs in all airbreathing

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animals. As a result, airflow cannot be steady and unidirectional, but it must be tidal: air to flow in and expelled gases flow out along the same channel. Because the lungs and airways can never be fully empty of air, they constantly have dead space. We can readily quantify the quantities of air exchanged while breathing, but we must employ an indirect approach to assess the void spaces held in the lungs and airways. The mystery of how birds breathe was addressed by implanting miniature O2 electrical sensors in various spots throughout the birds’ air sacs and airways. The birds then were exposed to pure O2 for a single breath, allowing researchers to trace that specific intake. The experiment proved that a single breath persists in a bird’s gas exchange system for two cycles of inhalation and exhalation, and that the air sacs function like bellows, expanding and compressing to maintain a constant and unidirectional flow of new air through the lungs. The benefits of the avian gas exchange system are analogous to those of fish gills. Fresh air is kept flowing unidirectionally over the gas exchange surfaces by the air sacs. As a result, a bird may continuously feed its gas exchange surfaces with fresh air with a pO2 near to that of the surrounding atmosphere. Even though the pO2 of the surrounding air is just marginally greater than those of the blood, O2 can diffuse from air to blood.

5.14. WHY DO ANIMALS NEED CIRCULATORY SYSTEM? A circulatory system is made up of a muscle pump (the heart), a fluid (blood), and a set of tubes (blood vessels) by which the fluid may be pumped around the body. The heart, blood, and vessels are also referred to as the cardiovascular system (Greek kardia, “heart,” and Latin vasculum, “vessel”). A cardiovascular system carries items all around the body.

5.14.1. Some Animals Do Not Have a Circulatory System Single-celled organisms meet all of their demands via direct interactions with their surroundings. These species are mainly found in watery or extremely damp terrestrial settings. Likewise, most multicellular aquatic species are tiny or thin enough that most of their cells are in contact with the outside world. Because nutrients, respiration gases, plus wastes may pass directly between the cells of their bodies and the environment, such animals may

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lack a circulatory system. Without a circulatory system, the cells of certain larger aquatic multicellular organisms are serviced by highly branching core canals termed gastrovascular networks, which basically transport the external environment inside the animal. For instance, all of the cells in a sponge are in touch with or extremely close to the water that surrounds the animal and flows through its central hollow. Larger organisms without a circulatory system, like sponges, jellyfish, and flatworms, are likely to be passive, sluggish, or perhaps even immobile. A circulatory system is essential for large, energetic animals.

Figure 5.5. A Picture of a Sponge. Source: Image by Flickr

5.14.2. Circulatory Systems Can Be Open or Closed Extracellular fluid supports the cells of big, mobile animals. All nutrients— oxygen, fuel, and vital molecules—are derived from that fluid, as are the waste materials of cell activity. The extracellular fluid is moved across the body by muscular chambers, or hearts, in circulatory systems. Extracellular fluid is like circulatory fluid in open circulatory systems and is referred to as hemolymph. This fluid exits the circulatory system’s vessels, percolates through cells as well as through tissues, and afterwards returns to the heart or circulatory system’s vessels to be pushed out again. Closed circulatory systems, on the other hand, entirely enclose the circulating fluid (blood) in a continuous network of vessels. Large molecules and blood cells remain inside the system, while water and low-molecular-weight solutes seep out of the tiniest channels, the

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capillaries, which are extremely permeable. Extracellular fluid refers to both the fluid within the circulatory system and the fluid outside it in animals with a closed circulatory system. Blood plasma is the fluid in the circulatory system; interstitial fluid is the fluid that surrounds the cells. A 70-kilogram individual has around 14 liters of total extracellular fluid volume. Blood plasma accounts for less than a fourth of it (approximately 3 liters). Circulatory systems regulate blood flow to tissues and organs, having two mutually reinforcing and complementary functions to keep blood composition stable by absorbing nutrients and removing wastes, give nutrition to and eliminate the waste from the body’s tissues.

5.15. CONCLUSION In size, animals are outdone on land by plants, among whose foliage they may often hide. The muscles that distinguish animals from plants or fungi are specializations of the actin and myosin microfilaments common to all eukaryotic cells. So integral is movement to the conception of animals that sponges, which lack muscle tissues, were long considered to be plants. Mobility requires the development of vastly more elaborate senses and internal communication than are found in plants or fungi. The type of nutrition is not as decisive as the type of mobility in distinguishing animals from the other two multicellular kingdoms. Diversity of form, in contrast to size, only impinges peripherally on human awareness of life and thus is less noticed. Only after their small movements were noticed in 1765 did the animal nature of sponges slowly come to be recognized. This is largely because animals have developed muscles and hence mobility, a characteristic that has stimulated the further development of tissues and organ systems. Animals differ from members of the two other kingdoms of multicellular eukaryotes, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology. Ancestral sponges, in fact, are in some ways not much more complex than aggregations of protozoans that feed in much the same way. A characteristic of members of the animal kingdom is the presence of muscles and the mobility they afford. Animals dominate human conceptions of life on Earth not simply by their size, abundance, and sheer diversity but also by their mobility, a trait that humans share.

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Leafy Place. n.d. Types of Animals: Classes, Species, Categories and More. [online] Available at: [Accessed 3 July 2022]. Lovegrove, B., 1999. Animal form and function. The Karoo, [online] pp.145-163. Available at: [Accessed 3 July 2022]. Sheets-Johnstone, M., 1998. On the Significance of Animate Form.  Creative Virtualities in Human Self-Interpretation-in-Culture, [online] pp.225-242. Available at: [Accessed 3 July 2022]. Van Valen, L., 1999. Animal | Definition, Types, & Facts. [online] Encyclopedia Britannica. Available at: [Accessed 3 July 2022]. Wells, P., 1991. The description of animal form and function. Livestock Production Science, [online] 27(1), pp.19-33. Available at: [Accessed 3 July 2022].

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THE DIVERSITY OF LIFE

CONTENTS 6.1. Introduction..................................................................................... 162 6.2. History of Life.................................................................................. 166 6.3. Estimates of Current Diversity.......................................................... 169 6.4. Living World Begin to Diversify....................................................... 172 6.5. Where Do Viruses Fit Into the Tree of Life?....................................... 173 6.6. Many RNA Viruses Probably Represent Escaped Genomic Components.................................................................. 174 6.7. How Do Eukaryotic Cells Arise?....................................................... 176 6.8. The Evolution of Seed Plants............................................................ 178 6.9. How Do Plants Support Our World?................................................ 181 6.10. Facts Form the Basis of Our Understanding of Evolution?............... 182 6.11. Mechanisms of Evolutionary Change............................................. 183 6.12. Natural Selection Result In Evolution............................................. 186 6.13. Natural Selection Can Change or Stabilize Populations.................. 186 6.14. Constraints on Evolution................................................................ 189 6.15. Conclusion.................................................................................... 191 References.............................................................................................. 192

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Genetic diversity is the heritable variation within a single species, including the differences among individuals in a local population. From an evolutionary perspective, this is the ultimate source of all kinds of diversity in the biosphere. On the other hand, ecosystem diversity is possibly the level of biodiversity most obvious to the lay observer, because of the immediate visual impact of the differences among aquatic and terrestrial landscapes or vegetation types, such as a pond and seashore, a conifer forest and an alpine meadow. However, its measurement suffers from major problems of standardization. Most approaches to biological diversity, therefore, focus on species (or taxonomic) diversity; this operational choice will be followed in this article. A first-level explanation of the diversity of life on Earth is the diversity of Earth itself.

6.1. INTRODUCTION There are two major aspects of the geographical diversity of the physical environment that allows living beings to become numerous. One aspect is habitat heterogeneity at local, regional, continental and even global scales. Organisms successfully thriving in a wide spectrum of different habitats are rare, the bulk of living species being instead confined, more or less strictly, to a narrow set of environmental conditions. The physical heterogeneity of the planet’s surface, however, does not explain why similar habitats in different continents, and even in different regions within the same continent, are inhabited by widely dissimilar species. This is explained, instead, by history. Physical or ecological barriers between similar habitat patches may interrupt gene flow to such an extent as to bring about allopatric speciation. Similar habitats in individual islands within an archipelago or on individual peaks within a rugged mountain range are commonly inhabited by related but different species of sedentary animals, such as land snails or wingless beetles. All these species, whose geographic range may be restricted to a few square kilometers, arose because of the physical or ecological barriers that interrupted the genetic flow between populations. In oceanic archipelagos, this condition of geographical isolation may affect the whole biota.

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For instance, the native fauna and flora of oceanic islands such as the Hawaiian chain are to a very large extent endemic: 89% for angiosperms and 99% for insects. Moreover, within this single island chain, a large number of species are confined to one single island, or even to a single district within one island, due to the habitat fragmentation caused by local topography or by recent lava flows. It has been estimated that the more than 10000 animal and plant species now inhabiting the Hawaiian archipelago evolved there from a few hundred successful colonizers, most of them of North American origin. The importance of geographical isolation in determining high levels of species diversity is also apparent in a comparison between marine and freshwater fishes. Of all fish species described to date (some 25 000), those living in the sea are less than twice as numerous as those living in inland waters, although the total volume of oceanic waters is about ten thousand times greater than the volume of inland waters. The relatively enormous diversity of freshwater fish species is explained by the fact that inland waters are fragmented into thousands of more or less completely isolated basins, a condition largely facilitating allopatric speciation. In more general terms, it has been estimated that only 15% of all living species described to date inhabit the sea. It is unlikely that future investigations will significantly alter this ratio. Likely explanations for this unbalanced distribution of diversity include the higher heterogeneity of continental environments and their higher structural (architectural) complexity with respect to the conditions prevailing in the oceans. At a higher taxonomic level, however, animal life is more diverse in the sea than on land. All animal phyla are represented in the sea and several phyla (e.g., echinoderms, ctenophores, sipunculans, brachiopods) are exclusively marine. This may be due, in part, to the fact that life originated in the sea and remained confined to this realm during much of its history; no less important, however, is the effect of the strict adaptations required for living in terrestrial (and, to a lesser degree, freshwater) environments, adaptations that cannot be met by animals with body designs like those of a sea urchin or a jellyfish. A second major explanation of the diversity of life is found in the multiple adaptations developed by most living beings in relation to the other

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organisms with which they interact, be these competitors, prey, predators, hosts or symbionts. The relevance of these interactions for the evolution of biological diversity is particularly conspicuous when two species interact so closely that each of them represents a major selective agent in the evolution of the other, thus offering a case for coevolution. Interesting examples of coevolution are found in the relationships between flowering plants and their insect pollinators: in many instances, the two partners are so closely specialized that size and shape of the insect’s mouthparts, temporal flight schedules, etc. are strictly matched by the shape of the corolla, the location of the nectaries, the length and shape of the stamens, and the timing of flower opening. Large plant families such as orchids (around 18,000 species) and legumes (around 16,500 species), and large genera such as Ficus (figs; around 800 species), owe much of their conspicuous species richness to their strict interactions with specialized pollinators. Interspecific relationships are also crucial in explaining the astonishing diversity found in several groups of parasites.

Most parasites attack a very restricted number of host species, sometimes just one. This explains, for example, the remarkable diversity found in Eimeria, a genus of sporozoan protists: more than one thousand species have been described to date and it has been estimated that in this genus there may exist some 35 000 species, each of them attacking a selected group (mostly a genus, or even a single species) of vertebrate (rarely invertebrate) hosts. The same will possibly apply to other groups of parasites, e.g., to several families of nematodes. A similar degree of host specificity is often found in small animals (many groups of insects and mites) and fungi (especially rusts, Puccinia and such groups) living on flowering plants. Adaptations to their hosts involve these parasites’ specializations to exploit only selected parts of the host plant, e.g., young leaves, mature leaves, stems, roots, seeds, etc., so that many dozen parasite species may attack the same host without directly interacting with each other. We must acknowledge, however, that neither geographical isolation nor specific adaptations to other organisms can help explain those extraordinary instances of biological diversity that are commonly known as species flocks. These are groups of dozens and even hundreds of species, all clearly derived from a single common ancestor from which they diversified without perceptible geographical isolation and now all living in the closest geographical proximity within a restricted geographical area.

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The most species-rich and best-investigated species flocks are those of the African cichlid fishes living in three large freshwater basins, Lake Victoria, Lake Tanganyika and Lake Malawi. Each of these lakes hosts a few hundred different cichlid species, each of them with its strikingly different morphological, ecological and behavioral adaptations, living alongside its closest relatives inhabiting the same lake. In the case of Lake Victoria, the cichlid species flock developed from a single ancestor since the last dramatic desiccation of the whole basin dated c.12 000 years ago, a very short time span to account for the origin of some three hundred species from a single ancestor. As insects represent more than one-half of the total biological diversity on Earth, it is sensible to ask the question, why are insects so numerous? The first explanation for their unique diversity is to be found in their size. Insects cannot be more than a few centimeters across, due to structural constraints such as the mechanical properties of their exoskeleton and the efficiency of gas diffusion in their tracheal system; on the other hand, their complex architecture cannot be easily accommodated in much less than 1 mm length. In fact, most insects are between 1 and 20 mm long. Insects not being individually too big, do not require large areas for populations to establish themselves, therefore no long-distance displacements are generally necessary for either feeding or reproduction. On the other hand, most insects are either too heavy or too fragile for long-distance passive transport. These conditions facilitate the establishment of isolated populations, a prerequisite for allopatric speciation. A second major cause of insect diversity is their feeding specialization. This is true for phytophagous species as well as for those living as parasitoids of other arthropods. In both cases, high degrees of host specificity are quite common. Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think. The formal biological definition of species is a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species, Homo sapiens. All humans can potentially interbreed with each other but not with members of any other species.

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In the real world, it isn’t always possible to make the observations needed to determine whether different organisms can interbreed. For one thing, many species reproduce asexually, so individuals never interbreed even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know whether different organisms could interbreed. Therefore, in practice, many biologists and virtually all paleontologists generally define species on the basis of morphology, rather than breeding behavior. Morphology refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, the more similar to one another different organisms appear, the greater the chance that they will be classified in the same species.

