Chemicals of Life

National Institute of Science Communication Dr K. S. Krishnan Marg New Delhi 110012, India

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Chemicals of Life Parol R. Sheth

©National Institute of Science Communication First Edition: January 2000 ISBN: 81-7236-198-X

Foundation of Biotechnology Series

Book No. 5 Volume Editor

Parvinder Chawla

Cover Design Illustrations

Pradip Banerjee Neeru Vijan, Malkhan Singh, Harjit Singh and Yogesh Kumar Supriya Gupta, Rohini Raina, Shiv Kumar Marhkan, Neeta Sahney and J. Singh C.M. Sundaram, S. Bhushan, S.c. Mamgain, G.C. Porel, Rajbir, Suraj Pal and Om Pal

Production Printing

Designed, Printed and Published by National Institute of Science Communication (CSIR) Under the project, Dissemination of Biotechnology Information sponsored by Department of Biotechnology (Govt. of India)

Price: Rs. 30/-

Foreword A variety of lifeless biomolecules are found in living organisms which serve as building blocks of the intricate structure of cells. Called the chemicals of life, they are present in four forms: proteins, nucleic acids, carbohydrates and lipids. The cellular machinery of living organisms is, in fact, organized around these biomolecules all of which have specific shapes and.functions. Collections of these inanimate chemicals of life constitute living organisms where they interact with one another to maintain and perpetuate the living state. The National Institute of Science Communication (NISCOM) is bringing out popular science books under a new series entitled 'Foundations of Biotechnology' as a part of the project on 'Dissemination of Biotechnological Information' sponsored by the Department of Biotechnology CD Bn, Govt. of India. This venture is yet another step taken by the Institute to make both students and laymen understand the science underlying the wonders achieved by applying hi-tech methods. Attractively illustrated and written in extremely simple and lucid style, these books would certainly help in percolating the awareness of biotechnology down to the school level. By introducing the vast subject of biotechnology especially to children of classes VII to X, it is hoped that many would be initiated to take up this subject for an adv~nced study as it is the time for them to decide upon their career options.

Keeping up with its major mandate of disseminating scientific information to large masses, NISCOM has undertaken this very important venture of popularizing the basic concepts essential to the understanding of sophisticated biotechniques. I firmly believe that these books would enhance the reader's curiousity to know more about this interesting multidisciplinary subject.

rl~~ (Manju-Sharma) Secretary to the Govt. of India Deptt. of Biotechnology

Preface Writing 'Chemicals of Life' was the most satisfying experience for me. It reminded me of my student days when I got introduced to biochemistry, a subject which deals with the study of biomolecules that control the biological functions of any living system. These chemicals of life occur in the cells of all plants and animals including us. Living things are full of chemical compounds including the many common elements such as hydrogen, nitrogen and oxygen. Biochemists try to determine the structure of such compounds and establish their biological functions. All organisms contain compounds which help form the various substances in a cell and enable it to function properly. Certain inorganic substances such as minerals and water also play a role in the cell's growth and maintenance. The book attempts to make both students and laymen understand the basis of living things - the chemicals of life without which there would be no life on this earth. The purpose of writing this book is to introduce the basic concepts of biochemistry, generate curiosity, inculcate interest in the subject and disseminate scientific information to masses.

Acknowledgements I am indebted to many persons who assisted me in the completion of this book. I heartily thank Mr S.K Nag, Chief Investigator, DBT project and Head, Popular Science Division of NISCOM for extending an invitation to write this book. My thanks are also due to Dr. , G.P. Phondke, Director, NISCOM. I appreciate the help rendered to me by Ms Parvinder Chawla for the excellent editing of the book. I cannot forget one name which needs a special mention, late Dr. Anil R Sheth, my "guru" and my Ph.D. guide who has imparted me constant inspiration. Thanks to Rajen, my loving husband for extending calmness and quiet confidence during the writing of this book. The acknowledgement would rather be incomplete without the mention of my son Miten who was ever willing to bear with my busy writing schedules. Bringing out any publication is not a one-man show. It is a joint effort of many individuals having expertise in varied fields. This includes the team of competent artists and production and printing staff of NISCOM. I am indebted to all of them for a splendid job.

ontents Origin of Life

1

Water - the Life Source

7

Proteins - the Vital, Functional Biomolecules

... 13

Nucleic Acids - the Threads of Life

... 26

Carbohydrates - the Energy Molecules

... 32

Lipids - the Large Oily Molecules

... 40

Vitamins, Minerals & Hormones

... 49

Oripin of ,Cife cules. When these molecules are isolated and ~l living things are composed of lifeless molexamined individually, they conform to all the physical and chemical laws that de.scribe the behaviour of inanimate matter. Yet, living organisms that contain the lifeless molecules, possess extraordinary attributes not shown by collections of inanimate matter. They are complicated and highly organized; they possess intricate internal structures and contain many kinds of complex molecules. Furthermore, living organisms occur in millions of different species. Each component part of a living organism appears to have a specific purpose or function. Even individual chemical compounds in cells have specific functions. Living organisms have the ability to extract, transform and use energy from their environment, either in the form of organic nutrients or the radiant energy of sunlight. With the help of these qualities, they can also build and maintain their own intricate energy-rich structures to do mechanical work, locomotion and transport materials. But the most extraordinary attribute ofliving organisms is their capacity for precise self-replication.

