Chapter 8 · How do Organisms Reproduce?

NCERT Exemplar Class 10 Science Chapter 8 Short Answer Questions

All 21 questions of NCERT Exemplar Class 10 Science Chapter 8 Short Answer Questions (How do Organisms Reproduce?), in the book's order, each with its answer and a step-by-step solution.

1. Q28
   In ​a bisexual ​flower ​inspite of the young stamens ​being removed artificially, the ​flower produces fruit. ​Provide a suitable explanation for the ​above situation.
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   The pistil is intact, so cross-pollination by insects or wind, followed by fertilisation, lets the ovary ripen into fruit.

   The stamens are the male part, so removing them stops only self-pollination; the pistil was left untouched. Pollen carried from another flower by an insect or by the wind landed on the stigma, which is cross-pollination. A pollen tube then grew down the style and the male gamete fertilised the egg in the ovule, and the fertilised ovary ripened into the fruit.

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2. Q29
   Can ​you consider ​cell ​division ​as a ​type of reproduction in unicellular ​organism? ​Give one reason.
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   Yes, because a unicellular organism's whole body is one cell, so dividing produces two new individuals, as in Amoeba.

   Yes. In a unicellular organism the whole body is one cell, so when that cell divides the parent itself becomes two new individuals: an Amoeba dividing by binary fission gives two Amoebae. Cell division therefore produces new individuals of the same kind, which is what reproduction means.

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3. Q30
   What ​is ​a clone? ​Why ​do ​offsprings formed by asexual reproduction exhibit ​remarkable similarity?
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   A clone is a group of genetically identical individuals from one parent; asexual offspring are alike because mitosis copies the parent's DNA.

   A clone is a group of individuals that are morphologically and genetically identical to one another and to the single parent they came from. Offspring of asexual reproduction form such a clone because they are produced from one parent by mitotic divisions alone: no gametes are made and no fusion takes place, so the DNA handed on is a copy of the very same parental DNA. The only differences are the very small variations produced by occasional inaccuracies in DNA copying.

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4. Q31
   Explain ​how, ​offspring and parents of ​organisms reproducing sexually ​have the same ​number of ​chromosomes?
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   Meiosis halves the chromosome number in gametes and fertilisation doubles it again, so parents and offspring both have $2n=46$.

   Because the halving at gamete formation is exactly cancelled by the doubling at fertilisation. Germ cells of each parent divide by meiosis, which halves the number, so every gamete is haploid at $n=23$ against the parents’ $2n=46$. At fertilisation one male gamete fuses with one female gamete, so the zygote is diploid again at $23+23=46$. The zygote then divides only by mitosis, which copies the chromosomes exactly, so every body cell of the offspring carries the same $2n=46$ as the parents.

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5. Q32
   Colonies ​of yeast fail to ​multiply ​in ​water, but ​multiply ​in sugar solution. Give ​one ​reason for this.
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   Yeast cannot make its own food; water has no nutrients, whereas sugar solution gives it glucose for energy and growth, so it multiplies.

   Yeast is a saprophytic fungus and cannot make its own food, so it has to take in ready-made organic nutrients. Plain water supplies none, so with no source of energy or of carbon the cells can neither grow nor bud and the colony does not multiply. A sugar solution supplies glucose, which the yeast respires to release energy and uses as raw material for new cell material, so the cells bud and the colony multiplies.

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6. Q33
   Why does bread mould ​grow ​profusely on a ​moist ​slice of bread ​rather than on a dry ​slice of bread?
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   Moisture lets spores germinate and hyphae digest and absorb food, so mould grows profusely on moist bread, not dry bread.

   Bread mould, Rhizopus, is a saprophytic fungus. Its spores germinate only when moisture is present, and its hyphae feed by secreting digestive enzymes onto the bread and absorbing the digested food in solution, both of which need water. A moist slice supplies that water, so the spores germinate and the mycelium spreads profusely. A dry slice has too little water, so the spores stay dormant, the enzymes cannot act and nothing dissolves, and the mould hardly grows.

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7. Q34
   Give two ​reasons ​for the ​appearance ​of variations ​among ​the ​progeny formed ​by sexual reproduction.
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   Meiosis gives each gamete a different combination of genes, and chance fertilisation joins any male gamete with any female gamete.

   (i) Gametes are made by meiosis, in which the maternal and paternal chromosomes exchange segments (crossing over) and are then assorted into the gametes independently of one another, so every gamete carries a different combination of the parent’s genes. (ii) Fertilisation is a chance event between two different parents: any one of the many genetically different male gametes may fuse with any one of the female gametes, so each zygote receives a fresh combination of DNA, half from each parent.

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8. Q35
   Would ​a ​Planaria cut ​vertically ​into two halves regenerate into ​two individuals? Complete Figure 8.2 D ​and E ​by indicating the regenerated ​regions.

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   Yes; each half regenerates the missing half from dividing cells, so one Planaria gives two whole individuals.

   Yes. Planaria has a very high power of regeneration because its body carries large numbers of unspecialised, actively dividing cells. A vertical cut leaves each half with part of the head and one eyespot, half the pharynx and part of every organ system, along with those cells. A mass of dividing cells forms over each cut surface and grows into the missing half, so each half becomes a complete worm and one animal gives two individuals. In the figure, D and E are completed by drawing in the missing half of each animal, the second eyespot and the other half of the pharynx and the body, so that D and E become two whole Planaria.

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9. Q36
   From the internet, gather information about the ​chromosome numbers of five animals and five plants. Correlate ​the number with the ​size of organism ​and answer the following questions.

   1. (a)Do larger ​organisms have more number ​of chromosomes/cells?
   2. (b)Can organism with fewer chromosomes reproduce more ​easily than organisms with more number of chromosomes?
   3. (c)More the number of chromosomes/cells greater is the DNA content. Justify.
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   Larger organisms do not have more chromosomes and fewer chromosomes do not ease reproduction; more chromosomes mean more DNA only within a species.

   (a) No. There is no correlation between body size and chromosome number. Among animals the fruit fly has $2n=8$, the frog $26$, the human $46$, the elephant $56$ and the much smaller dog $78$; among plants pea has $14$, onion $16$, maize $20$, rice $24$ and potato $48$. The dog is far smaller than the elephant yet carries more chromosomes. (b) No. How easily an organism reproduces depends on its mode of reproduction, its generation time, the age at which it matures and the number of gametes and offspring it produces, not on its chromosome number. All the homologous pairs are dealt with together in one meiosis, so extra pairs do not slow it. (c) The statement holds only within one species. DNA is carried on the chromosomes, so for cells of the same species more chromosomes does mean more DNA: a human body cell with $46$ holds twice the DNA of a gamete with $23$. Across species it fails, because chromosomes differ enormously in size — a lily has $24$ very long chromosomes while the small fern Ophioglossum reticulatum has over $1200$ short ones. So it is justified as a comparison of ploidy within a species, not as a general rule.

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10. Q37
    In ​tobacco plant, the male gametes have ​twenty ​four chromosomes. What is the ​number of chromosomes in the female gamete? ​What ​is the number of chromosomes in the ​zygote?
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    The female gamete has 24 chromosomes and the zygote has $24+24=48$ chromosomes.

    The female gamete also has $24$ chromosomes, because both gametes of a species are haploid. The zygote has $24+24=48$ chromosomes, the diploid number of the tobacco plant.

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11. Q38
    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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12. Q39
    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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13. Q40
    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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14. Q41
    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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15. Q42
    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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16. Q43
    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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17. Q44
    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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18. Q45
    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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19. Q46
    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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20. Q47
    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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21. Q48
    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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