Class 10 Science · Chapter 7

How do Organisms Reproduce?

185 Class 10 Science questions on How do Organisms Reproduce? from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 4 of 10

1. Q61 Exemplar · Short Answer Questions
   Why ​cannot fertilisation ​take place in ​flowers if ​pollination ​does not ​occur?
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   Only pollination lets a pollen tube grow and carry the male gamete to the egg; without it the gametes never meet.

   Because pollination is the only way the male gamete can begin its journey to the female gamete. The male gametes lie inside the pollen grain and cannot move on their own. Pollination transfers the pollen from the anther to the stigma; only then does the grain germinate and send a pollen tube down through the style into the ovary and up to an ovule, and only that tube can deliver the male gamete to the egg. Without pollination no pollen reaches the stigma, no pollen tube grows, the two gametes never meet, and fertilisation, which is their fusion, cannot happen.

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2. Q62 Exemplar · Short Answer Questions
   Is ​the chromosome number of zygote, ​embryonal ​cells and adult of ​a particular organism always constant? How is the constancy ​maintained in these three stages?
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   Yes: all three stages have the same diploid number, $2n=46$ in humans, maintained by mitosis; meiosis halves it only in gametes.

   Yes, it is constant. The zygote, the embryonal cells and the body cells of the adult all carry the same diploid number, $2n=46$ in a human being. The constancy is maintained because the zygote and every cell descended from it divide by mitosis: the DNA is first copied exactly and then shared equally, so each daughter cell receives a complete set and the number is neither halved nor doubled. Halving occurs only in the germ cells at gamete formation, by meiosis, and that halving is made good at once when two gametes fuse to form the zygote.

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3. Q63 Exemplar · Short Answer Questions
   Where ​is ​the zygote ​located ​in the flower ​after ​fertilization?
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   The zygote is located inside the ovule, within the ovary of the flower.

   The zygote lies inside the ovule, and the ovule is inside the ovary of the flower. It is formed there when the male gamete brought down by the pollen tube fuses with the egg in the ovule. The zygote then develops into the embryo, the ovule becomes the seed and the ovary becomes the fruit.

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4. Q64 Exemplar · Short Answer Questions
   Reproduction ​is linked ​to ​stability ​of population of ​a ​species. Justify the statement.
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   Reproduction replaces individuals that are lost, keeping numbers steady, and passes on the same body design with small variations that help survival.

   Individuals of every species are constantly lost to old age, disease, accident and predation, and reproduction is the only process that replaces them. Without it the number of individuals could only fall and the species would die out. When the rate at which new individuals are produced matches the rate at which they are lost, the size of the population stays roughly the same, and that is what stability means. Reproduction also copies the body design, the DNA, faithfully from one generation to the next, so the new individuals belong to the same species and are suited to the same habitat, while the small variations it produces let at least some of the population survive if the habitat changes. Reproduction therefore maintains both the number and the character of the population.

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5. Q65 Exemplar · Short Answer Questions
   How ​are ​general ​growth ​and sexual ​maturation ​different from ​each other?
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   General growth increases body size; sexual maturation, driven by sex hormones at puberty, makes the body able to reproduce.

   General growth is the increase in the size and mass of the body, in height, weight, the length of the limbs and the size of the organs. It is brought about by cell division and cell enlargement, goes on gradually from birth until the late teens, and is controlled mainly by the growth hormone and by nutrition; by itself it does not make an individual able to reproduce. Sexual maturation is the set of changes at puberty by which the reproductive system becomes functional. It occurs in one definite phase of life, at about $10$ to $12$ years in girls and $12$ to $14$ years in boys, is brought about by the sex hormones testosterone and oestrogen, and involves the maturing of the gonads so that gametes begin to be produced, together with the secondary sexual characters: hair in the armpits and the genital area, thickening of the voice and facial hair in boys, enlargement of the breasts and the beginning of menstruation in girls. In short, general growth changes how big the body is, while sexual maturation changes what the body is able to do.

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6. Q66 Exemplar · Short Answer Questions
   Trace the path of sperm during ejaculation ​and mention ​the gland ​and their functions associated ​with the male reproductive ​system.
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   Sperms pass from the testis through the epididymis, vas deferens and urethra; the seminal vesicles and prostate add nourishing fluids.

   Path of the sperm: testis, in whose seminiferous tubules the sperms are formed, then the epididymis, where they are stored and mature, then the vas deferens or sperm duct, which opens into the urethra, and finally out through the penis at ejaculation. Glands and their functions: the seminal vesicles pour in a fluid rich in fructose, which gives the sperms energy, makes them motile and forms the bulk of the semen; the prostate gland adds a slightly alkaline secretion that neutralises the acidity along the way, nourishes the sperms and makes their transport easier. The two secretions together with the sperms make up the semen.

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7. Q67 Exemplar · Short Answer Questions
   What changes ​are ​observed in ​the ​uterus if fertilisation does ​not occur?
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   The thick uterine lining breaks down and is shed through the vagina as menstruation, repeating about every $28$ days.

