Class 10 Science · Chapter 7

How do Organisms Reproduce?

434 Class 10 Science questions on How do Organisms Reproduce? from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 20 of 22

1. Q381 CBSE 2017
   What ​is ​vegetative ​propagation ? List ​its ​any four advantages.
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   New plants grow from a root, stem or leaf of the parent without seeds, giving earlier flowering and fruiting and keeping the parent's characters.

   Vegetative propagation is a method of asexual reproduction in plants in which new plants develop from vegetative parts of the parent plant, such as the root, stem or leaf, instead of from seeds (for example, the buds in the notches of a Bryophyllum leaf, or the eyes of a potato tuber). Advantages (any four): (1) Plants raised this way bear flowers and fruits earlier than those produced from seeds. (2) Plants that have lost the capacity to produce seeds, such as some varieties of banana, orange, rose and jasmine, can be propagated. (3) The new plants are genetically similar enough to the parent plant to have all its characteristics, so a desired variety is preserved. (4) Many plants can be raised quickly from one parent plant by layering, cutting or grafting, as is done for sugarcane, roses and grapes.

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2. Q382 CBSE 2017
   1. (a)What is meant by fragmentation in organisms ? Name a multicellular organism which reproduces by this method.
   2. (b)How does ​regeneration take ​place ​? Why ​is this process not possible ​in all animals ?
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   Spirogyra reproduces by fragmentation; regeneration needs cells that can rebuild the body, which complex animals lack.

   (a) Fragmentation is a method of asexual reproduction in which a multicellular organism with a simple body breaks up, on maturing, into two or more small pieces (fragments), and each fragment grows into a new individual. Example: Spirogyra, a filamentous green alga. (b) Regeneration is the ability of an organism to give rise to new individuals from its body parts: when a simple animal such as Planaria or Hydra is cut into pieces, many of the pieces grow into complete organisms. It is carried out by specialised cells, which divide to form a large mass of cells; different cells of this mass then change into the various cell types and tissues in an organised sequence called development. It is not possible in all animals because complex animals have highly specialised cells organised into tissues, organs and organ systems at definite positions in the body, and they do not have the specialised cells that can multiply and develop into a complete new body from a piece; they can at most heal wounds or regrow some lost parts.

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3. Q383 CBSE 2017
   What happens when :

   1. (a)Accidently, Planaria ​gets cut into many ​pieces ?
   2. (b)​Bryophyllum leaf falls on the ​wet soil ​?
   3. (c)​On maturation sporangia ​of Rhizopus bursts ?
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   Planaria pieces regenerate into new individuals, Bryophyllum buds grow into new plants, and Rhizopus spores are released to form new Rhizopus.

   (a) The pieces grow into new Planaria by regeneration: in each piece, specialised cells multiply and develop into the different cell types and tissues, rebuilding the missing parts, so each piece can form a complete Planaria. (b) The buds present in the notches along the margin of the Bryophyllum leaf develop roots and shoots on the wet soil and grow into new Bryophyllum plants (vegetative propagation by the leaf). (c) The spores inside are released and dispersed (spore formation). Each spore is protected by a thick wall, and when it lands on a moist surface such as bread it grows into a new Rhizopus.

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4. Q384 CBSE 2017
   How do variations arise in organisms ? ​“Variation is useful for the survival of species.” Justify ​this ​statement ​with ​the ​help of ​an ​example.
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   Variation arises from inexact DNA copying and from combining two parents' DNA; it lets some individuals, like heat-resistant bacteria, survive environmental change.

   Variations arise during reproduction in two ways: 1. The copying of DNA is not absolutely accurate, so the copies are similar to, but not always identical with, the original, and these small changes are passed on; this happens even in asexual reproduction. 2. In sexual reproduction each offspring receives DNA from two parents in a new combination, which produces much more variation. Example: in a population of bacteria living in temperate water, a few may, by variation, be able to tolerate more heat. If the temperature of the water rises, for example through global warming, most of the bacteria die, but the heat-resistant variants survive and multiply, so the population is not wiped out. Had all the bacteria been of the ordinary kind, the whole population could have died. Variation thus gives some individuals a chance to survive a change in their surroundings, so it is useful for the survival of species.

