7. How do Organisms Reproduce Class 10 NCERT Exemplar Solutions (CBSE 2026 Guide)

Class 10 Science Chapter 7: How do Organisms Reproduce : NCERT Exemplar Solutions with Detailed Explanation

Chapter 7: How do Organisms Reproduce

Multiple Choice Questions

1. In the list of organisms given below, those that reproduce by the asexual method are

(i) banana (ii) dog (iii) yeast (iv) Amoeba

(a) (ii) and (iv) (b) (i), (iii) and (iv) (c) (i) and (iv) (d) (ii), (iii) and (iv)

Answer: (b) (i), (iii) and (iv)

[ The organisms that reproduce by the asexual method in the given list are: (i) Banana (through rhizomes or suckers) (iii) Yeast (through budding or fission) (iv) Amoeba (through binary fission) ]

2. In a flower, the parts that produce male and female gametes (germ cells) are

(a) stamen and anther (b) filament and stigma

(c) anther and ovary (d) stamen and style

Answer: (c) Anther and ovary

[ In a flower, the parts that produce male and female gametes (germ cells) are the anther (produces pollen containing male gametes) and the ovary (contains ovules, which produce female gametes).]

3. Which of the following is the correct sequence of events of sexual reproduction in a flower?

(a) pollination, fertilisation, seedling, embryo

(b) seedling, embryo, fertilisation, pollination

(c) pollination, fertilisation, embryo, seedling

(d) embryo, seedling, pollination, fertilization

Answer: (c) Pollination, fertilisation, embryo, seedling

[ The correct sequence of events in sexual reproduction in a flower is pollination, fertilization, embryo development, and then the growth of the seedling.]

4. Offspring formed by asexual method of reproduction have greater similarity among themselves because

(i) asexual reproduction involves only one parent

(ii) asexual reproduction does not involve gametes

(iii) asexual reproduction occurs before sexual reproduction

(iv) asexual reproduction occurs after sexual reproduction

(a) (i) and (ii) (b) (i) and (iii) (c) (ii) and (iv) (d) (iii) and (iv)

Answer: (a) (i) and (ii)

[ Offspring formed by asexual reproduction have greater similarity among themselves because: (i) Asexual reproduction involves only one parent. (ii) Asexual reproduction does not involve gametes, and there is no mixing of genetic material from different individuals. ]

5. Characters transmitted from parents to offspring are present in

(a) cytoplasm (b) ribosome (c) golgi bodies (d) genes

Answer: (d) Genes

[ Characters transmitted from parents to offspring are present in genes. Genes are segments of DNA that carry the information needed to determine the traits and characteristics of an organism. They are passed from one generation to the next during reproduction.]

6. Characters that are transmitted from parents to offspring during reproduction show

(a) only similarities with parents

(b) only variations with parents

(c) both similarities and variations with parents

(d) neither similarities nor variations

Answer: (c) Both similarities and variations with parents

[ Characters transmitted from parents to offspring during reproduction can exhibit both similarities and variations with the parents. While certain traits may be inherited and show similarities, variations can also arise due to genetic recombination and mutations.]

7. A feature of reproduction that is common to Amoeba, Spirogyra and Yeast is that

(a) they reproduce asexually

(b) they are all unicellular

(c) they reproduce only sexually

(d) they are all multicellular

Answer: (a) They reproduce asexually

[ A feature of reproduction that is common to Amoeba, Spirogyra, and Yeast is that they reproduce asexually. They all have mechanisms for asexual reproduction, such as binary fission in Amoeba, fragmentation in Spirogyra, and budding in Yeast.]

8. In Spirogyra, asexual reproduction takes place by

(a) breaking up of filaments into smaller bits

(b) division of a cell into two cells

(c) division of a cell into many cells

(d) formation of young cells from older cells.

Answer: (a) Breaking up of filaments into smaller bits

[ In Spirogyra, asexual reproduction takes place by the breaking up of filaments into smaller bits, each of which can grow into a new filament under suitable conditions. ]

9. The ability of a cell to divide into several cells during reproduction in Plasmodium is called

(a) budding (b) reduction division (c) binary fission (d) multiple fission

Answer: (d) Multiple fission

[ The ability of a cell to divide into several cells during reproduction in Plasmodium is called multiple fission. This is a form of asexual reproduction in which a single cell divides into multiple daughter cells.]

