How do Organisms Reproduce?: Class 10 Science · Chapter 7

In the first week of the monsoon, a bare patch of ground near your school turns green almost overnight. Puddles fill with wriggling tadpoles, and a forgotten potato in the kitchen basket pushes out pale shoots. None of this new life came from nothing. Every blade of grass, every tadpole and every shoot came from a living thing of the same kind that was already there. Producing new individuals of one's own kind is reproduction (प्रजनन).

Here is a puzzle worth arguing about in class. A mango tree that never flowered could still live for a century, and a goat that never had kids would not die any sooner. Nutrition, respiration and excretion are different: stop any one of them and life ends. Yet flowers, seeds, eggs and babies cost a great deal of food and energy. So why do living things spend so much on reproduction?

Look out of the window and count the crows, neem trees and houseflies. We know these living things because there are thousands of each, and every crow looks much like every other crow. That resemblance is no accident: each crow hatched from an egg laid by a crow. Reproduction fills the world with copies of each kind, which is how we recognise a species at all. But are the copies exact?

7.1 Do Organisms Create Exact Copies of Themselves?

Two members of a species look alike because their bodies follow the same design. A tailor who cuts twenty kurtas from one paper pattern gets twenty kurtas of the same shape; alter the pattern and every kurta changes. A living thing carries its 'pattern' inside each of its cells.

That pattern is written in DNA (deoxyribonucleic acid), the long molecule in the chromosomes of a cell's nucleus that you met in Class 9. DNA works like a recipe book for proteins, and proteins do most of the building and running of a cell. Alter a recipe and a different protein comes out; enough such differences, and the body itself can turn out differently.

A new individual needs its own copy of the recipe book, so every kind of reproduction begins with copying DNA, which the cell does through a series of chemical reactions. A recipe book is useless without a kitchen, though: a copy of DNA on its own cannot keep anything alive. So the cell also doubles its working parts. Only then do the two DNA copies move apart and the cell divide, giving two cells that each have a full set of instructions and the equipment to use them (Fig. 7.1).

Copying, however, is never flawless. Think of copying a long recipe by hand: however careful you are, the odd letter comes out wrong. The reactions that copy DNA are extremely reliable, but not perfectly so, and now and then a small change creeps in. The copies are therefore close to the original without always matching it. Differences that arise like this are variations (विभिन्नताएँ).

A slip can be serious or trivial. A serious one garbles the instructions so badly that the new cell cannot use them, and it dies straight away. A trivial one leaves a cell that lives on, almost but not quite like its sister. Over many generations these surviving small differences pile up, and they are the raw material of evolution, the gradual change of living things. Chapter 8 follows how such differences are inherited.

7.1.1 The Importance of Variation

Each population has its own place and way of life in an ecosystem, called its niche: where it lives, what it eats and the conditions it can bear. A trout suited to an icy Himalayan stream would not last long in a warm village pond. Because DNA is copied faithfully, each new generation of trout is born with the same cold-water design as its parents, and the population can go on living in its stream year after year. In this sense, reliable reproduction keeps populations stable.

Niches, however, can change suddenly and for reasons no organism controls. In 1999 a super cyclone drove sea water far inland along the Odisha coast; a drought can turn a lake into cracked mud. Picture a population in which every member is identical: whatever kills one will kill all of them. Now picture one whose members differ slightly. A change that wipes out most of them may spare a few that happen to be different, and those few can rebuild the population (Fig. 7.2).

Notice two things. First, no creature changed because the pond turned salty; the survivors already carried their variation before the flood. Second, most variations are neither helpful nor harmful to the individual that carries them, and some are a disadvantage. So variation does not protect every individual. It works at the level of the population, making it likely that some individuals survive a change, and so the species does not die out.

7.2 Modes of Reproduction used by Single Organisms

A bacterium, a mould, a potato and a person all copy their DNA, but they pass it on in very different ways, and the reason is body design. A single cell can simply split; a body made of organs in fixed places cannot. The two activities below show two familiar living things multiplying in two very different ways.

7.2.1 Fission

A single-celled organism has the simplest possible method: it divides, and where there was one individual there are now two. Such splitting is fission. Most bacteria and many protozoa split down the middle into two equal daughter cells, which is binary fission (द्विखंडन).

In Amoeba, the nucleus divides first and then the cytoplasm pinches into two. An Amoeba has no fixed shape and no front or back, so this split can run across the cell in any direction (Fig. 7.3).

