Chapter 8 · How do Organisms Reproduce?

NCERT Exemplar Class 10 Science Chapter 8 Long Answer Questions

All 10 questions of NCERT Exemplar Class 10 Science Chapter 8 Long Answer Questions (How do Organisms Reproduce?), in the book's order, each with its answer and a step-by-step solution.

1. Q49
   Why are budding, ​fragmentation ​and regeneration all considered as asexual ​types of reproduction? ​With neat diagrams explain the ​process of regeneration in ​Planaria.
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   All three need one parent, no gametes and only mitosis; each cut Planaria piece regenerates into a complete, identical worm.

   Budding, fragmentation and regeneration are all asexual because each passes the same four tests: only one parent takes part; no gametes are formed; there is therefore no fusion of a male and a female gamete; and the cells divide only by mitosis, so the offspring carry exactly the same DNA as the parent. In budding, as in Hydra, an outgrowth called a bud arises on the parent body, develops the parent's structures and then detaches. In fragmentation, as in Spirogyra, the mature filament breaks into pieces and each piece grows into a new filament. In regeneration, as in Planaria, a body broken into pieces rebuilds what is missing so that every piece becomes a complete animal. Regeneration in Planaria goes through the stages drawn in the figure. The worm is cut into a head piece, a middle piece and a tail piece. At every cut surface the worm's specialised cells divide rapidly and pile up into a mass of new cells. The cells of that mass then change, in an organised sequence, into the tissues and organs that the piece lacks, a forward-facing cut surface building a new head with eyespots and a rear-facing one a new tail. Each piece thus becomes a small but complete Planaria, so cutting one worm into three gives three worms, all genetically identical to the original. Regeneration is still not the animal's normal way of multiplying, because it does not depend on being cut up in order to reproduce; it is asexual in its mechanism rather than a routine method of reproduction.

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2. Q50
   Write ​two ​points of ​difference between asexual ​and sexual types of reproduction. Describe why variations are observed ​in the offspring formed ​by sexual reproduction.
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   Asexual reproduction needs one parent; in sexual reproduction, two parents' DNA, meiosis and chance fertilisation make offspring genetically different.

   Two points of difference. First, asexual reproduction needs only one parent and forms no gametes, so no fertilisation takes place; sexual reproduction always involves two gametes, one male and one female, which fuse at fertilisation, and in most animals these come from two different parents. Second, asexual reproduction uses only mitosis, so the offspring are genetically identical to the parent, while sexual reproduction uses meiosis to form the gametes and fertilisation to combine them, so the offspring differ genetically from both parents and from one another. Variation appears in sexually produced offspring because three things act together. The offspring receives half its DNA from each parent, so it is a combination of two different genomes and can match neither. During meiosis each gamete receives only one chromosome out of every pair, and which member it receives is settled independently for each pair, so a single parent produces a large number of genetically different gametes; the two chromosomes of a pair may also exchange segments before they separate, creating chromosomes with new combinations of genes. And fertilisation is a matter of chance, since any one of the many male gametes may be the one that fuses with the female gamete. With only two chromosome pairs each parent could make $2^2=4$ kinds of gamete and the two parents together $4\times 4=16$ combinations; with the $23$ pairs a human has, the figures become $2^{23}=8,388,608$ kinds of gamete and $2^{46}$ combinations. A fourth and far smaller source is the occasional error made when DNA is copied, which occurs in asexual reproduction too.

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3. Q51
   Distinguish ​between pollination ​and fertilisation. Mention the ​site and product of fertilisation in ​a flower. Draw a neat, labelled diagram of ​a ​pistil ​showing pollen tube growth and its entry into ​the ovule.
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   Pollination is pollen transfer to the stigma; fertilisation is fusion of the male gamete and egg inside the ovule, forming a zygote.

   Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a pistil; fertilisation is the fusion of the male gamete, brought by the pollen tube, with the egg cell inside the ovule. Pollination moves a whole pollen grain and is carried out by an outside agent such as wind, water, an insect or a bird, whereas fertilisation joins two single cells and needs no outside agent, since the pollen tube itself delivers the male gamete. Pollination takes place on the stigma; fertilisation takes place inside the ovule. Pollination leaves only a pollen grain lodged on the stigma and may or may not be followed by anything more, while fertilisation always produces a zygote. Pollination is always the earlier event, and fertilisation cannot occur without it. The site of fertilisation in a flower is therefore the ovule, which lies inside the ovary of the pistil, and the product of fertilisation is the zygote. The zygote afterwards divides to form the embryo, the ovule develops into the seed and the ovary develops into the fruit, while the sepals, petals, stamens, style and stigma usually wither and fall. The labelled diagram is given in the figure: a compatible pollen grain on the stigma germinates, a pollen tube grows out of it and down through the style, enters the ovule through the micropyle and releases the male gamete, which fuses with the egg cell there.

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4. Q52
   Distinguish ​between ​a ​gamete ​and zygote. Explain their ​roles ​in ​sexual ​reproduction.
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   A gamete is haploid and cannot develop alone; the zygote is diploid, formed by their fusion, and grows into the new individual.

   A gamete is a haploid reproductive cell — a sperm, or the male gamete carried in a pollen grain, or an egg — formed by meiosis in a reproductive organ and carrying one set of chromosomes, $n$, which is $23$ in a human. A zygote is the single diploid cell formed when a male and a female gamete fuse at fertilisation; it carries two sets of chromosomes, $2n$, which is $46$ in a human. A gamete comes from one parent, cannot develop on its own, and dies if it does not fuse; a zygote carries DNA from both parents and begins dividing at once. Their roles follow from this. The gamete carries one parent's genetic material into the next generation; because it is haploid, the fusion of two gametes restores the full chromosome number instead of doubling it, so meiosis and fertilisation together keep the chromosome number of the species constant; because meiosis deals the chromosome pairs afresh each time, every gamete carries a different combination, which is where variation among offspring begins; and the two kinds are specialised in opposite ways, the male gamete small and mobile so that it can travel, the female gamete large and stocked with food for the early embryo. The zygote is the first cell of the new individual, so the whole organism is built from it; it holds the complete diploid set as a new combination of both parents' characters; and it divides repeatedly by mitosis while its cells become different tissues, which is how the embryo and finally the whole individual are formed.

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5. Q53
   Draw ​the diagram ​of a flower ​and ​label the four ​whorls. Write the names of ​gamete producing organs in ​the flower.
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   Calyx, corolla, androecium and gynoecium; gametes are produced in the anther of the stamen (male) and the ovule of the pistil (female).

   The labelled diagram is given in the figure. A flower is built of four whorls borne on the receptacle at the top of the pedicel. Counted from the outside inwards they are: the calyx, the whorl of sepals, which are small and usually green and protect the flower while it is a bud; the corolla, the whorl of petals, which are larger and brightly coloured and attract insects and birds for pollination; the androecium, the whorl of stamens, which is the male reproductive whorl, each stamen being a thin filament carrying an anther at its top; and the gynoecium, that is the pistil or carpel, which is the female reproductive whorl and consists of a stigma at the top, a style below it and a swollen ovary at the base containing the ovules. The calyx and the corolla are called accessory whorls, because a flower can still set seed without them, while the androecium and the gynoecium are the essential or reproductive whorls. The gamete-producing organs are the stamen (male) and the pistil or carpel (female). In the stamen it is the anther, the swollen upper part, that produces the pollen grains, each of which carries the male gametes; in the pistil it is the ovule, lying inside the ovary at the base, that holds the female gamete, the egg cell. The filament, the style and the stigma produce no gametes, so the exact answer is: male gametes from the anther of the stamen, the female gamete from the ovule in the ovary of the pistil.

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6. Q54
   What is ​placenta? ​Mention ​its ​role ​during pregnancy?
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   The placenta is a disc-shaped tissue linking embryo and mother; it supplies food and oxygen, removes wastes, secretes hormones and anchors the embryo.