2

Chemicals of Life

Philosophers believe that living organisms are endowed with a mysterious and divine life-force - "vitalism". But modern science seeks a rational explanation to determine how the collections of inanimate molecules that constitute living organisms interact with one another to maintain and perpetuate the living state. Let us see how these lifeless molecules came into being. It is a long story. This is the chemical evolution which refers to the origin and development of organic molecules from the inorganic precursors in the presence of energy. We now know that the earth was first formed about 4,800 million years ago. It is believed that chemical evolution took piace on the earth for at least the first 1,000 million years of its life. The first living cells arose perhaps about 3,500 million years ago. Then began the process of biological evolution which still continues. It is believed that the earliest living cells used organic compounds of the rich organic "soup", as building blocks for their own structures. Gradually, through the ages, the organic compounds of the primitive sea were consumed and the living organisms began to learn how to make their own organic biomolecules. Scientists suggest that the universe came into being some 20 billion years ago. An explosion hurled hot, energy-rich subatomic particles in space. Gradually, the universe cooled, these elementary particles combined to form positively charged nuclei to which negatively charged electrons were attracted. Thus were created the hundred or more chemical elements. Only 27 of these natural elements are essential for different •..

I

Origin of life

Living organisms arose from lifeless molecules

3

4

Chemicals of life

forms of life. Carbon, hydrogen, nitrogen and oxygen are the major elements that combine to form a great variety of molecules. These are the biomolecules found in living organisms. Even the simplest and smallest cells, the bacteria, contain a very large number of different organic molecules. It is a mind-boggling fact that a single cell of the common bacterium Escherichia coli contains about 5,000 different kinds of organic compounds. And all these biomolecules have specific functions in cells. Living cells are self-regulating chemical engines, tuned to operate on the principle of maximum economy. They capture, store and transport energy in a chemical form, largely as adenosine triphosphate (ATP). ATP functions as the major carrier of chemical energy in the cells. ATP can transfer its energy to certain other biomolecules and become the energydepleted adenosine diphosphate (ADP). The molecular machinery of the cells is made up of organic compounds of carbon. Perhaps, the versatility of the element carbon is the major factor in the selection of carbon compounds during the origin and evolution of living organisms. Water is the most abundant single compound in all types of cells and organisms. Inorganic salts and mineral elements also constitute a small fraction of solid matter in the cells. However, nearly all the solid matter in all kinds ofcells, is organic and is present in four forms: proteins, nucleic acids; DNA and RNA, carbohydrates and lipids. These four classes of biomolecules are all relatively large structures with high

5

Origin of Life

Lipids Proteins

Nucleic acids

Carbohydrates

The four classes of biomolecules

6

Chemicals of life

molecular weights and are therefore called macromolecules. Surprisingly, over 90 per cent of the solid organic matter of living organisms, containing many thousands of different macromolecules, is constructed from only about three dozen different kinds of simple, small organic molecules. These simple organic compounds of which living organisms are constructed, are unique to life.They differ in shape and size and their chemical reactivity enable them not only to serve as building blocks of the intricate structure of cells,- but also to participate in their dynamic, self-sustaining transformations of energy and matter. They are the chemicals of life and they perform the most difficult and intricate tasks. Nearly all the biomolecules are the derivatives of hydrocarbons, compounds of carbon and hydrogen. One or more hydrogen atoms of hydrocarbons may then be replaced by different kinds of functional groups to yield different families such as alcohols, amines, acids etc. The functional groups attached to the carbon chains determine their chemical properties. Most ofthe biomolecules are polyfunctional containing two or more different kinds offunctional groups. For example, amino acids have an amino group and a carboxyl group. The chemistry of living organisms is organised around these biomolecules all of which have specific shapes and dimensions.