6.2. HISTORY OF LIFE A few major events punctuated the history of life on Earth. Some of these events were more or less particularly instrumental in the resulting biological diversity. For a couple of billion years (roughly speaking, 3000 to 1000 million years ago), life was represented only by prokaryotic, mostly unicellular forms, later accompanied by the first, still unicellular, eukaryotes. Within this long-time span, two evolutionary transitions proved to be of fundamental importance for the subsequent history of biological diversity: the origin of sex, with which it becomes meaningful to speak of biological species, and, later, the origin of multicellularity, a prerequisite for the evolution of complex and potentially diverse body plans such as those of animals and plants. The few multicellular organisms found in rocks older than one billion years are simple algal threads composed of chain-linked single cells. The first unequivocal metazoan-type fossils are those of the Vendian or Ediacaran age, c.620 to 550 million years ago. Their genealogical relations to modern phyla, however, are much disputed. According to some paleontologists they represent an early, independent experiment in multicellularity, not belonging to the ancestry of the true metazoans. These true metazoans suddenly appear at the base of the Cambrian strata, c.550 million years ago, in what has been described as the Cambrian explosion of life. Whether this stratigraphic evidence actually records an abrupt rise in biodiversity or merely the consequence of the development of the first

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mineralized (fossilizable) skeletons is still a matter of dispute. However, this Cambrian event is the single most dramatic event in the history of biodiversity documented in the fossil record. The Cambrian explosion was soon followed by the diversification of the marine biota into four main components: the infauna living within soft substrates, the epifauna living at the surface of soft and especially hard substrates, the plankton and, somehow later, the nekton, that is the complex of actively swimming animals, many of them predators, including fishes and large arthropods such as the eurypterids. The most recent animal phyla were already present in the Cambrian, some of them (e.g., arthropods) with a great number of different species and body plans. A further crucial event in the history of life was the invasion of land by plants (Middle Silurian), arthropods (Upper Silurian) and vertebrates (Upper Devonian). Plants colonized terrestrial habitats by developing rigid stems bearing photosynthetic leaves and reproductive organs, a root system to anchor the stem and a vascular system to conduct water and minerals; terrestrial animals modified body surface and respiratory organs in order to keep water loss to a minimum. The limited availability of water also caused both plants and animals to adopt new reproductive strategies. Animals were obliged to abandon external fertilization and to adopt spermatophores or to evolve internal fertilization. The susceptibility to desiccation of eggs and embryos was prevented either by laying the eggs in water (thus retaining, or developing anew, an amphibian life style), or otherwise. Many insects developed ovipositors to lay eggs in living plant tissue, but the two definitive answers to the danger of desiccation were found later, either in viviparity or in the production of better encased eggs, such as those of amniote vertebrates. In parallel, flagellated male gametes requiring water to travel to the female gametes were abandoned by the evolutionary line leading to the flowering plants. Arachnids and myriapods were already present in the Upper Silurian, whereas the oldest record for insects only dates from the Lower Devonian, and the other major group of nonmarine invertebrates, the pulmonated snails, is only known from the Carboniferous. More or less at the same time (end of early Carboniferous), insects had developed flight ability.

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Vertebrates came a bit later on the scene than arthropods. The earliest known land-dwelling vertebrates or tetrapods date from the Upper Devonian and the earliest flying vertebrates did not evolve before the late Triassic. These were pterosaurs, later to be joined by birds, in the late Jurassic, whereas the first flying mammals (bats) did not evolve before the Eocene. By that time, however, most recent orders of mammals had already differentiated from mammal origins in the late Triassic. A latecomer to the evolutionary scene is the angiosperms, the earliest fossil evidence for this group only dating from the early Cretaceous. Following this late appearance, however, the flowering plants experienced a rapid burst of differentiation, largely triggered by the simultaneous explosion of insect diversity. The Palaeo-gene was the time of origin of grasses (Gramineae), a plant family whose enormous success is due largely to their habit of continuous growth. Finally, the Neogene was the age of herbaceous plants with the explosion of families such as the Compositae (daisies and sunflowers), but also of the passerine birds, whose diversification is probably related, on one hand, to the diversification of seed-bearing plants and, on the other hand, to their frequent specialization to chasing flying insects. Other groups that gently increased in diversity during the Neogene include frogs and snakes. The history of biological diversity, however, is not just one of uninterrupted increase, it was also punctuated by some major critical events known as mass extinctions. One of the major mass extinctions happened towards the end of the Devonian. Apparently, it did not affect the vascular plants which had already placed their foot on land, but in the sea, it had catastrophic consequences on the reef communities and affected with particular severity trilobites, ammonoids, brachiopods and placoderms. The most severe of all mass extinctions, however, was probably the event that marks the end of the Permian (and of the Paleozoic era), when 70 to 90% of marine invertebrate species became extinct within a short time span. Whole previously successful groups such as trilobites, tabulate and rugose corals and fusulinid foraminifera disappeared completely; others, such as brachiopods, bryozoans, ammonoids and the stalked echinoderms, were severely affected. The next major event was the K–T extinction, at the boundary between the Cretaceous and the Tertiary. This is the most widely investigated extinction, marked by the disappearance of two well-known and very diverse groups, the ammonoids

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and dinosaurs. Other groups that went extinct by the end of the Cretaceous include two groups of large aquatic reptiles, the plesiosaurs and the mosasaurs, and the rudists, a family of large reef-building bivalves with heavy, oddshaped shells. The K–T extinction also severely affected the planktonic realm, especially foraminifera, radiolarians and coccolithophores.

6.3. ESTIMATES OF CURRENT DIVERSITY The single most commonly used descriptor of species diversity is species number. This number is correlated, indeed, with some measures of ecological diversity, such as the complexity of food webs, or topographic diversity. However, species richness is, at best, just a measure of one aspect of the global diversity of life. To improve the information content of biodiversity estimates, it has been suggested that we need to incorporate measures of phylogenetic relatedness among the species present in a given area, so to approach a more informative description of ‘character richness’ in the sample. But even estimating species diversity on Earth is not easy. This is due only in part to the incompleteness of our current inventory of biodiversity. There are serious problems, indeed, even with that part of biological diversity that has been already described and named. A first problem arises because of the lack of comprehensive and reliable monographs for many, if not most, of the major groups of living beings. For example, there is no recent world catalog for popular groups such as beetles (Coleoptera), or butterflies and moths (Lepidoptera): that means that the current estimates of 400 000 described species in the first group and 150 000 in the second may well be some 20% wrong. The major difficulty is not so much retrieving all existing species names from a very scattered literature, but identifying all synonymies; that is, all cases where two or more different names apply to one and the same species. Synonymizing requires a critical appraisal of old and new evidence and, as such, requires the time-consuming work of many dedicated specialists. Even for a well-researched group such as the flowering plants, less than 20% of the currently recognized species have been treated in the genus- or familylevel monographs during the twentieth century. A more subtle but far from trivial problem derives from the uncertainties in the definition of the basic unit of biodiversity. The circumscription of species may be very different, indeed, if one adopts a biological or a phylogenetic species concept.

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This is well exemplified by a 1992 study of the birds-of-paradise, a well-known group where traditional classifications, based on the biological species concept, acknowledge the existence of 40 to 42 species, whereas not less than 90 phylogenetic species may be distinguished in the same group. Still worse is the case of organisms with uniparental reproduction, where the biological species concept simply does not apply, by definition. Examples are offered, in the northern temperate regions, by the brambles (Rubus), the hawthorns (Crataegus) and the dandelions (Taraxacum). In each of these three genera, hundreds of species names have been given to slightly different morphotypes, which are perhaps morphologically distinct, but often, occurring together in the same spot, do not behave as different ecological units within the local community. For the strict advocates of the biological species concept, these living beings simply demonstrate that not all aspects of life are articulated in a biological species; this would imply that describing biodiversity only in terms of species counts is, in principle, unsatisfactory. From a geographical point of view, there are some prominent hot spots of biological diversity. For example, the four areas of highest diversity for higher plants are Latin America, where one-third of the world flora is at home with some 85 000 species thus far recorded, China (30 000 species, some 12% of the world total), Mexico (26 000) and Indonesia (20 000). A latitudinal gradient of biodiversity, with species number decreasing from the Equator to the Poles, is broadly observable despite the existence of many plant and animal groups whose distribution is center in the temperate areas, such as the Rosaceae, the Cruciferae and the aphids. These latitudinal differences in species diversity are observed at the local as well as at the regional levels. For instance, on one hectare of tropical forest in Ecuador, there may be as many as 473 tree species, whereas in a temperate forest a mere handful of tree species (if not just one species, as in many forest in cold temperate areas) may cover hundreds of square kilometers. Historical factors, as well as present-day conditions, concur with the explanation of the higher species diversity in the tropics. For instance, it has been suggested that during the Pleistocene, the Amazonian forest became fragmented into a large number of small areas that acted as refugia for the forest fauna and provided an opportunity for intensive allopatric speciation. Later on, when these forest fragments joined together again to form the present-day forest, the species that had differentiated in the separate refugia

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had a chance to expand and to become sympatric. How much their present coexistence depends on the high complexity of the ecosystem or on its high productivity is still a matter of dispute. The current level of knowledge varies greatly between different groups. In the case of birds, if we disregard the problems arising from adopting different species concepts, we can reasonably expect that no more than a few dozen species remain to be described. In the case of mammals, however, recent descriptions of previously unknown species are not limited to small inconspicuous species of rodents, shrews and bats, but also include, quite unexpectedly, some large animals such as the bilkis gazelle (Gazella bilkis) from Yemen, described in 1985, Madagascar’s golden lemur (Hapalemur aureus), described in 1986, and four large ruminants from the Vietnamese forests, three of them representing completely new genera, first described in the 1990s: the saola or spindle horn (Pseudoryx nghetinghensis), the linh duong (Pseudonovibos spiralis), a giant muntjac deer (Megamuntiacus vuquangensis) and another muntjac (Muntiacus truongsonensis). It is for invertebrates, however, that a truly dramatic increase in species description has taken place in the last few decades. An impressive example is provided by arachnids and crustaceans: in these groups, the number of new species described between 1960 and 1970 equals the total number of species described in the same groups during the previous two centuries. This example easily suggests that a large percentage of existing species have not yet been described. Currently, at least 15 000 species are annually described as new. Different approaches have been followed to obtain estimates of the number of existing species. These methods generally focus either on less intensively investigated taxonomic groups, such as bacteria, fungi, nematodes, mites and insects, or on some exceptionally species-rich habitats. The two aspects, however, are closely interrelated. For example, insects are the main component of species diversity in the tropical forest canopy, as nematodes are in the deep-sea floor. A sample of arthropods collected on just 10 trees in Borneo included 24000 specimens, belonging to more than 2800 species. One of the most abundant and diverse groups were the tiny parasitic chalcid wasps: among the 1455 specimens belonging to this group, 739 different species could be counted, 437 of these being represented by just one specimen each.

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These and similar data have led to an estimation of the total number of arthropod species in the tropical forests worldwide at somewhere between 10 million and 80 million. Much more conservative estimates, however, have been obtained following different approaches. It may be sensible, for instance, to compare the number of described and undescribed species collected by prolonged sampling efforts in biologically rich and hitherto less investigated areas. Thus, in a very extensive collection of Hemiptera from a topographically diverse area of tropical rainforest in Sulawesi (in Indonesia), the described species amounted to more than one-third of the total, thus suggesting that the total number of extant species of the same group, and of insects at large, would only be close to 2.5 million. Besides the tropical forest canopy, possibly the richest reservoir of uncharted biodiversity is the deep sea, despite the relatively low amount of energy flowing through it. Estimating this aspect of biodiversity, however, is even more problematic than in the case of tropical insects. Global estimates of existing biodiversity are thus quite uncertain. Figures ranging from5 to 130 million species have been recently offered for the gross total. Species counts are the simplest but not the only possible way of describing biodiversity at either the local or regional level. Interesting comparisons between ecosystems may be obtained, for instance, by considering the local distributions of species in terms of the average size of adult individuals or in terms of relative or absolute abundance of the species.

6.4. LIVING WORLD BEGIN TO DIVERSIFY You may think that you have little in common with unicellular prokaryotes. But multicellular eukaryotes like yourself actually share many attributes with Bacteria and Archaea. For example, all three of you: • • • • •

Conduct glycolysis Use DNA as the genetic material that encodes proteins Produce those proteins by transcription and translation using a similar genetic code Replicate DNA semi-conservatively Have plasma membranes and ribosomes in abundance

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These features support the conclusion that all living organisms are related to one another. If life had multiple origins, there would be little reason to expect all organisms to use overwhelmingly similar genetic codes or to share structures as unique as ribosomes. Furthermore, similarities in DNA sequences of universal genes (such as those that encode the structural components of ribosomes) confirm the monophyly of life. Despite the commonalities found across all three domains, major differences have evolved as well. Let’s first distinguish between Eukarya and the two prokaryotic domains. Note that “domain” is a subjective term used for the largest groups of life. There is no objective definition of a domain, any more than there is of a kingdom or a family.

6.5. WHERE DO VIRUSES FIT INTO THE TREE OF LIFE? Some biologists do not think of viruses as living organisms, primarily because they are not cellular and must depend on cellular organisms for basic life functions such as replication and metabolism. But viruses are derived from the cells of other living organisms. They use the same essential forms of genetic storage and transmission as do cellular organisms. Viruses infect all cellular forms of life, including bacteria, archaea, and eukaryotes. They replicate, mutate, evolve, and interact with other organisms, often causing serious diseases when they infect their hosts. They are also numerically among the most abundant organisms on the planet. And, finally, viruses clearly evolve independently of other organisms, so it is almost impossible not to treat them as a part of life. Several factors make virus phylogeny difficult to resolve. The tiny size of many viral genomes restricts the phylogenetic analyses that can be conducted to relate viruses to cellular organisms. The rapid mutational rate, which results in the rapid evolution of viral genomes, tends to cloud evolutionary relationships across long periods of time. There are no known viral fossils (viruses are too small and delicate to fossilize), so the paleontological record offers no clues as to viral origins. Finally, viruses are highly diverse, and several lines of evidence support the hypothesis that viruses have evolved repeatedly within each of the major groups of life.

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6.6. MANY RNA VIRUSES PROBABLY REPRESENT ESCAPED GENOMIC COMPONENTS Although viruses are now obligate parasites of cellular species, they may once have been cellular components involved in basic cellular functions— that is, they may be “escaped” components of cellular life that now evolve independently of their hosts.

6.6.1. Negative-Sense Single-Stranded RNA Viruses A case in point is a class of viruses whose genome is composed of singlestranded RNA that is the complement (negative-sense) of the mRNA needed for protein translation. Many of these negative-sense single-stranded RNA viruses have only a few genes, including an RNA-dependent RNA polymerase that allows them to make mRNA from their negative-sense RNA genome. Modern cellular organisms cannot generate mRNA in this manner (at least in the absence of viral infections), but scientists speculate that singlestranded RNA genomes may have been common in the distant past, before DNA became the primary molecule for genetic information storage. A selfreplicating RNA polymerase gene that begins to replicate independently of a cellular genome could conceivably acquire a few additional protein-coding genes through recombination with its host’s DNA. If one or more of these genes were to foster the development of a protein coat, the virus might then survive outside the host and infect new hosts. It is believed that this scenario has been repeated many times independently across the tree of life, given that many of the negative-sense single-stranded RNA viruses that infect organisms from bacteria to humans are not closely related to one another. In other words, negative-sense single-stranded RNA viruses do not represent a distinct taxonomic group but exemplify a particular process of cellular escape that probably happened many different times. Examples of familiar negative-sense single-stranded RNA viruses include the viruses that cause measles, mumps, rabies, and influenza.