10. The correct sequence of reproductive stages seen in flowering plants is

(a) gametes, zygote, embryo, seedling

(b) zygote, gametes, embryo, seedling

(c) seedling, embryo, zygote, gametes

(d) gametes, embryo, zygote, seedling

Answer: (a) Gametes, zygote, embryo, seedling

[ The correct sequence of reproductive stages in flowering plants is gametes (pollen and egg cells) → zygote (formed after fertilization) → embryo (develops from the zygote)→ seedling (develops from the germinating seed).]

11. The number of chromosomes in parents and offsprings of a particular species remains constant due to

(a) doubling of chromosomes after zygote formation

(b) halving of chromosomes during gamete formation

(c) doubling of chromosomes after gamete formation

(d) halving of chromosomes after gamete formation

Answer: (b) halving of chromosomes during gamete formation

[ Halving of chromosomes during gamete formation ensures that the number of chromosomes remains constant in parents and offspring. This process occurs during meiosis, where gametes are produced with half the chromosome number. ]

12. In Rhizopus, tubular thread-like structures bearing sporangia at their tips are called

(a) filaments (b) hyphae (c) rhizoids (d) roots

Answer: (b) Hyphae

[ In Rhizopus, tubular thread-like structures bearing sporangia at their tips are called hyphae. These structures play a crucial role in the asexual reproduction of Rhizopus by producing and releasing spores from the sporangia.]

13. Vegetative propagation refers to formation of new plants from

(a) stem, roots and flowers (b) stem, roots and leaves

(c) stem, flowers and fruits (d) stem, leaves and flowers

Answer: (b) Stem, roots, and leaves

[ Vegetative propagation refers to the formation of new plants from structures such as stems, roots, and leaves.]

14. Factors responsible for the rapid spread of bread mould on slices of bread are

(i) large number of spores

(ii) availability of moisture and nutrients in bread

(iii) presence of tubular branched hyphae

(iv) formation of round shaped sporangia

(a) (i) and (iii) (b) (ii) and iv) (c) (i) and (ii) (d) (iii) and (iv)

Answer: (c) (i) and (ii)

[ The factors responsible for the rapid spread of bread mold on slices of bread are: (i) Large number of spores (ii) Availability of moisture and nutrients in bread ]

15. Length of pollen tube depends on the distance between

(a) pollen grain and upper surface of stigma

(b) pollen grain on upper surface of stigma and ovule

(c) pollen grain in anther and upper surface of stigma

(d) upper surface of stigma and lower part of style

Answer: (d) Upper surface of stigma and lower part of style

[ The length of the pollen tube depends on the distance between the upper surface of the stigma and the lower part of the style. This is the path the pollen tube must traverse to reach the ovule for fertilization in the process of plant reproduction.]

16. Which of the following statements are true for flowers?

(i) Flowers are always bisexual

(ii) They are the sexual reproductive organs

(iii) They are produced in all groups of plants

(iv) After fertilisation they give rise to fruits

(a) (i) and (iv) (b) (ii) and (iii) (c) (i) and (iii) (d) (ii) and (iv)

Answer: (d) (ii) and (iv)

[ (ii) Flowers are the sexual reproductive organs. (iv) After fertilization, they give rise to fruits.

Flowers can be either bisexual (containing both male and female reproductive structures) or unisexual (containing either male or female reproductive structures) ]

17. Which among the following statements are true for unisexual flowers?

(i) They possess both stamen and pistil

(ii) They possess either stamen or pistil

(iii) They exhibit cross pollination

(iv) Unisexual flowers possessing only stamens cannot produce fruits

(a) (i) and (iv) (b) (ii), (iii) and (iv) (c) (iii) and (iv) (d) (i), (iii) and (iv)

Answer : (b) (ii), (iii) and (iv)

[ Unisexual flowers have either stamen or pistil, so they are not bisexual. They mostly depend on cross pollination. Staminate flowers lack ovary, so they cannot form fruits.]

18. Which among the following statements are true for sexual reproduction in flowering plants?

(i) It requires two types of gametes

(ii) Fertilisation is a compulsory event

(iii) It always results in formation of zygote

(iv) Offspring formed are clones

(a) (i) and (iv) (b) (i), (ii) and (iv) (c) (i), (ii) and (iii) (d) (i), (ii) and (iv)

Answer: (c) (i), (ii) and (iii)

[ Sexual reproduction needs male and female gametes, fertilisation must occur, and it forms a zygote. Offspring are not clones because genetic variation is produced.]

