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.

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Exercises (11) Intext Questions (Page 114) (2) Intext Questions (Page 119) (5) Intext Questions (Page 126) (5)

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Multiple Choice Questions (27) Short Answer Questions (21) Long Answer Questions (10)

CBSE Class 10 Science previous year papers, with answers →

Questions · page 13 of 22

Q241 CBSE 2023
1. (a)What happens when the egg is not fertilised ?
2. (b)How is sperm genetically different from a human egg / ova ?
3. (c)List any three contraceptive methods practised for family planning. Mention how these methods work.
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An unfertilised egg causes menstruation; the sperm decides the child's sex; barrier, hormonal and surgical methods prevent pregnancy.
(a) If the egg is not fertilised, it lives for about one day. The uterus, which prepares every month to receive a fertilised egg, has a thick and spongy lining; this lining is no longer needed, so it slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation. It takes place roughly every month and usually lasts about two to eight days. (b) A sperm and an egg each carry half the chromosomes of a body cell: 23, that is, 22 autosomes and one sex chromosome. Every egg carries an X chromosome ( $22 + X$ ), whereas a sperm carries either an X or a Y chromosome ( $22 + X$ or $22 + Y$ ). So the sperm decides the sex of the child: an X-bearing sperm gives a girl (XX) and a Y-bearing sperm gives a boy (XY). (c) Three contraceptive methods: 1. Barrier (mechanical) methods: a condom on the penis, or a similar covering in the vagina, stops the sperm from reaching the egg; the condom also helps prevent the transmission of many sexually transmitted infections. 2. Hormonal methods: oral pills change the hormonal balance of the body so that eggs are not released, and fertilisation cannot occur. 3. Surgical methods: in vasectomy the vas deferens is blocked so that sperm transfer is prevented; in tubectomy the fallopian tubes are blocked so that the egg cannot reach the uterus. (Devices such as the loop or copper-T placed in the uterus are another method.)

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Q242 CBSE 2023
1. (a)Name the two types of pollination and differentiate between them.
2. (b)Explain the post fertilization changes that occur in the ovary of a flower.
3. (c)Given below is a diagram of a germinating seed. Label the parts that (i) gives rise to future shoot. (ii) gives rise to future root system. (iii) stores food.
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Pollination is self or cross; the ovule becomes the seed and the ovary the fruit; plumule forms the shoot, radicle the root.
(a) The two types are self-pollination and cross-pollination. In self-pollination the pollen is transferred from the stamen to the stigma of the same flower, so it does not have to be carried to another flower, and the offspring show less variation. In cross-pollination the pollen is transferred from one flower to the stigma of another, usually on a different plant of the same species; it needs an agent such as wind, water or animals, and when it brings together gametes from two different plants the offspring show more variation. (b) After fertilisation the zygote divides several times to form an embryo within the ovule; the ovule develops a tough coat and is gradually converted into a seed; and the ovary grows rapidly and ripens to form a fruit. Meanwhile the petals, sepals, stamens, style and stigma may shrivel and fall off. (c) (i) A, the plumule, gives rise to the future shoot. (ii) B, the radicle, gives rise to the future root system. (iii) C, the cotyledon, stores food.