6.6.2. Positive-Sense Single-Stranded RNA Viruses The genome of another type of single-stranded RNA viruses is composed of positive-sense RNA. Positive-sense genomes are already set for translation; unlike in negative-sense RNA, no replication of the genome into the complement strand is needed before protein translation can take

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place. Positive-sense single-stranded RNA viruses are the most abundant and diverse class of viruses. Most of the viruses that cause diseases in crop plants are in this group. When they infect plants, these viruses kill patches of cells in the leaves or stems of the plants, leaving live cells amid a patchwork of discolored dead plant tissue (giving them the name mosaic or mottle viruses). Other viruses in this group infect bacteria, fungi, and animals. Human diseases caused by positive-sense single-stranded RNA viruses include polio, hepatitis C, and the common cold. As is true of the other functionally defined groups of viruses, these viruses appear to have evolved multiple times across the tree of life from different groups of cellular ancestors.

6.6.3. RNA Retroviruses The RNA retroviruses are best known as the group that includes the human immunodeficiency viruses (HIV). Like the previous two categories of viruses, RNA retroviruses have genomes composed of single-stranded RNA and likely evolved as escaped cellular components. Retroviruses are so named because they regenerate themselves by reverse transcription. When the retroviruses enter the nucleus of their vertebrate host, viral reverse transcriptase produces cDNA from the viral RNA genome and then replicates the single-stranded cDNA to produce double-stranded DNA. Another virally encoded enzyme called integrase catalyzes the integration of the new piece of double-stranded viral DNA into the host’s genome. The viral genome is then replicated along with the host cell’s DNA; the integrated retroviral DNA is known as a provirus. Many components of cellular species (such as retrotransposons) resemble components of retroviruses. Retroviruses are only known to infect vertebrates, although genomic elements that resemble portions of these viruses are a component of the genomes of a wide variety of organisms including bacteria, plants, and many animals. Several retroviruses are associated with the development of various forms of cancer, as cells infected with these viruses are likely to undergo uncontrolled replication. Phylogenetic research revealed that many of the groups classified as “protists” are not closely related to one another. The term “protist” refers to a convenience term rather than a genuine taxonomic category. The majority are minuscule, but a few are enormous: gigantic kelps, for example, may grow to be as long as a football field. Some protist groups are connected to mammals and fungi, whereas others are related to terrestrial plants

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very distantly. The monophyly of eukaryotes is supported by cellular characteristics. Late Devonian rocks include the first fossil records of seed plants. The live seed plants are classified into two groups: gymnosperms and angiosperms (flowering plants). The sporophyte became less reliant on the gametophyte as plant development progressed. The invention of the seed allowed seed plants to colonize drier locations. Heterosporous seed plants exist. They generate two kinds of spores: male and female. Male spores split mitotically within the spore wall to generate a pollen grain, a multicellular male gametophyte.

6.6.4. Double-Stranded RNA Viruses Double-stranded RNA virusesmight have developed over and again from single-abandoned RNA precursors — or maybe the other way around. These infections, which are not firmly connected with each other, taint organic entities from all through the tree of life. For instance, numerous significant plant diseases are brought about by double-stranded RNA viruses, while other infections of this kind reason many instances of loose motions in people.

6.7. HOW DO EUKARYOTIC CELLS ARISE? Numerous individuals from the space Eukarya are recognizable to us. The world has no issue perceiving trees, mushrooms, and insects as plants, microbes, and animals, separately. Nonetheless, a stunning combination of different eukaryotes — generally minute living beings — don’t squeeze into any of these three groups.

Figure 6.1. A typical animal cell with labeled organelles. Source: Image by Wikimedia commons

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Eukaryotes that are not plants, creatures, or parasites have generally been “dropped” into the class protists. Phylogenetic examinations, nonetheless, are clear and predictable in showing that many of the groups that fall under the rubric of “protists” as a matter of fact are not firmly connected with each other, however, are paraphyletic. In this manner “protist” doesn’t portray a formal scientific categorization, but is a comfort term — a shorthand approach to saying “each one of the eukaryotes that are not plants, creatures, or parasites.”

6.7.1. The Diversity Of Protists Is Reflected In Both Morphology And Phylogeny Regarding their developmental connections, as well as in numerous parts of their science, protists are more different than any of the three better realized eukaryote groups. A few protists are motile, while others don’t move; some are photosynthetic, others heterotrophic; most are unicellular, yet some are multicellular. Most are tiny, however, a couple are huge: monster kelps, for instance, can develop to be longer than a football field. They allude to the unicellular types of protists as microbial eukaryotes, however, we ought to remember that there are huge, multicellular protists too. The phylogeny of the significant eukaryote heredities stays a subject of exploration and discussion. A few groups of protists are firmly connected with animals, while others are firmly connected with the land plants, despite everything others are simply remotely connected with any of these natural eukaryotes.

6.7.2. Cellular Features Support The Monophyly of Eukaryotes Eukaryotic cells vary in numerous ways from prokaryotic cells, and these remarkable characters of eukaryotes lead us to reason that eukaryotes are monophyletic. As such, there was a solitary eukaryotic ancestor which developed into the various genealogies of protists, as well as plants, animals, and organisms. Given the idea of transformative cycles, the numerous synapomorphies of eukaryotes without a doubt didn’t emerge all the while. One can make a few sensible surmising’s about the main occasions that prompted the development of another cell type, remembering that the worldwide climate

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went through a colossal change — from low to high accessibility of free oxygen — over the span of these occasions. Remember, in any case, that these deductions, albeit sensible and grounded, are yet speculative; the speculation here is a rare example of that researchers are presently considering. It portrays here, an unmistakable hypothesis on the beginning of the eukaryotic cell as a structure for contemplating the difficult inquiry of eukaryotic starting points.

6.8. THE EVOLUTION OF SEED PLANTS 6.8.1. Seed Plants Become Today’s Dominant Vegetation? In the Devonian, a development had showed up: a few plants grew widely thickened woody stems, which came about because of the multiplication of xylem. This kind of development in the distance across stems and roots is called auxiliary development. Among the principal plants with this transformation were seedless vascular plants called pro-gymnosperms, all types of which are currently wiped out. The earliest fossil proof of seed plants is found in late Devonian rocks. Like the pro-gymnosperms, these seed plants were woody. They had fernlike foliage yet had seeds connected to their leaves. Toward the finish of the Permian, different groups of seed plants became predominant. The living seed plants fall into two significant groups, the gymnosperms (like pines and cycads) and the angiosperms (blooming plants). There are a few contending phylogenetic speculations with respect to the significant groups of gymnosperms compared with the angiosperms, the most generally upheld connections. Every living gymnosperm and numerous angiosperms show optional development. The life cycle patterns of all seed plants additionally share highlights, as it is seen.

6.8.2. Features of the Seed Plant Life Cycle Protect Gametes and Embryos A pattern in plant development: the sporophyte turned out to be less subject to the gametophyte, which decreased compared to the sporophyte. This pattern went on with the presence of the seed plants, whose gametophyte age is decreased significantly farther than it is in the greeneries.

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The haploid gametophyte grows part of the way or altogether while joined to and healthfully reliant upon the diploid sporophyte. Among the seed plants, hands down the earliest groups of gymnosperms (like cycads and ginkgos) had swimming sperm. Later groups of gymnosperms and angiosperms showed different methods for uniting eggs and sperm. The product of this striking transformative pattern in seed plants was autonomy from the fluid water that earlier plants needed to help sperm in arriving at the egg. The coming of the seed established the chance to colonize drier regions and spread over the earth. Seed plants are heterosporous; that is, they produce two sorts of spores, one that turns into the male gametophyte and one that turns into the female gametophyte. They form separate microsporangia and megasporangia on structures that are gathered on short structures, for example, the stamens and pistils of an angiosperm flower. Inside the microsporangium, the meiotic products are microspores, what divi4de mitotically inside the spore wall one or a couple of times to form a multicellular male gametophyte called a pollen grain. Pollen grains are let out of the microsporangium to be spread by wind or by an animal pollinator. The mass of the pollen grain contains sporopollenin, the most natural compound known, which safeguards the pollen grain against lack of hydration and harm — one more benefit regarding endurance in the earthly climate. Sporopollenin in spore walls added to the effective colonization of the earthly climate by the earliest land plants. Rather than the pollen grains delivered by the microsporangia, the megaspores of seed plants are not shed. All things being equal, they form into female gametophytes inside the megasporangia. These megagametophytes are subject to the sporophyte for food and water. In most seed plant species, only one of the meiotic products in a megasporangium survives. This nucleus separates mitotically, and the subsequent cells divide again to create a multicellular female gametophyte. The megasporangium is encircled by sterile sporophytic structures, which form an integument that safeguards the megasporangium and its cells. Together, the megasporangium and integument comprise the ovule, which will form into a seed after fertilization.

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6.8.3. The Seed Is a Complex, Well-Protected Package A seed contains tissues of three types. A seed coat creates from the integument — tissues of the diploid sporophyte parent that encompass the megasporangium. Inside the megasporangium is the haploid female gametophytic tissue from the future, which contains a source of nutrients for the embryo. (This tissue is genuinely broad in most gymnosperm seeds. In angiosperm seeds, it is extraordinarily diminished, and sustenance for the embryo is provided by a tissue called endosperm, which is portrayed underneath.) In the focal point of the seed is the third era, the embryoof the new diploid sporophyte. The seed of a gymnosperm or an angiosperm is a very much protected resting stage. The seeds of certain species might stay dormant yet remain suitable (equipped for development) for many years, germinating just when conditions are good for the development of the sporophyte, as occurred with the 2,000-year-old Judean date seed. Conversely, the embryos of seedless plants form directly into sporophytes, which either survive or die, contingent upon natural circumstances. Spores of a few seedless plants might stay dormant and reasonable for significant stretches of time; however, seeds give a safer and enduring dormant stage. During the dormant stage, the seed coat shields the embryo from drying and may likewise safeguard it against potential hunters that would somehow eat the embryo and its nutrients. Many seeds have primary transformations that advance their dispersal by wind or, on a more regular basis, by animals. At the point when the young sporophyte resumes development, it draws on the food stores in the seed. The formation of seeds is a significant justification behind the gigantic developmental outcome of the seed plants, which are the predominant life types of most current land plants.

6.8.4. A Change in Anatomy Enabled Seed Plants to Grow To Great Heights The eldest seed plants created wood — broadly multiplied xylem — which gave them the help to become taller than different plants around them, hence catching all the lighter for photosynthesis. The more young part of wood is very much adjusted for water transport, while more established wood becomes obstructed with pitches or different materials.

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Albeit at this point not utilitarian in transport, the more seasoned wood keeps on offering support for the plant. Not all seed plants are woody. With seed plant development, many seed plants lost the woody development ability; in any case, other worthwhile properties assisted them with developing in many places.

6.9. HOW DO PLANTS SUPPORT OUR WORLD? When life moved onto land, it was plants that molded the climate, and the present climate is overwhelmed by angiosperms. Members of the two biggest angiosperm clades are all over. The monocots incorporate grasses, cattails, lilies, orchids, and palms. The eudicots incorporate by far most of natural seed plants, including most spices (i.e., nonwoody plants), plants, trees, and bushes. Among the eudicots are such different plants as oaks, willows, beans, snapdragons, and sunflowers. Plants make significant commitments to biological system processes by which the climate keeps up with resources that benefit people. These advantages remember the impacts of plants on soil, water, the environment, and the environment. Plants produce oxygen and eliminate carbon dioxide from the climate, as well as assume significant parts in forming and recharging the fertility of soils. Plants assist with holding soil, giving protection against erosion by wind and water. They likewise moderate the neighborhood environment in different ways, like by expanding moistness, giving shade, and obstructing wind.

6.9.1. Seed Plants Are Our Primary Food Source Plants are essential makers; that is, their photosynthesis traps energy and carbon, making those resources accessible for their own requirements as well as for the herbivores and omnivores that eat them, for the carnivores and omnivores that eat the herbivores, and for the prokaryotes and organisms that total the food chain. The earliest strides in human development included developing angiosperms to give a solid food supply. Today, twelve types of seed plants stand between mankind and starvation: rice, coconut, wheat, corn (likewise called maize), potato, yam, cassava (additionally called custard or manioc),

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sugarcane, sugar beet, soybean, normal bean (Phaseolus vulgaris), and banana. Many other seed plants are developed for food, yet none position with these twelve in significance. Without a doubt, the greater part of the world’s human population infers most of its food energy from the seeds of a solitary plant: rice, Oryza sativa. Rice is especially significant in the Far East, where it has been developed for over 8,000 years. Individuals additionally use rice straw in numerous ways, for example, as covering for rooftops, food and bedding for domesticated animals, and attire. Rice structures, as well, have many purposes, going from compost to fuel. Another fundamentally significant angiosperm is the coconut (Cocos nucifera). In certain societies of the seaside jungles, this monocot tree is known as the “tree of life.” Every over-the-ground part of the plant — its seeds, organic product, stem (trunk), leaves, buds, and, surprisingly, its sap — is useful and worth to people. Many individuals get most of their protein from the “meat” of coconut seeds, and the seed’s “milk” is crucial in regions where water is scant or ill-suited to drink.

6.10. FACTS FORM THE BASIS OF OUR UNDERSTANDING OF EVOLUTION? The living scene is continually evolving. Scholars notice a considerable lot of these progressions straightforwardly, both in research center tests and in normal populations. Numerous different changes are kept in the fossil record of life. One can quantify the rate at which new transformations emerge, notice the spread of new hereditary variations through a population, and see the impacts of hereditary change on the structure and capability of living beings. As such, development is a reality that can notice straightforwardly. Scholars likewise have collected an enormous group of proof about how transformative changes happen, and about what developmental changes have happened previously. The comprehension and utilization of the systems of developmental change to natural issues is known as transformative hypothesis. The transformative hypothesis has numerous valuable applications, like the development of flu immunizations. Researchers use development to study, comprehend, and treat infections; to foster better agrarian harvests and practices; and to foster modern cycles that produce new particles with helpful properties.

