Chapter 7 · How do Organisms Reproduce?

NCERT Class 10 Science Chapter 7 Exercises

All 11 questions of NCERT Class 10 Science Chapter 7 Exercises (How do Organisms Reproduce?), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Asexual reproduction ​takes ​place ​through ​budding ​in

   1. (a)Amoeba.
   2. (b)Yeast.
   3. (c)Plasmodium.
   4. (d)Leishmania.
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   (b) Yeast.

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2. Q2
   Which ​of the following is ​not ​a part of the ​female ​reproductive system ​in human beings?

   1. (a)Ovary
   2. (b)Uterus
   3. (c)Vas deferens
   4. (d)Fallopian tube
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   (c) Vas deferens

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3. Q3
   The anther contains

   1. (a)sepals.
   2. (b)ovules.
   3. (c)pistil.
   4. (d)pollen grains.
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   (d) pollen grains.

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4. Q4
   What ​are ​the advantages of ​sexual ​reproduction ​over ​asexual reproduction?
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   Sexual reproduction gives more variation by combining two parents' DNA, helping a species survive changes in its niche and evolve into new species.

   Sexual reproduction has these advantages over asexual reproduction. (i) It combines the DNA of two different individuals, so each offspring receives one set of genes from each parent and carries a combination that neither parent had. (ii) It therefore produces far more variation than asexual reproduction, in which the offspring are near-identical copies of a single parent and the only variation comes from the small inaccuracies of DNA copying. (iii) That variation is what allows a species to survive a change in its niche: if a population were uniform, a drastic change such as a rise in temperature could destroy all of it, whereas in a varied population some individuals may happen to suit the new conditions and survive to rebuild it. (iv) Useful variations are inherited and accumulate over generations, so sexual reproduction speeds up adaptation and evolution and can give rise to new species. (v) The chromosome number is nevertheless kept constant, because each germ cell carries only half the usual number and the full number is restored at fertilisation — so the extra variation costs nothing in stability.

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5. Q5
   What are ​the ​functions performed ​by ​the ​testis in ​human beings?
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   The testis forms sperms and secretes testosterone, which regulates sperm formation and brings about male puberty changes.

   The testis performs two functions in human beings. (i) It produces the male germ cells, the sperms. Their formation begins at puberty; each sperm is a small cell consisting mainly of genetic material with a long tail that helps it move towards the female germ cell. The sperms are carried away by the vas deferens, which joins the urethra, and secretions of the seminal vesicles and the prostate gland provide them with a fluid that makes their transport easier and gives them nutrition. (ii) It secretes the male sex hormone testosterone into the blood. Testosterone regulates the formation of the sperms and brings about the changes in appearance seen at puberty in boys, such as thicker hair on the face and a deepening of the voice. The testes lie in the scrotum outside the abdominal cavity because sperm formation requires a temperature lower than that of the body.

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6. Q6
   Why does menstruation occur?
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   The uterine lining prepared each month for a fertilised egg breaks down and is shed when the egg is not fertilised.

   Menstruation occurs because the uterus prepares itself every month for a pregnancy that does not always follow. One of the ovaries releases an egg each month, and while that egg is travelling down the fallopian tube the lining of the uterus becomes thick and spongy and richly supplied with blood, so that it could receive a fertilised egg and nourish the embryo for months. If the egg is fertilised, the embryo settles in this lining and grows there, and the lining is not shed. If the egg is not fertilised, it lives only about a day, and the thick lining is then of no use. It therefore slowly breaks down and comes out through the vagina as blood and mucous. This is menstruation. It lasts about two to eight days, and because an egg is released every month the whole cycle repeats about every twenty-eight days.

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7. Q7
   Draw ​a labelled ​diagram of the longitudinal ​section ​of ​a ​flower.
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   Draw the flower cut lengthwise and label pedicel, receptacle, sepal, petal, stamen (filament, anther) and carpel (stigma, style, ovary, ovules).

   The labelled longitudinal section is drawn in the figure. Working from the outside inwards, the flower shows four whorls borne on the receptacle at the top of the pedicel (the stalk). (i) The sepals are the small green leaf-like structures of the outermost whorl, which protect the flower while it is in bud. (ii) The petals lie just inside them and are larger and brightly coloured, which attracts insects. (iii) The stamens are the male reproductive parts; each has a thin stalk called the filament carrying a swollen anther at its top, and the anther contains the pollen grains. (iv) The carpel, or pistil, stands at the centre and is the female reproductive part; it has a sticky stigma at the top on which pollen lands, a narrow style below it, and a swollen ovary at the base. Because the flower is cut lengthwise, the ovary is shown opened, revealing the ovules inside it. After fertilisation the ovule becomes the seed and the ovary becomes the fruit.