Leishmania, the parasite behind kala-azar (spread by sandfly bites), has a fixed, slim shape with a long, lashing flagellum at its front end. Because its body has a definite layout, it cannot split just anywhere: it always divides lengthwise, in line with the flagellum, so that each daughter ends up with a flagellum of its own.

The malaria parasite, Plasmodium, works differently. Its nucleus divides again and again first, so that one cell holds many nuclei. Then each nucleus gathers a little cytoplasm around itself, and the cell releases many daughter cells at the same time. This is multiple fission (बहुखंडन). Yeast, as Activity 7.1 showed, reproduces by budding instead.

7.2.2 Fragmentation

That clue explains Spirogyra's method. A full-grown filament simply comes apart into short lengths, and each length goes on growing and dividing into a complete filament. Splitting a body into pieces that each become a new individual is fragmentation (खंडन). It works only because every Spirogyra cell can do every job (Fig. 7.4).

Try the same trick with a dog or a mango tree and it fails. Their cells are specialists, grouped into tissues and organs that must sit in exact places: a heart in the chest, roots below the ground. A single heart cell or leaf cell cannot rebuild that whole arrangement, so such organisms need other methods.

In such bodies, reproduction is handed to particular cells, just as digestion or movement is. Whatever they are, these cells need a special power: to multiply and then turn into every other kind of cell the new body will need. The next methods show such cells at work.

7.2.3 Regeneration

A Planaria is a small flatworm, about a centimetre long, that glides under stones in clean streams. Slice one into several pieces and, within two weeks or so, each piece has rebuilt whatever it lacked and become a complete worm. Hydra, a tiny freshwater relative of the jellyfish, has the same talent. Rebuilding a whole organism from a fragment of its body is regeneration (पुनरुद्भवन) (Fig. 7.5).

How does a piece know what to grow? The work is done by a reserve of special cells that keep the power to multiply. After a cut they gather at the wound and divide into a small lump of new cells. Cells in the lump then specialise one after another, in a set order, into whatever the piece lacks: eyes and brain for a new head, gut and muscle for a new tail. This orderly building of a body is called development.

Regeneration is a repair kit rather than a way of reproducing: no Planaria in a stream waits to be chopped up before it multiplies. A lizard shows the difference neatly. It can grow back a tail it has shed, but the shed tail never grows into a new lizard.

7.2.4 Budding

Hydra also uses those multiplying cells for ordinary reproduction, with no injury involved. When food is plentiful, cells at one point on its body wall keep dividing, and a knob grows out: the bud. Over a few days the knob lengthens, opens a mouth and sprouts its own tentacles, so a small Hydra hangs from the side of the large one. Then it pinches off and lives independently (Fig. 7.6).

This is budding (मुकुलन). The yeast of Activity 7.1 buds too, though its body is a single cell and its bud is a small new cell. In both cases the offspring begins as an outgrowth of the parent.

7.2.5 Vegetative Propagation

Snip a leafy piece off a money plant, stand it in water, and within two weeks it has roots: it is becoming a new plant. Growing new plants from a root, stem or leaf, with no seed involved, is vegetative propagation (कायिक प्रवर्धन); few animals could manage anything like it. Some plants do it unaided. The thick, fleshy leaf of Bryophyllum (patharchatta) carries small buds in the notches along its edge, and when such a leaf drops on moist soil, the buds put out roots and grow into new plants (Fig. 7.7).

Farmers and gardeners use the same ability on purpose. Sugarcane is planted as short pieces of stem, called setts, each with a few nodes. Roses and grapevines are grown from stem cuttings. In layering, a low branch of jasmine or bougainvillea is bent down and covered with soil until it roots, and only then is it cut from the parent. In grafting, a shoot of a good mango variety is joined on to the rooted stem of a hardy one, and the two grow as one plant (Table 7.4).

Why not just sow seeds? Think like a mango grower. A grafted sapling starts bearing fruit within three to five years, while a tree raised from a stone may take twice as long. The graft's mangoes taste exactly like those of the parent tree, because its DNA is nearly the same, whereas a seedling mixes features from two parents and may disappoint. And for plants that no longer make seeds, such as the bananas we buy and seedless grapes, vegetative propagation is the only option.

7.2.6 Spore Formation

The bread mould from Activity 7.2 is a fungus called Rhizopus, and it shows a division of labour. Its white, cottony threads, the hyphae, spread through the bread and digest it: they are the feeding part. Reproduction is left to special structures, the tiny upright stalks with round heads.