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Information on developmental standards has assisted researchers with understanding how life developed and how species communicate. It additionally permits us to make expectations about the natural world. In regular discourse, individuals will often utilize “hypothesis” to mean an untested speculation, or even a theory. In any case, the expression “transformative hypothesis” alludes to no single speculation, and it unquestionably isn’t a mystery. As utilized in science, “hypothesis” alludes to the whole group of work on the comprehension and utilization of a field of information. At the point when it alludes to the “gravitational hypothesis,” it is not inferring that gravity is an untested thought. Nobody questions that gravity exists — one can see its belongings surrounding. All things considered, it is alluding to how to interpret the components that outcome in gravitational force, and the utilization of that comprehension to make forecasts about the communications of actual items. Drop this book, and it will fall at a predictable rate, as indicated by the gravitational hypothesis. Likewise, when it alludes to the transformative hypothesis, it is alluding to how it might interpret the components that outcome in natural changes in populations over the long haul, and the utilization of that comprehension to decipher changes and connections of natural living beings. That organic populations advance through time isn’t questioned by researchers. It can, and do, notice developmental change consistently. One can straightforwardly notice the development of flu infections, yet the developmental hypothesis permits us to apply that data to the issue of growing more successful antibodies. A few instruments of transformative change are perceived, and established researchers are constantly extending how they might interpret how and when these systems apply to specific organic issues. Concentrating on the components of development and their endless applications is the dynamic and invigorating field of developmental hypothesis.

6.11. MECHANISMS OF EVOLUTIONARY CHANGE Transformative systems are powers that change the hereditary construction of a population. Solid Weinberg harmony is an invalid speculation that expects those powers are missing.

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The realized transformative instruments incorporate change, quality stream, hereditary float, nonrandom mating, and determination — every one of which goes against one of the five fundamental suppositions of HardyWeinberg harmony. Scientists have proactively examined Darwin’s central clarification for development, to be a specific regular choice. Albeit regular choice is as a rule a significant part of development, even Darwin perceived that it was not by any means the only component of development, and numerous extra transformative powers have been found since Darwin’s opportunity. This led to examination of a portion of different systems that result in development.

6.11.1. Mutations Generate Genetic Variation 0The beginning of hereditary variation is change. A mutation is any changing of the nucleotide groupings of a living being’s DNA. The course of DNA replication is somewhat flawed, and changes show up pretty much every time a genome is repeated. Changes happen arbitrarily concerning a living being’s versatile requirements; it is determined following up on this irregular variety of those outcomes in variation. Most changes are either unsafe to their carriers or nonpartisan. A couple are gainful, notwithstanding, and beforehand harmful or nonpartisan alleles might become favorable if conditions change. Moreover, transformations can reestablish to a population hereditary variety that other developmental cycles have eliminated.

Figure 6.2. Potential evolutionary outcomes of hybridization. Source : Image by Wikimedia commons

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In this manner, mutations both create and assist with keeping up with hereditary variation in populations. Mutation rates can be high, as with the flu infections, yet in numerous life forms the change rate is exceptionally coming up short (10-8 to 10-9 changes for each base pair of DNA per age). Indeed, even low general mutation rates, be that as it may, are adequate to make a significant hereditary variety, on the grounds that every one of countless genes might change, and populations frequently contain huge quantities of individuals. For instance, if the likelihood of a point transformation (duplication, deletion, or substitution of a single base) were 10-9 for every base pair per age, then every human gamete, the DNA of which contains 3 × 109 base pairs, would average three new point mutations (3 × 109 × 10-9 = 3) — and every zygote would have a normal of six new transformations. The ongoing human population of around 7 billion individuals would be supposed to have around 42 billion new changes that were absent one generation before. So, even though the change rate in people is very low, human populations contain colossal hereditary minor changes. One of the circumstances for Hardy-Weinberg balance is that there be no change. Albeit this condition is never stringently met, the rate at which changes emerge at a solitary locus is typically low to the point that transformations without help from anyone else bring about just little deviations from the Hardy-Weinberg balance. On the off chance that enormous deviations are found, it is typically fitting to remove transformation as the reason and to search for proof of other developmental components following up on the populace.

6.11.2. Gene Flow May Change Allele Frequencies Not many populations are totally isolated from different populations of similar species. Relocation of people and developments of gametes between populations — a peculiarity called gene pool — can change allele frequencies in a population. If the individuals survive and reproduce in their new area, they might add new alleles to the populace’s genetic stock, or they might change the frequencies of alleles currently present on the off chance that they come from a population with varying allele frequencies. For a population to be at Hardy-Weinberg balance, there should be no gene from populations with various allele frequencies.

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6.12. NATURAL SELECTION RESULT IN EVOLUTION Even though development is characterized as changes in the gene frequencies of a population starting with one generation and then onto the next, normal choice follows up on the mix — the actual elements expressed by an organism with a given genotype — as opposed to directly on the genotype. The conceptive commitment of a heterozygote to subsequent generations comparative with the commitments of different aggregates is called its wellness. Changes in conceptive rate don’t be guaranteed to change the hereditary construction of a population. For instance, if all individuals in a population experience a similar expansion in conceptive rate (during an earth good year, for example), the gene pool of the population won’t change. Changes in genes of offspring are liable for increments and decreases in the size of a population, however, just changes in the overall progress of various heterozygotes in a population lead to changes in allele frequencies starting with one generation then onto the next. The wellness of individuals of a specific mix is an element of the likelihood of those individuals enduring duplicated by the normal number of offspring they produce over their lifetimes. At the end of the day, the wellness is not entirely set in stone by the general paces of fitness and generation of individuals with that phenotype.

6.13. NATURAL SELECTION CAN CHANGE OR STABILIZE POPULATIONS To improve on our conversation as of recently, individuals have considered just characters impacted by alleles at a solitary locus. be that as it may, most characters are affected by alleles at more than one locus. Such characters are probably going to show quantitative as opposed to subjective variety. For instance, the distribution of body sizes of individuals in a population, a person that is impacted by qualities at numerous loci as well as by the environment, is probably going to look like the ringer molded bends. Normal choice can follow up on characters with quantitative variety in any of a few unique ways, delivering very various outcomes: • •

Balancing out determination protects the typical qualities of a population by leaning toward normal individuals. Directional determination changes the qualities of a population by leaning toward individuals that shift in one unit from the mean of the population.

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Problematic determination changes the qualities of a population by leaning toward individuals that shift in the two units from the mean of the population

6.13.1. Stabilizing Selection If the smallest and largest individuals in a population contribute very few offspring to the next generation than do individuals closer to the average size, then stabilizing selection is operating on size. Stabilizing selection decreases variation in populations, but it does not change the mean. Natural selection often acts in this way, countering increases in variation brought about by sexual recombination, mutation, or migration. Rates of evolution in many species are slow because natural selection is often stabilizing. Stabilizing selection operates, for example, on human birth weight. Babies born lighter or heavier than the population mean die at higher rates than babies whose weights are close to the mean . In discussions of specific genes, stabilizing selection is often called purifying selection, because there is selection against any deleterious mutations to the usual gene sequence.

6.13.2. Directional Selection Directional selection operates when individuals at one extreme of a character distribution contribute more offspring to the next generation as compared to other individuals, shifting the average value of that character in the population toward that extreme. In the case of a single gene locus, directional selection may eventually occur in favoring a particular genetic variant (which is also known as positive selection for that variant). By favoring one phenotype over another, directional selection results in an increase of the frequencies of alleles that produce the favored phenotype. The individual then falls under stabilizational selection. Many instances of directional selection have been noticed straightforwardly, and long-haul models have large amounts of the fossil record. The long horns of Texas Longhorn cows are an illustration of a gene that has developed through directional selection. Texas Longhorns are relatives of steers that Christopher Columbus brought to the New World. Columbus got a couple of steers in the Canary Islands and carried them to the island of Hispaniola in 1493. The dairy cattle

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immediately increased, and their relatives were taken to the central area of Mexico. As the Spanish investigated what might later become Texas and the southwestern United States, they carried a portion of these dairy cattle with them, some of which got away and formed wild populations. Populations of these wild dairy cattle expanded enormously throughout the following couple of hundred years, however, there was weighty predation from bears, mountain lions, and wolves, particularly on the young calves. Cows with longer horns were more efficient in safeguarding their calves against attacks, and over the course of the following couple of hundred years, the typical horn length of dairy cattle in the wild groups expanded impressively. Moreover, the animals developed protection from endemic illnesses of the Southwest, as well as higher fertility and life span. Texas Longhorn cows frequently live and deliver calves very much into their twenties, or about two times the length of many types of steers that have been falsely chosen by individuals for qualities like high fat content or high milk production (which are instances of artificial directional selection).

6.13.3. Disruptive Selection At the point when disruptive selection works, individuals at inverse limits of a character distribution offer more posterity to the cutting edge than do individuals near the mean, which increases variation in the population in the dark-bellied seed cracker (Pyrenestes ostrinus), a West African finch, shows how disruptive selection can impact populations in nature. The seeds of two sorts of sedges (swamp plants) are the most copious food sources for these finches during part of the year. Birds with big beaks can promptly break the hard seeds of the sedge Scleria verrucosa. Birds with little beaks can break S. verrucosa seeds just with trouble; nonetheless, they feed more productively on the delicate seeds of S. goossensii than do birds with bigger beaks. Young finches whose beaks deviate extraordinarily from the two overwhelming bill sizes don’t get by as well as finches whose beaks are near one of the two sizes addressed by the extremes. Since there are not many bountiful food sources in the environment those with middle sized-beaks are less productive in utilizing both key food sources. Disruptive selection hence keeps a bimodal beak size distribution.

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6.14. CONSTRAINTS ON EVOLUTION It would be mixed up to expect that mutations can deliver any characteristic it could envision. Development is compelled in numerous ways. The absence of suitable genetic variation, for instance, forestalls the improvement of numerous possibly positive characteristics. There is no such thing as on the off chance that the allele for a given characteristic in a population, that a gene can’t develop regardless of whether it would be exceptionally preferred by regular determination. Most potential blends of genes and genotypes have never existed in any population, and thus have never been tried under regular selection. Constraints are forced on organisms by the dictates of physics and chemistry. The size of cells, for example, is constrained by the stringencies of surface area-to-volume ratios. The ways in which proteins can fold are restricted by the bonding capacities of their constituent molecules. And the energy transfers that fuel life must operate within the laws of thermodynamics.

6.14.1. Developmental Processes Constrain Evolution Developmental constraints on development are foremost on the grounds that all transformative developments are alterations of beforehand existing designs. Human specialists trying to control a plane can begin “without any preparation” to make a totally new sort of motor (fueled by fly drive), to supplant the past kind (controlled by propellers). Developmental changes, notwithstanding, can’t occur along these lines. Current aggregates of organisms are obliged by current circumstances and past specific pressures. A striking illustration of such formative requirements is given by the development of fish that invest most of their energy in the ocean depths. One quality, the base dwelling skates, and beams, share a typical precursor with sharks, whose bodies were at that point ventrally smoothed and whose skeletal casing is made of adaptable ligament. Skates and beams developed a body type that further leveled their paunches, permitting them to swim along the sea depths. On the other hand, plaice, sole, and flop are base dwelling relatives of profound bellied, along the side leveled progenitors with hard skeletons. The main way these fishes can lie level is to flounder over on their sides. Their capacity to swim is in this way reduced, however, their bodies can lie still and are very much disguised.

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During development, one eye of these flatfishes moves so the two eyes are situated on the similar side of the body. Such changes in eye position have developed a few times, and changes have occurred in the two courses (that is, both left-and right-eyed flatfishes have developed freely). Little changes in the position of one eye presumably assisted native flatfishes with seeing better, bringing about the level body structures tracked down today. This way of creating a straightened body may not be ideal, yet the fishes’ formative capacities compel the pathways that development can take.

6.14.2. Trade-Offs Constrain Evolution Transformations as often as possible force both health costs and advantages. For a transformation to develop, the health benefits it gives should surpass the health costs it forces — as such, the tradeoff should be advantageous. For instance, there are metabolic expenses related to creating and keeping up with specific obvious highlights (like tusks or horns) that males use to rival different males for access to females. The way that these highlights are normal in numerous species proposes that the advantages got from having them should offset the expenses. Because of compromises, numerous attributes that are versatile in one setting might be maladaptive in another. Consider the rough-skinned newt, Taricha granulosa, and one of its predators, the normal strap snake, Thamnophis sirtalis. The newt has in its skin a powerful neurotoxin called tetrodotoxin (TTX). TTX incapacitates nerves and muscles by obstructing sodium channels. Most vertebrates — including many garter snakes — will die if they eat a newt. In any case, a few snakes can eat such newts and survive. In certain populations of garter snakes, TTX-resistant sodium pumps have developed in the nerves and muscles. Be that as it may, the snakes take care of this trait. For a few hours after eating a newt, TTX- resistant snakes can move just leisurely, and they never move as quickly as sensitive snakes. Along these lines, resistant snakes are more helpless against their own hunters than are TTX-sensitive snakes that just don’t experience harmful newts. Without anyone else, in any case, they don’t empower us to foresee long haul transformative changes. Long haul examples of developmental change can be emphatically affected by occasions that happen so rarely (a shooting star influence, for instance) or so sluggishly (mainland float) that they are probably not going to be seen during momentary examinations. The

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manners by which transformative cycles act might change after some time with changing natural circumstances. Indeed, even among the relatives of solitary hereditary animal types, various genealogies might develop every which way. Accordingly, extra kinds of proof, exhibiting the impacts of uncommon and surprising occasions on patterns in the fossil record, should be assembled on the off chance that it wishes to figure out the course of development more than billions of years.

6.15. CONCLUSION Changes as frequently as conceivable power both health costs and benefits. The fundamental way few fishes can lie level is to struggle over on their sides. Little changes in the place of one eye apparently helped ancestral flatfishes with seeing better. Newt stores in its skin a strong neurotoxin called tetrodotoxin (TTX). Most vertebrates — including many garter snakes — will pass on the off chance that they eat a newt. In specific peoples of ally snakes, TTX-safe sodium redirects have progressed in the nerves and muscles. Long stretch instances of formative change can be unequivocally impacted by events that occur so once in a blue moon (a falling star impact, for example) or so languidly (central area float) that they are most likely not going to be seen during transitory assessments. Further, the chapter covered a few aspects of diversity and plausible associated mechanisms for a reader to understand the basics.

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Minelli, A., 2005. Diversity of Life. eLS, [online] Available at: [Accessed 3 July 2022]. Epstein, P., 1993. The Diversity of Life. JAMA: The Journal of the American Medical Association, [online] 269(15), p.2006. Available at: [Accessed 4 July 2022]. Hardin, G. and Wilson, E., 1993. The Diversity of Life. Population and Development Review, [online] 19(1), p.183. Available at: [Accessed 4 July 2022]. Rodd, R., 1992. On the Diversity of Life. Biology, Ethics, and Animals, [online] pp.13-41. Available at: [Accessed 4 July 2022]. Taylor, M., 1994. Diversity of life. Nature, [online] 368(6469), pp.362363. Available at: [Accessed 4 July 2022].