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Q243 CBSE 2023
1. (i)Where are testes located in the human males and why ? State two function of the testes.
2. (ii)In the human female, one of the ovaries releases an egg every month. State the changes that take place if (1) the egg is fertilized, and (2) the egg is not fertilized. (iii) What is done during the surgical method in males and females to prevent pregnancy ?
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The scrotum keeps the testes cool; a fertilised egg implants, otherwise menstruation follows; vasectomy and tubectomy block the sperm duct or fallopian tube.
(i) The testes lie outside the abdominal cavity, in a small muscular pouch called the scrotum, because the formation of sperms requires a temperature about $2$ to $3^{\circ}$ C lower than the normal body temperature. Two functions: (1) they produce the male gametes, the sperms, and (2) they secrete the male sex hormone testosterone, which brings about the secondary sexual characters and regulates sperm production. (ii)(1) If the egg is fertilized, a zygote is formed; it divides as it moves down the oviduct and the embryo becomes implanted in the thick spongy lining of the uterus. A placenta develops for nutrition, oxygen and the removal of wastes, and the pregnancy continues for about nine months. (2) If the egg is not fertilized, it survives only about one day. The thickened uterine lining is no longer needed, so it breaks down and is shed along with blood and mucus through the vagina. This is menstruation; it lasts about $2$ to $8$ days and recurs about every $28$ days. (iii) In males the surgical method is vasectomy: a small portion of each vas deferens (sperm duct) is cut out and the cut ends are tied, so sperms cannot reach the semen. In females it is tubectomy: a small portion of each fallopian tube (oviduct) is cut out and the ends tied, so the egg cannot meet a sperm or reach the uterus. Neither operation removes the gonad, so hormone secretion is unaffected.

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Q244 CBSE 2023
1. (i)Define vegetative propagation. List its any two advantages.
2. (ii)Write in tabular form two differences between binary fission and multiple fission giving one example for each.
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Vegetative propagation grows a new plant from a vegetative part without gametes; binary fission gives two daughter cells, multiple fission gives many.
(i) Vegetative propagation is the type of asexual reproduction in which new plants are produced from a vegetative part of the parent plant - a root, a stem, a leaf or a bud - without the formation or fusion of gametes and without any seed. Two advantages: (1) All the new plants are genetically similar enough to the parent plant to have all its characteristics, so a desirable character of the parent plant is preserved. (2) Plants that have lost the capacity to produce seeds, such as banana, orange, jasmine and rose, can still be propagated. (A further advantage: plants raised by vegetative propagation bear flowers and fruits earlier than plants raised from seed.) (ii) Differences between binary fission and multiple fission: Binary fission: the parent cell splits into **two** daughter cells. | Multiple fission: the parent cell splits into **many** daughter cells at the same time. Binary fission: the nucleus divides only once. | Multiple fission: the nucleus divides many times before the cytoplasm divides around each nucleus. Example of binary fission: Amoeba (also Leishmania, bacteria). | Example of multiple fission: Plasmodium.

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Q245 CBSE 2023
(a) What is regeneration ? Which types of cells are used by such multicellular organisms to regenerate ? Name the organism which exhibits this process. (b) State one function of each of the following parts of the human male reproductive system :
1. (i)Vas deferens
2. (ii)Testes
3. (iii)Prostate glands
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Regeneration by specialised cells occurs in Hydra and Planaria; the vas deferens carries sperms, the testes make them and the prostate nourishes them.
(a) Regeneration is the process in which some fully differentiated simple multicellular organisms give rise to complete new individuals from their own body parts: if such an organism is cut or broken into many pieces, each piece grows into a complete organism. It is carried out by **specialised** cells, which first proliferate to make large numbers of cells and then differentiate into the various cell types and tissues of the new body. Hydra and Planaria are organisms that exhibit this process. (b)(i) Vas deferens: it carries the sperms away from the testis to the urethra. (ii) Testes: they produce the sperms, that is the male gametes, and secrete the male sex hormone testosterone. (iii) Prostate glands: their secretion, added to that of the seminal vesicles, nourishes the sperms and provides the fluid medium that makes their transport easier.