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5. Q385 CBSE 2017
   Name the two types of ​mammalian gametes. How are these different ​from each other ​? ​Name the type ​of reproduction ​they are involved in. ​Write the advantage ​of this type of reproduction.
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   Sperm and egg are the gametes; the sperm is small and motile, the egg large with stored food; sexual reproduction gives variation.

   The two types of mammalian gametes are the male gamete, the sperm, and the female gamete, the egg (ovum). Differences: the sperm is small and motile — it swims with its tail — whereas the egg is much larger and non-motile; the egg contains a store of food for the early embryo, while the sperm has almost none; sperms are produced in very large numbers by the testes, whereas the ovaries release far fewer eggs (in humans, usually one a month). They are involved in sexual reproduction. Advantage: sexual reproduction combines DNA from two different individuals, so the offspring show new combinations of variations; this variation makes it more likely that some individuals of the species survive when the environment changes.

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6. Q386 CBSE 2017
   Reproduction is one of the ​most important ​characteristics of living ​beings. Give ​three reasons in ​support of the statement.
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   Reproduction keeps the species going, passes on DNA and body design to offspring, and produces variation that helps survival and evolution.

   (1) It keeps the species going. Individuals die — of age, disease or accident — and reproduction produces new individuals of the same kind to take their place, so the species continues from one generation to the next. An individual does not need to reproduce to stay alive, but its kind does. (2) It passes on the DNA, and with it the body design. In reproduction DNA copied from the parent, or from both parents, passes to the offspring, so the offspring look very much like their parents. This consistency of DNA copying keeps the body-design features that let a population live in its particular place in the ecosystem, which is why reproduction is linked to the stability of populations. (3) It produces variation. DNA copying is not absolutely accurate, and sexual reproduction combines DNA from two parents, so offspring differ slightly. If the surroundings of a population change drastically, some of these variants may survive when others cannot, so variation is useful for the survival of species over time and is the basis of evolution.

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7. Q387 CBSE 2017
   What is ​pollination ​? List in ​tabular ​form any ​two distinguishing main points ​between its ​two types.
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   Pollination is transfer of pollen from anther to stigma; self-pollination uses pollen of the same plant, cross-pollination pollen of another plant.

   Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a pistil. Its two types are self-pollination and cross-pollination. Self-pollination: the pollen comes from the same flower, or from another flower of the same plant | Cross-pollination: the pollen comes from a flower on another plant of the same species. Self-pollination: often needs no outside agent | Cross-pollination: needs an agent such as wind, water or animals (insects, birds). Self-pollination: less variation in the offspring | Cross-pollination: more variation in the offspring.

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8. Q388 CBSE 2017
   State ​the basic requirement for sexual reproduction ? Write ​the ​importance of ​such ​reproductions in ​nature.
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   Gametes of two parents must fuse; the resulting variation helps the species survive changes and is the basis of evolution.

   Basic requirement: male and female gametes (germ cells) must be formed and must fuse, in fertilisation, to form a zygote. The gametes usually come from two different parents, a male and a female; each gamete carries half the number of chromosomes, so the zygote gets the full number. Importance in nature: (i) The offspring combine DNA from two parents, so sexual reproduction produces more variation in a population than asexual reproduction does. (ii) Variation helps a species survive changes in its surroundings: some individuals may have variations that let them survive where others would die. (iii) Variations accumulated over many generations are the basis of evolution and of the origin of new species.

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9. Q389 CBSE 2017
   What is regeneration ​? Give one ​example of ​an organism that shows ​this ​process and one organism that does ​not. Why ​does regeneration not occur in the latter ?
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   Regeneration is regrowth of whole individuals from body pieces (Planaria); humans cannot, as complex bodies lack cells that rebuild all tissues and organs.