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NERVOUS SYSTEM, SENSORY SYSTEM AND IMMUNE SYSTEM

CONTENTS 7.1. Introduction..................................................................................... 194 7.2. The Nervous System Consists of Neurons and Supporting Cells........ 198 7.3. Nerve Impulses are Produced on the Axon Membrane..................... 200 7.4. The Central Nervous System Consists of the Brain............................ 202 7.5. Language and Other Functions........................................................ 206 7.6. The Autonomic Nervous System....................................................... 208 7.7. Animals Employ a Wide Variety of Sensory Receptors...................... 208 7.8. Sensing Muscle Contraction and Blood Pressure.............................. 210 7.9. Sensing Taste, Smell, and Body Position........................................... 212 7.10. Evolution of the Immune System.................................................... 215 7.11. Many of the Body’S Most Effective Defenses are Nonspecific......... 216 7.12. Cells of the Specific Immune System.............................................. 221 7.13. Initiating the Immune Response..................................................... 222 7.14. The Immune System Can Be Defeated............................................ 223 7.15. Conclusion.................................................................................... 224 References.............................................................................................. 226

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To perform its functions, the immune system must ensure continuous reception and processing of information about the antigenic state of the organism. This perception must allow an evaluation of how serious is any divergence from the norm (the notion of “norm” of course varies during the development of the organism and between different organisms).

7.1. INTRODUCTION The immune system initiates effector functions aimed to form an adequate defensive response. In the context of its “goals”, the immune system can be considered as a sensory organ receiving and processing specific information. Although this concept has been formulated by others, it has not however contributed to a new understanding of the laws regulating the functions of the immune system functions, nor has the immune system been put on a par with other sensory systems. Moreover, the immune system has sometimes been declared an isolated and peculiar system and thereby been denied any analogy with neuronal networks altogether. In contrast to this firmly rooted view, we want to demonstrate some results of establishing common laws that regulate the functioning of the immune system as well as other sensory systems of the organism.

7.1.1. Nervous System The nervous system is an organized set of cells specializing in the transmission of electrochemical impulses from sensory receptors to the place of a reaction via a network. All living organisms can sense changes in themselves and in their surroundings. Changes in the external environment include changes in light, temperature, sound, motion, and odor, whereas changes in the internal environment include variations in the posture of the head and limbs and also changes in the internal organs. To live, both internal and external changes must always be noticed and assessed. Since life on this planet evolved and the environment has become more complicated, organisms’ existence relied on their ability to adjust to changes in the circumstances. A swift response or reaction was one requirement for survival. Because chemical communication between cells was too sluggish for life, a mechanism emerged that allowed for rapid responsiveness. That system

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was the nervous system, which is based on the practically instantaneous transmission of electrical impulses from one part of the body to another via specialized nerve cells known as neurons. There are two kinds of nervous systems: diffuse and concentrated. There is no brain in the diffuse system, which is seen in lower invertebrates, and neurons are dispersed all through the creature in a netlike arrangement. A part of the nervous system has a prominent role in planning information and guiding reactions in upper invertebrates and vertebrates’ centralized systems. This centralization is completed in vertebrates, who have a fully formed brain and spinal cord. Nerve fibers in the peripheral nervous system transport impulses from and to the brain and spine.

Figure 7.1. Human Nervous System diagram. Source: Image by Wikimedia Commons

7.1.2. Sensory System Animals use their senses, also known as sensory reception or sense perception, to identify and react to events in their internal and external habitats. Animal senses are best explained due to the type of physical energy, or modalities, involved.

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The light senses (photoreception; i.e., vision), mechanical senses (mechanoreception; i.e., touch, balance, and hearing), chemical senses (chemoreception; i.e., taste and smell), and electric sense (electroreception) of some fish are the four primary modalities. There are interior senses that employ the same modalities as the outward senses. Proprioceptors, for example, are mechanical sensors that assess muscle lengths and joint locations, whereas interoceptors monitor blood pressure. Similarly, chemoreceptors monitor blood CO2 (carbon dioxide) and pH levels, as well as a range of sensors that react to the existence of hormones and metabolites at the molecular level. All nerve cell input to the CNS takes the same form, as action potentials generated by afferent (inward-conducting) sensory neurons. Because distinct sensory neurons transmit to different areas of the brain, they are linked with various sensory modalities. The strength of the experience is determined by the frequency with which the sensory neuron conducts action potentials. The brain distinguishes a sunset, a symphony, and a searing agony solely by the identification of the sensory neuron conveying the action potentials and the frequency of these impulses. Thus, when the auditory nerve is stimulated artificially, the brain interprets the sensation as sound. However, when the optic nerve is artificially stimulated in the same way and to the same degree, the brain sees a flash of light.

7.1.3. Immune System The immune system is a sophisticated array of defense reactions seen in humans and other advanced animals that aid in repelling infections (pathogens). Nonspecific, innate immunity and specific, acquired immunity are the two cooperating defensive mechanisms that impart disease immunity. Nonspecific defense systems reject all microbes uniformly, whereas specific immune responses are tuned to specialized types of invaders. Both systems collaborate to prevent pathogens from entering and multiplying within the body. These immune pathways also aid in the elimination of cancer-causing cells in the body. The immune system is a collection of distinct cell types and soluble chemicals that work together to eliminate any alien (non-self) materials. Immune system cells are classified as lymphoid or myeloid and are created by pluripotent stem cells.

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Natural killer cells (NK-cells), T and B lymphocytes are lymphoid lineage immune cells, whereas the myeloid lineage includes all other immune cells. The diverse immunoglobulin isotypes and complement system proteins represent the non-cellular components of the immune system.

Figure 7.2. The NK cell releases perforin and granzyme, leading to cancer cell apoptosis. Source: Image by Wikimedia Commons

The immune system rejects any organ transplants brought into the body from separate individuals, even though it is important to keep a person (receiver) alive, but accepts a tissue transplanted through one location of the same individual into another. This suggests that the immune response does not distinguish between good and harmful items; rather, the immune system’s rule in dealing with things is to accept self and reject non-self-matters. This is its primary function and primary task. Although the immune system minimizes the size of the destructions it produces, it always leaves some lasting harm as a result of its immunological reactions. Immune systems responses are described as any immunological reactions undertaken by the immune system’s cellular and humoral elements that are elicited in reaction to a foreign stimulus, such as the invasion of an infectious microbe that succeeds in breaching any of the body’s preventive physical, biochemical, or physiological barriers. All the remarkable changes that take place at the site of microbe invasion reflect the characteristics of inflammation, which include an increase in body

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temperature, swelling, redness at the site of invasion, and pain, as well as the production of specific immunoglobulins, antigen-reactive lymphocytes, and induction of immune tolerance, which all imply the initiation of the immunological components as well as the occurring of immune responses. The expected outcome of any evoked immunological response is the elimination of infectious microorganisms with minimum permanent harm to self-tissues due to the activation of these immunological responses.

7.2. THE NERVOUS SYSTEM CONSISTS OF NEURONS AND SUPPORTING CELLS 7.2.1. Neuron Organization An animal’s capacity to react to environmental circumstances is required. A fly avoids being swatted; a crayfish’s antennae detect food and the crayfish swims toward it. It must contain sensory receptors that can detect the stimuli and motor effectors that can react to it in order to do so. The nervous system connects sensory receptors and motor effectors mostly in invertebrate phyla and all vertebrate classes. The nervous system is made up of neurons and supporting cells, as detailed in Chapter 49. Sensory (or afferent) neurons transmit impulses from sensory receptors to the central nervous system (CNS), whereas motor (or efferent) neurons transmit impulses from the CNS to effectors (muscles and glands). Aside from sensory and motor neurons, most invertebrates and all vertebrates have a third type of neuron in their nervous systems: association neurons (or interneurons). These neurons are found in the brain and spinal cord of vertebrates, where they assist give more sophisticated reflexes and higher associative capabilities such as learning and memory. Sensory neurons transport impulses into the CNS, while motor neurons send impulses away from the CNS. The peripheral nervous system (PNS) of vertebrates is made up of sensory and motor neurons working together. Somatic motor neurons induce skeletal muscle contraction, whereas autonomic motor neurons govern the activity of smooth muscles, heart muscle, and glands. The sympathetic and parasympathetic nervous systems work in tandem to balance the autonomic motor neurons. Regardless of their appearance, most neurons have the same functional architecture. The nucleus is located in the cell body, which is an expanded

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area. One or even more cytoplasmic projections known as dendrites protrude from the cell body. Motor and association neurons have a plethora of highly branching dendrites, allowing them to receive information from multiple sources at the same time. A few neurons have dendritic spines, which are extensions from the dendrites that enhance the surface area accessible to receive impulses. The information arriving at the cell body’s dendrites is integrated at the cell body’s surface. If the ensuing membrane excitation is adequate, impulses are generated and carried away from the cell body along an axon. Although an axon may create short terminal branches to activate a number of cells, each neuron has a single axon leaving its cell body. The axons that govern the muscles in your feet are almost a meter long, while the axons that reach from the brain to the pelvis in a giraffe are nearly three meters long.! Neuroglia, or supporting cells, provide structural and functional support to neurons. These cells are 10 times more numerous than neurons and perform a range of roles including delivering nourishment to neurons, eliminating waste from neurons, regulating axon movement, and providing immunological functions. Schwann cells and oligodendrocytes, which generate myelin sheaths that coat the axons of many neurons, are two of the most significant types of neuroglia in vertebrates. In the PNS, Schwann cells generate myelin, whereas oligodendrocytes make myelin in the CNS. Throughout development, these cells coil around each axon numerous times to produce the myelin sheath, an insulating coating made up of several layers of membrane. Myelinated axons have myelin sheaths, but unmyelinated axons do not. The white matter of the CNS is made up of myelinated axons, whereas the grey matter is made up of unmyelinated dendrites and cell bodies. In the PNS, myelinated and unmyelinated axons are packed together to create nerves, quite like wires in a cable.

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7.3. NERVE IMPULSES ARE PRODUCED ON THE AXON MEMBRANE 7.3.1. The Resting Membrane Potential Neurons interact by transferring electrical characteristics of the plasma membrane from one cell to another. The neuron’s design facilitates the propagation of electrical messages known as nerve impulses. To comprehend how well these impulses are formed and transferred throughout the nervous system, one must first explore some of the electrical characteristics of plasma membranes. The battery in a vehicle or a flashlight splits electrical charges in between the two poles. There is stated to be a potential difference, or voltage, between both the poles, with one pole being positive and the other negative. Likewise, a potential difference exists across the plasma membrane of every cell. The negative pole of the membrane is the side exposed to the cytoplasm, while the positive pole is the side exposed to the extracellular fluid. The membrane potential is the name given to this potential difference. Because of three causes, the interior of the cell is more charged negatively than the outside: (1) Negatively charged large molecules such as proteins and nucleic acids are more plentiful inside the cell and cannot diffuse out. These compounds are known as fixed anions. (2) The sodium-potassium pump enters the cell with just two potassium ions (K+) for every three sodium ions (Na+) pumped out. This not only adds to the electrical potential but also forms concentration gradients for Na+ and K+. (3) Ion channels allow more K+ to diffuse out of the cell than Na+ to enter. Gates are portions of the channel protein that open and seal the pore of Na+ and K+ channels in the plasma membrane. Based on the membrane potential, the gates in neuron axons and muscle fibers close or open. As a consequence, these channels are known as voltage-gated ion channels.

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Figure 7.3.  Na+ influx via voltage-gated ion channels during depolarization. Source: Image by Wikimedia Commons

7.3.2. Action Potentials Generation of Action Potentials When the plasma membrane is gently fully discharged, an oscilloscope shows a little upward deflection of the line, which quickly decays returning to the resting membrane potential. Because the amplitudes of these modest changes in membrane potential depend on the intensity of the stimulus, they are referred to as graded potentials. Graded potentials can be either depolarizing or hyperpolarizing, and they can combine to increase or lessen their effects, much as two waves combine to form one larger one when they meet in synchrony, or they cancel each other out when a trough hits a crest. Summation refers to the capacity of graded potentials to combine. Yet, after a certain amount of depolarization is reached (about –55 mV in human axons), a nerve impulse, or action potential, is produced. The threshold is the amount of depolarization required to generate an action potential. Depolarization that approaches or surpasses the barrier activates both the Na+ and K+ channels, but the Na+ channels open first. The fast diffusion of Na+ into the cell increases the membrane potential toward the equilibrium potential for Na+ (+60 mV—recall that the positive sign signifies that the membrane switches polarity as Na+ rushes in).

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Saltatory Conduction Because action potentials are performed without decrement (without diminishing in amplitude), the last action potential at the axon’s end has the same size as the initial action potential. If the axon has a big diameter or is myelinated, the velocity of conduction increases. Because action potentials in myelinated axons are exclusively created at the nodes of Ranvier, they carry impulses faster than nonmyelinated axons. One action potential still acts as the depolarization trigger for the next, but depolarization must propagate from one node to the next before voltagegated channels may be opened. As a result, the impulses appear to hop from node to node in a mechanism known as saltatory conduction (Latin saltare, “to leap”). Conduction (Latin for “jumping”). Return to the “wave” analogy used in the previous section to illustrate action potential propagation for a minute to observe how saltatory conduction speeds up nerve transmission. The “wave” spreads through a full stadium’s seats as spectators rising up in one area cause individuals in the adjacent section to rise up in turn. Because the “wave” skips parts of empty bleachers, it actually moves quicker across the stadium as there are more vacant sections. The wave does not even have to wait for the missing persons to stand, instead “jumping” the gaps—much how saltatory conduction leaps the nonconduction “gaps” of myelin between exposed nodes.

7.4. THE CENTRAL NERVOUS SYSTEM CONSISTS OF THE BRAIN 7.4.1. The Evolution of the Vertebrate Brain Sponges have been the only significant multicellular animal phylum that lacks nerves. Cnidarians have the most basic neurological systems: all neurons are identical and are joined together in a web, or nerve net. There is little associated activity, there is no control over complicated behaviors, and there is minimal coordination. The free-living flatworms, phylum Platyhelminthes, are the simplest organisms having associative action in the neural system. Two nerve cords run down the bodies of these flatworms, while peripheral nerves stretch forth to the muscles.

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The two nerve cords merge at the front of the body, generating an expanded mass of neural tissue which also contains associative neurons and synapses that connect neurons. This primordial “brain” is a primitive central nervous system that allows for considerably more intricate regulation of muscle responses than cnidarians can. All future evolutionary modifications in nervous systems may be seen as elaborations on traits already present in flatworms. Earthworms, for instance, have a central nervous system that is linked to all other sections of the animal by peripheral nerves. Furthermore, in arthropods, the central coordination of complex responses is becoming increasingly focused on the front end of the nerve cord. As this area developed, it became home to an increasing number of associative interneurons and tracts, which are highways inside the brain that connect associative components. Fossils of the inside braincases of ancient agnathans, fish which swam 500 million years ago, have exposed a number of the initial phases of the vertebrate brain’s evolution. Despite their modest size, these brains already contained the three parts that define modern vertebrate brains: (1) The hindbrain, or rhombencephalon; (2) The midbrain, or mesencephalon; and (3) The forebrain or prosencephalon. The hindbrain was the most important component of these early brains, and it is still the case in fishes today. The hindbrain, which is made up of the cerebellum, pons, and medulla oblongata, can be thought of as an extension of the spinal cord that is largely responsible for coordinating motor reflexes. Tracts with a great number of axons go up and down the spinal cord like wires to the hindbrain. In turn, the hindbrain integrates the many sensory impulses from the muscles and organizes the pattern of muscular responses. The cerebellum (“little cerebrum”), a tiny extension of the hindbrain, is responsible for most of this coordination. The cerebellum is bigger in vertebrates than in fishes because it plays an increasingly essential role as a coordinating center for movement. The cerebellum processes data on the current position and movement of each limb, the state of relaxation or contraction of the muscles involved, and the overall position and relationship of the body to the outside world in all vertebrates.

