Life ; From Cell to Cell 8172360371, 9788172360375

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LIFE: FROM CELL TO CELL

BALPHONDKE

Publications & Information Directorate (CSIR) Dr. K.S. Krishnan Marg New Delhi -110 012

Life: From Cell to Cell BalPhondke

© Publications & Information Directorate First Edition: January 1991 Second Edition: June 1992 Reprinted: October 1995 ISBN: 81-7236-037-1

Cover Design Illustrations

Pradip Banerjee Pradip Banerjee, Neeru Sharma, Shushila Vohra, P.R. Mehta, Mohan Singh and Neeru Vijan

Designed, Printed and Published by Publications & Information Directorate (CSIR) Dr. K.5. Krishnan Marg, New Delhi -110012

Price: Rs. 20/-

To DEEP AU and HARSHAL Who Have Set Out On A New Ufe Together

-Baba

Acknowledgements Like human beings books too have their own gestation periods. This book has had a long one. It was first conceived a few years ago, remained in a state of suspended animation and was brought to life again l\OW. Naturally, it needed considerable reshaping. During these various stages, it was read by friends like Ram Sathe, Achyut Thatte, Biman Basu and Raju Bhisey. Their numerous comments and suggestions, not always followed, have resulted in adding sparkle to the book. The responsibility for the many murky spots that might have still remained is, however, entirely mine. The impetus for bringing it out of cold storage was provided by Drs. H.R. Bhojwani and Sushil Kumar who strongly supported the proposal of producing a set of short popular science monographs as a part of the programme in celebration of the Golden Jubilee Year of the Council of Scientific and Industrial Research, 1991 - 1992. This book is being published as a prototype of this series of monographs. Though a book is sired by the author, it is nursed by a number of midwives who look after several aspects of production like copy-editing, composing, art work, proof reading etc. Kollegala Sharma, Pradeep Banerjee, A.S. Rajasekar, Neeru Sharma, Chandrashekhar, Radhe Shiam and V. Ramachandran under the watchful eyes of K. Satyanarayana and S.s. Saksena have provided this postnatal infant care. To formally thank them would perhaps be insulting because for them it has been a labour of love. But not to mention them would be ingratitude.

Bal Phondke

Preface Life on the planet earth spans a multisplendorous spectrum. At one end is the tiny bacterium, almost a non-entity, made up of a single cell. At the other end is our species, the most evolved animal, a marvel of multicellular organization. Yet even the human being, comprising several seemingly complex organ systems as it is, starts out as but a single fertilized cell. Collecting half of the master design of life from the mother and the other half from the father this cell multiplies by that simple process, common throughout the living kingdom, of division. That primordial germ cell Initially merely growing in number and then specializing, without growth, in different tasks, constitutes and sustains the adult human being. For existence, or what is commonly understood as living, the body has to depend upon a coordinated orchestration of the different constituent organ systems. There is, however, unity in this apparent diversity. The fundamental unit of all the organ systems is a cell and all the cells perform like a chemical factory. Their specific needs of raw materials and energy may be different and their finished products diverse, but their basic structure and style of functioning do not differ. Only that to meet the differing needs the cells adapt themselves to the ambient conditions by assuming different forms, sizes, rates of production. And that initial germ cell goes through all these throes so that the adult organism it has developed into produces that seed which on fusing with its counterpart from the other sex could yield yet another germ cell for the perpetuation of the species, indeed life. Life is thus an eternal journey from cell to cell.

dng and Communication ternal of Workers ce of Creation inage System River athe Chemical Factory Engine

69 54 50 41 22 12 1 63 28 35

The Building Blocks of Life

...

The Building Blocks of Life The Cell uppose we take a piece of glass and go on breaking it to make smaUer and smaller pieces. Can we go on forever! Obviously not. As several scientists have stated we will reach the stage of an atom, which is the smallest representative unit of matter. If we likewise approach living organisms we will come across the cell which is the fundamental unit of life just as the atom is the basic unit of all matter. At one time atom was considered as the smallest indivisible unit of matter. Subsequent scientific studies have shown that this is not so. The atom is made up of still smaller constituents organised in a specific manner. The constituents by themselves do not possess the properties of the atom. Only when they get organised in a set arrangement they assume the atomic properties. The cell too is made up of several constituent units; but by themselves they cannot sustain life. Onl y by coming together in an organised and definite pa ttern inside a cell can they make up a living entity. Two German scientists - M.J. Schleiden (1804-1881), a botanist, and Theodor Schwarm (1810-1882), a zoologistenunciated the cell theory of life in 1838. Like most scientific theories, this statement, that living organisms are cellular in structure, represented a generalised and consolidated conclusion derived from a number of observations made by earlier scientists. The first hint of the cellularity of living organisms came from the observation of Robert Hooke (1635-1703), an English physicist, who saw in a piece of cork

2

LIFE: FROM CELL TO CELL

Robert Hooke ...

world could be undertaken.

whichisalivingtissue in the plant, an array of tiny porelike structures. These looked very much like parts of a honey-comb. He called them cells, meaning small rooms. Hooke was able to see the cells by using a primitive microscope. This was in 1665. However, this was only a casual observation. So, others did not take due note of it until almost the end of the eighteenth century. It was around this time that good microscopes became available and a thorough examination of the microscopic

This change in scenario was mainly due to the Dutch lens grinder Anton van Leeuwenhoek (1632-1723). He had ...his primitive microscope and the piece of cork Inset: Honey-comb structure of cork

THE BUILDING BLOCKS OF LIFE

3

Anton van Leeuwenhoek with his microscope

assembled a microscope. Like a child with a new toy he started looking through this at almost anything he could find. He found several tiny living beings which were hitherto invisible. He called them animalcules and wrote lengthy exhaustive letters to the Royal Society in London, describing in detail what he saw. Leeuwenhoek's studies of cells opened the door to a completely new concept of the make-up of all living beings, and his work inspired others to look closely at plant and animal tissues. These scientists described cells and discussed their significance. Schleiden and Schwann put all these loose threads together and wove a meaningful pattern. By stating in clear terms that the cell was both the structural

4

LIFE: FROM CELL TO CELL

and the functional unit of the organisation and development of life, they laid the foundation of modem biology. Twenty years later carne another important landmark. Rudolf Virchow (1821-1902), the great German physician, formulated the hypothesis that cells corne only from pre-existing cells. The real significance of this pronouncement became clear when it was realised that sperm (the male seed) and ovum (the female seed) are also cells. Life, thus, could be seen as an endless journey from cell to cell. Birth, growth, development, evolution, disease, ageing and death, which are the characteristic features of life, could all be seen now as varied aspects of cellular behaviour. The cell is a complete living structure; indeed, there are numerous organisms which are made up of just a single cell. These unicellular organisms include bacteria, fungi and some algae. They show all the distinctive qualities of life such as growth, development and reproduction. On the other hand, the higher organisms in the plant and animal kingdom, may be a grass or a small animal, are made up of a stupendously large number of cells. The human body, for example, contains 50 trillion (50,000,000,000,000) cells. In an unicellular organism every individual is capable of carrying out all the functions that are vital to life. In a multicellular organism, on the other hand, cells fall in to specialised groups. They acquire proficiency in carrying out a specific function, at times sacrificing their inherent ability to perform other equally essential functions. Consequently, such cells have to depend on other specialised cells to make up for their deficiencies. An individual cell from a multicellular organism, thus, cannot maintain life independently but only as a part of a complex but coherently functioning network. It is a common observation that indi viduals acting totally independently can only constitute a disorderly crowd. But when each C'ne undertakes to become a specialised member in an organised collaborative venture, together they establish a civilised society. This is equally true of cells.

5

THE BUILDING BLOCKS OF LIFE

Mitochondrion

ICell

membrane

Nucleus

Cell -

the building block of life has an organized structure

6

LIFE: FROM CELL TO CELL

All cells with a similarity in characteristic features and in the specialised jobs they are expected to carry out join together to form different types of tissues like bone, blood, epithelium, etc. In turn, such tissues with closely related functions group together in an organised fashion to form organs. Although each organ can execute its own assigned work independently, only when it collaborates with other organs can its output become meaningful for maintaining normal life. For example, a heart can pump blood efficiently. But it needs help from the lungs for enriching the blood with oxygen. It requires arteries to transport the blood to the rest of the body, and veins to bring back the impure blood. It is clear, then, that certain organs work in unison, forming an organ system which looks after one of the vital functions necessary to support life. The systems, in turn, interact with each other to keep the body in good health.