   Regeneration is the ability of a fully differentiated organism to give rise to new individual organisms from its body parts: if the organism is cut into pieces, many of the pieces grow into complete individuals. It is carried out by specialised cells, which multiply and then develop into the various cell types and tissues. An organism that shows regeneration: Planaria (or Hydra). An organism that does not: a human being (or another complex animal, such as a dog). Reason: complex animals have a highly organised body. Their specialised cells form tissues, the tissues form organs and organ systems, and these lie at definite positions in the body. They lack cells that can multiply and develop into all the tissues and organs of a new body, so a cut-off part cannot rebuild the whole organism.

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10. Q390 CBSE 2017
    What ​is ​vegetative propagation ​? ​State ​two ​advantages and two ​disadvantages of this method.
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    Vegetative propagation grows new plants from a parent's root, stem or leaf; it gives earlier flowering but little variation.

    Vegetative propagation is a mode of asexual reproduction in which new plants develop from a vegetative part of a parent plant — its root, stem or leaf — under appropriate conditions, without seeds. Examples: buds in the notches of a Bryophyllum leaf grow into new plants; the methods of layering and grafting raise new plants from stems. Advantages (any two): (1) Plants that have lost the capacity to produce seeds, such as banana, orange, rose and jasmine, can be propagated. (2) Plants raised this way bear flowers and fruits earlier than those produced from seeds. (3) The new plants are genetically similar enough to the parent plant to have all its characteristics, so a desirable variety is kept. Disadvantages (any two): (1) The new plants show very little variation, so they are less able to cope with a change in their surroundings, and a disease or pest that attacks one plant is likely to attack them all. (2) Undesirable characters of the parent plant are passed on along with the desirable ones.

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11. Q391 CBSE 2017
    State ​in sequence four ​main ​steps ​of sexual ​reproduction ​and write two advantages of such reproductions.
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    Gametes form and fuse at fertilisation to make a zygote, which develops into an embryo; advantages: more variation, survival and evolution.

    Four main steps, in sequence: 1. Formation of gametes: the male and female parents form gametes, each with half the number of chromosomes of the body cells. 2. Fertilisation: a male gamete and a female gamete fuse. 3. Formation of the zygote: the fused cell, the zygote, has the full number of chromosomes again (in humans, $23+23=46$). 4. Development of the embryo: the zygote divides repeatedly to form an embryo, which develops into a new individual. Two advantages of sexual reproduction: First, it produces more variation in the offspring, because the DNA of two parents is combined in new ways. Second, this variation helps the species to survive changes in its environment, and it is the basis of evolution.

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12. Q392 CBSE 2017
    State the ​changes that take place in the ​uterus when :

    1. (a)​Implantation ​of ​embryo ​has occurred.
    2. (b)Female ​gamete / egg is not fertilised.
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    After implantation the lining is kept and a placenta forms; if the egg is not fertilised, the lining is shed as menstruation.

    (a) When implantation has occurred: the thick, spongy lining of the uterus, richly supplied with blood, is retained (so there is no menstruation) and nourishes the embryo. A special tissue, the placenta, develops in the uterine wall: villi on the embryo's side lie in blood spaces on the mother's side, so glucose and oxygen pass from the mother's blood to the embryo and the embryo's wastes pass into the mother's blood. The uterus enlarges as the embryo develops, over about nine months. (b) When the egg is not fertilised: the thickened lining is no longer needed, so it slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation; it lasts about 2 to 8 days and occurs roughly every month.

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13. Q393 CBSE 2017
    What is ​contraception ? ​Name ​any two methods. How does the use of these ​methods have a direct ​effect on the health and prosperity of a family ? ​State ​any three points.
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    Contraception prevents pregnancy, e.g. by condoms or oral pills; it protects health and raises a family's standard of living.