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Q246 CBSE 2023
(i) Name the parts in the human female reproductive system where the following functions take place :
1. (1)Maturation of eggs
2. (2)Fusion of the egg and the sperm
3. (3)Implantation of the zygote (ii) What happens to the egg (1) when it is fertilised ? (2) when it is not fertilised ?
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Eggs mature in the ovary, fertilisation occurs in the oviduct, implantation in the uterus; an unfertilised egg leads to menstruation.
(i) (1) Maturation of eggs - the **ovary**; (2) Fusion of the egg and the sperm - the **oviduct** (fallopian tube); (3) Implantation of the zygote - the **uterus**. (ii) (1) When the egg is fertilised: a zygote is formed in the oviduct. It divides repeatedly to form a ball of cells, the embryo, which travels to the uterus and is implanted in its thick spongy lining. The placenta develops and supplies the embryo with glucose and oxygen and removes its wastes; the embryo grows into a foetus and, after about nine months, the child is born. (2) When the egg is not fertilised: it lives for only about one day and dies. The thickened uterine lining, no longer needed, breaks down slowly and is shed with blood and mucous through the vagina. This is menstruation, which lasts about two to eight days and recurs about every twenty-eight days.

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Q247 CBSE 2023
(i) What happens when :
1. (1)Leaves of Bryophyllum fall on the soil ?
2. (2)Planaria is cut into many pieces ?
3. (3)Sporangia of Rhizopus on maturation liberate spores ? Mention the modes of reproduction in each of the above three cases. (ii) Write the changes that occur in a flower once the fertilisation has taken place.
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Bryophyllum (vegetative propagation), Planaria (regeneration) and Rhizopus (spore formation) reproduce asexually; after fertilisation the ovule becomes the seed and the ovary the fruit.
(i)(1) The margin of a Bryophyllum leaf carries vegetative buds in its notches; when the leaf falls on damp soil each bud grows into a new Bryophyllum plant. Mode: vegetative propagation. (2) Each cut piece of Planaria regrows the parts it is missing and becomes a complete new Planaria. Mode: regeneration. (3) The liberated spores, each protected by a thick wall, are carried by air; on reaching a moist surface each germinates into a new Rhizopus. Mode: spore formation. All three are asexual, involving one parent and no fusion of gametes. (ii) After fertilisation the zygote divides and develops into the embryo; the ovule develops into the seed, its wall hardening into a seed coat; the ovary grows and ripens into the fruit, which protects the seeds and helps disperse them; and the petals, sepals, stamens, style and stigma usually wither and fall off (in a few fruits such as the tomato the sepals persist).

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Q248 CBSE 2023
1. (i)State in brief the functions of the following organs in the human female reproductive system : (I) Ovary (II) Fallopian tube (III) Uterus
2. (ii)What is menstruation ? Why does it occur ?
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Ovary makes eggs and hormones, fallopian tube is the fertilisation site, uterus nourishes the embryo; menstruation sheds the unneeded uterine lining.
(i)(I) Ovary: it produces the female gamete, the ovum or egg - thousands of immature eggs are present at birth and one matures and is released about every $28$ days - and it also secretes the female hormones oestrogen and progesterone, which bring about the secondary sexual characters and control the uterine cycle. (i)(II) Fallopian tube (oviduct): it carries the ovum from the ovary towards the uterus, and it is the site of fertilisation - the sperm meets the egg here and the zygote is formed; the two tubes unite into the uterus. (i)(III) Uterus: its lining thickens every cycle to receive a fertilised egg; the embryo is implanted in that lining and is nourished through the placenta, and the uterus holds and protects the developing foetus until birth. (ii) Menstruation is the monthly discharge of blood and mucus through the vagina, consisting of the broken-down lining of the uterus together with the unfertilised egg; it lasts about $2$ to $8$ days and recurs about every $28$ days. It occurs because the uterine lining, which had thickened and become rich in blood vessels in readiness to receive a fertilised egg, is not needed when fertilisation does not take place; it therefore breaks down and is shed.