Water -

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different from that of the inanimate matter of the chemical of living matter is very earth's crust.composition The difference in the elementary composition of the earth's crust is even more striking when we consider the composition by weight of the dry or solid portion of living matter, excluding its water content. Water in living systems make up 70 per cent or more of the weight of most forms of life. Carbon makes up from 50 to 60 per cent by weight of the solid matter of living cells, nitrogen almost 8 to 10 per cent, oxygen about 25 to 30 per cent and hydrogen almost 3 to 4 per cent. In contrast, carbon, hydrogen, oxygen and nitrogen, the elements predominating in living organism together make up much less than 1 per cent of the mass of the earth's crust. But on the other hand, eight of the ten most abundant elements in the human body are also among the ten most abundant elements in seawater. Perhaps, seawater was the liquid medium in which living organisms first arose in the early history of the earth. Water is thus the great mother of us all. It pervades aU portions of every cell. Water is the medium in which transport of nutrients, the reactions

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Chemicals of life

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What is human body composed of? Gosh, 65%of it is just water! (inset)

Water - the Life Force

9

of metabolism and the transfer of chemical energy occurs. In fact, all aspects of cell structure and function . are necessarily adapted to the physical and chemical properties of water. We take water for granted as aN inert, bland liquid convenient for practical purposes. However, we cannot live without water. You probably think your body feels firm and solid because it is full of strong bones and muscles. But, in fact, 65 per cent of our body is made up of water. A 60 kilogram man has approximately 34 litres of water in his tissues. Our blood is mostly made of water. Water also helps to keep our muscles and joints running smoothly. Some of this water is lost because we breathe it out. We lose water when we sweat and pass out urine. We need to drink about 2-3 litres of water everyday. However, a person's water requirements vary considerably according to the climate, dietetic habits, activities and body structure. Water is a simple compound containing two parts of hydrogen with one of oxygen. The structural formula of water is H20. The water that we drink from the tap comes a long way. It has travelled as rain through the atmosphere, has flowed along the ground into a river or it may have spent part of its journey underground. On the way, it picks up many substances which dissolve in it. Some substances which dissolve in water are minerals. These are chemicals in the rocks. Water contains traces of calcium, sodium, magnesium and iron depending upon the soil from which it is obtained. These are minerals that are essential for good health.

10

Chemicals of life

These minerals are often called trace elements. Fluoride is one such trace element that may be found in water. It helps us to grow strong teeth. Soft water contains small amounts of minerals and it lathers easily while hard water contains a higher proportion of calcium salts and does not lather easily. Why has the nature chosen water and not any other solvent or liquid to be a part of any living organism? Water is a chemically stable substance but has rather unusual properties. It has a higher melting point, boiling point and heat of vaporisation than most common liquids. This fact indicates that there are strong forces of attraction between adjacent water molecules because of the strong hydrogen bonding. Why should liquid water show such strong intermolecular attraction? The answer lies in the structure ofwater molecule. Each of its two hydrogen atoms shares an electron pair with the oxygen atom. The polarity and hydrogen bonding properties makes water a potent solvent. Water ionises very slightly to form H+ and OH- ions. In dilute aqueous solutions, the concentrations of hydrogen and hydroxyl ions are inversely related. The hydrogen ion concentration of biological systems is usually expressed in terms of pH. The ionisation products H+ and oH-profoundly influence the properties of many important components of cells, such as proteins, nucleic acids, lipids and enzymes. The catalytic or speeding up .activity of enzymes is strongly influenced by pH. Living organisms have been smart enough to exploit the unusual properties of water. The high specific heat of water is useful to the cell since it allows water to act

Water - the Life Force

';J- Hydrogen

11

bond

Water molecules form hydrogen bonds with one another

as a "heat buffer", keeping the temperature of an organism constant even when the air temperature fluctuates. Vie humans use the property of high heat of vaporisation of water as a means of losing excess body heat by evaporation of sweat. Plants use the property of hydrogen bonding as a means of transporting dissolved nutrients from roots up to the leaves during the process of transpiration. Even the fact that ice has a lower

12

Chemicals of life

density than liquid water and, therefore floats has important biological consequences in the life cycles of aquatic organisms. Most importantly, the biological properties of macromolecules, the proteins and nucleic acids, derive from their interactions with water molecules of the surrounding medium. All the important functions of the body depend on the presence of a proper amount of water. For example, blood plasma contains 92 per cent of water, the red blood cells contain 70 per cent of water, the digestive secretions or juices are mainly water and urine too contains about 97 per cent of water. Healthy kidneys regulate body water efficiently. With an excess there is increased formation of urine and during deprivation the secretion of urine is diminished. The colour of the urine is a practical guide to the adequacy of fluid intake; in a healthy person, pale yellow urine indicates an adequate intake. In acute vomiting and diarrhoea, excessive loss of water and electrolytes may have to be replaced. Water intake also affects the bowels. The commonest cause of constipation is inadequate intake of water. Almost all food-stuffs except pure fat contain varying amounts of water. All liquids, including tea, coffee, aerated drinks, contain water. Fruits and vegetables also contain a high proportion of water. Water which we consume in various forms is absorbed only slightly from the stomach but it is rapidly absorbed from the small intestine and to a lesser extent from the large intestine. A balance is maintained between the intake and the excretion of water. Any excess water is excreted.