19. In Figure 8.1, the parts A, B and C are sequentially

(a) cotyledon, plumule and radicle

(b) plumule, radicle and cotyledon

(c) plumule, cotyledon and radicle

(d) radicle, cotyledon and plumule

Answer: (c) plumule, cotyledon and radicle

[ A → Plumule (future shoot) ; B → Cotyledon (food store) ; C → Radicle (Future root)]

20. Offspring formed as a result of sexual reproduction exhibit more variations because

(a) sexual reproduction is a lengthy process

(b) genetic material comes from two parents of the same species

(c) genetic material comes from two parents of different species

(d) genetic material comes from many parents

Answer: (b) Genetic material comes from two parents of the same species.

[ In sexual reproduction, genes from both parents combine, causing reshuffling of traits and variation in offspring.]

21. Reproduction is essential for living organisms in order to

(a) keep the individual organism alive

(b) fulfill their energy requirement

(c) maintain growth

(d) continue the species generation after generation

Answer: (d) Continue the species generation after generation

[ Reproduction is essential for living organisms to ensure the continuation of their species from one generation to the next.]

22. During adolescence, several changes occur in the human body. Mark one change associated with sexual maturation in boys

(a) loss of milk teeth (b) increase in height (c) cracking of voice (d) weight gain

Answer: (c) Cracking of voice

[ The cracking or deepening of the voice is a change associated with sexual maturation in boys during adolescence. This change is a result of the growth of the larynx (voice box) and the lengthening of the vocal cords, which is influenced by the increased production of testosterone during puberty.]

23. In human females, an event that reflects onset of reproductive phase is

(a) growth of body (b) changes in hair pattern

(c) change in voice (d) menstruation

Answer: (d) Menstruation

[ Menstruation marks the start of the reproductive phase in human females, as it indicates the beginning of the menstrual cycle and the potential for pregnancy.]

24. In human males, the testes lie in the scrotum, because it helps in the

(a) process of mating (b) formation of sperm

(c) easy transfer of gametes (d) all the above

Answer: (b) Formation of sperm

[ The main reason the testes are located in the scrotum in human males is to regulate the temperature for sperm production. The slightly lower temperature in the scrotum compared to the internal body temperature is conducive to the formation of sperm.]

25. Which among the following is not the function of testes at puberty?

(i) formation of germ cells (ii) secretion of testosterone

(iii) development of placenta (iv) secretion of estrogen

(a) (i) and (ii) (b) (ii) and (iii) (c) (iii) and (iv) (d) (i) and (iv)

Answer: (c) (iii) and (iv)

[ The functions of the testes at puberty include the formation of germ cells (sperm), and the secretion of testosterone. However, the development of the placenta and the secretion of estrogen are not functions of the testes; these are associated with female reproductive physiology.]

26. The correct sequence of organs in the male reproductive system for transport of sperms is

(a) testis → vasdeferens → urethra

(b) testis → ureter → urethra

(c) testis → urethra → ureter

(d) testis → vasdeferens → ureter

Answer: (a) Testis → Vas deferens → Urethra

[ Sperms are produced in the testes, travel through the vas deferens, and exit through the urethra during ejaculation.]

27. Which among the following diseases is not sexually transmitted?

(a) Syphillis (b) Hepatitis (c) HIV - AIDS (d) Gonorrhoea

Answer: (b) Hepatitis

[ Hepatitis is not exclusively a sexually transmitted disease.]

Short Answer Questions

28. In a bisexual flower inspite of the young stamens being removed artificially, the flower produces fruit. Provide a suitable explanation for the above situation.

Answer: In a bisexual flower, even if the young stamens are removed, the flower can still produce fruit due to the presence of functional pistils. If pollination occurs from an external pollen source, fertilization can take place, leading to the development of fruit despite the absence of its own stamens.

29. Can you consider cell division as a type of reproduction in unicellular organism? Give one reason.

Answer: Yes. In unicellular organisms, one cell divides to form two new identical individuals. So cell division itself increases the number of organisms, which is reproduction.

30. What is a clone? Why do offsprings formed by asexual reproduction exhibit remarkable similarity?

Answer: A clone is an organism produced from a single parent having identical genetic material.