Shapes of Cells If we look at the living world around us, we find an amazing variety of form and function. The tall slender palm tree stands in striking contrast to a short spreading mulberry bush. The giraffe has a long neck, almost as tall as the tree-tops. The squat hippopotamus's neck is almost non-existent. The tough skin of a rhinoceros lets one appreciate the silky softness of a deer- skin. Yet, all these structures have one thing in common. They are all made of cells. The body of an animal consists of several different organs. Each has a different structure as well as function. If we look at the cells of these organs we will find a bewildering variety of shapes, sizes and otherfeatures. Yet all this array originates from a single fertilised cell. This cannot happen unless that single cell possesses the faculty of responding to different needs and environments by undergoing suitable changes, whenever necessary, in shape and size.

THE BUILDING BLOCKS OF LIFE

7

Cells come in a variety of shapes

A sphere is considered to be the most beautiful and best proportioned of all geometrical structures. It is, therefore, not surprising that a number of cells are spherical. Many unicellular organisms like yeasts and bacteria are, indeed, of this shape. So are the eggs of certain marine animals when released in water. But bacteria also assume other shapes such as rods, spirals or even commas. Some algae like desmids or dinoflagellates corne in very weird forms. The familiar amoeba, in fact, has no unique shape. It is, rather, a fluid mass which can turn into any shape, according to the demand of the particular situation and environment in which it finds itself. Another algae found in warm marine waters, the Acetabularia, is a remarkable unicellular organism. It appears almost like a flower with a long slender stalk. Even within a single multicellular organism, different cells take different shapes. Moreover, the same cell may alter its shape to suit a particular need at any time. For example, when

8

LIFE: FROM CELL TO CELL

Acetabu/aria -

a one cell plant

life begins with a fertilized egg, which is a single cell, it is spherical. But when it starts dividing and growing, the edges start flattening even as early as the stage when the growing mass consists of but four cells. Later, when the cells start specialising to form a unique organ structure, they can adapt themselves to the availability of space. Therefore, those cells which form the capillaries for blood flow, assume a shape which is long and narrow, like a tube. Even the cells of the same tissue have a different shape, depending upon the organ to which they belong. That is why the muscles along the legs and arms, where quick movements are necessary, are elongated and striated. On the other hand, those surrounding the stomach wall are smooth, rounder and shorter. The major force which dictates the shape of the cell is, however, the functional requirement. A red blood-cell is spherical when viewed from the front but flattened and

THE BUILDING BLOCKS OF LIFE

9

concave when looked at from the side. Its function, of course, is to transport oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. Its thin shape allows easy and free exchange of these gases through the membrane. The round edge permits it to slide smoothly and even squeeze through the long and narrow capillaries .•A nerve cell has to communicate all the information gathered by the sensory organs such as the eye, ear or the nose to the brain and direct the command given by the brain to the muscles. Unless it becomes thread-like, with an ability to branch out when necessary, would it be able to perform its task?

Cell Size As with shape, cells also come in different sizes. The smallest cells visible under an ordinary light microscope have a diameter of the order of 0.2micron. (micron is one thousand th of a millimeter). There are even smaller cells. Louis Pasteur (1822 - 1895) had discovered the existence of a cell which is known as PPLO (Pleuropneumonia-like organism). Pasteur found it to be responsible for a type of cattle disease similar to pleurisy. But he was unable to isolate or grow it. It was invisible even under a microscope since it is only a tenth of a micron in diameter. At the other end of the size-spectrum is the egg of an ostrich. This is the largest cell we know, measuring 15 cm around the outside. The ratio of the linear dimensions of the largest to the smallest cell is 75000 : 1. When the volumes are compared the ratio is much greater at 750003: 1. The range of cell sizes inside a single organism like the human being is also exceptionally wide. At one extreme is the tiny, almost spherical, leucocyte (white blood-cell) having a diameter of 3-4 micron. On the other hand, the neuron, which is a nerve-cell, can extend to a length of over one metre. The factors which determine the size of the cell are, again, the needs of the cell with respect to the particular task it has to

10

LIFE: FROM CELL TO CELL

Red blood cell _

•

•

•

x 100 again Influenza bacillus - _

•

--.•

Cells come in different sizes

carry out. A hen's egg, whose average external dimensions are 60 x 45 mm, is larger beca use it stores food in an enormous yolk for the developing embryo. The human egg, in comparison, develops within the body and draws all its nutrition-

THE BUILDING BLOCKS OF LIFE

11

Louis Pasteur

al requirements from the mother. Its size of 0.1 mm in diameter is, therefore, quite adequate. Within all this apparently wide diversity there is a unique close relationship between structure and function. Basically, all cells are instruments of energy transformation. They have to convert the energy obtained from food and nutrients to a form essential for maintaining life. The precise shape and size of a cell are, therefore, dictated by the mode in which the cell can do this most efficiently. This compatibility between form and function will become clear when we look at the structure of a cell in detail.

A Visit to a Chemical Factory very biological activity involves a chemical reaction. The activities which are so familiar to us as to go almost unnoticed, like breathing, walking, seeing, tasting, thinking, in other words, merely existing all need energy. This energy becomes available only through a chemical reaction taking place within a cell. A cell has, therefore, been thought of as a chemical factory; a general purpose factory manufacturing all types of products necessary for life. This will obviously be true of single-celled organisms. Alternatively, it can be geared to produce only a particular type of material which is the case in respect of cells in a multicellular organism. But the basic organization of the factory is more or less the same. For our knowledge of the structure of the cell, we owe a great deal to the meticulous studies carried out by cell biologists using the electron microscope as well as to the work of biochemists doing elaborate chemical analysis on isolated components of the cell.

The Manager The Manager of this factory is the nucleus. This is the most prominent part of the cell. It is bounded by a double layer of membranes called the nuclear envelope that separates it from the rest of the cell, which is referred to as cytoplasm. If appropriately stained with dyes, a thread-like material can be seen within the nucleus. This is chromatin, which is an aggregate of two types of large molecules, DNA (deoxyribonucleic acid) and protein. As the time of cell

13

A VISIT TO A CHEMICAL FACTORY

division approaches, batches of chromatin become tightly coiled and become visible as chromosomes. The transmission of chromosomes to its daughter cells, formed as a consequence of division accounts for transfer and perpetuation of hereditary traits from one generation to the next. The organisms cells of which have a well defined and organized nucleus, are called eukaryotes. All the higher plants and animals belong to this category. Most of the unicellular organisms and lower forms of life have cells which lack this organized nucleus. They are known as prokaryotes. One can also see within the nucleus one or more round bodies. This is the nucleolus, which is involved in the production and export of certain materials to the cytoplasm .

Tne

.

eSU!n

The DNA within the chromosomes contains all the necessary heredi tary information. It is like the master architect who holds the grand design for the distinctive features not only for that cell but for the entire organism of which the cell is just a component. The proteins which constitute the building blocks of the organism are man ufactured according to the design handed down by the DNA. The contractor entrusted with the job of translating and expressing that design in the

The molecule of DNA has a helical structure

14

LIFE: FROM CELL TO CELL

A cell is like a factory.

A VISIT TO A CHEMICAL FACTORY

15

16

LIFE: FROM CELL TO CELL

final ready product - proteins - is the RNA (ribonucleic acid) molecule. There are different types of RNA molecules; some carry the message from the DNA to the site of manufacture; some bring together the raw material (basic blocks) needed for the assembly. The assembly line where the proteins are manufactured is the ribosomes. These are specific aggregates of proteins and RNA and are located in the cytoplasm. In the mycoplasma and bacterial cells they lie free whereas in the higher cells they are mostly attached to the membranous network known as the endoplasmic reticulum.

The Assembly Line The endoplasmic reticulum (ER) is the sprawling network of membranes which extends from the nuclear envelope right upto the outer periphery of the cell. A part of the ER is studded with ribosomes giving it a rough appearance. This rough ER is involved in the manufacture, storage and export of proteins. Part of the ER which is devoid of ribosomes is smooth. This takes care of the production of fats (lipids) and fat-soluble hormones. Membranes, in general, abound in a cell. One such complex of membranes, distinct from the ER, is the Golgi apparatus, named after its discoverer. The term dictyosome is also used to describe this structure. It acts as a marshalling yard for directing traffic of packaged proteins meant for export or internal use.