Each head is a sporangium (plural sporangia), a case packed with hundreds of microscopic spores (बीजाणु). When it bursts, the spores drift off in the air. Every spore has a thick, tough wall that keeps it from drying out, so it can wait a long time. When it lands on a moist surface with food, such as damp bread, it germinates and grows into new hyphae (Fig. 7.8).

Every method in this section, from fission to spores, needs only one parent, and the offspring receive copies of that parent's DNA. Reproduction from a single individual like this is called asexual reproduction (अलैंगिक जनन) (Table 7.7).

7.3 Sexual Reproduction

Many organisms cannot reproduce alone. A papaya orchard of only female trees sets little or no fruit unless a few male trees grow nearby. In the monsoon, male frogs croak to call females, and eggs are laid only after a pair has met. In these and countless other species, a new individual begins only when two parents, a male and a female, each contribute. This is sexual reproduction (लैंगिक जनन).

Why go to the trouble of finding a partner, when Amoeba, Hydra and potatoes manage on their own? What can the sexual mode offer that the asexual one cannot? The answer brings us back to DNA copying and variation.

7.3.1 Why the Sexual Mode of Reproduction?

Recall the story so far. Copying slips create variation, and variation is a population's insurance against a changing world (Section 7.1.1). If that is so, a species would gain from a way of producing more variation, and faster.

Copying slips alone are a slow source, because cells copy DNA very carefully and only a few new changes appear in each generation. Careless copying is no answer either: most badly copied DNA would not work at all, and the cells carrying it would die.

The solution is to reshuffle variations that already exist. Picture two card players, each holding a different hand built up over many games. If each passes half of their cards to a new player, that player holds a hand nobody has held before, yet every card in it has already been played without disaster. In the same way, every living individual carries its own collection of variations, gathered over many generations; because the individual is alive and healthy, none of them can be seriously harmful. Sexual reproduction combines DNA from two such individuals, so each offspring is a brand-new combination.

There is a catch. Suppose each parent simply handed over all of its DNA. A child would then carry twice the DNA of either parent, a grandchild four times as much, and so on. Cells cannot run properly with ever more sets of instructions piling up (Example 7.1).

The fix is to halve before combining. Special cells in the reproductive organs divide by meiosis (अर्धसूत्री विभाजन), a division that gives each new cell only half the usual number of chromosomes, and so half the DNA. These half-sets are the germ cells, or gametes (युग्मक). Fusion of a male and a female gamete makes one cell with a full set again, the zygote (युग्मनज) (Fig. 7.10).

Building a whole body from one cell takes energy, so the zygote needs a packed lunch to start with. Simple organisms such as some algae make two gametes that look alike. In more complex organisms the two have divided the work. One gamete carries the food store: it is large, does not move, and is called the female gamete, or egg. The other carries little more than DNA: it is tiny, can swim or be carried to the egg, and is called the male gamete, or sperm. Making two such different gametes is why male and female reproductive organs differ and, in many species, why male and female bodies differ too.

7.3.2 Sexual Reproduction in Flowering Plants

Look into a hibiscus (gudhal) flower and you can see the whole reproductive system of a flowering plant, or angiosperm. Working inwards: green sepals, then bright petals, then the stamens, and in the centre the pistil. Only the stamens and the pistil make germ cells, so they are the reproductive organs (Fig. 7.11).

Flowers like those of hibiscus, pea and rose have both stamens and a pistil: they are bisexual. Some plants make unisexual flowers, with only stamens or only a pistil. A papaya tree usually bears flowers of one kind only, so there are male trees and female trees, while cucumber and bitter gourd vines carry both kinds of flower on the same plant.

Each stamen, a male part, is a slender filament with an anther at its tip. The anther makes pollen grains, the powdery dust that leaves a yellow-orange smudge on your nose when you sniff a lily.

The pistil at the centre is the female part. Read it from the top: the stigma, a tip that is often sticky and catches pollen; the style, a stalk that holds the stigma up; and the ovary, the swollen base. Cut open the ovary of a pea flower and you would find tiny ovules, each containing one egg cell, the female gamete.

For a seed to form, a male germ cell from a pollen grain must join the egg cell inside an ovule. Their union is fertilisation (निषेचन), and the new cell, a zygote, can develop into a plant. There is a gap to cross first: pollen sits in the anthers, but the eggs lie sealed inside the ovary.