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large molecules. And amongst these, proteins 'TIeconstitute major classes ofbiomolecules in cellsmatter are very the largest fraction of living in all types of cells. They constitute 50 per cent or more of the dry weight of cells in an organism. There are thousands of different proteins in each species of an organism and there are perhaps 10 million different species. Proteins consist of very long polypeptide chains having from 100 to over 1,000 amino acid units joined by peptide linkages. They are relatively large structures with high molecular weights ranging from 5,000 to over one million. The name protein in Greek is proteios which means llfirstllOF- "foremostll. They are the direct products and effectors of gene action in all forms of life and are the most versatile of all biological molecules. Their building blocks are amino acids just as glucose is the building block of starch or lipids are the building blocks of fatty acids or deoxynucleotides and ribonucleotides are the building blocks of DNA and RNA The amino acids include two functional groups; an arnino

Chemicals of life

14

Amino acid Protein

Amino acids are the building blocks of proteins

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Proteins - the Vital, Functional Biomolecules

15

group and a carboxyl group in a basic hydrocarbon chain. Regardless of the function or biological activity, proteins are built from the same basic set of 20 standard amino acids which by themselves have no intrinsic biological activity. Then, you may wonder as to what is it that gives one protein an enzymatic activity, another protein a hormone activity and still others antibody activity. How do these differ chemically? Quite simply, proteins differ from each other because each has a distinctive sequence of its amino acid 'units'. The amino acids are the alphabets protein structures, since they can be arranged in an almost infinite number of sequences to make an infinite number of proteins. All proteins contain carbon, hydrogen, nitrogen and oxygen while some also contain iron, phosphorous and sulphur. The amino acids within are linked together int(}long chains called polypeptides. A few of these are straight but most are bent into three dimensional shapes. Proteins can be divided into two major classes on the basis of their shape and certain physical characteristics - globular proteins and fibrous proteins. In globular proteins the polypeptide chain or chains are tightly folded into compact spherical or globular shapes. These are soluble in water and they diffuse readily. Nearly all of the 2,000 or more enZYlnes are globular proteins, as are the blood transport proteins, antibodies -the body's defence proteins, and nutrient storage proteins.

16

Chemicals of llie

Fibrous proteins are insoluble in water and are long and stringy molecules with the polypeptide chains extended along one axis rather than folded into a globular shape. Most fibrous proteins serve as structural proteins and have a protective role to play. Amongst all the proteins present in the body, the fibrous proteins may constitute one-half or more of the total body proteins in higher animals. They provide external protection since they are the major components of the outer layer of skin, hair, feathers, nails and horns. Fibrous proteins also provide support, shape and form, since they are major organic components of connective tissues, including cartilage, tendons, bone and deeper layers of the skin. These stringy proteins are three dimensional but they have simpler structures than those of the globular proteins. There are four kinds of fibrous proteins that have protective or structural function in animals. These include a-keratin, ~-keratin, collagen and elastin. The a-keratins are the characteristic insoluble, tough proteins found in hair, wool, feathers, scales, horns, hooves and tortoise shells. The best example of ~-keratin proteins is the silk fibroin. Collagen is the most abundant protein in vertebrates. It is found in tendons, silk fibres, blood vessels, bone and cartilage. Collagen fibres do not stretch yet they have great tensile strength. On partial hydrolysis, collagen is converted into gelatin, a soluble digestible mixture of polypeptides. Elastin is a characteristic protein of connective tissues which has elastic properties. Myosin, actin and tubulin are units of intracellular filamentous

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red blood cell production in the bone marrow, the centre part of the bone.

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Chemicals of Life

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Other globular proteins are enzymes whose biological function is to speed up the chemical reactions. They possess a specific catalytic activity. Most of the chemical reactions of organic molecules are catalysed or speeded up by enzymes. There is a family of over 2000 different enzymes, each capable of catalysing a different kind of chemical reaction. Although enzymes are proteins, some must be attached to certain non-protein molecules in order to function. Many of these non-protein molecules are memls, such as copper, iron or magnesium. These occur as trace elements in nature. Others are organic compounds called coenzymes. If a coenzyme is tightly attached to the protein part of an enzyme, the unit is called a prosthetic group. Neither the coenzyme nor the protein part of a prosthetic group can function alone.