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Q249 CBSE 2023
1. (a)What is meant by pollination ? Name and differentiate between the two types of pollination.
2. (b)What happens to the pollen which falls on a suitable stigma ?
3. (c)Mention the post-fertilisation changes that occur in (i) Ovary and (ii) Ovule in the flower.
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Pollination moves pollen from anther to stigma; a pollen tube leads to fertilisation, then the ovary becomes the fruit and the ovule the seed.
(a) Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a carpel. Its two types are **self**-pollination, in which the pollen is transferred to the stigma of the **same** flower, and **cross**-pollination, in which the pollen is transferred from one flower to the stigma of **another** flower, usually on a different plant of the same species. Cross-pollination needs an agent such as wind, water, an insect or a bird to carry the pollen, and when it brings together gametes from two different plants it produces more variation in the offspring. (b) On a suitable stigma the pollen grain germinates and puts out a pollen tube, which grows down through the style and enters the ovule in the ovary; the male germ cell travels down this tube and fuses with the female germ cell (the egg) inside the ovule, so fertilisation occurs and a zygote is formed. (c)(i) The ovary grows and ripens into the **fruit**, its wall becoming the pericarp, while the petals, sepals, stamens and stigma usually wither and fall off. (c)(ii) The ovule develops into the **seed**: the zygote inside it divides to form the embryo and the coverings of the ovule harden into the seed coat.

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Q250 CBSE 2023
(i) Explain by giving one example each :
1. (1)Unisexual flowers
2. (2)Bisexual flowers (ii) Name the labelled parts A, B, C and D in the diagram given below. (iii) “Pollination may occur without fertilisation but fertilisation will not take place without pollination.” Give reason to justify this statement.
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Unisexual flowers have one organ, bisexual flowers both; A is the pollen grain, B stigma, C pollen tube, D egg; fertilisation needs pollination.
(i) (1) Unisexual flowers contain only one of the two reproductive organs - either the stamens or the pistil, not both. Example: papaya (also watermelon). (2) Bisexual flowers contain both the stamens and the pistil in the same flower. Example: Hibiscus (also mustard). (ii) A - Pollen grain; B - Stigma; C - Pollen tube; D - Female germ cell (the egg) inside the ovule. (iii) Pollination is only the transfer of pollen from the anther to the stigma, whereas fertilisation is the fusion of the male germ cell of the pollen with the female germ cell of the ovule. After pollination the grain must still germinate, grow a pollen tube down the style and reach the ovule; if the pollen is of another species, is non-viable, or conditions are unfavourable, this sequence stops before the ovule is reached, so pollination can occur without fertilisation. But without pollination no pollen grain is on the stigma, so no pollen tube grows, the male germ cell never reaches the ovule, and fertilisation cannot take place. Pollination is necessary for fertilisation but not sufficient for it.

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Q251 CBSE 2023
All the reproductive methods of living organisms are broadly categorized into two types : 1. Asexual reproduction, and 2. Sexual reproduction. Asexual reproduction involves the participation of a single parent without the formation of gametes, fertilisation and transfer of genetic material. This method is a common means of rapidly increasing offsprings under favourable conditions.
1. (a)Name the type of fission that occurs in Leishmania and Plasmodium.[1]
2. (b)Write one advantage of sexual mode of reproduction over asexual reproduction.[1]
3. (c)(i)Give reasons why : (i) Colonies of yeast fail to multiply in water but multiply in sugar solution. (ii) Rhizopus individuals do not grow on a dry slice of bread.[2]
4. (c)(ii)OR : Name the filamentous structures a student could identify when he collected water from a pond that appeared dark green. How do these organisms multiply ? Explain.[2]
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Leishmania: binary fission; Plasmodium: multiple fission; sexual reproduction gives variation; yeast needs sugar to bud; Rhizopus needs moisture; Spirogyra fragments.
(a) Leishmania divides by binary fission, which takes place in a definite orientation with respect to its whip-like flagellum; Plasmodium divides by multiple fission, producing many daughter cells at one time. (b) Sexual reproduction combines the DNA of two parents, so the offspring show variations; these variations let the species adapt to changes in the environment and are the raw material of evolution. (c)(i) Yeast multiplies by budding, and forming buds needs energy and raw material. Water supplies no nutrient, so the colony cannot grow; sugar solution supplies glucose, which the yeast uses for energy and growth, so it buds rapidly. Rhizopus spores need moisture to germinate and its hyphae absorb food in solution, so on a dry slice of bread no germination or growth is possible. (c)(ii) OR — The filamentous structures are Spirogyra, a filamentous green alga. It multiplies by fragmentation: on maturing, the filament breaks up into two or more fragments and each fragment grows by cell division into a new complete filament.