    Contraception is the deliberate use of methods or devices to prevent pregnancy. Two methods: (1) the barrier method, such as a condom, which stops the sperm from reaching the egg; (2) oral contraceptive pills, which change the hormonal balance so that eggs are not released. (Others: a Copper-T or loop placed in the uterus; surgical methods such as vasectomy and tubectomy.) Effects on the health and prosperity of a family: 1. Births can be spaced and the number of children limited, which protects the mother's health, since each pregnancy makes major demands on her body and mind. 2. Condoms also help prevent the transmission of many sexually transmitted diseases, such as HIV-AIDS, gonorrhoea and syphilis, keeping the family healthier. 3. A smaller family can give each child better food, education and health care with the same income, which raises the family's standard of living.

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14. Q394 CBSE 2017
    List three techniques that have been developed to prevent pregnancy. Which one of these ​techniques is not meant for males ? ​How does ​the use of ​these techniques have a ​direct impact on ​the health and ​prosperity of a family ?
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    Barrier, hormonal and surgical methods prevent pregnancy; oral pills are not meant for males; planned families improve a family's health and prosperity.

    Three techniques: (1) Barrier (mechanical) methods — a condom worn on the penis, or a similar covering worn in the vagina, stops sperms from reaching the egg. (2) Hormonal (chemical) methods — oral pills change the hormonal balance of a woman's body so that eggs are not released. (3) Surgical methods — the vas deferens of the male is blocked (vasectomy), or the fallopian tubes of the female are blocked (tubectomy). (Devices such as the loop or copper-T placed in the uterus are another technique.) Not meant for males: the oral pills, which stop the release of eggs by a woman's ovaries. (The copper-T, placed in the uterus, is also used only by women.) Impact on the family: Health — births can be spaced and limited, so the mother's body is not worn down by frequent pregnancies, which make major demands on a woman's body and mind; the condom also helps to prevent the spread of sexually transmitted infections between the partners. Prosperity — a small, planned family shares its income among fewer members, so each child can be given better food, health care and education, and the family's standard of living rises.

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15. Q395 CBSE 2017
    List the two ​types of ​reproduction. Which ​one of the two ​is responsible for bringing in more ​variations ​in its progeny and how ?
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    Asexual and sexual reproduction; sexual gives more variation because the zygote combines DNA from two different parents.

    The two types of reproduction are asexual reproduction (one parent) and sexual reproduction (two parents). Sexual reproduction brings more variations into the progeny. How: in sexual reproduction each parent forms gametes (germ cells) carrying half the DNA - one set of chromosomes. When a male gamete fuses with a female gamete at fertilisation, the zygote receives DNA from two different individuals, whose DNA already differs, so it gets a new combination of variations that neither parent had. In asexual reproduction the DNA of a single parent is copied, and the only variations come from small inaccuracies in copying, so the offspring remain very similar to the parent.

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16. Q396 CBSE 2017
    Write ​the functions ​of each ​of the following parts ​in ​flowering plants :

    1. (i)Pistil
    2. (ii)Stamen
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    The pistil is the female part (stigma, style, ovary with ovules) and the stamen is the male part, whose anther makes pollen.

    (i) Pistil: it is the female reproductive part of the flower. The stigma receives the pollen grains; the style is the passage through which the pollen tube grows down to the ovary; the ovary contains the ovules, each with an egg cell (the female gamete), and fertilisation takes place in the ovule. After fertilisation the ovules develop into seeds and the ovary into the fruit. (ii) Stamen: it is the male reproductive part of the flower. Its anther produces the pollen grains, which contain the male germ cells (male gametes), and its filament holds the anther up.

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17. Q397 CBSE 2017
    In our country ​the legally prescribed minimum ​age for marriage is 18 years for females and 21 years for males. Why is ​it essential to fix the ​minimum age for ​marriage ? Give three reasons to ​justify your ​answer. List three methods for preventing unwanted pregnancy.
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    A minimum age lets body and mind mature and allows education; unwanted pregnancy is prevented by condoms, oral pills or a loop or Copper-T.