Proteins - the Vital, Functional Biomolecules

21

Without enzymes, the chemical reactions in all living things would occur slowly or not at all, and no life would be possible. Although enzymes of different plants and animals have different protein structures, they function in similar ways. Enzyme molecules function by altering other molecules. They combine with the altered molecules to form a complex structure in which chemical reactions takes place. The enzyme remains unchanged and simply separates from the product of the reaction. Thus, enzymes serve as biological catalysts. A single enzyme molecule can perform its entire function a million times a minute. The chemical reactions occur thousands or even million times faster with enzymes than without them. All living cells make enzymes. Our body has thousands of enzymes and each kind does a specific job. Without enzymes, we cannot breathe, see, move or digest food. What happens to the food we eat? Enzymes in the digestive system breakdown food for use in the body. The pancreatic enzymes trypsin, chymotrypsin and elastase act on proteins in the duodenum. They convert the bigger molecules into small peptide chains known as the oligopeptides and amino acids. In the small intestine, oligopeptides are further digested by the intestinal cells. The intestinal enzymes, peptidases, further break down the amino acids into single amino acids and are then absorbed. Absorbed amino acids reach the liver through blood vessels where some amino acids are converted back to proteins. Others reach the tissues through blood circulation and are utilised for protein synthesis. Pepsin secreted by the

22

Chemicals of life

walls of the stomach, acts on proteins. Amylase secreted by the salivary glands in the mouth splits carbohydrates into simpler chemicals. Lipase is secreted by pancreas into the small intestine where it breaks down fats. Photosynthesis in plants also depends on the action of enzymes. Many enzymes breakdown complex substances into simpler ones, others build complex compounds from simple ones. Most enzymes remain in the cells where they were formed, but some work elsewhere. For example, lipase from pancreas travels to the small intestine to breakdown fats. An enzyme's structure can easily be destroyed by heat, acids or alkalis. Scientists believe that a high body temperature such as 42°C may cause death because the heat makes vital enzymes inactive. Many deadly poisons damage important enzymes. Hereditary diseases may occur in people born without certain enzymes or the presence of an abnormality in the synthesis of enzymes. When cells are injured, by impairment of blood supply or by inflammation, cer~ tain enzymes leak into the blood. Measurement ofthe activity of such enzymes can help diagnose important medical disorders such as, anaemia, cancer, heart disorders etc. Enzymes can also be used in therapy; to help clean wounds, dissolve blood clots, check allergic reactions and so on. If your diet lacks adequate amounts of coenzymes; the vitamins, the enzymes cannot function properly and various body disorders may develop.

23

Proteins - the Vital, Functional Biomolecules

Seeds ofmany plants store nutrient proteins that are required for the growth of the embryonic plant, such as the seed proteins of wheat, corn and rice. These are the nutrient and storage proteins. Other examples are the eggwhite protein ovalbumin or casein, the major protein of milk. Animal tissues contain ferritin that stores iron.

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Living organisms as well as our body has special soldiers for defence. The func- Structure of an antibody tion of defence proteins is to defend organisms against invasion by other species or protect them from injury. There are immunoglobulins or antibodies which are specialised proteins made by the white blood cells. The defence proteins can recognise and conquer the invading bacteria, viruses or foreign proteins from another species. Fibrinogen and thrombin prevent blood loss in an injury and help in forming clots. Snake venoms, bacterial toxins and toxic plant proteins such as ricin, also function as defence proteins. The regulatory proteins are the ones that help regulate cellular or physiological activity. This group includes many hormones, such as insulin, which regulates the sugar metabolism, growth hormone and other hormones of the pituitary gland, situated in the

24

Chemicals of Life

brain, parathyroid hormone which regulates calcium and phosphate transport. The functions of certain proteins are rather exotic. Have you heard of monellin? It is a protein of an African plant and has an intensely sweet taste. The blood plasma of some Antarctic fish contains "antifreeze" proteins which protect their blood from freezing. The wing hinges of some insects are made of resilin, a protein that has perfect elastic properties. Extraordinarily, they are all made from the same amino acids and yet they have very different properties and functions. Proteins which perform the same function in different species, for example haemoglobin which has the same oxygen-transport function in different vertebrates, are known as homologous proteins. Homologous proteins from different species usually have polypeptide chains that are identical or nearly identical in length. These proteins show sequence homology, that is, certain critical positions in the polypeptide chains of homologous proteins contain the same amino acids, regardless of the species. In other positions of homologous proteins, the amino acid may differ. The more closely related the species, the more identical would be the amino acids sequences of their homologous proteins. Thus, the sequences of homologous proteins indicate that organisl:ns containing them arose from a common ancestor but underwent changes as different species diverged during evolution. Proteins are necessary for the growth and repair of tissues.For example, the inner layer ofthe intestine, the

Proteins - the Vital, Functional Biomolecules

25

epithelium, is shed at regular intervals and proteins are required for its regeneration. Fats and carbohydrates cannot be substituted for proteins as they do not contain nitrogen. Proteins from the food, supply raw materials for the formation of digestive juices, hormones, plasma proteins, haemoglobin, vitamins and enzymes. Each gram of protein supplies 4 kilocalories. Therefore, it is very important to eat a balanced diet. Protein-rich foods such as meat, fish, poultry, eggs and milk are 'first class' proteins. Pulses, wheat, millet and vegetables are a must for growing strong and healthy.