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This information is collected in the cerebellum, synthesized, and the resultant directives are sent to efferent channels. The remainder of the brain in fishes is devoted to sensory reception and processing. The optic lobes receive and process visual information in the midbrain, whereas the forebrain processes olfactory (smell) information. Fish’s brains continue to develop throughout their lifetimes. This continuing development stands in stark contrast to the brains of other vertebrate groups, which typically finish development by infancy. The human brain continues to develop throughout childhood, but no new neurons are made once development has stopped, with the exception of the small hippocampus, which regulates which memories are stored in longterm memory and which are forgotten.

7.4.2. The Human Forebrain The cerebrum of the human mind is so enormous that it looks to surround the remainder of the brain. It is divided into right and left hemispheres of the brain that are linked by a tract known as the corpus callosum. Frontal, parietal, temporal, and occipital lobes are subdivided from the hemispheres. Each hemisphere gets sensory input from the opposing, or contralateral, side of the body and controls that side largely. As a result, for example, a touch on the right hand is largely conveyed to the left hemisphere, which might also subsequently trigger the movement of that hand in reaction to the contact. Damage to one hemisphere caused by a stroke frequently results in the loss of feeling and paralysis on the opposite side of the body.

Figure 7.4. Four lobes of Human Forebrain. Source: Image by Wikimedia Commons

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Cerebral Cortex Most of the cerebrum’s neuronal activity takes place within the same layer of grey matter which is only a few millimeters thick on its outer surface. This layer, known as the cerebral cortex, is densely packed with nerve cells. In humans, it includes approximately 10 billion nerve cells, accounting for nearly 10% of all neurons in the brain. The surface of the cerebral cortex is heavily convoluted; this is especially true in the human brain, where convolutions increase the surface area of the cortex thrice. The cerebral cortex’s actions are classified into three types: motor, sensory, and associative. The main motor cortex is located on the posterior edge of the frontal lobe, right in front of the central sulcus (convolution), along the gyrus (crease). Each point on its surface corresponds to a particular component of the body’s movement.

The main somatosensory cortex is located on the anterior margin of the parietal lobe, just beyond the central sulcus. Each point in this region receives information from sensory neurons in a specific section of the body that serve cutaneous and muscular sensations. Because of the necessity for manual dexterity and speech, large regions of the motor cortex and primary somatosensory cortex are devoted to the fingers, lips, and tongue. The auditory cortex is located within the temporal lobe, and different parts of this cortex are responsible for different sound frequencies. The visual cortex is located in the occipital lobe, with separate sites processing information from different places on the retina, which correspond to specific spots in the eyes’ visual fields. The association cortex is the area of the cerebral cortex that is not occupied by the motor and sensory cortices. The association cortex, the seat of higher mental functions, is most extensive in primates, particularly humans, where it accounts for 95 percent of the cerebral cortex’s surface.

Basal Ganglia Several clusters of cell bodies and dendrites that generate grey matter islands are buried deep within the cerebrum’s white matter. The basal ganglia are clusters of neuron cell bodies that receive sensory input from ascending pathways and motor orders from the cerebral cortex and cerebellum.

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The basal ganglia send signals down the spinal cord, where they help govern bodily movements. Damage to certain parts of the basal ganglia can cause the resting tremor of muscles seen in persons with Parkinson’s disease.

7.5. LANGUAGE AND OTHER FUNCTIONS 7.5.1. Arousal and Sleep Reticular formation is a widespread collection of neurons found in the brain stem. The reticular activating system, one component of this structure, regulates awareness and alertness. This system examines the information entering the brain and detects key stimuli. It is fed by all sensory channels. When the reticular activating system is stimulated to arousal, the degree of activity in numerous sections of the brain increases. Anesthetics and barbiturates inhibit neural pathways from the reticular formation to the cortex and other brain areas. The reticular activating system regulates both sleep and wakefulness. Sleeping in a dark environment is easier than sleeping in a light one because there are fewer visual inputs to excite the reticular activating system. Furthermore, serotonin, a previously described neurotransmitter, reduces activity in this system. Serotonin induces a decrease in brain activity, which leads to sleep. Sleep does not imply a loss of consciousness. It is, however, an ongoing process with numerous states that may be exposed by measuring the electrical activity of the brain in an electroencephalogram (EEG). The EEG of a calm but awake individual with closed eyelids consists mostly of big, Slow waves with a frequency range of 8 to 13 hertz (cycles per second). These are known as alpha waves. Because many sensory inputs are being received, processed, and translated into motor movements in an attentive participant with eyes open, the EEG waves are faster (beta waves are detected at frequencies ranging from 13 to 30 hertz) and desynchronized. In various periods of sleep, theta waves (4 to 7 hertz) and delta waves (0.5 to 4 hertz) can be seen. With the start of sleepiness, the first change in the EEG is a slowdown and reduction in the total amplitude of the waves. Slow-wave sleep includes numerous stages, although they are all distinguished by decreased arousability, skeletal muscle tone, heart rate, blood pressure, and breathing rate. The EEG resembles that of a calm, awake individual during REM sleep (called after the fast eye movements that occur

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during this stage), and the heart rate, blood pressure, and breathing rate all increase. Individuals in REM sleep, paradoxically, are more difficult to rouse and are more prone to arise voluntarily. Dreaming occurs during REM sleep, and the fast eye movements match those performed by the eyes when awake, implying that dreamers “watch” their dreams...

7.5.2. Language and Spatial Recognition Even though two cerebral hemispheres appear to be anatomically identical, they are responsible for distinct functions. Language is the most fully researched instance of this lateralization of function. In 90 percent of right-handed people and approximately two-thirds of left-handed people, the left hemisphere is the “dominant” hemisphere for language—the hemisphere in which most brain processing linked to language is undertaken. The dominant hemisphere has two language regions. Wernicke’s region, situated in the parietal lobe between the major auditory and visual areas, is vital for language comprehension and speech formation. Broca’s region, which is located near the section of the motor cortex that controls the face, is in charge of producing the motor output required for linguistic communication. Language abnormalities known as aphasias can be caused by damage to certain brain locations. For example, if Wernicke’s region is injured, the person’s speech is quick and fluid but without meaning; words are tossed together as in a “word salad.”

7.5.3. Memory and Learning The foundation of memory and learning is one of the brain’s big mysteries. There is no one area of the brain that appears to house all aspects of memory. Since very substantial cortical injury does not selectively eliminate memories, specific cortical areas cannot be identified for specific experiences. Memory is not completely lost if sections of the brain, particularly the temporal lobes, are destroyed. Many memories survive the damage, and the capacity to access them gradually returns over time. As a result, researchers who have attempted to examine the physical principles behind memory have frequently felt as if they were clutching at a

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shadow. The fundamental mechanisms by which memories are created have been extensively studied.

7.6. THE AUTONOMIC NERVOUS SYSTEM The sympathetic and parasympathetic nervous systems, as well as the medulla oblongata of the hindbrain, make up the autonomic nervous system. Despite differences, the sympathetic and parasympathetic nervous systems share some characteristics. Two neurons are involved in both efferent motor pathways: one has its cell body in the central nervous system (CNS) and sends an axon to an autonomic ganglion, while the other has its cell body in the autonomic ganglion and sends an axon to synapse with a smooth muscle, cardiac muscle, or gland cell. A preganglionic neuron is the first neuron, and that always releases ACh at its synapse. The second neuron is a postganglionic neuron, which releases ACh in the parasympathetic division and norepinephrine in the sympathetic division. Preganglionic neurons of the sympathetic division arise in the thoracic and lumbar areas of the spinal cord. The majority of these neurons’ axons synapse in two parallel chains of ganglia just outside the spinal cord. These structures are commonly referred to as the sympathetic chain of ganglia. The sympathetic chain comprises postganglionic neuron cell bodies, and it is the axons from these neurons that innervate the various visceral organs. However, there are notable exceptions to this general rule. Most crucially, some preganglionic sympathetic neurons’ axons bypass the sympathetic chain and instead terminate within the adrenal gland. The adrenal gland is divided into two parts: the cortex and the medulla. the inner part and an outer part Epinephrine (adrenaline), a hormone secreted by the adrenal medulla in response to sympathetic nerve innervation,

7.7. ANIMALS EMPLOY A WIDE VARIETY OF SENSORY RECEPTORS 7.7.1. Categories of Sensory Receptors and Their Actions Sensory information is transmitted to the CNS and perceived in four steps: (1) Stimulation—a physical stimulus impinges on a sensory neuron or an accessory structure;

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(2) Transduction—the stimulus energy is used to generate electrochemical nerve impulses in the sensory neuron’s dendrites; (3) Transmission—the sensory neuron’s axon conducts action potentials along an afferent pathway to the CNS; and (4) Interpretation—the brain generates a sensory perception from the electrochemical events produced by afferent stimulation. We see (along with hearing, touching, tasting, and smelling) with our brains, not our sense organs. Sensory receptors differ in terms of the type of external stimulation that most effectively activates their sensory dendrites. Environmental stimuli are classified into three types: (1) Mechanical forces, which activate mechanoreceptors, (2) Chemicals, which excite chemoreceptors, and (3) Electromagnetic and thermal energy, which stimulate a range of receptors, including photoreceptors in the eyes. The most basic sense receptors are free nerve terminals that react to bending or stretching of the sensory neuron membrane, changes in temperature, or substances like oxygen in the extracellular fluid. Other sensory receptors are more complicated, comprising sensory neurons interacting with specific epithelium cells.

7.7.2. Sensing the External and Internal Environments Exteroceptors Exteroceptors are receptors that detect stimuli in the surrounding environment. Before vertebrates inhabited land, almost all of their external senses originated in water. As a result, many terrestrial vertebrate senses favor stimuli that travel well in water, employing receptors that have survived the shift from sea to land. Mammalian hearing, for example, turns an aerial signal into a watery one by employing receptors that originated in water. A few vertebrate sensory systems that work well in water, such as fish electrical organs, do not work in the air and are not found in terrestrial vertebrates. Some land inhabitants, on the other hand, have sensory systems, such as infrared sensors, that cannot operate in the sea.

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Sensory systems can convey various amounts of information about their surroundings. Certain sensory systems merely give sufficient intelligence to understand how an object is present; these draw an animal’s attention to the object but provide little or no clue of its location. Other sensory systems offer information about an object’s position, allowing the animal to move toward it. Other sensory systems allow the brain to create a three-dimensional view of an item and its surroundings.

Interoceptors Interoceptors detect sensations that originate within the body. Internal sensors detect muscle length and tension, limb position, pain, blood chemistry, blood volume and pressure, and body temperature. Many of these receptors are simpler than those that monitor the external environment and are thought to resemble primordial sensory receptors. In the next sections, students will look at the many types of interoceptors and exteroceptors and how they sense different sorts of stimuli.

7.7.3. Sensory Transduction Because they have stimulus-gated ion channels in their membranes, sensory cells may react to stimuli. Depending on the sensory system, the sensory stimulation induces these ion channels to open or shut. A sensory stimulation causes a change in the membrane potential of the receptor cell in this way. In most circumstances, the sensory stimulation causes a depolarization of the receptor cell, similar to the excitatory postsynaptic potential (EPSP) produced in a postsynaptic cell in response to neurotransmitters. A receptor potential is a depolarization that happens in a sensory receptor in response to the stimulus.

7.8. SENSING MUSCLE CONTRACTION AND BLOOD PRESSURE Mechanoreceptors are sensory cells that have ion channels that respond to mechanical forces applied to the membrane. When the membrane is mechanically distorted, these channels open, causing a depolarization (receptor potential) that leads the sensory neuron to generate action potentials.

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7.8.1. Muscle Length and Tension Buried within the skeletal muscles of all vertebrates except the bony fishes are muscle spindles, sensory stretch receptors that lie in parallel with the rest of the fibers in the muscle. Each spindle consists of several thin muscle fibers wrapped together and innervated by a sensory neuron, which becomes activated when the muscle, and therefore the spindle, is stretched. Muscle spindles, together with other receptors in tendons and joints, are known as proprioceptors, which are sensory receptors that provide information about the relative position or movement of the animal’s body parts. The sensory neurons conduct action potentials into the spinal cord, where they synapse with somatic motor neurons that innervate the muscle. A muscle’s contraction causes the tendons that are connected to it to tighten. Another type of proprioceptor, the Golgi tendon organs, monitors this tension; if it becomes too high, they induce a response that inhibits the motor neurons that innervate the muscle. This reaction serves to keep muscles from contracting so hard that they harm the tendons to which they are linked.

7.8.2. Blood Pressure Blood pressure is measured at two separate locations in the body. The carotid sinus is an expansion of the internal carotid arteries, which provide blood to the brain. The other is the aortic arch, which is a section of the aorta near its exit from the heart. Both locations include a highly branched network of afferent neurons called baroreceptors that detect tension in the walls of blood arteries. When blood pressure drops, so does the frequency of impulses produce by baroreceptors. In response to this decreased input, the CNS stimulates the sympathetic division of the autonomic nervous system, generating an increase in heart rate and vasoconstriction. Both effects contribute to blood pressure elevation, hence preserving homeostasis. A rise in blood pressure, on the other hand, decreases sympathetic activity and increases parasympathetic activity, slowing the heart and reducing blood pressure.

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7.9. SENSING TASTE, SMELL, AND BODY POSITION Chemoreceptors are sensory cells that contain membrane proteins that can bind to certain chemicals in the extracellular fluid. The membrane of the sensory neuron depolarizes in response to this chemical interaction, resulting in the generation of action potentials. Chemoreceptors are involved in the perceptions of taste and smell, as well as the chemical composition of the blood and cerebrospinal fluid.