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Q252 CBSE 2022
Name the part/organ of the human male reproductive system
1. (a)which is a common passage for both sperms as well as urine.
2. (b)where formation of germ cells or sperms takes place.
3. (c)which is blocked to prevent the transfer of sperm.
4. (d)which provides nutrition to the sperms.
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The parts are the urethra, testes, vas deferens, and the seminal vesicles with the prostate gland.
(a) Urethra. (b) Testes (testis). (c) Vas deferens (sperm duct); blocking it is the surgical method of contraception called vasectomy. (d) Seminal vesicles and the prostate gland: their secretions form the fluid in which the sperms travel, and this fluid provides nutrition to the sperms.

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Q253 CBSE 2022

Differentiate between the process of binary fission and multiple fission giving an example of each.

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Binary fission splits one cell into two (Amoeba); multiple fission splits it into many daughter cells (Plasmodium).
(1) Number of daughter cells: in binary fission one parent cell divides into two daughter cells; in multiple fission one parent cell divides into many daughter cells at the same time. (2) Division of the nucleus: in binary fission, as in Amoeba, the nucleus divides once and then the cytoplasm divides; in multiple fission, as in Plasmodium, the nucleus divides repeatedly to form many nuclei before the cell divides. (3) Examples: binary fission in Amoeba (also Leishmania); multiple fission in Plasmodium, the malarial parasite.

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Q254 CBSE 2022
1. (a)Which of the following flowers will have higher possibility of self-pollination ? Mustard, Papaya, Watermelon, Hibiscus
2. (b)List the two reproductive parts of a bisexual flower.
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Mustard and hibiscus, being bisexual, are likely to self-pollinate; a bisexual flower's reproductive parts are the stamens and the pistil.
(a) Mustard and hibiscus. Their flowers are bisexual: each flower has both stamens and a pistil, so pollen from its own anthers can reach its own stigma. Papaya and watermelon have unisexual flowers, with either stamens or a pistil, so their pollen has to be carried to a different flower. (b) The two reproductive parts of a bisexual flower are the stamens, the male part (anther and filament), which produce the pollen grains, and the pistil or carpel, the female part (stigma, style and ovary), whose ovary contains the ovules.

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Q255 CBSE 2022
Give reasons :
1. (i)Placenta is extremely essential for foetal development.
2. (ii)Uterine lining becomes thick and spongy after fertilisation.
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(i) The placenta supplies the embryo with food and oxygen and removes its wastes; (ii) the thick, spongy lining receives and nourishes the embryo.
(i) The embryo gets its nutrition from the mother’s blood through the placenta, a disc embedded in the uterine wall. The placenta has villi on the embryo’s side surrounded by blood spaces on the mother’s side, giving a large surface area through which glucose and oxygen pass from the mother to the embryo, and the embryo’s waste substances pass into the mother’s blood to be removed. Without the placenta the foetus could not obtain food and oxygen or get rid of its wastes, so it could not develop. (ii) After fertilisation the zygote is implanted in the lining of the uterus. The lining is thick, spongy and richly supplied with blood so that it can receive the embryo and nourish it as it grows; the placenta develops in it. (The uterus prepares this lining every month; if the egg is not fertilised, the lining breaks down and is shed in menstruation.)

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Q256 CBSE 2022

What is Pistil (Carpel) ? State the main function of any two of its parts.