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8. Q8
   What ​are the ​different methods ​of ​contraception?
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   Contraception uses barrier methods, oral pills, intra-uterine devices (loop, copper-T) and surgical methods (vasectomy, tubectomy).

   The methods of contraception fall into four groups, according to the link in reproduction that each one breaks. (i) Barrier methods use a physical device to stop the sperm from reaching the egg — condoms used on the penis, and similar coverings for use in the vagina. These also protect both partners from sexually transmitted diseases such as gonorrhoea, syphilis and HIV-AIDS. (ii) Chemical methods: drugs taken orally by females, the oral pills, change the hormonal balance of the body so that eggs are not released and fertilisation cannot occur; they may have side effects. (iii) Intra-uterine contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy; they can cause side effects due to irritation of the uterus. (iv) Surgical methods block the duct itself: in a vasectomy the vas deferens in the male is blocked, so sperms cannot be transferred, and in a tubectomy the fallopian tube in the female is blocked, so the egg cannot reach the uterus. These are safe in the long run, though the surgery itself can cause infections and other problems if it is not done properly. The surgical removal of an unwanted pregnancy is not a method of contraception, since contraception acts before a pregnancy begins.

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9. Q9
   How ​are the ​modes ​for reproduction ​different in unicellular and multicellular ​organisms?
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   Unicellular organisms reproduce by dividing the whole cell; multicellular ones use body parts or, in complex forms, specialised germ cells.

   The difference follows from how the body is built. In a unicellular organism the whole body is a single cell, so dividing that cell already produces two complete organisms and no special reproductive structure is needed. The modes are therefore all forms of cell division: binary fission into two equal daughters (Amoeba, and Leishmania splitting along a definite axis), multiple fission into many daughters at once (Plasmodium), and budding, in which a bud on the parent cell grows and detaches (yeast). In a multicellular organism the cells are organised into specialised tissues and organs, so a cell taken from the body cannot by itself rebuild the whole organism and simple division will not do. Simpler multicellular organisms can still use body-based methods — fragmentation in Spirogyra, regeneration in Hydra and Planaria through specialised cells that grow into all the tissues, budding in Hydra, spore formation in Rhizopus, and vegetative propagation from a root, stem or leaf in plants such as sugarcane, roses and Bryophyllum. As the body design becomes more complex, reproduction is taken over by specialised reproductive tissue that makes germ cells; two germ cells fuse to form a zygote, and the zygote develops into the new individual. In short: a unicellular organism reproduces by dividing its whole body, whereas a multicellular organism reproduces through a part of its body set aside for the purpose.

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10. Q10
    How ​does ​reproduction help in ​providing stability to populations ​of species?
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    Reproduction replaces lost individuals, keeping numbers steady, and copies DNA with small variations, so some individuals survive when the niche changes.

    Populations of a species occupy well-defined places, or niches, in an ecosystem, and reproduction gives such populations stability in two ways. (i) It keeps the numbers steady. Individuals are constantly lost through age, disease and predation, and without reproduction the number in an area could only fall. By producing new individuals to replace those that die, reproduction keeps the size of the population consistent, at the level the food and conditions of the niche can support. (ii) It keeps the species suited to its place and able to survive change. Reproduction works by copying DNA, and the DNA carries the design of the body, so the offspring have essentially the same body design as their parents and can use the same niche. At the same time DNA copying is never absolutely accurate, so small variations appear in every generation. If a population were entirely uniform, a drastic change in the niche — a rise in temperature, the drying up of a pool, a new disease — could wipe out all of it at once. Because the population is varied, some individuals may happen to suit the new conditions; they survive, reproduce, and the population is rebuilt. So reproduction stabilises a population both in its numbers and in its continued existence over time.

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11. Q11
    What ​could ​be the ​reasons ​for adopting ​contraceptive ​methods?
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    To avoid unwanted pregnancy, protect the mother's health, prevent sexually transmitted diseases and control population growth.

    Contraceptive methods are adopted for reasons that reach from one couple outwards to the whole country. (i) To avoid an unwanted pregnancy. Being sexually mature does not mean a couple is ready, physically, emotionally or economically, to become parents, and contraception lets them decide whether and when to have a child. (ii) To space and limit pregnancies for the health of the mother. Pregnancy and childbirth make heavy demands on a woman's body, and pregnancies that follow closely on one another leave no time for recovery, so both her health and the care available to the children already born suffer. (iii) To protect against sexually transmitted diseases. Bacterial infections such as gonorrhoea and syphilis, and viral ones such as HIV-AIDS, are transmitted during sexual activity, and barrier methods such as condoms prevent this as well as preventing pregnancy. (iv) To control the growth of the population. The rate at which a population increases depends on the birth rate against the death rate; better health and living conditions have lowered the death rate, so a high birth rate makes the population rise rapidly and press on the country's food, land, water, housing and employment. Keeping the birth rate down is therefore necessary if the standard of living is to be maintained.

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