7.9.1. Taste In vertebrates, taste buds—collections of chemo-sensitive epithelial cells connected with afferent neurons—mediate the experience of taste. Taste buds are distributed throughout the body of a fish. These are the most sensitive chemoreceptors known in vertebrates. They are especially sensitive to amino acids; for example, a catfish can tell the difference between two distinct amino acids at a concentration of fewer than 100 parts per billion (1 g in 10,000 L of water)! Bottom-feeding fish rely on their capacity to taste the surrounding water to detect the availability of food in an often-murky environment. Taste buds are found in the epithelium of the tongue and oral cavity, within elevated regions known as papillae, in all terrestrial vertebrates. Salty, sweet, sour, and bitter taste buds are found in humans. The actions of sodium (Na+) provide a salty flavour, whereas the effects of hydrogen (H+) produce a sour taste. The structure of organic compounds that cause sweet and bitter tastes, such as sugars and quinine, varies. Taste buds that respond well to various tastes are concentrated in specific parts of the tongue: sweet at the tip, sour on the sides, bitter in the rear and salty over the majority of the tongue’s surface. The human complex taste experience is the consequence of various combinations of impulses in sensory neurons from these four types of taste buds, as well as information connected to smell. The influence of scent on taste may well be easily illustrated by eating an onion with the nose open and then with the nose blocked. Many arthropods, like vertebrates, contain taste chemoreceptors. Flies, for example, have taste receptors in sensory hairs on their feet due to their style of food hunting. The sensory hairs include a variety of chemoreceptors that detect sugars, salts, and other substances. By combining impulses from

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these chemoreceptors, they can sense a wide range of flavors. f they trip over potential by combining impulses from these chemoreceptors, they can sense a wide range of flavors. The proboscis (the tubular feeding device) expands to feed if they step on possible food.

7.9.2. Smell Olfaction, or the sense of smell in terrestrial vertebrates, is mediated by chemoreceptors placed in the upper region of the nasal passages. These receptors are bipolar neurons with dendrites that terminate in cilia tassels that extend into the nasal mucosa and an axon that project straight into the cerebral cortex. A terrestrial vertebrate utilizes its sense of smell to sample the chemical world around it in the same way that a fish uses its sense of taste. Because terrestrial animals are surrounded by air rather than water, their sense of smell has evolved to detect airborne particles (albeit these particles must dissolve in extracellular fluid before activating the olfactory receptors. Many mammals have exceptionally keen senses of smell, so much so that a single odorant molecule may be all that is required to stimulate a specific receptor. Though humans could only identify four types of taste, they can sense thousands of different odors. According to a new study, there might be up to a thousand distinct genes coding for various scent receptor proteins. The specific collection of olfactory neurons that respond to a certain odor may serve as a “fingerprint” that the brain may use to identify the odor.

7.9.3. Internal Chemoreceptors Sensory receptors in the body detect a wide range of chemical properties of blood or blood-derived fluids, including cerebrospinal fluid. These receptors include the aorta and carotid body peripheral chemoreceptors, which are predominantly sensitive to plasma pH, and the medulla oblongata central chemoreceptors, which are responsive to cerebrospinal fluid pH. These receptors were explored with the control of breathing. If the breathing pace is excessively slow, the level of plasma CO2 rises, creating more carbonic acid and lowering blood pH. Carbon dioxide can also enter the cerebral fluid and reduce the pH, triggering the central chemoreceptors. The chemoreceptor activation indirectly impacts the brain stem’s respiratory control center, increasing breathing rate. The aortic bodies can

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also respond to a decrease in blood oxygen concentrations, but this impact is usually minor until a person travels to a high altitude.

7.9.4. The Lateral Line System The lateral line system gives fish a sense of “distance touch,” allowing them to detect changes in pressure and low-frequency vibrations. This helps a fish to locate prey and swim in unison with the rest of its school. It also allows a blind cave fish to feel its surroundings by tracking variations in water flow patterns via the lateral line sensors. The lateral line system is present in frog larvae, but it is destroyed after metamorphosis and is not seen in any terrestrial animal. The lateral line system’s sense complements the fish’s sense of hearing, which is done by a distinct sensory apparatus.

Figure 7.5. Sketch of the lateral line sensory organ, used by fish to sense pressure differences in the water. Source: Image by Wikimedia Commons

In the anatomy and mechanics of hearing, the lateral line system is made up of sensory organs located within a longitudinal canal in the fish’s skin that runs down either side of the body and multiple canals in the head. The sensory structures are referred to as hair cells because they contain hairlike features on their surface that protrude into a gelatinous membrane known as a cupula (Latin for “little cup”).

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Sensory neurons innervate the hair cells and provide signals to the brain. Hair cells feature multiple hairlike processes of roughly the same length known as stereocilia and one longer process known as a kinocilium. Sound waves in the fish’s surroundings force the cupula to shift, causing the hairs to bend. The accompanying sensory neurons are excited and create a receptor potential when the stereocilia bend in the direction of the kinocilium. The frequency of action potentials generated by the sensory neuron increases as a result. When the stereocilia are bent in the opposite direction, the sensory neuron activity is suppressed.

7.9.5. The Ears and Hearing Fish use their lateral line system to detect vibrating pressure waves in water. Terrestrial animals perceive identical vibrational pressure waves in the air via similar hair cell mechanoreceptors in the inner ear. Hearing functions better in water than in the air because water transfers pressure waves more effectively. Despite this limitation, terrestrial vertebrates utilise hearing extensively to monitor their habitats, communicate with other members of their species, and recognise potential sources of danger. Auditory stimuli move much further and faster than chemical stimuli, and auditory sensors give superior directional information than chemoreceptors. However, auditory stimuli alone convey very little detail concerning distance.

7.10. EVOLUTION OF THE IMMUNE SYSTEM All species have defense systems against the invasion of smaller organisms and viruses. Bacteria fight themselves against viral invasion by using restriction endonucleases, enzymes that destroy any foreign DNA that lacks the bacterium’s particular pattern of DNA methylation. Multicellular species have a more challenging defensive challenge since their bodies frequently take up complete viruses, bacteria, or fungus rather than bare DNA. Invertebrates handle this challenge by labeling their cells’ surfaces with proteins that function as “self” labels. Invertebrates with special amoeboid cells assault and engulf any invading cells that lack such labels. Invertebrates utilize a negative test to identify foreign cells and viruses by checking for the lack of certain markers.

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This approach provides an extremely effective surveillance system for invertebrates, but it has one major flaw: any microbe or virus having a surface protein that resembles the invertebrate self-identifier will not be detected as alien. Invertebrates have no resistance against such a “clone” intruder. Elie Metchnikoff, a Russian biologist, was the first to identify that invertebrates have immunological systems in 1882. He collected the small translucent larva of a common starfish on a beach in Sicily. He stabbed it carefully with a rose thorn. The next morning, he noticed a swarm of small cells covering the surface of the thorn, as if trying to devour it. The cells were striving to defend the larva by phagocytosing the intruder. Metchnikoff was given the Nobel Prize in Physiology or Medicine in 1908 for the discovery of what has become termed the cellular immune response, together with Paul Ehrlich for work on the second important component of immunological defense, the antibody or humoral immune response. Many components of the invertebrate immune response are shared with the vertebrate immune response.

7.11. MANY OF THE BODY’S MOST EFFECTIVE DEFENSES ARE NONSPECIFIC 7.11.1. Skin: The First Line of Defense The vertebrate is protected against infection in the same way as knights protected mediaeval towns. “Walls and moats” prevent admission; “roaming patrols” fight outsiders; and “sentries” confront anybody strolling about and summon patrols if a valid “ID” is not supplied. 1. Moats and walls: the skin, the vertebrate body’s outermost covering, is the initial barrier against microbial penetration. Mucous membranes in the respiratory and gastrointestinal systems also are major barriers to invasion. 2. Patrols on the go. If the body’s initial line of defense is breached, it will launch a cellular counterattack, employing a battery of cells and substances that destroy bacteria. These defenses kick in very quickly once the infection begins. 3. Sentry. Finally, the body is protected by mobile cells that patrol the circulation, inspecting the surfaces of any cells they come across. They are a component of the immune system. One type

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of immune cell assaults and destroys any foreign cell, whilst the other type marks the foreign cell or virus for removal by roving patrols.

The Skin as a Barrier to Infection The skin is the biggest part of the vertebrate body, comprising about 15% of an adult human’s overall mass. The skin not just protects the body by establishing a virtually impenetrable barrier but also enhances this protection with chemical weapons on the surface. The pH of the skin’s surface is 3 to 5, acidic enough to hinder the development of many germs. Sweat also contains the enzyme lysozyme, which digests bacterial cell walls. In addition to protecting the body against virus and microbe invasion, the skin prevents excessive water loss to the air through evaporation. The skin’s epidermis is around 10 to 30 cells thick, or about the thickness of this page. The stratum corneum, or outer layer, comprises cells that are constantly abraded, damaged, and worn by friction and stress throughout the body’s numerous functions. The body reacts to this harm by changing cells rather than fixing them. Cells are lost continually from the stratum corneum and replaced by new ones created in the deepest layer of the epidermis, the stratum basale, which includes some of its most actively proliferating cells in the vertebrate body. The cells that develop in this layer move upward and enter a large intermediate stratum spinosum layer. As they go higher, they generate the protein keratin, which gives skin its toughness and water-resistant. Such new cells ultimately reach the stratum corneum, where they usually stay for approximately a month before being lost and replaced by fresher cells from below. Psoriasis, which affects around 4 million Americans, is a chronic skin illness in which epidermal cells are replaced every 3 to 4 days, almost eight times quicker than usual.

Other External Surfaces In addition to the skin, there are two more possible entrance points for viruses and microorganisms: the gastrointestinal tract and the respiratory system. Remember that the digestive and respiratory systems are both accessible to the outside world, and their surface must guard the body against outside intruders.

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Several microbes are found in food, but most are destroyed by saliva (which also includes lysozyme), the stomach’s highly acidic environment, and digesting enzymes in the intestine. Inhaled air contains microorganisms as well. The cells that line the smaller bronchi and bronchioles create a coating of sticky mucus that captures most bacteria before they reach the warm, wet lungs, which would offer perfect breeding grounds. Other cells that line these channels contain cilia that constantly sweep mucus toward the glottis. It can then be eaten, transporting possible intruders from the lungs to the digestive tract. An infectious agent known as a pathogen would sometimes infiltrate the digestive and respiratory systems, causing the body to activate defensive mechanisms like vomiting, diarrhea, coughing, and sneezing to remove the pathogens.

7.11.2. Cellular Counterattack: The Second Line of Defense The surface defenses of the vertebrate body are quite efficient, although they are periodically compromised, enabling intruders to penetrate. At this moment, the body defends itself using a slew of nonspecific cellular and chemical devices. This is referred to as the second line of defense. These gadgets all have one feature: they respond to any microbial infection without waiting to identify the intruder. Although these nonspecific immune response cells and chemicals circulate throughout the body, there is a primary place for the collecting and distribution of immune system cells, which is known as the lymphatic system. The lymphatic system is a network of lymphatic capillaries, ducts, nodes, and lymphatic organs that, among other things, store cells and other substances utilized in the immune response. These cells are spread throughout the body to combat infections and are also kept in lymph nodes where foreign intruders can be removed when bodily fluids flow via them.

Cells That Kill Invading Microbes White blood cells called leukocytes travel throughout the body and fight invading germs within tissues. There are three types of these cells, and each destroys invading bacteria in a different way. Macrophages (“great eaters”) are enormous, irregularly shaped cells that destroy microorganisms by consuming them by phagocytosis, similar to how an amoeba ingests a food particle. The membrane-bound vacuole harboring the bacteria merges with a lysosome within the macrophage. Fusion stimulates lysosomal enzymes,

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which destroy the bacterium by releasing massive amounts of oxygen-free radicals. Macrophages also absorb viruses, cellular debris, and dust particles in the lungs. Macrophages circulate in the extracellular fluid continually, and their phagocytic functions augment those of the specialist phagocytic cells found in the liver, spleen, and bone marrow. Monocytes (an undifferentiated leukocyte) in the blood squeeze through capillaries to penetrate connective tissues in reaction to infection. Monocytes are converted into extra macrophages at the site of infection. Neutrophils are leukocytes that, like macrophages, phagocytose germs and destroy them. Furthermore, neutrophils produce chemicals (some of which are similar to home bleach) that kill both other bacteria and neutrophils. Natural killer cells don’t directly target invading microorganisms. These instead destroy virus-infected cells in the body. And kill through piercing the target cell’s plasma membrane instead of phagocytosis. Perforins are proteins that are secreted by natural killer cells that embed into the target cell’s membrane, generating a pore. Water rushes into the target cell, causing it to expand and rupture. Natural killer cells fight cancer cells as well, frequently before they grow into a recognizable tumor. Natural killer cells are one of the body’s most powerful cancer-fighting defenses.

7.11.3. The Immune Response: The Third Line of Defense Few others can go through infancy without getting infected with something. Chicken pox, for example, is an infection that most of us get before reaching our adolescence. It is called a childhood illness because most of us get it as children and never get it again. You are typically immune to the sickness after you have experienced it. This immunity is provided by certain immune defense systems.

Discovery of the Immune Response In 1796, an English country doctor called Edward Jenner did an experiment that marked the start of immunology research. Smallpox was a frequent and lethal illness back then. Jenner did notice, though, that milkmaids who had contracted a much milder form of “the pox” known as cowpox (probably from cows) seldom contracted smallpox. Jenner went out to investigate the theory that cowpox provided immunity against smallpox. He infected them with cowpox, and many of

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them developed immunity against smallpox, just as he expected. Smallpox and cowpox are now known to be produced by two distinct viruses with similar surfaces. Jenner’s patients who were injected with the cowpox virus developed a response that protected them from later infection with the smallpox virus. Vaccination refers to Jenner’s technique of injecting an innocuous germ to impart resistance to a deadly one. Modern efforts to create resistance to malaria, herpes and other illnesses sometimes include antigen delivery using a harmless vaccinia virus similar to the cowpox virus. Many years passed before anybody discovered how exposure to an infectious pathogen might give disease resistance. The famed French scientist Louis Pasteur took an important step in answering this question more than a half-century later. Pasteur was researching fowl cholera when he obtained a culture of germs from ill chickens that, when injected into healthy birds, would cause the disease. He mistakenly placed his bacterial culture on a shelf before leaving on a twoweek vacation. Once he returned, he injected this ancient culture into healthy birds and discovered that it had been weakened; the injected birds got just mildly unwell and eventually recovered. Surprisingly, such birds did not become ill when infected with fresh chicken cholera. They stayed healthy even after being fed huge quantities of active fowl cholera germs, which caused the sickness in control hens. Clearly, something in the bacterium might elicit immunity as long as the germs did not kill the animals first. We now know that chemicals projecting from the surfaces of bacterial cells elicited active immunity in chickens.