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The ribosome moves slowly over the mRNA template step by step with the help of tRNA. The finished polypeptide chain is then released and the protein is now complete. Life isn't easy on the inside, it is nluch n10re complicated than you can imagine. How can just four bases. determine the order of 20 amino acids in a protein

Nucleic Acids -

the Threads of Life

31

chain? The answer lies in the triplet code. In other words, a group of three bases in a certain order forms the coding segment known as the codon, for a specific amino acid. Each codon is given a three-letter name that corresponds to the abbreviation of the names of its bases. It is surprising that the genetic code is universal - from bacteria to human beings. And the order of bases - the genetic code in the DNA molecule is passed on from one generation to the next. It makes an elephant give birth to an elephant and not a tiger. It makes a human being give birth to another human being and not a monkey. It is this order that determines the colour of your eyes, the shape of your ears and thousands of other traits. Sometimes things go wrong. And this is what precisely happens when changes occur in the sequences of bases in genes. These are what we callmutations. Mutations can occur through rare accidents during the duplication of DNA ]be wrong base may mistakenly become attached to a DNA half-ladder. Mutations can also occur in DNA that has been damaged by X-raysor certain chemicals. A mutation that occurs in a body cell affects only the person who carries it. However, a mutation in a sex cell can be transmitted from one generation to the next. A mutation may result in the formation of a faulty protein. For example, a mutation for the gene for haemoglobin in the blood can produce haemoglobin with a reduced abilityto transport oxygen causing sickle cell anaemia. Many such genetic disorders are known today which are caused due to defective proteins.

tlte Energ!/ Molecules

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drates is another important group of macromoleaddition to proteins and nucleic cules, present in living cells. acids, Most carbohyof the carbohydrates found in nature occur as polysaccharides with high molecular weight. Polysaccharides such as starch have molecular weights running in millions. Because polysaccharides are built from only a single kind of unit or from two different alternating units, they cannot carry encoded genetic information as nucleic acids. Carbohydrates in the form of sugar and starch represent a major part of the total caloric intake; almost 55 per cent or more for humans as well as for animals and even for many microorganisms. Green plants and other photosynthetic organisms utilize solar energy to synthesize carbohydrates from carbondioxide and water. Starch and other carbo- hydrates made by photosynthesis become the ultimate energy and carbon sources for non-photosyntheticcells of animals and microbes. Carbohydrates are polyhydroxy aldehydes or ketones or substances that yield

Carbohydrates - the Energy Molecules

33

such compounds on hydrolysis. In these carbon 'hydrates', the ratio of carbon to hydrogen to oxygen is 1:2:1. Although many carbohydrates conform to this ratio, others do not show this ratio and some also contain nitrogen, phosphorous or sulphur. There are three major classes of carbohydrates: monosaccharides, oligosaccharides and polysaccharides. Monosaccharides are simple sugars consisting of single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the 6-carbon sugar glucose. Monosaccharides are colourless, cry.stalline solids that are freely soluble in water but insoluble in oils or non-polar solvents. Most have a sweet taste. The backbone of monosaccharides is an unbranched single bonded carbon chain. Both mono and disa~charides have names ending with the suffix-ose. Monosaccharides having 4, 5, 6 and 7 carbon atoms in their backbones are called tetroses, pentoses, hexoses and heptoses. The principal monosaccharides include glucose, fructose and galactose. Glucose, a mildly sweet sugar is the most important carbohydrate in the blood. It is also called blood sugar.Fructose, an extremely sweet sugar comes from fruits and vegetables. Large amounts of glucose and fructose are found in honey. Galactose occurs in food only as a part of a disaccharide called lactose found in\milk. Oligosaccharides consist of short chains of J)1onosaccharide units joined together by covalent bonds. Most oligosaccharides having three or more

Chemicals of life

34

(a)

Glycogen

(c)

Structure of a monosaccharide (a), disaccharide (b) and a polysacccharide (c)

units do not occur free but are je>inedas side chains to polypeptides in glycoproteins and proteoglycans. The most abundant oligosaccharides, however, are the diasaccharides, which have two monosaccharide units as in cane sugar which consists of 6-carbon sugars, glu-