The first step across that gap is pollination (परागण): getting pollen from an anther on to a stigma. When a flower's pollen lands on its own stigma, we call it self-pollination. When pollen travels to the stigma of a different flower, it is cross-pollination, and since pollen cannot move by itself, it needs a carrier: the wind for maize and grasses, water for some water plants, and animals such as bees, butterflies, sunbirds and bats for many others.

Landing on the stigma is only the start. The pollen grain absorbs moisture and sugars from the stigma and sends out a thin pollen tube. The tube grows down through the style, carrying the male germ cell, until it reaches an ovule in the ovary. There it releases the male germ cell, which fuses with the egg cell (Fig. 7.12).

Fertilisation sets off a makeover, summarised in Table 7.9. Inside each ovule the zygote divides to build an embryo, a miniature plant, and the ovule hardens into a seed. Around the seeds, the ovary enlarges and ripens into the fruit: a mango, a pea pod and a tomato are all ripened ovaries. Flower parts that are no longer needed wither, though a few may survive on the fruit; the green cap of a brinjal and the crown of a pomegranate are leftover sepals.

A seed is an excellent package. It holds the embryo together with a food store, inside a coat that protects it from drying out and from damage. A seed can wait for months until conditions are right, and wind, water or animals can carry it far from its parent. When it gets water, air and warmth, the embryo starts growing into a seedling. This is germination (अंकुरण).

7.3.3 Reproduction in Human Beings

Humans reproduce sexually, and to understand how, it helps to look first at your own body. Compare a photo of yourself at five with one from today: you are taller and heavier, your shoe size has gone up again and again, and last year's school uniform no longer fits. All of this is growth: the same body, only larger.

Somewhere around the start of the teens, something different begins. A ten-year-old who grows ten centimetres is simply a bigger child, but the new changes reshape the body itself and bring new features and new feelings. Some of them happen in both boys and girls, while others differ, as the comparison below shows.

Puberty is not a switch that flips on a birthday. It is a slow process spread over several years, and its order and speed differ from person to person. Two friends of the same age can be years apart: one may have a deep voice at 12 while another's is unchanged at 15, and both are normal. Bodies also end up looking different, just as heights and voices differ; there is no single 'correct' size or shape for any part of the body. Together, these changes are called sexual maturation.

Why does this happen in the teens and not at the age of five? Think of the body as running on a budget. Through childhood, most of its food and energy go into building a bigger body. Making germ cells needs its own special tissues, just as plants need special parts for pollen and eggs, and the body puts off this costly project. Once the fastest growth is over, resources are freed and the reproductive tissues mature. This stage of adolescence is puberty (यौवनारंभ).

Each of these changes prepares the body for some part of sexual reproduction:

• Gametes must meet. Flowering plants send their male germ cells out of the plant inside pollen. Humans, like many animals, transfer sperms directly into the female's body, which needs a penis that can become erect.
• Maturity must be recognisable. Before mating, individuals need to be able to tell who is sexually mature; features such as the new patterns of hair act as signals.
• Pregnancy and feeding need preparation. A mother carries her baby for about nine months and then feeds it on breast milk, so the uterus and the breasts develop during puberty to make this possible.

7.3.3(a) Male Reproductive System

Think of the male system as a factory with a delivery route: one part makes the sperms, and a set of tubes and glands carries them out of the body (Fig. 7.14).

The two testes (singular testis) are the sperm factories. Unlike most organs, they sit outside the abdomen, in a pouch of skin called the scrotum. The reason is temperature: sperms develop properly only two or three degrees below normal body temperature. The testes have a second job too. They release testosterone, the hormone from Chapter 6, which keeps sperm production going and causes the puberty changes seen in boys.

Sperms leave each testis through a long tube, the vas deferens, which loops up into the body. On their way out, two kinds of glands pour in fluid: the seminal vesicles and the prostate gland. These secretions carry the sperms along and feed them. Each vas deferens finally opens into the urethra, the tube that also drains urine from the bladder, and the urethra runs through the penis to the outside. Urine and sperms therefore share the last part of the route.

A sperm is a tiny swimming cell: a head that carries its DNA, and a long, whip-like tail that drives it towards the egg.

7.3.3(b) Female Reproductive System

The two ovaries, one on each side of the uterus, make the eggs and also some hormones. Unlike the testes, which keep making new sperms, the ovaries hold a stock of hundreds of thousands of immature eggs from before birth. After puberty a few start to ripen each cycle, and usually just one egg is released each month, from one ovary or the other (Fig. 7.15).