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The pistil is the female part of a flower; the stigma receives pollen, and the ovary holds the ovules and later becomes the fruit.
The pistil (carpel) is the female reproductive part of a flower, present at its centre. It has three parts: the stigma, the style and the ovary. Main functions (any two): 1. Stigma: the terminal part, which may be sticky, receives the pollen grains during pollination. 2. Style: the elongated middle part through which the pollen tube grows down to reach the ovary, carrying the male germ-cell. 3. Ovary: the swollen bottom part contains the ovules, each with an egg cell (female gamete); fertilisation takes place in the ovule, and after fertilisation the ovary develops into the fruit.

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Q257 CBSE 2022

What is DNA ? "The DNA copies generated may be similar but may not be identical to the original." Give reason to justify this statement.

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DNA is the chemical in chromosomes that carries inherited information; copying is never absolutely reliable, so copies are similar but not always identical.
DNA (deoxyribonucleic acid) is the chemical present in the chromosomes in the nucleus of a cell that carries the information for the inheritance of features from parents to the next generation; it is the information source for making proteins. Reason: during reproduction a cell builds a copy of its DNA by chemical reactions, and no biochemical reaction is absolutely reliable. So some small changes (variations) creep in each time DNA is copied, and the copies formed are similar to each other but may not be identical to the original DNA. (A copy with a drastic change cannot work with the cell's apparatus and the new cell dies; cells whose copies carry small changes survive, and this gives variation.)

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Q258 CBSE 2022

What is Stamen ? State what happens when a pollen lands on a suitable stigma.

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The stamen is the male reproductive part; pollen on a suitable stigma grows a tube to the ovule, where fertilisation forms a zygote.
The stamen is the male reproductive part of a flower. It consists of an anther, which produces the pollen grains, borne on a stalk called the filament. When a pollen grain lands on a suitable stigma: 1. It germinates: a pollen tube grows out of the pollen grain. 2. The pollen tube grows down through the style and reaches the ovary, where it enters an ovule, carrying the male germ-cell. 3. The male germ-cell fuses with the female germ-cell (egg) in the ovule. This fusion is fertilisation, and it gives a zygote, which is capable of growing into a new plant. After fertilisation the zygote develops into an embryo, the ovule becomes a seed and the ovary ripens into a fruit.

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Q259 CBSE 2022

What is variation ? Why is variation beneficial to the species but not necessary for an individual ?

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Variation means differences among individuals of a species; it helps the species survive changes, but an individual can live without it.
Variation is the occurrence of differences in characters among the individuals of a species - between parents and offspring, and among the offspring themselves. It arises during reproduction. Beneficial to the species: if the niche of a population changes drastically (for example, the water in which bacteria live becomes much warmer through global warming), a population of identical individuals could be wiped out. If some individuals carry variations suited to the new conditions (heat-resistant bacteria), they survive and reproduce, and the species continues. Variation thus helps a species to survive over time, and it is the basis for evolution. Not necessary for an individual: an individual can live and reproduce in its existing conditions without any new variation, and a particular variation may be useful, useless or even harmful to the individual that carries it. Its value to the species shows chiefly when the environment changes, through the survival of some individuals of the population over generations.

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Q260 CBSE 2022

Which one of the two multicellular organisms — Spirogyra and Planaria reproduces by regeneration and why ? Give an example of any other organism which can also reproduce by the same process.

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Planaria regenerates, because specialised cells in each piece rebuild the lost parts; Spirogyra only fragments. Another example: Hydra.
Planaria reproduces by regeneration. Planaria is a multicellular animal with a fully differentiated body, with a distinct head and tail. If its body is cut into pieces, specialised cells in each piece multiply and form a mass of cells, and these cells change into the various kinds of cells and tissues in an organised sequence, so each piece rebuilds the parts it has lost and grows into a complete Planaria. Spirogyra does not do this: it is a simple filament of similar cells that, on maturing, just breaks into pieces, each of which grows by cell division into a new filament. That is fragmentation, not regeneration. Another organism that reproduces by regeneration: Hydra.

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