The Power 110use In order to work, a factory needs power. The boiler-house which generates steam provides this. The power-house of the cell is the mitochondrion. The mitochondria were the first organelles to be identified. They are often rod shaped, varying in size from 0.2 to 5.0 ~. More than one such power-house may exist in a cell. A rat liver cell of 25 ~ diameter may contain as many as 1000 mitochondria.

17

A VISIT TO A CHEMICAL FACTORY

The mitochondrion

A double layer of lipid and protein, apparently similar to the nuclear membrane and the outer cell membrane, covers the mitochondria. The inner membrane is variously folded into cristae, giving it an almost maze-like appearance. The outer membrane is fairly elastic, enabling the mitochondria to swell or con tract as required.

Broken-down fragments of food such as proteins, lipids and carbohydrates, enter the mitochondria. There, they combine with oxyg~n to produce carbon dioxide, water and most importantly, energy. A substantial fraction of the energy is repacked in a chemical form, mostly as an energy-rich molecule of ATP (Adenosine tri-phosphate). Since oxygen is used and phosphorus is added, in this process, to form ATP,

The cell-membrane is double walled

18

LIFE: FROM CELL TO CELL

it is called oxidative phosphorylation. Since carbon dioxide is given out, some people consider it to be a respiratory process. It is not surprising that there is a correlation between mitochondrial location and intense cellular activity. The energy it produces would be needed most at the active sites. Thus, we see that in muscles the mitochondria are close to the muscle fibres which convert the chemical energy to a mechanical form. The mitochondria also contain some DNA and ribosomes. So at times it has been thought of as an independent state within the cellular empire. In some plant cells there is another site for energy generation. This is the chloroplast which contains chlorophyll. Energy is generated by the process of photosynthesis.

The House-Keeper In any factory, there has to be machinery for destroying certain leftovers or unwanted goods. Similarly, hostile elements which gain entry would also have to be attacked. Some raw material which comes in, may have to be broken down to a suitable form that can be fed into the processing equipment. All such tasks in a cell are performed by lysosomes, which are membrane-bound sac-like structures. They contain a number of catalytic proteins-the enzymes-which can specifically digest large molecules. It has been suggested that the self-destruction which follows cell-death also occurs as a result of the rupture of lysosomes which lets loose all the enzymes.

The Compound Wall The cell is separated from its environment by the cell membrane. It acts like a gate-keeper permitting entry only to

A VISIT TO A CHEMICAL FAcrORY

19

certain material while keeping out others. This selective permeability is its characteristic feature. A cell membrane is made up of a two molecular layer of The water-seeking (hydrophilic or polar) charged part of the molecule is on the outside and the water - repelling (hydrophobic) part oriented inwards. This layer is fluid and can undergo alterations in its structural organisation to suit the demands of a situation. Embedded in lipoprotein molecules.

Pinocytosis is}ike cell drinking a liquid.

20

LIFE: FROM CELL TO CELL

Like cells recognize each other

A VISIT TO A CHEMICAL FACTORY

this fluid mosaic mass are proteins or enzymes, traversing the entire depth of the layer.

21

some of them

For a number of years the cell membrane was considered to be a mere compound wall. Recently, however, it has been found to participate actively in a variety of cellular functions. Two such functions are pinocytosis and phagocytosis. The former name is derived form the Greek words for 'drink' and 'cell'. The process is almost like the cell drinking a liquid. A channel is created in the membrane through which liquid enters and then the membrane pinches off pockets that are incorporated in the cytoplasm. By phagocytosis the cell 'eats' solid materials which it first engulfs by extending parts of a membrane-somewhat like holding out two arms. The material is then drawn inside for the enzymes to digest. The membrane also seems to form a communication system to 'talk' to other cells. Thereby the cells exhibit an ability to 'recognise' one another. If heart cells and retinal(eye) cells from a chick. embryo are made into single cells and dispersed in a medium, the heart cells seekout and aggregate with qeart cells and the retinal cells with retinal cells. Their 'talk' is more complex. If the heart cells are kept apart they will initially beat at random but soon, like a well organised orchestra, they start keeping the same rhythm. The plant cells and many unicellular organisms have an additional envelope surrounding the cell membrane. This is the cell wall which is made up mostly of cellulose-like substances which are derivatives of sugar compounds. A system of internal scaffold ing called cytoskeleton

is used for maintaining cell shape and is responsible for movement of cells as well as movement of particles inside the cell. These are made of three components; microfilaments, microtubules and intermediate filaments.

The Dance of Creation s we saw earlier, the human body contains approximately 5 x 1013 (50,000,000,000,000) cells. Yet each of us started as a single cell. Rudolph Virchow's theory of biogenesis states that a cell can originate only from a pre-existing cell. It is clear, then, that the 50 trillion cells in an adult human being can arise only from a single cell. The increase in the number is brought about by the process of cell division. Anyone who has watched a living cell divide is sure to be fascinated by the organised way in which the various cellular constituents behave, placing themselves in an ordered composition. It is as if one is witnessing the dance drama of creation.

The Drama of Growth In a drama the part played by characters on the stage is easily

seen. But there are usually a number of other participants who work behind the scenes and whose contribution is equally important. The same is true of cell division. The drama takes place mainly in the nucleus but the cytoplasm also undergoes significant changes. The preparation for this drama of cell division-mitosis begins while the cell is still in its resting stage or interphase. At this time the nucleus is large. The nucleic acids and proteins which constitute the chromosomes are synthesised and the chromosomes themselves are replicated. However, they are still not distinct and appear like a tangled ball of

THE DANCE OF CREATION

23

wool. They become visibly distinct as long, thin threads during the next act in the drama when the cell enters the next stage: the prophase. The chromosomes are divided along their entire length into two halves called the chromatids. The entire length of the two chromatids is held together at a single point Growth takes place by increase in cell number. The number increases by division of cells. Cell division-mitosis-occurs in stages giving rise to two cells)each with one set of all chromosomes.

1

24

LIFE: FROM CELL TO CELL

called centromere. The chromosomes then start becoming thicker and shorter so that each one can not only be seen clearly but even identified individually. The nucleoli diminish in size and, by the end of prophase, disappear. In late prophase the nuclear membrane also disintegrates and at the same time a radiating structure is seen nearby. This is the centrosome with 'astral rays' radiating from it. This gives rise to the spindle, which is like the strings that control the dancing puppets. The puppets turn out to be the chromosomes. These first move through the cytoplasm and then by their centromeres get attached to the spindle threads-which are formed by microtubules. The chromosomes then posi tion themselves along the equator ready to start their dance. This stage is the metaphase. Anaphase follows with the division of centromeres so that each chromatid gets one centromere. Slowly the chromatids are pulled apart to opposite poles till the two groups of chromosomes pack themselves densely at the two poles. The next stage, telophase, is almost the prophase in reverse. The nuclear membrane reforms. The spindle disintegrates. The chromosomes uncoil to again become slender threads. Simultaneously, the cell membrane around the equator starts cleaving until two daughter cells are formed which move apart. Cell division is, of course, part of the process of growth. Although the dance of the chromosomes, the formation of the spindle, and the generation of the daughter cells are visible events, cell division also involves assimilation of raw material from outside, by using which new building blocks are created. Energy is generated. Before the new cells engage in the same drama, growth takes place. The daughter cells are exact replicas of their parent cell. This ensures that all the structural characteristics are preserved through the generation. Since the daughter cells contain the exact complement and number of chromosomes, they are equipped with the identical blueprint of the master design.

THE DANCE OF CREA nON

ance of

25

reation

In the higher animals which reproduce by sexual means, another type of cell division, meiosis, takes place. In mitosis

the daughter cells have the same number of chromosomes as the parent cells. If the sexual cells, the gametes (that is, the male seed, the sperm, or the female seed, the egg) were formed by mitosis, they would each contain the same number of Meiosis-

the process of cell division by which sexual cells are produced.