A released egg is swept into the funnel-shaped end of the nearest oviduct (fallopian tube). The two oviducts lead inwards to the uterus (womb), a hollow, muscular organ about the size of a fist that can stretch enormously during pregnancy. Its narrow neck, the cervix, leads down into the vagina, the canal that opens to the outside.

Sperms enter the female body during sexual intercourse, when they are released into the vagina. Of the millions released, only a few hundred complete the swim through the cervix and uterus into the oviducts, and only one can fertilise an egg. So fertilisation happens in the oviduct, not in the uterus. Over the next few days the zygote divides again and again while drifting down the oviduct, becoming a ball of cells, the embryo. On reaching the uterus it buries itself in the thick lining: this is implantation. The embryo then develops its organs, and from about the ninth week of pregnancy it is called a foetus.

An embryo cannot eat or breathe, so it depends entirely on its mother. The link between them is the placenta (अपरा), a flat, round organ that grows where the embryo has implanted, built partly from the embryo's tissue and partly from the uterine wall (Fig. 7.16).

The embryo's side of the placenta is covered with countless tiny, finger-like villi. These dip into pools of the mother's blood in the uterine wall, so every villus is bathed in her blood. Together the villi offer an enormous surface area. Across it, glucose and oxygen pass from the mother's blood into the embryo's, while carbon dioxide and other wastes pass back for her body to remove. The two bloods never mix: a thin barrier always separates them.

After about nine months (roughly 40 weeks counted from the start of the mother's last period) the baby is ready to be born. The thick muscular wall of the uterus then tightens and relaxes again and again, in waves, until the baby is pushed out through the widened cervix and the vagina.

7.3.3(c) What happens when the Egg is not Fertilised?

Each month the uterus prepares for a guest that may never arrive. Its lining grows thick and soft, packed with blood vessels, ready to feed an embryo. Usually no sperm meets the egg, and an unfertilised egg dies within about a day. The prepared lining then has no purpose, so the body sheds it: the lining breaks up, and its blood and mucus flow out through the vagina over a few days. This monthly flow is menstruation (ऋतुस्राव), commonly called a period (Fig. 7.17).

A period usually lasts from two to eight days. Then the lining starts building up again, another egg is released, and the cycle repeats roughly once a month: about every 28 days on average, though anything from about 21 to 35 days is normal. The cycle pauses during pregnancy and stops for good in middle age.

7.3.3(d) Reproductive Health

Because puberty unfolds over years while the body is still growing, looking grown-up is not the same as being ready. A teenager's body may not yet be fit for pregnancy, and the mind may not be prepared for a sexual relationship or the work of raising a child. Deciding when one is ready is among the biggest decisions a person makes.

Pressure can come from all sides: classmates who dare you to 'prove' you are grown up, relatives who want a wedding and grandchildren soon, and public campaigns that ask couples to keep families small. Pulled in different directions, it is easy to drift into choices that are not truly yours. Reliable facts about health help you decide for yourself, and remember that in India child marriage is against the law.

Sexual contact involves the closest possible contact between two bodies and their fluids, so it is an easy route for germs. Infections passed on mainly in this way are called sexually transmitted infections (STIs). Gonorrhoea and syphilis are caused by bacteria; genital warts and HIV, the virus behind AIDS, are caused by viruses. Many STIs show no signs for a long time, so a person can pass one on without knowing. A condom, a thin sheath worn over the penis during sex, lowers the risk of many STIs, though no method makes it zero.

Any act of sexual intercourse may result in pregnancy. Carrying a baby for nine months, giving birth and caring for a newborn strain a woman's body and mind, and a girl or woman who is too young or unprepared can be seriously harmed. Ways of preventing pregnancy are called contraception (गर्भनिरोधन); the table sorts them by how they work (Table 7.11).

Doctors can also end a pregnancy, which Indian law allows only under strict conditions. A cruel misuse of this is sex-selective abortion: some families illegally find out the sex of a foetus and have it aborted if it is a girl. Testing to learn the sex of a foetus was banned by a law passed in 1994, yet the number of girls for every thousand boys under the age of six fell between the 2001 and 2011 censuses. Female foeticide is a crime and a grave injustice. Every missing girl leaves society poorer and less balanced, and campaigns such as Beti Bachao Beti Padhao work to protect and educate girls.

Whether a population grows or shrinks is simple arithmetic: births add people and deaths remove them. India's population passed 140 crore in 2023, and people disagree about what that means. One view is that every extra person makes it harder to lift living standards for all. Another holds that the real problem is how unequally land, money, schooling and health care are shared, and that fairer sharing matters more than the head count.