    Why a minimum age is essential (three reasons): (1) Before this age the body, particularly the reproductive system of a girl, is still developing. Pregnancy and childbirth make major demands on a woman’s body and mind, and an early pregnancy can harm the health of the young mother and of her baby. (2) Sexual maturation is gradual; being sexually mature does not mean that the mind is ready for marriage or for the responsibility of bringing up children. The minimum age gives time to become mentally and emotionally mature. (3) Marrying later gives time to complete education and become economically independent, and it tends to mean fewer children, which helps to check the growth of the population. Three methods for preventing unwanted pregnancy: (1) Mechanical barriers: a condom on the penis, or a similar covering worn in the vagina, stops sperms from reaching the egg. (2) Oral pills, which change the hormonal balance of the body so that eggs are not released. (3) A loop or Copper-T placed in the uterus. Surgical methods (vasectomy, in which the vas deferens is blocked, and tubectomy, in which the fallopian tubes are blocked) are also used.

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18. Q398 CBSE 2017
    List ​any four steps ​involved ​in sexual reproduction and ​write its ​two ​advantages.
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    Gametes form, are transferred, fuse into a zygote and develop into an embryo; variation is the advantage, aiding survival and evolution.

    Four steps in sexual reproduction: (1) Formation of gametes: male and female gametes are formed in the reproductive organs, each with half the number of chromosomes of a body cell. (2) Transfer of gametes: the male gamete is brought to the female gamete (pollination in flowering plants; transfer of sperms into the female body in humans). (3) Fertilisation: the male and female gametes fuse to form a zygote, which has the full number of chromosomes. (4) Development: the zygote divides repeatedly to form an embryo, which grows into a new individual. Two advantages: (i) The offspring get DNA from two parents, so sexual reproduction produces more variation than asexual reproduction. (ii) Variation helps a species survive when conditions change and, accumulated over generations, is the basis of evolution.

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19. Q399 CBSE 2017
    Describe reproduction by ​spores ​in ​Rhizopus.
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    Rhizopus forms spores in sporangia; they are released, carried by air, and germinate on a moist surface into new Rhizopus.

    Rhizopus (bread mould) reproduces asexually by spores. Its body is a network of thread-like hyphae growing on moist bread. Some hyphae grow upwards as thin stalks, each bearing a tiny round, knob-like structure at its tip called a sporangium. Inside the sporangium a large number of tiny spores are formed. When the sporangium matures, it bursts and releases the spores, which are light and are carried by air. Each spore is covered by a thick wall that protects it until it comes into contact with a moist surface; there it germinates and grows into a new Rhizopus individual.

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20. Q400 CBSE 2017
    (a) Write the functions of the following parts of human ​male reproductive ​system ​:

    1. (i)​Testis ​;
    2. (ii)Vas deferens ;
    3. (iii)Urethra ;
    4. (iv)Prostate (b) List any ​two common pubertal ​changes that appear in human ​males.
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    Testis makes sperms and testosterone; vas deferens carries sperms; urethra passes sperms and urine; prostate adds fluid; puberty deepens the voice.

    (a) (i) Testis: produces the male germ cells, the sperms, and secretes the male hormone testosterone. (ii) Vas deferens: carries the sperms from the testis to the urethra. (iii) Urethra: forms the common passage for both the sperms and urine, which leave the body through it by way of the penis. (iv) Prostate: adds its secretion (with the seminal vesicles), so that the sperms are in a fluid which makes their transport easier and also provides nutrition. (b) Any two of: (1) thick hair grows on the face (moustache and beard) and in the armpits and the genital area; (2) the voice begins to crack and becomes deeper; (3) the penis occasionally becomes enlarged and erect; (4) the skin becomes oily and pimples may appear.

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