5

chromosomes as the parent cell. When the egg is fertilised it fuses with the sperm resulting in the zygote. If each of the fusing partners, the sperm and the egg, have the full number of chromosomes, the zygote would contain double the number of chromosomes. So the cells of the next generation would all have double the number. Each subsequent generation would result in further doubling of the number of chromosomes. This would result in a chaotic situation. Obviously, a compensatory mechanism is essential. This is provided by two nuclear divisions but only one division of chromosomes which is the characteristic feature of meiosis. Consequently, in the gametes the chromosome number is halved. The human cells contain 46 chromosomes. These exist in 23 pairs. The members of 22 pairs are totally identical or homologous. The 23rd pair is responsible for deciding the sex of the individual. In the female this pair is homologous, the chromosomes being XX. The male contains a nonhomologous pair: XY. The egg cells produced by the female by the

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Human cells contain 23 pairs of chromosomes

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LIFE: FROM CELL TO CELL

26

H

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)) 11 ~, 1 i 1 t H H H fi

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THE DANCE OF CREA nON

27

process of meiosis contain not 46 but 23 chromosomes, each egg cell carrying a single X chromosome. The sperm cells also have 23 chromosomes; half the sperm cells carrying an X chromosome and half of them a Y. Meiosis consists of two divisions. In the first division prophase is a lengthy and elaborate stage during which the homologous pairs are joined together so that during the first anaphase the two members of the pair rather than the chromatids of the same chromosome move in opposite directions. The second division is essentially similar to mitosis, with the constituent chromatids separating from each other. But, unlike mitosis, no duplication of chromosomes takes place in their second division. A human egg weighs only about a millionth of a gram. The sperm which fertilizes it adds another five billionth of a gram. Yet the child which develops from it in nine months weighs about 3500 g. This increase in mass of one billion times is achieved by the processes of growth and differentiation. Multiplication of cell number is an obvious part of growth but it also involves a complicated pattern with different centres growing at different rates and different times. Simultaneously with the multiplication some of these generalised cells are transformed into specialised ones so that they can undertake specific jobs. This is differentiation. This is achieved at the expense of the ability to divide. Therefore, in general, the more specialised or differentiated a cell, the less it is likely to divide.

Teams of Workers irds of a feather flock together, goes a saying. This is true in the case of cells as well. Cells with similar specialised features have means of recognising each other. These look alikes gather together to form tissues. Those cells which specialize in forming substances that hold the body structure together make up the connective tissue. Their special job is to join one part organ of the body to other. The cells that constitute the fibrous connective tissue are called fibroblasts. These tissues manufacture fibrous proteins which form coils, thus becoming tough and elastic at the same time. Two chief proteins in the connective tissue are collagen and elastin. The fibroblasts also produce mucopolysaccharides which act like a cement between two cells, holding them to gether.

The Outer Cover The most visible tissue is, of course, the skin or epithelial tissue. Its special task is to cover and protect other tissues and organs from the environment. It is made up of three parts. The outer epidermis is three to four layers thick and can assume different shapes and forms as demanded. To shield the cornea of the eye it becomes a transparent film but to protect the fingertips it literally becomes as tough as a nail. Despite its specialization the epidermis retains the ability to grow. It is self-renewing. Cells produced in the bottom layer push upwards. Thus a complete new coat for the body is formed every month. The lower layer or dermis acts mainly as a shock absorber. The lowest region is occupied by the subcutaneous tissue

TEAMS OF WORKERS

29

Skin's special task is to cover and protect other tissues

which teems with nerve tissue, blood vessels, sweat glands, oil-producing sebaceous glands and roots of hair. The skin also acts as a reporter, communicating information about the environment to the brain via the nerve carriers. The sensory stimuli in the skin are set off by changes in pressure, or temperature, tissue damage, etc.

Th The framework which houses all the organs is provided by another specialised form of tissue-the skeleton. The skeleton is a tower of bones. The 206 bones of an adult come in a great

LIFE: FROM CELL TO CELL

30

~I

The skeleton is a tower of bones

variety of forms, each perfectly suited to its task. That is why the skull is built up of flat plates but the backbone or spine of hollow rings. To house the middle ear, they become tiny and soft but to provide the pedestal of the leg they become hard and long. Bones are joined together by ligaments and through tendons they hook on to muscles. Like any tissue,the bone tissue too adopts a structure that is best suited to the function it has to perform. That is why it comes in two basic types: cancellous, which is light and porous, as in the spine; and compact, which is dense and strong as in leg bones. The bones contain virtually all the minerals found in the body, such as calcium, phosphorus, copper, cobalt, etc. Bone tissue infact plays a crucial role in maintaining prope,· calcium balance. The meeting point between bones, constitutes a joint. Joints allow the bod y to move in a controlled manner. Some joints are strong but then their mobility is restricted. Other joints are not as strong but are very mobile. The vertebrae forming the spine constitute a strong and slightly mobile joint. In these, there is a thick pad of fibro-

31

TEAMS OF WORKERS

Bones in the leg are dense and strong

cartilage between the bones which are held together by strong fibrous ligaments. The pad acts as a shock absorber. Mobile joints are usually complex. They have to withstand friction of movement. Their bone surfaces are, therefore, covered with smooth cartilage. The edge of the joint has a Bone joint in elbow

32

LIFE: FROM CELL TO CELL

Bone joints in backbone and skull serve different functions

TEAMS OF WORKERS

33

strong fibrous capsule surrounding the synovial sac, which secretes a fluid that acts as a lubricant. The mobile joints are of different types, depending upon the nature of the movement. That is why the elbow joint is like a hinge, whereas the one in the shoulder is like a ball and socket. The neck forms a pivot joint allowing rotation, but the wrist which needs bending and circular, but not rotatory motion, forms an ellipsoid joint. The hollows of all bones contain the soft porridge-like marrow which continuously produces red as well as white blood cells, thus providing a replacement for the ever-dying blood cells.

The Cables Cells specialising in contraction form the tissue of the muscles. The body's 600 odd muscles are the cables whose pull on bones makes motion possible. Some muscles are made up of long bundles of fibres. These are the striated muscles. Most of the skeletal muscles which perform only as a result of a conscious effort ~ the voluntary muscles ~ are of this type. The involuntary muscles, for instance, those which aid in digestion, or breathing, are round and smooth, and cannot contract with speed. The heart, too, is a muscle and possesses features of both types. Muscles usually work in pairs ~ne pulling and the other pushing. They contract and then relax. The muscle fibrils consist of rows of two filament-like proteins ~ actin and myosin. They face each other like two combs with their teeth running into each other. When the brain issues its order in the form of an electrochemical impulse, it induces these two to glide towards each other, producing the contraction. The energy required for this work is supplied to the muscles in the form of glucose which is burnt up with the oxygen. When oxygen is in low supply it can use an alternative chemical reaction which however, produces lactic acid. If this

34

LIFE: FROM CELL TO CELL

Muscles are cables pulling bones to move

accumulates, the muscles find it difficult to contract, giving the sensation of fatigue. The great team of muscles and bones have given man stature and mobility. Such cooperation between tissues results in the formation of some vital teams, which is what organs are. These too work together in groups, forming the various systems.

Fuelling the Engine

ave you seen a stearn engine? It uses up fuel in the form of coal and water to generate energy which allows it to haul an entire trainload. The human body too is like an engine which bums up fuel in the form of food, water and air to convert it into energy which enables it to maintain life. The stearn engine is made up of a number of small parts. They are organised together into systems depending upon the exact job they execute during the normal working of the engines. The body is similarly made up of various organs which combine together to form systems specialising in specific tasks. The fuel that is taken in has to be stored and then broken down and burnt to be converted to a suitable form of energy. This is the work of the digestive system. The energy thus generated has to be transported to all parts of the body, to each and every single cell, so that the tiny 'chemical factory' keeps working efficiently. This is the task of the circulatory and respiratory system. The waste products generated during the burning of fuel have to be thrown out. The kidney, and to a certain extent the liver, carries out this function. But the human body is much more complex and efficient than an ordinary stearn engine. It has its own control and communication network in the form of the central nervous system. Moreover, a characteristic feature of a living_ organism, such as a human being, is that it produces its own progeny. The reproductive system helps in this. The body is thus an intricate and delicate machine composed of several apparently different organ systems working in total harmony.