   The placenta is a special disc-shaped tissue that develops inside the uterus after the embryo has implanted in the uterine lining. It is a joint structure, formed partly from the tissues of the embryo and partly from the tissues of the mother's uterine wall, and the embryo is joined to it by the umbilical cord. On the embryo's side it carries a very large number of finger-like projections called villi, which project into the wall of the uterus and lie in spaces filled with the mother's blood, so the villi provide a large surface area for exchange. Its roles during pregnancy are these. It provides nutrition, since glucose, amino acids and other nutrients pass from the mother's blood into the embryo's blood. It is the embryo's organ of respiration, since oxygen diffuses from the mother's blood into the embryo's blood and carbon dioxide passes back the other way; the embryo's own lungs cannot work before birth and there is no air in the uterus. It carries out excretion, since the waste substances produced by the developing embryo, such as urea, pass into the mother's blood and are removed by her kidneys. It acts as an endocrine gland, secreting hormones such as progesterone and oestrogen which keep the lining of the uterus intact and so maintain the pregnancy. And it anchors the embryo firmly to the wall of the uterus. Throughout all of this the mother's blood and the embryo's blood remain separate and never mix; every substance crosses the thin barrier between them by diffusion, moving from where it is more concentrated to where it is less.

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7. Q55
   What are ​various ​ways ​to ​avoid pregnancy? Elaborate any ​one method.
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   Pregnancy is avoided by barrier, chemical, intra-uterine or surgical methods; a condom keeps sperms out of the vagina.

   A pregnancy begins only if an egg is released, sperms enter the female reproductive tract, a sperm meets the egg in the oviduct, and the embryo that results implants in the uterus. Any method that breaks one of those links avoids pregnancy, and the methods fall into four groups according to which link they break. Not having the sexual act is the only completely certain way. Barrier methods put a mechanical barrier in the way so that the sperms cannot reach the egg: a condom worn on the penis, or a similar covering used in the vagina. Hormonal or chemical methods, chiefly the oral contraceptive pills taken by the woman, change the hormonal balance of the body so that eggs are not released and fertilisation therefore cannot occur; being drugs, they may have side effects. Intra-uterine contraceptive devices such as the loop and the copper-T are placed in the uterus by a doctor and change the conditions there so that no pregnancy is established, though they may cause side effects by irritating the uterus. Surgical methods block the passage of the gametes: vasectomy, in which the vas deferens is blocked in the male so that sperms do not reach the semen, and tubectomy, in which the fallopian tube is blocked in the female so that the egg and the sperms cannot meet; both are intended to be permanent and are safe in the long run, though the surgery can cause infections or other problems if it is not done properly. Taking the barrier method in detail: a condom is a thin sheath, usually of rubber, worn over the penis during the sexual act, and it collects the semen so that no sperm is released into the vagina. With no sperm in the female tract, none can reach the oviduct, the egg cannot be fertilised and no pregnancy can begin, so this method acts at the earliest point in the chain. Because no drug is taken and no hormone is altered it has no effect on the rest of the body, it is easily available and needs no doctor, and it is the only group of methods that also lowers the risk of sexually transmitted infections, including HIV-AIDS, for both partners, since the same barrier stops the exchange of body fluids by which those pathogens spread. Its limitations are that it must be used correctly every time, that it may slip or tear and is therefore not completely certain, and that each one can be used only once.

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8. Q56
   How ​does ​fertilisation ​take place? Fertilisation ​occurs ​once in a month. Comment.
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   A sperm fuses with the egg in the oviduct; ovulation, not fertilisation, is monthly, so fertilisation occurs at most once a month.

   Fertilisation is the fusion of a sperm with an egg, and in a human it happens as follows. About once every 28 days one of the two ovaries releases a mature egg, which is drawn into the funnel-shaped opening of the oviduct, or fallopian tube. During the sexual act semen containing millions of sperms is deposited in the vagina; the sperms swim through the cervix and the uterus and up into the oviducts. The egg travels slowly down the oviduct while the sperms swim up it, and the two meet in the upper part of the oviduct, near the ovary, which is the site of fertilisation. Many sperms may reach the egg but only one enters it, and its nucleus, carrying $23$ chromosomes, fuses with the nucleus of the egg, which also carries $23$; the single cell formed is the zygote, with $46$. As soon as one sperm has entered, the membrane of the egg changes so that no other can. The zygote then divides by mitosis as it is carried down the oviduct, and about a week later the resulting ball of cells implants in the thickened lining of the uterus, where the placenta develops. On the statement that fertilisation occurs once in a month: what actually occurs once a month is ovulation, and because only one egg is released in a cycle there can be at most one fertilisation in that cycle. But fertilisation itself takes place only if sperms are in the oviduct while the egg is still alive, and the egg lives only about a day after it is released; in most cycles no sperm reaches it, the egg dies, and the thickened uterine lining breaks down and is shed as menstruation. The accurate statement is therefore that fertilisation can occur at most once in a month, not that it does occur once in a month. Once it has occurred, ovulation and menstruation both stop for the whole of the pregnancy, so it does not recur monthly at all. Occasionally more than one egg is released, and if both are fertilised, non-identical twins are born.