Key Concepts of Specific Immunity An antigen is a chemical that causes an immunological reaction. Antigens are huge, complex molecules, such as proteins, that are foreign to the body and are often found on the surface of infections. A big antigen may be composed of multiple pieces, each of which elicits a distinct immunological response. The many sections are known as antigenic determinant sites in this example, and each functions as a distinct antigen. On the surface of certain lymphocytes are receptor proteins that identify an antigen and guide a particular immune response against either the antigen or the cell that contains the antigen.

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B lymphocytes produce antibodies in response to antigens. Antibody proteins are released into the blood and other bodily fluids, providing humoral immunity. (The term humor is used here in its old connotation, referring to a bodily fluid.) T cells are lymphocytes that do not make antibodies but instead target the cells that carry the particular antigens. Cell-mediated immunity is therefore produced by these cells. The body is protected by specialized immune responses in two ways. To begin, a human can develop immunity by being exposed to a pathogen (disease-causing substance) and perhaps contracting the disease. This is acquired immunity, similar to the resistance to chicken pox gained after suffering the disease as a child. This technique is also known as active immunity. Second, an individual can get immunity by acquiring antibodies from another person. This happened to you when you were born when antibodies produced by your mother were given to you via the placenta. Passive immunity refers to immunity obtained in this manner.

7.12. CELLS OF THE SPECIFIC IMMUNE SYSTEM White blood cells, or leukocytes, play a role in the body’s immunological defense processes. Leukocytes comprise phagocytic neutrophils, eosinophils, basophils, and monocytes, as well as two kinds of lymphocytes (T cells and B cells), which are not phagocytic but are crucial to the specific immunological response, the third line of defense. T cells control the cell-mediated response, while B cells control the humoral response. The cells travel from the bone marrow to the thymus (thus the abbreviation “T”), a gland located immediately above the heart. They learn to recognize bacteria and viruses based on antigens found on their surfaces. Tens of millions of T lymphocytes are produced, each specialized in the recognition of a specific antigen. No intruder can avoid being detected by a few T cells. T cells are classified into four types: inducer T cells, helper T cells (often represented by the letter TH), cytotoxic (“cell-poisoning”) T cells (often represented by the letter TC), and suppressor T cells (often represented by the letter Ts).

Virus-infected cells are lysed, and suppressor T cells stop the immunological response. Unlike T cells, B cells mature in the bone marrow rather than the thymus. (B cells are so termed because they were discovered in a chicken area known as the bursa.) B lymphocytes are discharged from the bone marrow

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and circulate in the blood and lymph. Individual B cells, like T cells, are specialized to identify certain foreign antigens. When a B cell encounters the antigen to which it is responding, it begins to divide quickly, and its children differentiate into plasma cells and memory cells. Each plasma cell is a little factory that produces antibodies that attach flags to that antigen wherever it occurs in the body, designating any cell containing the antigen for death. Pasteur observed immunity as a function of such antibodies and the ongoing existence of the B cells that created them.

7.13. INITIATING THE IMMUNE RESPONSE Assume you’ve recently come down with a fever to learn how well the third line of defense works. Tiny water droplets absorbed into your respiratory system carry influenza viruses into your body. The viruses attack and destroy mucous membrane cells if they avoid being entangled in the mucus lining the respiratory membranes (first line of protection) and being consumed by macrophages (second line of defense). At about this stage, macrophages launch an immunological defense. Macrophages examine the surfaces of all cells Macrophages come into contact with. Most vertebrate cells have glycoproteins on their surfaces, which are created by a collection of genes known as the major histocompatibility complex (MHC). These glycoproteins are known as MHC proteins or, more specifically, human leukocyte antigens (HLA). The genes that encode MHC proteins are extremely polymorphic (having numerous forms); for example, the human MHC proteins are determined by the most polymorphic known genes, with about 170 alleles apiece. Because no two people have the same mix of alleles, the MHC proteins are unique to each person, much like fingerprints. As a consequence, the MHC proteins on tissue cells act as self-identifiers, allowing the individual’s immune system to discriminate between its own cells and foreign cells, a process known as self-versus-nonself identification. T cells in the immune system use MHC proteins on the cell surface to determine whether a cell is a self or nonself. When a foreign particle, such as a virus, enters the body, it is partially digested by cells. The viral antigens are processed within the cells and transported to the plasma membrane’s surface.

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Antigen-presenting cells are the cells that carry out this activity. The processed antigens form a complex with the MHC proteins at the membrane this allows T cells to detect antigens provided to them. Antigens linked with MHC proteins are therefore specific to each person, much like fingerprints.

7.14. THE IMMUNE SYSTEM CAN BE DEFEATED 7.14.1. T cell Destruction: AIDS Attacking the immunological process itself is one method of overcoming the vertebrate immune system. CD4+ T cells are helper and inducer T cells. As a result, any infection that inactivates CD4+ T cells renders the immune system incapable of responding to any foreign antigen. Because of this, acquired immune deficiency syndrome (AIDS) is a fatal illness. Since it identifies the CD4 coreceptors associated with these cells, the AIDS retrovirus, known as human immunodeficiency virus (HIV), undertakes a direct attack on CD4+ T cells. In at least three ways, HIV’s attack on CD4+ T cells cripples the immune system. First, HIV-infected cells die only after replicating viruses that infect other CD4+ T cells, until the whole CD4+ T cell population is eliminated. CD4+ T cells comprise about 60 to 80 percent of circulating T cells in a healthy person; in AIDS patients, CD4+ T cells are frequently too scarce to detect. Second, HIV infects CD4+ T cells, causing them to release a soluble suppressor factor that prevents other T cells from reacting to the HIV antigen. Finally, HIV may inhibit MHC gene transcription, making it difficult to recognize and destroy infected CD4+ T cells and therefore preserve those cells from any lingering immune system remnants.. Such reactions to HIV infection have the cumulative effect of wiping away the human immune defense. With no immune system, any of a number of ordinarily common illnesses can be lethal. Cancer mortality becomes far more likely when there is no ability to recognize and eliminate cancer cells as they form. Indeed, AIDS was originally identified as a disease as a result of a cluster of cases of an extremely rare type of malignancy. Cancer kills more AIDS patients than any other cause. Since HIV very lately has become a human disease vector, presumably by transfer from chimps in Central Africa to humans, it is abundantly obvious that AIDS is one of the most devastating illnesses in human history. AIDS

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has a one-hundred percent death rate; no patient with AIDS symptoms has ever survived and over a few years without therapy. Strong therapy can extend life, but how longer is unknown. Nevertheless, the disease is not particularly infectious since it is spread from one person to another by the transfer of internal bodily fluids, most notably sperm and blood during transfusions. Not all people who have been exposed to HIV (as shown by anti-HIV antibodies in their blood) have contracted the illness. Until lately, only one successful therapy for halting the course of the disease entailed the use of medications like AZT that impede the function of reverse transcriptase, the enzyme required by the virus to generate DNA from RNA. However, a new type of medication has just been accessible that inhibits protease, an enzyme required for viral assembly. Treatments that involve a mix of reverse transcriptase inhibitors and protease inhibitors appear to reduce HIV levels, but they are quite expensive. Attempts to generate an AIDS vaccine continue, both by splicing sections of the HIV surface protein gene into the vaccinia virus and by developing a safe strain of HIV. Though intriguing, such techniques still have to be effective and are hindered by the fact that various strains of HIV appear to have distinct surface antigens. HIV, like the influenza virus, engages in antigen shuffling, making vaccine development problematic.

7.15. CONCLUSION A nervous system is an organized group of cells specialized for the conduction of electrochemical stimuli from sensory receptors through a network to the site at which a response occurs. In the centralized systems of higher invertebrates and vertebrates, a portion of the nervous system has a dominant role in coordinating information and directing responses. This centralization reaches its culmination in vertebrates with a welldeveloped brain and spinal cord. That system was the nervous system, which is based upon the almost instantaneous transmission of electrical impulses from one region of the body to another along with specialized nerve cells called neurons. Since communication from one cell to another by chemical means was too slow to be adequate for survival, a system evolved that allowed for faster reactions. Once detected, these internal and external changes must be analyzed and acted upon in order to survive.

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All living organisms are able to detect changes within themselves and in their environments. In the diffuse type of system, found in lower invertebrates, there is no brain, and neurons are distributed throughout the organism in a netlike pattern. This chapter also discussed various aspects of the invertebrate and vertebrate systems (the latter received more focus) for a reader to grasp the basics.

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REFERENCES 1.

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3.

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5.

Erulkar, S., 1998. nervous system | Definition, Function, Structure, & Facts. [online] Encyclopedia Britannica. Available at: [Accessed 3 July 2022]. HASAN LINJAWI, D., n.d. IMMUNOLOGY. [online] Mlinjawi.kau. edu.sa. Available at: [Accessed 3 July 2022]. Humphrey, J., 2001. immune system. [online] Encyclopedia Britannica. Available at: [Accessed 3 July 2022]. Okoye, A. and Picker, L., 2013. CD4+T-cell depletion in HIV infection: mechanisms of immunological failure. Immunological Reviews, [online] 254(1), pp.54-64. Available at: [Accessed 3 July 2022]. Sadava, D., Craig Heller, H., Hillis, D. and Berenbaum, M., 2009. Life: The Science of Biology. 9th ed. [ebook] WHFreeman,2009. Available at: [Accessed 3 July 2022].

INDEX

A

C

Acquired immune deficiency syndrome (AIDS) 223 algae 98, 103 ammonoids 168 animals 33, 36, 37, 43, 47, 61, 62 antigen-reactive lymphocytes 198 Arabidopsis thaliana 68 Arctic tundra 37 Atoms 3, 5, 7

carbohydrates 37, 39, 41, 46, 56, 57, 63 carbon dioxide 67 cell theory 33, 34, 53 cellular biology 2, 29 cellular biosynthesis 67 cellular contents 66 cell wall 98, 103, 104, 105, 106, 108, 109, 113, 120 cerebral cortex 205, 213 chemical bond 5 Chemoreceptors 212, 213 chloroplasts 66, 67, 77 Chordata 132, 133, 135 chromosome 67, 68, 77, 78, 81, 82, 91, 92, 93, 95 circulatory system 6 cod 133 communication 131, 142, 143, 159 coral reef 37 cortex thrice 205 ctenophores 163 cytoplasm 66, 67, 68, 76 cytosol 67

B bacteria 33, 37 biodiversity 162, 166, 169, 170, 172 biological diversity 162, 164, 165, 166, 168, 169, 170 Biology 1, 2, 11, 21, 22, 29, 30 biosphere 7 birch 99 Blood pressure 211 brachiopods 163, 168 brains 36 bryozoans 168

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D Deoxyribonucleic acid (DNA) 5 deserts 98 developmental biology 2, 29 Disaccharides 41 E echinoderms 163, 168 ecosystem 7, 8 ecosystem diversity 162 electroencephalogram (EEG) 206 endoplasmic reticulum 66 environment 32, 35, 36, 37, 47, 51, 52, 54, 55, 56, 57, 58, 59, 60 eukaryotes 7, 10, 11, 20 evolutionary biology 2, 29 excitatory postsynaptic potential (EPSP) 210 F feeding 131 ferns 98 flowers 98, 99, 101, 110, 112, 119, 121, 122, 124, 125, 126 food 5, 21 food webs 169 freshwater 163, 165 fungi 37 G Gazella bilkis 171 gene 67, 69, 74, 75, 81, 88, 89, 90, 91, 93, 94, 95 Genetic diversity 162 genetics 2, 29 genome 68, 89 geographical diversity 162 global diversity 169

Golgi apparatus 66 gymnosperms 98, 102, 110, 112, 126 H Hapalemur aureus 171 Hearts 36 Homo sapiens 165 hornworts 98 human immunodeficiency virus (HIV) 223 human leukocyte antigens (HLA) 222 hydrogen atoms 41, 45 I immune system 194, 196, 197, 216, 218, 222, 223, 226 Immune system cells 196 innate immunity 196 J jellyfish 163 L lipids 37, 39, 42, 44, 45, 46, 48, 54, 55, 56, 57, 63 liverworts 98 living organisms 2, 6, 7, 10, 11, 29 lycopods 98 lysosomes 66, 84 M mackerel 133 macromolecule 5 major histocompatibility complex (MHC) 222 mammals 131, 132, 133, 134, 135, 140, 144, 148, 152

Index

Mechanoreceptors 210 Megamuntiacus vuquangensis 171 meristem 98, 107, 112, 113, 114, 122, 126 metabolic functions 5 Microorganisms 2, 29 microscopic bacteria 3 mitochondria 66, 67, 77 Mobility 130, 131, 159 molecule 5, 7, 8, 9, 15, 19 Monosaccharides 41, 42 Morphology 166, 177 mosses 98 multicellular eukaryotes 130, 159 multicellular eukaryotic organisms 130 Muntiacus truongsonensis 171 mushrooms 32 Mycoplasma genitalium 68 Mycoplasmas 66 myofilament-based mobility 131 N Natural killer cells (NK-cells) 197 nematodes 164, 171 Nerve fibers 195 nervous system 194, 195, 198, 200, 203, 208, 211, 224, 226 neurological systems 131, 142 neuronal networks 194 neurons 195, 196, 198, 199, 202, 203, 204, 205, 206, 208, 209, 211, 212, 213, 215, 224, 225 nucleic acids 37, 39, 42, 48, 49, 50, 51, 63 nucleotides 9

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O oak 99 Oligosaccharides 41, 42 Organelles 66 Organs 6, 8 organ system 7 P peculiar system 194 peripheral nervous system 195 photosynthesis 66 Phytotaxa 98 plant biology 98 plant cells 66, 84, 98, 103, 104, 105, 106 Plants 37, 57 plasma membrane 200, 201, 219, 222 pollen grains 98, 120, 122, 123 pollock 133 polymers 37, 38, 39, 41, 43, 44, 48, 58 Polysaccharides 41, 43 Prokaryotes 6, 10 proteins 37, 39, 40, 42, 43, 48, 50, 54, 55, 56, 60, 61, 62, 63 Pseudonovibos spiralis 171 Pseudoryx nghetinghensis 171 R ribonucleic acid (RNA) 68, 74 rivers 98 Root system 98 S salmon 133 saltatory conduction 202 Schwann cells 199

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seed 32 sensory reception 195, 204 sensory systems 194, 209, 210 single-celled organisms 3, 6, 10, 21 Single-celled prokaryotes 7 sipunculans 163 sodium ions 200 species diversity 163, 169, 170, 171 spinal cord 195, 198, 203, 206, 208, 211, 224 sporozoan protists 164 stem 98, 101, 102, 110, 113, 114, 119, 126 stomachs 36 sunlight 66 T tiniest organisms 4 tissue 7

Tissue system 98 topographic diversity 169 tropical rainforest 37 tuna 133, 141, 156 U unicellular eukaryotes 130 V vertebrates 195, 198, 199, 203, 209, 211, 212, 213, 215, 224 virology 2, 29 viruses 32, 62 W water 7, 18, 27 Y yeast 66, 70, 79