LIFE: FROM CELL TO CELL

36

Food burnt in the digestive system provides body with energy

Burning the Fuel The fuel of the human being comes in a variety of forms. The main constituents, however, are only a few. These are proteins, carbohydrates (such as starches and sugars) and iipids (such as fatty substances). Whatever the form of the food, it first gets broken down into small fragments by the

FUELLING

THE ENGINE

37

teeth. Thirtytwo of these work in pairs like the two arms of a pincer. They also contribute to the enjoyment of eating, because food swallowed as a whole would not have much taste. Contrary to common belief, the tooth has quite an intricate structure. The outer covering is the enamel, made up of calcium phosphate, the hardest mineral in the body. It is unresponsive to any sensation and can withstand the pressures of chewing. Under the enamel comes the dentine, a substance related ~obones. Below this is the heart-land of pulp, which is a relatively soft material containing nerves and blood vessels. The whole tooth is anchored in the tailor-made socket by cementum, which is a bony tissue. As food is chewed by the teeth, it is moved about by the nimble and muscular tongue which sees that the food does not escape from between the teeth. The lips and cheek stand guard along the outer rim of the teeth. The tongue has another function too. It helps man speak. It is covered with small conical projections called papillae which contain tiny cells that react to the chemical nature of the food and give rise to the sensation of taste. These are the taste buds. The chewed food is then mixed up with saliva to turn it into a soft mushy mixture. Saliva is secreted by several glands situated in the mouth and contains an enzyme, amylase. Enzymes, which are biological catalysts help speed up biochemical reactions resul ting in the conversion of one chemical into another more useful one. They playa crucial role in maintaining life. Amylase, for example, converts starch to sugar. The tongue by some dextrous movements pushes the chewed food into the long muscular tube of the oesophagus or foodpipe. To ensure that the food takes only this route, the muscular action involved in swallowing blocks off the other routes. That is why the food mixture does not come out of the nose or enter the windpipe. At this stage t!1evoluntary part of food digestion ends and the digestive system gets into action with out any conscious efforts on the part of the eater.

38

LIFE: FROM CELL TO CELL

Villi make up most of innerside of small intestine

Richly muscled, the oesophagus is able to produce wavelike pushes to finish the job of delivering the food to the stomach through a valve which regulates the rate of delivery. The food is deposited one layer at a time. The muscular contractions of the stomach thoroughly mix the food deposit with the gastric juices secreted by the 35 million glands lining the stomach wall. These juices consist mainl y of hydrochloric acid, which can break down any substance, qnd the enzyme pepsin, which cuts up proteins into their constituents, the amino acids. The thick gruel thus formed is ready to enter the top part of the small intestine, the duodenum. But the gatekeeper valve, the pylorus, restricts the entry to a small portion only. This is to ensure that the strong acid in the mush

FUELLING THE ENGINE

39

does not bum the soft tissue. That is why it is immediately neutralised by an alkaline juice. If the acid is so strong why does it not eat up the stomach itself? It would, but for the mucous lining providing a protective barrier.

The almost 9-metre-Iong coiled looping tube of the intestine is the real steam generator. It is an elaborate food processing factory, changing food into the normal components of the blood. The fat is converted into fatty acids and glycerol, the carbohydrates into glucose and proteins into amino acid~. Only the fibres of cellulose are spared and thrown away. Everythin~else is passed to the blood or lymph stream. The intestine has very able assistants to help it carry out its job. The pancreas produces the alkaline juice demanded by the duodenum. The demand is made in the form of a substance called secretin which, when poured into the blood stream, prods the pancreas into action. The pancreas also produces a strong chemical arsenal of enzymes: trypsin to chop up proteins, fipase to tear apart fats, and amylase to fragment carbohydrates. The liver contributes with bile, passed on through the gall bladder, which breaks down large fat globules into small water-soluble ones. The liver is also a very strict storemaster. If excess glucose is produced it immediately converts it into glycogen and stores it away for future use. Apart from these glands, help also comes from billions of bacteria that normally reside in the lower part of the small intestine like the jejunum and ileum. These are such unselfish friends that many of them sacrifice their lives to help their host. That is why the waste product or faeces contains dead bacteria in addition to the undigested parts of food such as cellulose.

40

LIFE: FROM CELL TO CELL

The intestine is lined with innumerable finger-like projections, the villi, which present an absorptive surface of9 square metres. These take the amino acids and sugars from the processed food and pass them to the blood stream. The products of fat are given to the lymph stream. Finally, the large intestine, which has a bigger diameter, slowly absorbs all the water to be handed over to the blood and the semisolid wastes are excreted through the rectum. One could say that the smoke and useless hot water is thrown out by the engine and the useful steam is ready to be presented to the piston.

The Living River 11unicelll!lar organisms draw their food and energy by the simple process, of diffusion a process where materials simply pass through the cell membrane. Waste products go out in the same manner. This is easily possible since these cells bathe.in an ocean of nutrients. But in a multicellular organisms, several cells are seated deep within the body and thus are far removed from contact with the exterior environment. The situation is analogous to that of a farm located far away from a source of water like a river. The river water can, nonetheless, be made available by building a canal system. (The human body has formulated a similar solution: a river of blood which transports the solid, liquid and gaseous nutrients. Through an elaborate system of canals these are then made available to each and every cell which draws its own ration, again, by simple diffusion.

The Pump The heart, which is a cone-shaped organ, roughly the size of a fist, plays the leading role in keeping the river flowing. Poets have romanticised this organ but in reality it is just a pump which sends the blood spurting through the 90,000kilometre-Iong network of blood vessels all through the life span of an individual. The heart is made of muscle and divided into left and right halves by a barrier - the septum. Each half is further divided into two chambers, the upper or the atrium and the lower or the ventricle. Blood carrying the gaseous waste product of carbon dioxide from cells all over the body pours into the right atrium through the great veins or venae cave. The

LIFE: FROM CELL TO CELL

42

/

The living river of blood flows through the body via a network the river loading and unloading gaseous merchandise.

THE LIVING RIVER

of canals. Cargo ships of red blood cells move through Battleships of white blood cells patrol the river.

43

LIFE: FROM CELL TO CELL

44

Heart is a living pump

superior vena cava brings blood from the upper body while the inferior vena cava carries blood from the lower parts. This inflow causes extension of the muscular wall which is then induced to contract, pouring the contents into the ventricle. The distention of the ventricular muscle thus brought about results in its contraction. When it does this the first surge of blood back towards the atrium shuts the tricuspid valve between the two chambers. The blood is thus forced through the other opening into the large blood vessel, the pulmonary artery, which carries it to the lungs. Here the carbon dioxide is exchanged for oxygen and the oxygen-rich blood flows back to the heart via the pulmonary vein and into the left atrium. From the left atrium the oxygenated blood passes into the left ventricle through the mitral valve which gets closed when the contraction of the ventricle tries to force the blood backwards. The blood, therefore gets pushed into the biggest artery, the aorta, which carries the fresh blood all over the body. To keep it flowing in that direction another valve is situated between the left ventricle and the aorta, the aortic valve.

45

THE LIVING RIVER

The circulatory system

The human heart beats at a rate of 60 to 80 times a minute. This would imply that the heart of a 60 year old man would have beat a total of 2 billion times. And still it does not show signs of fatigue unless its owner has abused it. That shows the strength, efficiency and perseverance of the living entities. At each beat, the heart ejects 130 millilitres of blood so tha t in one min u te the quietly working heart pum ps five Iitres of blood. The pushing out of blood by the contraction of the ventricles takes 3/10 of a second. This is followed by a rest period of 5/10 of a second. During exercise the need of the body's muscles for blood is greater. So the heart beats faster.

46

LIFE: FROM CELL TO CELL

The rhythm of the heart is maintained by electrical activity generated by the heart cells themselves.

The Purificaiion The solid and liquid nourishment which is transported through the blood stream is made available by the digestive system, while the gaseous nourishment is provided by the respiratory system. Oxygen is available freely in the air. This is drawn in mainly through the nose and occasionally through the mouth by the process of breathing. The air passes through the windpipe which divides into two bronchial tubes, one for each of the lungs. The internal architecture of the lungs is like a branching tree hanging upside down. The largest branches are the bronchi, which give rise to the bronchioles, about 1/4 mm in diame~er. These act like air passages. The ends of the bronchioles are studded with grape-like bunches of air sacs - the alveoli. There are some 250 million of these. Each alveolus is covered with a cobweb of capillaries, the tiny blood vessels. These are so narrow that the blood cells have to pass through them in single file. Although this passage takes only about a second it is sufficient for the gaseous exchange to take place. Breathing under automaticml'scular control takes place 16 times a minute. At each breath about a litre of air is taken in, although only two-thirds of it reaches the lungs. For an efficient performance the lungs need moist, warm and clean air. Producing this very special air in the space of a few centimetres is quite a feat. The tear glands that bathe the eyes and the moisture-secreting glands in the nose and throat produce as much as a litre of fluid a day to humidify the air. Surface blood vessels along the route take care of the heating job. Enzymes like lysozyme in the nose and throat kill some of the bacteria. The cleaning process begins in the nose with the hair trapping dust particles. Sticky mucus there and in the throat and bronchial passages filter out finer particles. But the

THE LIVING RIVER

47

The lungs

real cleaning job is done by cilia - microscopic hair-like organelles -lining the bronchi which wave like wheat in the wind about 12 times a second. The cellular factory is thus kept continuously supplied with the raw materials needed for its production line as well as its energy requirements.