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9. Q57
   Reproduction is essentially ​a phenomenon ​that is ​not for ​survival of ​an individual but for the stability of a species. ​Justify.
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   Reproduction does not keep an individual alive, but it replaces dying individuals and gives variations that let the species survive changes.

   The statement makes two claims and both can be justified. First, reproduction is not needed for the survival of an individual. What keeps an individual alive is nutrition, respiration, transport and excretion; reproduction is not among them, and an organism that never reproduces carries all four of them out normally. A worker bee and a mule never reproduce and are perfectly healthy, and nothing in their bodies fails for want of it. Reproduction is in fact costly to the individual, using energy and materials that could have gone into its own maintenance; a mustard plant dies once it has flowered and set seed, and a drone honeybee dies after mating, so reproducing can shorten a life rather than prolong it. Second, reproduction is essential to the stability of the species, in three ways. Every individual dies eventually, so a population that produced no offspring would shrink with every death and reach zero within one lifetime; reproduction replaces the individuals that die and keeps the population in existence. Reproduction is also the copying of DNA, the blueprint of the body design, and because the copies are accurate the offspring resemble their parents, so the species stays a recognisable kind of organism from generation to generation. And because that copying is never perfect, and because sexual reproduction combines the DNA of two parents, small variations appear in every generation. Those variations are what carry a species through a change in its surroundings. If the water in which a population of bacteria lives becomes much hotter, a population of identical individuals could be destroyed entirely, but in a varied population a few individuals may already tolerate heat; they survive, reproduce and rebuild the population, and the species persists. The benefit in every one of these cases belongs to the population across generations, and to no single organism, which is exactly what the statement asserts.

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10. Q58
    Describe ​sexually transmitted ​diseases ​and ​mention the ways to ​prevent them.
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    Diseases spread mainly by sexual contact (gonorrhoea, syphilis, warts, HIV-AIDS); prevented by condoms, one uninfected partner, screened blood and sterile needles.

    Sexually transmitted diseases, also called sexually transmitted infections, are diseases caused by pathogens that pass from an infected person to a healthy one mainly during the sexual act, because that act brings the two bodies into very close contact and involves the exchange of body fluids. Some of them are bacterial: gonorrhoea and syphilis are the common examples, and both can be cured by antibiotics if they are diagnosed and treated early. Others are viral: warts, caused by the human papilloma virus, and HIV-AIDS, caused by the human immunodeficiency virus. Antibiotics have no effect on viruses, and HIV infection has no cure. Untreated bacterial infections cause pain, discharge and ulcers, can spread to the reproductive organs and leave both men and women infertile, and syphilis can damage many organs if it is left untreated. HIV infects and destroys the cells of the immune system, so over several years the body loses its ability to fight infection and the person suffers repeated infections that a healthy person would resist; this condition is AIDS. These infections also spread by other routes: through infected blood and blood products, through needles and syringes used by more than one person, and from an infected mother to her child during pregnancy, at birth or through breast-feeding. They are prevented by blocking those routes. Using a mechanical barrier, a condom, during the sexual act stops the exchange of body fluids and lowers the risk for both partners, and keeping to a single uninfected partner reduces the chance of meeting an infected person at all. Blood for transfusion must be screened, and needles and syringes must be sterile and used once only and never shared. Pregnant women should be tested and treated so that the infection is not passed on to the baby, and a vaccine exists against the human papilloma virus that causes warts. Early diagnosis and treatment of both partners cures the bacterial infections and stops them being passed on further, whereas treating one partner alone leads to re-infection. Finally, accurate awareness matters, because these diseases are not spread by shaking hands, hugging, sharing food or utensils, using the same toilet, or living and working beside an infected person, and an infected person therefore need not be avoided or isolated.

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