   The lining of the uterus had already grown thick and spongy and rich in blood vessels, ready to receive and nourish a fertilised egg. If fertilisation does not occur the egg lives for only about a day, and the lining is no longer needed. It slowly breaks down and is shed: the lining, together with blood and mucus and the unfertilised egg, passes out through the vagina. This is menstruation. The flow lasts about two to eight days, and the cycle is repeated about every $28$ days, the uterus building a fresh lining each time.

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8. Q68 Exemplar · Short Answer Questions
   What ​changes are ​observed ​in the ​uterus subsequent to implantation ​of ​young embryo?
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   The uterine lining is retained, a placenta develops for exchange between mother and embryo, and the uterus enlarges as the foetus grows.

   Three changes follow. First, the thick uterine lining is kept instead of being shed, so menstruation stops for the whole of the pregnancy. Second, a special tissue called the placenta develops where the embryo meets the uterine wall: it is a disc embedded in the wall, carrying villi on the embryo’s side which lie in blood spaces on the mother’s side. The large surface area so created lets glucose and oxygen pass from the mother’s blood to the embryo and the embryo’s wastes pass into the mother’s blood; the embryo is joined to the placenta by the umbilical cord. Third, the uterus itself grows enormously, its muscular wall stretching as the embryo develops into a foetus over about nine months, until those muscles contract rhythmically at birth to push the child out.

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9. Q69 Exemplar · Short Answer Questions
   What ​are ​the ​benefits of ​using mechanical barriers ​during ​sexual act?
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   They prevent pregnancy by keeping sperms from the egg and prevent sexually transmitted infections such as HIV-AIDS.

   Mechanical barriers, the condom worn on the penis and the covering placed in the vagina, work by keeping the sperms and the body fluids of the two partners physically apart, and this gives two benefits. First, they prevent pregnancy: the sperms cannot reach the egg, so fertilisation cannot take place. The method is simple, needs no medicine, is reversible and has none of the hormonal side-effects that contraceptive pills can have. Second, they prevent the spread of sexually transmitted infections such as syphilis, gonorrhoea and HIV-AIDS, because the barrier stops the exchange of body fluids and the direct contact by which those infections pass.

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10. Q70 Exemplar · Short Answer Questions
    In the given Figure 8.3 label the ​parts and mention their functions

    1. (a)Production of egg
    2. (b)Site ​of fertilisation
    3. (c)​Site ​of ​implantation
    4. (d)​Entry of the ​sperms

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    Ovary makes the egg, oviduct is the site of fertilisation, uterus is the site of implantation, and the vagina is where sperms enter.

    (a) Production of egg — the ovary. There are two, one on each side. Each produces the ovum, one mature egg being released about every $28$ days, and they also secrete the female sex hormones oestrogen and progesterone, which bring about the secondary sexual characters and control the uterine cycle. (b) Site of fertilisation — the oviduct or fallopian tube, in its upper part. Its fringed funnel collects the egg released from the ovary, the sperm meets and fuses with the egg here, and the tube then carries the zygote down to the uterus. (c) Site of implantation — the uterus. The embryo becomes embedded in its thick, spongy lining; the placenta develops here, the uterus nourishes and protects the foetus, and its muscular wall contracts at birth to deliver the baby. (d) Entry of the sperms — the vagina, which opens into the uterus through the cervix. It receives the sperms during the sexual act and is the passage by which they enter the female tract; it is also the birth canal and the passage for the menstrual flow.

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11. Q71 Exemplar · Short Answer Questions
    What would ​be the ratio of ​chromosome ​number between an egg and ​its zygote? How is ​the ​sperm genetically different from the ​egg?
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    Egg : zygote $=1:2$; the egg always carries X, the sperm X or Y, and they carry different parents' genes.

    The ratio is $1:2$. The egg is a haploid gamete carrying $n$ chromosomes and the zygote it forms on fusing with a sperm is diploid with $2n$, so egg : zygote $=n:2n=1:2$, which is $23:46$ in human beings. The sperm differs from the egg in the sex chromosome it carries. Both carry the same haploid number, but the female has the pair XX, so every egg receives an X, while the male has the pair XY, so meiosis puts an X into half the sperms and a Y into the other half. The egg is therefore of one genetic kind, $22+X$, and the sperm of two, $22+X$ or $22+Y$, and it is the sperm that decides the sex of the child. The two also differ in origin, the sperm carrying the father’s genes and the egg the mother’s, so their combinations of alleles differ.

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12. Q72 Exemplar · Long Answer Questions
    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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13. Q73 Exemplar · Long Answer Questions
    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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14. Q74 Exemplar · Long Answer Questions
    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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15. Q75 Exemplar · Long Answer Questions
    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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16. Q76 Exemplar · Long Answer Questions
    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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17. Q77 Exemplar · Long Answer Questions
    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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18. Q78 Exemplar · Long Answer Questions
    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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19. Q79 Exemplar · Long Answer Questions
    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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20. Q80 Exemplar · Long Answer Questions
    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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