Carbohydrates - the Energy Molecules

35

cose and fructose joined in covalent bonds. Among the most important disaccharides are sucrose, lactose and maltose. Sucrose is table or household sugar which comes from sugarcane and juices of the sugar beet plant. A molecule of sucrose consists of a molecule of glucose linked to a molecule of fructose. Lactose also called as milk sugar, makes up about 5 per cent of cow's milk. A molecule of lactose consists of a molecule of glucose and a molecule of galactose. Maltose or malt sugar remains after the brewing process. It is used to flavour some sweets. A molecule of n1altose consists of two molecules of glucose. Polysaccharides consist of long chains having hundreds or thousands of monosaccharide units. Some polysaccharides such as cellulose have single chains whereas others such as glycogen have branched chains. The most abundant polysaccharides are starch, glycogen and cellulose of the plant world. A molecule of starch consists of hundreds or even thousands of glucose molecules joined end to end. It is the chief form of carbohydrate stored by plants. Starch occurs in food such as beans, maize, potatoes and wheat. Cellulose and glycogen also consist of many glucose molecules. Cellulose makes up much of the cell walls of plants. Cellulose is the only carbohydrate that cannot be digested by the human body and has no food value. However, it helps maintain the health and tone of the intestines and thus aids digestion. Cattle, goats and many other animals that eat plants, have bacteria in their digestive systems that can breakdown cellulose. The bodies of such animals use the digested cellulose

Chemicals of llie

36

as fuel. Glycogen, sometimes called animal starch, is the chief form of stored carbohydrates in animals. Glycoproteins are hybrid molecules. They are proteins that are linked to carbohydrates, elther single monosaccharides or relatively short oligosaccharides. Glycolipids is another class of hybrid molecules which contain carbo.hydrate groups. The most remarkable glycoprotein is the anti-freeze protein present in the blood of some Arctic and Antarctic fish and in winter flounder and codfish of the eastern coast of North America. These proteins depress the freezing point of water, allowing the fish to tolerate the low temperatures of polar seawater. Animal cell surface too contains glycoproteins: in the. cells lining the intestine there lies a very thick carbohydrate-rich coat called the glycocalyx or "fuzzy coat". One of the best known membrane glycoproteins is the glycophorin of the red blood cell membrane which contains almost 50 per cent carbohydrate molecules attached to a polypeptide chain. How does our body use carbohydrates? Only monosaccharides can enter the bloodstream directly from the digestive system. The rest of the carbohydrates have to be br.oken down into simple sugars. Digestion of carbohydrates begins in the mouth. Chewing breaks down the cellulose envelope and makes starch and sugar readily available for subsequent digestion. Saliva contains a starch-~plitting enzyme ptyalin or salivary amylase which converts starch into simple sugars and maltose. The digestive action of ptyalin is \

Carbohydrates -

Glucose

Sucrose

37

the Energy Molecules

Starch

Lipids

Proteins

I

Mineral salts & Vitamins Water Cellulose

Saliva

Gastric juice

Pancreatic juice

Intestinal juice

CD

CIl

oU

,:g

a

u.

CIl

'"

o 2

Various enzymes secreted by different glands aid in digestion of food

38

Chemicals of life

continued in the stomach until acidity of the gastric juice rises and stops the action. Carbohydrates are mainly digested in the small intestine. The enzyme, amylase of the pancreatic juice rapidly converts starch into maltose. Sucrase or invertase, maltase and lactase, enzymes present in the intestinal cells convert disaccharides into monosaccharides. The breaking down process occurs in the small intestine after which the blood transports the simple sugars to the liver. The liver changes fructose and galactose into glucose, which is then carried by the blood to all the cells of the body. The cells use glucose as fuel for the muscles and nerves to build and repair body tissues. The liver changes excess glucose into glycogen and stores it. Only when the level of sugar in the blood is low, the liver changes glycogen back into glucose and releases it into the, blood. Glycogen is also stored in the muscles as an emergency reserve of energy. Some of the glycogen is changed back into glucose when the body needs energy quickly. Carbohydrates are sometimes mistakenly termed as "junk" foods without recognising their nutritive value as suppliers of energy. Apart from supplYing ready fuel, carbohydrates also playa role in the proper functioning of the liver, the central nervous system and heart and muscle contraction. The mechanism of the control of blood sugar levels is associated with the hormone insulin secreted by the pancreas.

Carbohydrates - the Energy Molecules

39

Carbohydrates form our staple food; rice, wheat, bajra, jowar, pulses - moong dal, tuvar dal etc. Foods high in carbohydrate content include bananas, bread, macaroni, potatoes, sweet potatoes, sugar, honey and jaggery. Some sources of carbohydrates such as fruits, vegetables and whole cereal grains also contain important vitamins and minerals. Most sweets and soft drinks have a high sugar content, however, they serve only as a source of energy for the body and so do not provide the health benefits of the other carbohydrate foods.