48

LIFE: FROM CELL TO CELL

The intricate canai system of the flow can easily provide an ideal model for any city water supply. The Grand Canal is, of course, the aorta which after leaving the left ventricle divides into two major branches, the ascending and descending aorta. These branch further into smaller arteries which branch further into capillaries. The capillaries obviously have to be thin and narrow so that Blood cells are of different they can supply each cussizes and shapes tomer cell with the nutrients and pick up its garbage of wastes. The capillaries further along the route coalesce to become venules which join further to become veins. The heart pumps the blood in surges. The muscular walls of the arteries absorb this shock and also smoothen the flow by the time it reaches the various parts of the body. The return flow is thus regularised and smooth. Valves located at regular intervals along the veins ensure that the flow is in one direction only.

hip Through this Iiving river sail the cellular ships. The ships are of different types. There are the cargo ships of red cells which haul the gaseous "

THE LIVING RIVER

49

merchandise. When they reach the capillaries adjoining a tissue cell, in the distant parts of the body, they unload oxygen and in its place get loaded with carbon dioxide. The rest of the nutrients are available in dissolved form in the river itself. The leucocytes are mainly defence ships which form a roving patrol. They are armed with the missiles of antibodies which they fire against the enemy. The enemy, bacteria or viruses, which could bring disease, comes in different forms. The missiles are also tailor-made, a separate type to act against every different enemy. Others like macrophages carry Gut scavenging operations of cleaning the debris which might remain after an action against the enemy.

The Drainage System The Wast here is no factory which manufactures a product without generating any wastes. This is true of our cellular factories too. The raw materials for the cellular chemical factory come in the form of oxygen and the components of food. As a result of the oxidation process, which combines these raw materials, a number of products are formed. Among these are carlxm dioxide and water. The protein molecules contain nitrogen and some sulphur, iron, The urinary system drains out liquid wastes

THE DRAINAGE SYSTEM

51

etc., in addition to the constituents of carbohydrates and fats, such as carbon, hydrogen and oxygen. So the digestion of the proteins gives rise, additionally, to nitrogen-containing compounds, mainly urea. These are waste products like the ash which remains after coal is burnt. The gaseous waste product of carbon dioxide is hauled away by the red blood cells and handed over to the alveoli in the lungs to be discarded through expiration. But the other wastes are dumped into the living river of blood. Such blood, though like river Ganga is sacred yet polluted by sewage, cannot be allowed to go upto the heart and lungs because the potentially toxic wastes can damage these tissues beyond repair. The blood, therefore, passes through the kidneys where it gets filtered.

The Filter. Although a kidney weighs only about 150 grammes in an adult human being, it contains more than a million little filtering units called nephrons. These look like worms with a long head and a twisted tail. The blood capillaries, reaching the nephrons, coil and twist around themselves to form a Cross sectional view of a kidney mass like a ball of wool which is, therefore, aptly named the glomerulus. The blood flowing through the glomerulus suddenly finds itself in a region of greatly enhanced cross-sectional larea. Quite naturally, the flow slows down giving ample time for water, ions and dissolved substances, like urea to pass through to the kidney tubules. At any time, as much as a

LIFE: FROM CELL TO CELL

52

Inside of a nephron -

the drain

quarter of the total blood supply may be passing through the kidneys. The kidneys carry out the filtration very judiciously. One of the functions of filtration is to help maintain the essential balance of various salts in the blood. These are needed but only in the right amounts. The kidneys secrete certain hormones, under the influence of the pituitary gland. These ensure that a proper balance of blood constituents as well as blood pressure are maintained. Since the blood gets constantly filtered, urine is produced continuously. Microscopic droplets of the fluid from each of the nephrons pass out to be stored in the bladder. The bladder, when full, exerts a pressure on the muscular wall which begins to contract rhythmically. This opens up the sphincter at the bottom, and the urine passes into the urethra and from there to the outside.

THE DRAINAGE SYSTEM

53

The rate of formation of urine varies, depending on the concentrations of various substances in the blood and the need to maintain the various balances. For example, in extreme cold, in order to maintain the temperature, the blood supply to the skin is reduced while the blood supply to internal organs is increased to preserve internal heat. Naturally, more blood passes through the kidneys and so more urine is produced.

e

v

f

•

lSOnS

The liver also helps in these cleaning and waste disposal processes. It produces a lot of urea which is then passed on to the kidneys. When a man gets excited or angry the adrenal glands produce a lot of hormones. The liver destroys the excess. Another major function of the liver is that of detoxification. Substances like nicotine from cigarettes, caffeine from coffee, alcohol from liquor, and other drugs cannot be allowed to accumulate in the blood. The liver tames them by destroying their toxic properties. The liver is also a thrifty housekeeper. During exercise the muscles burn up large amounts of glucose, producing deadly lactic acid. Instead of discarding it, the liver converts it into glycogen and stores it. This can later be used as fuel by reconversion to glucose. The same is true of red blood cells. Each second, millions of them die and have to be destroyed. The liver does this but salvages the break-down products for use in building new ones. Some of the debris is used for the formation of the bitter greenish-yellow digestive juice of bile. But for these ever-vigilant cleaners, the body's living river of blood would become highly polluted and would not be able to sustain life.

Communication lthough an engine is a self.,.running machine, without a driver it would lose its sense of direction and purpose. The engine of the human body has a built-in driver. This driver, the brain, commands, controls and regulates the smooth, efficient and purposeful functioning of the body.

The Command Headquarters The brain and the spinal cord make up the central nervous system. The peripheral nervous system consists of the elaborate network of nerves carrying information to and from the central nervous system. Together they control all the body's activities. The brain is a soft jelly-like structure which resides in the well-protected fortress of the skull. It is bathed in a watery fluid, the cerebrospinal fluid, which brings in nutrients, takes away wastes and also acts as a shock absorber. Another protective mechanism is the blood brain barrier which functions like a security officer, allowing entry only to beneficial materials like glucose but keeping out harmful bacteria and toxins. There are three parts to the brain. The forebrain consists of the cerebrum, thalamus and the limbic system. The brain stem makes up the mid brain while the cerebellum, pons and medulla constitute the hind brain.

COMMAND AND COMMUNICATION

55

Cross section of the Brain

Seventy per cent of the brain is formed by the cerebrum which is made up of two hemispheres. The right hemisphere is concerned with the left side of the body and vice versa. The left hemisphere controls speech, writing, mathematical ability and logical behaviour. But it is the right half which allows one to make three-dimensional judgements; this is also responsible for an individual's intelligence and artistic development. Different parts of the brain supervise different functions of the body. The cerebrum looks after the movement and function of all the limbs. The areas concerned with a particular organ have now been mapped out. Consciousness is also due to the cerebrum; it is the constant acti vity of the cerebral cells. The limbic system forms the enormous storehouse of

56

LIFE: FROM CELL TO CELL

The map of homunculus showing the region of the brain which controls movement of different body parts

memory. The hypothalamus with the aid of the endocrine system controls functions such as appetite, thirst and body temperature. Together with the limbic system, it also regulates emotions such as pleasure or anger. The constant, continuous running of the body machine is due to the smooth functioning of the medulla oblongata. It maintains all the essential regulatory mechanisms of the body such as respiration, blood circulation, blood pressure, alertness and sleep. The cerebellum provides help by coordinating all these activities. It also ensures that all movements are smooth and regular. This is done by information relayed to it from the medulla oblongata. Just as an editor of a newspaper assesses all the relevant pieces of information received and puts them together to formulate a meaningful

COMMAND AND COMMUNICA nON

57

news story, the cerebellum assesses this information and coordinates different nerves to produce regulated movement. The brain is the most important organ of the body. It is responsible not only for an individual's physical actions but also for his mental capacity. No wonder, then, that even though it accounts for only two per cent of the body-weight it requires twenty per cent of the body's oxygen and blood supply. Even if there is temporary stoppage of blood supply, unconsciousness occurs.

The Signal Corps A pilot is able to fly an aeroplane with ease because he constantly obtains information about the functioning of the various parts of the engines which is displayed on the hundreds of dials in the cockpit. It addition, he keeps himself aware of external factors, such as climate, through radio contact, with the ground centres. The pilot of the body, the brain, also has a similar network made up of nerve fibres which reach each and every corner of the body. Additionally, sensory organs like the eyes, ears, nose, skin and tongue continuously send information about the external environment. The sensory nerves carry information from the organs to the brain while the motor nerves bring the brain's orders to the muscles responsible for the movement of these organs. The nerves are made up of neurons. They look like a spider with his cobweb. The 'spider' is the cell nucleus and the fibres of the cobweb are the dendrites. The dendrites pick up messages and pass them to the nucleus where they are analysed and passed to the axons. The axons connect wi th the dendri tes of the next neuron. This connection, however, is not a physical one but a chemical one because between the axon of the cell and the dendrite of the next there is a gap, the synapse. Normally these cells have potassium ions inside and sodium ions outside the membrane. When a message comes, the cell

58

LIFE: FROM CELL TO CELL

Neurons release chemical messengers into synaptic gap

COMMAND AND COMMUNICATION

59

is excited and there is an exchange of ions. This generates an electrical potential. This wave travels down to the synapse and releases chemicals such as noradrenaline or acetylcholine. These chemicals can cross the synapse to bind to their respective receptors on the dendrites of the next cell which now gets excited. Thus, the message gets passed on upto the brain. This electrochemicaL conduction of the message along the nerve fibre takes place extremely fast. Moreover, there are enzymes to destroy the chemicals in the synapse once their task is over so that the neuron is ready to receive another message. The muscular movement of any organ or limb needs a fast response to a particular stimulus. But to keep the biochemical machine of the body functioning smoothly, the responses have to occur slowly and specifically. The brain brings about these relatively slow actions by sending orders not in an electrochemical fashion but in a chemical form. These chemicals are the hormones. Some are secreted directly by the pituitary and pineal glands, closely associated with the brain. Others are released by several other endocrine glands loca ted in different parts of the body. The hypothalamus acts as a link between the nervous system and the endocrine system. The messages from the inside of thE body are picked up by the receptor neurons attached to the various parts. But messages about the external surroundings are given initially by the sensory organs to the neurons connected with them.

eporter The eye works like a camera, bringing in images of objects into the field of view. The corn~a is the outer window which allows the light to enter. The amount of light allowed to enter is controlled by the pupil which can vary its size depending on the intensity of light. Even the lens is flexible, for its shape can be altered by the ciliary muscles. This permits finer adjustments in the focal length of the lens, resulting in a sharp image. The screen on which the image is projected is the retina

60

LIFE: FROM CELL TO CELL

The Eyes acting like camera and the Ears like microphone collect and send information to the Brain acting like a computer terminal

which is made of two types of light-sensitive cells, the cones and rods. There are 130 million of these in the proportion of 18 rods to 1 cone. The cones are sensitive to the colours red, green and blue, while the rods respond to shades of grey. The rods contain the chemical rhodopsin which is sensitive to light. When light falls on it, a complicated chemical reaction takes place to fix the image. The two eyes work together to provide binocular vision. It is only because of this doubling of information reaching the visual cortex of the brain that the latter is able to construct a three-dimensional image. The lachrymal glands adjoining the eyeball secrete tears which act as a lubricant as well as a protection from harmful micro-organisms. Dust or small particles are also drained away by the tears. Irritants or emotion causes excessive secre-

COMMAND AND COMMUNICA nON

61

The inner view afEye

tion which floods the drainage system in the eye, resulting in the flow of tears. If the eye is the camera of the body, the ear is the microphone which brings in information on the surroundings by receiving sound waves. The sound is directed by the outer ear to the tympanic membrane. This is ovoid in shape and tightly stretched. It transmits the sound to the three small interlocking bones attached to it, the malleus, incus and stapes. These bones then vib:-ate in unison and amplify the sound twenty times. This is then transmitted to the inner ear, the cochlea, which looks like a snail. In fact, it is a fluid-filled coiled tube. The vibrations of the fluid finally reach the organ

62

LIFE: FROM CELL TO CELL

TheEnr- a cross-sectional view

of corti which looks remarkably like the inner parts of a piano. Different hair cells of this organ are stimulated by sound waves of different wavelengths. In turn, this activates one of the 30,000 neurons to carry the message to the brain.

Life is Elernal ife is eternal. Death is merely the inevitable degeneration of the particular house it had temporarily occupied. Life goes on because it is capable of recreating life. This is easily seen, as a living organism can give rise to a replica of its own. This is reproduction. Unicellular organisms accomplish this merely by cell division. In a multicellular organism an elaborate network of organs exists for the purpose of reproduction. Here the team work between organs extends even beyond an individual. For the reproductive organs of a male and a female have to work in unison to create a new life.

ed

V o

L:;J -----..

New life begins with an union between an male ovum seed). (the female An egg egg)is and produced a spermin (the the ovaries of a woman. These almond-shaped organs are dormant in young females, but days or so. The ovaries consist of a great many immature Under every the inin adults produce egg onecells. egg once 28 fluence of certain hormones one of them

matures into an egg which starts its journey through the fallopian tubes towards a possible union with the sperm. The ovaries also produce several other hormones which give a woman her feminine characteristics. The egg and sperms

The counterpart of the ovary in the male is the testis. The ovaries lie inside the body, but the testes hang outside. This

64

LIFE: FROM CELL TO CELL

is because the main product of this organ, the sperm, is easily inactivated at normal body temperature. It, therefore, prefers a slightly cooler atmosphere produced by the body's elaborate 'air-conditioning' system, based on the principle of sweating or evaporation. The testes, somewhat like the kidneys, contain an intricate network of tubules. In thes,seminiferous tubules some 50 million sperm cells are manufactured every day. The sperm cell is the smallest cell of the body. It has a flailing tail which gives it mobility to swim towards the The reproductive system of a male egg. But the important portion is its head, which contains all the hereditary information that would be passed to the new living being. Like the ovaries, the testes produce all the hormones which characterise a male. Again like the ovaries, they remain dormant until the individual attains the age of puberty.

eginning of ~ ew Life When an egg unites with a sperm their respective genetic contents residing in their chromosomes combine, forming the full complement needed to begin a new living individual. This is fertilization. The first cell which heralds the construction of a whole new being is formed. It needs food for its growth. It can get that temporarily from the supply that the egg carries. But that is not enough and a more secure source of supply as well as a more congenial environment is necessary. This is provided by the womb.

65

LIFE IS ETERNAL

The reproductive system of a female

The womb is an optimistic organ. It keeps itself ready every month in the expectation of receiving a fertilized egg. Its velvet-smooth lining thickens to provide a cushion for the egg. A number of new tissues, glands and blood vessels are formed to provide nourishment. The opening for the sperm the cervix, steps up mucous secretion to ease the sperm's entry. If no fertilized ovum appears, all these tissues, including the extra layers of lining, are discarded. This is the menstrual flow. The onset of this flow, the menarche, is a signal that the woman has reached reproductive age. The menstrual flow occurs every 28 days or so in synchrony with the release of the egg by the ovaries. It will continue till menopause.

The male sperm formed in the testes can be ejected out through the penis. During intercourse it is introduced into the woman's vagina and quickly enters the womb through the cervix to proceed to the fallopian tubes. It is here that it can meet the egg and fertilize it. By the time the fertilized egg

66

LIFE: FROM CELL TO CELL

At 4 weeks

At 28 weeks

At 9 weeks

At 34 weeks At

14 weeks

At 20 weeks At 38 weeks

Growth of a foetus

reaches the womb a few cell divisions have taken place, exhausting the food supply that the egg carried. So the egg gets anchored to the lining of the womb. A reddish pancake-

LIFE IS ETERNAL

67

like organ then sprouts from the egg. This is the placenta which enacts several roles, protecting, feeding and nurturing the growing foetus (the new life). As the foetus grows in size the womb also gets bigger and bigger. The foetus's lifeline is the umbilical cord which contains two arteries and a vein. The arteries carry wastes from the foetus to the placenta where they diffuse into the mother's blood stream. The vein brings all the nutrients which get filtered across the placental membrane.