£ipids -

tlte £arpe

Oil!! Molecules

'ne

the lipids. They play an important role in cell other important groupLipids of macromolecules are structure and function. are concentrated source of food energy and yield about twice as many calories as an equal weight of protein or carbohydrate. Many kinds of organisms store food in lipid form. For example, the seeds of many plants store lipids as food reserves for their embryos. The bone marrow, the tissues beneath the skin and those surrounding the body organs consist mostly of stored lipids. Lipids dislike water. Hence they are, water-insoluble. They are oily or greasy organic substances that can be extracted from the cells and tissues by non-polar solvents such as chloroform and ether. The most abundantly found lipids are the fats or triacylglycerols which are the major fuels for most organisms. Indeed, they are the most important storage form of chemical energy. The other class of lipids includes the polar lipids that are the major components of cell membranes. Membranes are simply not inert "skins" surrounding

Lipids - the Large Oily Molecules

41

the cells. They contain many important enzymes and transport systems. Moreover, on the outer surface of cell membrane are located many different recognition or receptor sites that can recognize other cells. These can bind hormones, and sense other types of signals from the external environment. Most of the properties of cell membranes are reflections of their polar lipid content. There are several classes of lipids and each has a specific/biological function. Fatty acids are the characteristic building- block components of most lipids. Fatty acids are long-chain organic acids having from 4 to 24

Fatty acids . ....

Hydrophobic tail

Hydrophilic head, Water .....

.:

Lipids are oily and water insoluble

42

Chemicals of life

carbon atoms. They have a singly carboxyl or acid group and a long non-polar hydrocarbon 'tail' which gives these lipids their water-insoluble and oily nature. Nearly all fatty acids in nature have an even number of carbon atoms. Amongst these, those with 16 and 18 carbons are the most abundant. Lipids are classified into two classes, according to their structure. Simple lipids contain only carbon, hydrogen and oxygen. They consist of an alcohol in combination with certain organic acids containing a variable number of carbon atoms. The most common type of simple lipid, the triacylglycerols or triglycerides (fat) contain one molecule of an alcohol called glycerol and three molecules of fatty acid. These fats include butter, lard or pig fat, tallow or beef and mutton fat, blubber or whale fat, castor oil, coconut oil and olive oil.Waxes are another group of simple lipids containing an alcohol moleculeJhat is larger than the glycerol molecule. Triacylglycerols are the major components of storage or depot fat in plant and animal cells but are not comD;lonly found in membranes. Simple triacylglycerols are tristearoyglycerol (tristearin); tripalmitoyglycerol (tripalmitin) and trioleyglycerol (triolein), which contain stearic acid, palmitic acid and oleic acid respectively. Triglycerols containing two or more different fatty acids are called mixed triacylglycerols. Triacylglycerols undergo hydrolysis when boiled with acids or bases or when acted upon by the enzyme lipase. The primary chemical reaction that is involved in making household soap from triacylglycerols is hy-

lipids - the Large Oily Molecules

43

drolysis of triacylglycerols with sodium hydroxide or potassium hydroxide called saponification or soap formation. In most animal and plant cells, triacylglycerols occur as microscopic oily droplets, finely dispersed and emulsified. Large amounts oftriacylglycerols are stored in the adipocytes or fat cells; the Adipose tissue constitutes specialised cells of the confat storage cells nective tissue of animals, in the form of fat droplets which fill almost the entire cell volume. Fat cells are found under the skin, in the abdominal cavity and in the mammary glands. In fat people, many kilograms of triacylglycerols are deposited in fat cells of the body, sufficient to supply basal energy needs of the body for several months. In contrast, our body can store less than a day's energy supply in the form of glycogen. Triacylglycerols yield over twice as much energy gram for gram, as carbohydrates. In some animals, triacylp-Iycerolsstored under the skin serve a double purpose, .both as important energy storage depots and an insulation against very low temperatures. Seals, walruses, penguins and other Arctic and Antarctic aninlals are amply padded with triacylglycerols. Waxes are included in the simple lipid class of mole-

cules. In vertebrates, waxes are secreted by skin glands

44

Chemicals of life

as a protective coating to keep the skin pliable, lubricated and waterproof. Hair, wool and fur are also coated with waxy secretions. Birds, particularly waterfowl, secrete waxes in their preen glands to·make their feathers water-repellent. The leaves of many plants are coated with a protective layer of waxes. Waxes are formed as well as used in very large amounts in marine life, especially in plankton organisms. Here, wax serves as the chief storage form of caloric fuel. Since some whales, herring, salmon and many other marine species consume planktons in large amounts, waxes are major food and storage lipids in oceanic food chains. Complex lipids have a more complicated structure than simple lipids. They include phospholipids that contain phosphorous, steroids that are made up of four rings of carbon atoms joined together, and other compounds such as glycolipids which contain lipids with one or more sugar molecules. Other complex lipids include fat-soluble vitamins such as vitamins A,D,E and K and terpenes, and yellow pigments like carotene. Phospholipids are found in all bacteria and in the cells of plants and animals. They are most plentiful in sperm, eggs, embryos and brain cells. A phospholipid molecule contains a molecule of glycerol, a phosphate ion and two molecules of fatty acid. Most phospholipids also contain a compound with nitrogen in it. Some contain inositol, a substance found in vitamin B complex. Phospholipids are often called as polar or charged lipids as their structures have polar heads and nonit