Eternal Journey When the foetus is fully mature, the muscular wall of the womb under hormonal influence contracts vigorously, increasing the diameter of the cervix from the size of a finger-tip to a full 15 cm in diameter. And so the temporary tenant of the womb is set out in the world. A new living being is born which in its own time can give rise to that tiny cell which can grow into another living being. Life thus forms an endless, eternal journey from cell to cell. \

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LIFE: FROM CELL TO CELL

Glossary Actin: The fibrous protein in muscle cells. Alveoli: Air sacs in the lungs where gaseous exchange with

the blood takes place. Amino acids: The constituent part of proteins. Anaphase: The penultimate stage in cell division. Antibodies: Special protein molecules which defend the body

against infection. Aorta: The biggest artery in the body. Artery: A blood vessel which carries blood from the heart to all parts of the body Atrium: The upper chamber of the heart. Axons: Parts of the nerve cell which carry information away from the cell. Bladder: The organ that stores urine. Bronchi: Constituent units of the lung branching out from the windpipe. Bronchioles: Smaller branches of bronchi. Cancellous: A type of bone which is spongy in nature. Capillaries: The smallest blood vessel. Carbohydrates: Starch-like molecules made up or carbon, hydrogen and oxygen. Sugars belong to this group of molecules. Cartilage: A connective tissue joining two bones. Cementum: The bony socket in which whole tooth is anchored. Centromere: Cen tral globe-like part of a chromosome through which it hooks on to the spindle. Cerebellum: The hind brain. Cerebrospinal fluid: The fluid in which the brain is bathed. Cerebrum: Part of the forebrain controlling movement and function of all limbs. Cervix: The opening of the womb. Chlorophyll: A green-coloured molecule which helps to trap light energy. Chloroplast: Part of the plant cell where photosynthesis takes place.

GLOSSARY

69

Chromatins: The constituent threads of the chromosomes. Cilia: Tiny hair-like structures lining the air tract. Cochlea: The inner ear. Collagen: A protein of connective tissue. Compact bone: The dense type bone of limbs. Cornea: The transparent membrane on the outside of the eye. Corti: The innermost part of the ear. Cristae: Intricate folds of the membranes in,side a

mi tochondrion. Cytoplasm: The fluid part of a cell surrounding the nucleus. Cytoskeleton An arrangement of microtubules and microfilaments holding together the cell structure. Dendrites: Fibre-like parts of a nerve cell. Dentine: Hard bone like material which is a part of the tooth. Dermis: The middle layer of the skin tissue. Dictyosome: Golgi apparatus. Differentiation: Process by which cells become specialized. DNA: Deoxyribonucleic acid--the molecule of heredity. Duodenum: The initial portion of the small intestine. Elastin: The second major protein in connective tissues. Ellipsoid joint: The bone jointin the wrists that allows bending and circular motion. Embryo: An organism in the very earl y stages of development; in man, from conception upto the second month of pregnancy. Enamel: The outer covering of the tooth. Endoplasmic reticulum: The elaborate system of membranes inside the cytoplasm. Enzyme: Protein molecule which acts as a catalyst in the chemical reactions in the body. Epidermis: The outer layer of the skin tissue. Epithelium: Skin. Eukaryotes: Organisms whose cells have a well defined nucleus. Evolution: A continuing process of change from one state of

form to another.

Fallopian tubes: Tubes connecting the ovary to the womb.

LIFE: FROM CELL TO CELL

70

Fertilization: The union of a sperm and an egg. Fibroblast: Elongated flattened cell present in the

tissue.

Foetus:

connective

The unborn baby. containing bile. cells.

Gall bladder: A duct Gametes: The sexual

Cup-shaped beginning of the nephron. Glucose: The simple sugar--product of carbohydrate tion.

Glomerulus:

diges-

One of the breakdown products of lipids. A special organelle in the cytoplasm. Hormones: Endocrine gland secretions that influence specific organs. Ileum: The lower part of the small intestine. Incus: One of the three small vibrating bones in the outer ear attached to the tympanic membrane. Interphase: The first or resting phase of cell division. Jejunum: That part of the small intestine which intervenes between the duodenum and the ileum. Joint: The meeting point of two bones. Lachrymal gland: The gland adjoining the eyes which produces tears. Leucocyte: White blood cell. Limbic system: Part of the forebrain which stores memory. Lipids: Fats. Lipoprotein: A compound molecule containing lipids and proteins. Liver: An important organ removing poisons from blood. Lipase: Fat digesting enzyme. Lysosome: A sac-like structure in the cell which contains enzymes necessary for digestion. Lysozymes: Enzymes that digest cells, bacteria and viruses. Macrophages: 'Ea ter cells': large w hite blood cells which digest dead bacteria. Malleus: One of the three small vibrating bones in the outer ear attached to the tympanic membrane. Glycerol:

Golgi apparatus:

GLOSSARY

71

Marrow: Porridge-like substance found in the hollow of the

bones.

Medulla: Part of the hind brain which regulates essential body

functions. Meiosis: The cell division in sexual organs. Menarche: The onset of menstruation in the female.

Menopause: The cessation of menstruation in the female. Metaphase: The central stage in cell division. Mitochondria: Part of a cell responsible for generation

of

energy. Mitosis: Cell division in non-sexual organs. Mitral. valve: The valve between the left atrium and the left

ventricle of the heart. Motor nerves: Nerves connected to muscles which being information from the brain. Mucopolysaccharides: Substances derived from long chains of sugar molecules which act as a cement between cells. Myosin: The second fibrous protein in muscle cells. Nephrons: Filtering units of the kidney. Neuron: The nerve cell. Nucleolus: Round bodies inside a nucleus. Nucleus: Central controlling organ of a cell. Oesophagus: The food canal. Ovanj: Female s-exual organ which produces the egg. Ovum: The Egg. Papillae: The taste buds. PhagoClJtosis: The process by which the cell digests solid particles. Photosynthesis: Process by which plant cells use energy obtained from light to manufacture carbohydrates from water and C02. Pinocytosis: The process, akin to drinking, by which the cell draws in liquid nutrient. Pituitary: The small endocrine gland on the mid-brain. Placenta: The organ through which nutrients and wastes pass between the mother and the foetus. Prophase: The second stage of cell division.

72

LIFE: FROM CELL TO CELL

Pleurisy: An infectious disease of the lung. Proteins: Molecules which constitute the building blocks of

orgamsms. Pulmonary artery: Blood vessel carrying blood from the heart

to the lungs. Pulmonary vein: Blood vessel carrying blood from the lungs

to the heart. Pupil: A part of the eye. Pylorus: The valve between the stomach and the intestine. Retina: The screen in the eye on which an image is formed. Ribosome: Site of protein manufacture (located in the cytoplasm). Seminiferous tubules: Site of sperm production. Sensory nerve: Nerve which carries information from the sensory organ to the brain. Septum: The barrier between the left and the right halves of the heart. Skeleton: The framework of bones supporting the body. Sperm: The male seed. Sphincter: Muscular disc located between the urinary bladder and the urethra and which controls the flow of urine. Stapes: One of the three small vibrating bones in the outer ear, attached to the lympanic membrane. Subcutaneous: Under the skin. Synapse: The gap between two adjacent neurons. Synovial sac: A duct secreting the viscous fluid which lubricates the joints. Telophase: Terminal stage of cell division. Testes: Male sexual organ which produces sperm. Thalamus: Part of the forebrain controlling normal body functions. Tissue: A collection of similar cells joined together. Tricuspid valve: The valve between the right atrium and the right ventricle of the heart. Trypsin: Protein digesting enzyme. Tympanic membrane: The covering on the ear drum which resonates to the received sound.

GLOSSARY

73

Unicellular: Made up of a single cell. Urea: Nitrogen-containing compound produced as a waste

by the body. Urethra: The canal through which urine passes to the outside. Ventricles: Lower chambers of heart Venules: The small blood vessels which form veins. Vein: A blood vessel carrying blood from all parts of the body to the heart. Vena cava: The biggest vein in the body. Vertebrae: The elements of back-bone. Villi: The finger like structures on surface of the intestine. Zygote: The fertilized egg.

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