Asexual reproduction involves only one parent and no fusion of gametes, so there is no mixing of genes. Hence offspring are genetically identical and show strong similarity.

31. Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes?

Answer: In sexually reproducing organisms, offspring inherit half the chromosomes from each parent via specialized reproductive cells called gametes (sperm and egg). Gametes are formed through meiosis, which reduces the chromosome number by half. During fertilization, the fusion of male and female gametes restores the original chromosome number in the offspring, ensuring it matches that of the parents.

32. Colonies of yeast fail to multiply in water, but multiply in sugar solution. Give one reason for this.

Answer: Colonies of yeast fail to multiply in water because they lack an energy source for growth and reproduction. In a sugar solution, yeast cells have access to sugars, which they can metabolize to obtain energy, allowing them to grow and multiply.

33. Why does bread mould grow profusely on a moist slice of bread rather than on a dry slice of bread?

Answer: Bread mould grows rapidly on moist bread because moisture provides suitable conditions for spores to germinate and absorb nutrients. Dry bread lacks enough water for proper growth and reproduction.

34. Give two reasons for the appearance of variations among the progeny formed by sexual reproduction.

Answer: Variations appear in sexually reproduced offspring because:

(a) Genetic material from two different parents mixes during fertilisation.

(b) Genes get rearranged in different combinations during formation of gametes.

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

Answer: Yes, a Planaria cut vertically into two halves can regenerate into two individuals.

In Figure D and E, the left half (D) regenerates a new right side including the head and tail, and the right half (E) regenerates a new left side similarly.

Planaria have remarkable regenerative abilities, allowing each half to develop into a complete organism, as long as the cut sections contain essential parts like a portion of the head and tail.

36. 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.

(a) Do larger organisms have more number of chromosomes/cells?

(b) Can organism with fewer chromosomes reproduce more easily than organisms with more number of chromosomes?

(c) More the number of chromosomes/cells greater is the DNA content. Justify.

Answer: Animals (Diploid chromosome number = 2n)

Human – 46 chromosomes (23 pairs)

Cat – 38 chromosomes

Chicken – 78 chromosomes

Carp (fish) – 100 chromosomes

Great white shark – 82 chromosomes

Plants (Diploid chromosome number = 2n)

Rice (Oryza sativa) – 24 chromosomes

Potato – 48 chromosomes

Wheat – 42 chromosomes (hexaploid)

Tomato – 24 chromosomes

Black mulberry – 308 chromosomes (154 pairs)

(a) No, larger organisms do not necessarily have more chromosomes. Chromosome number is species-specific, not related to body size.

(b) No, reproduction does not depend on number of chromosomes. It depends on reproductive mechanisms and compatibility, not chromosome count.

(c) Yes, more chromosomes generally mean more DNA because each chromosome contains a DNA molecule. So, organisms with more chromosomes/cells have greater total DNA content.

37. 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?

Answer: In tobacco plants, if the male gametes have 24 chromosomes, the female gametes will also have 24 chromosomes, as both male and female gametes are haploid.

When these gametes fuse during fertilization, they form a zygote, which is diploid. Therefore, the number of chromosomes in the zygote will be the sum of the chromosomes from both gametes:

24 (male gamete) + 24 (female gamete) = 48 chromosomes in the zygote.

38. Why cannot fertilisation take place in flowers if pollination does not occur?

Answer: Fertilisation requires the fusion of male and female gametes. Pollination transfers pollen (containing male gametes) from the anther to the stigma. Without pollination, male gametes cannot reach the female gamete inside the ovule. Hence, no fusion occurs, and fertilisation cannot take place.

39. 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?

Answer: Yes, the chromosome number of the zygote, embryonal cells, and adult cells of a particular organism remains constant.

This constancy is maintained because cell division during growth takes place by mitosis. In mitosis, each daughter cell receives the same number of chromosomes as the parent cell. Therefore, the chromosome number remains unchanged from the zygote stage to the embryonic stage and finally in the adult organism. For example, if a zygote has 46 chromosomes, the embryonal cells and adult body cells will also have 46 chromosomes.

40. Where is the zygote located in the flower after fertilization?

Answer: After fertilization, the zygote is located inside the ovule, which is present in the ovary of the flower.

41. Reproduction is linked to stability of population of a species. Justify the statement.

Answer: Reproduction is linked to the stability of the population of a species because it helps in the continuous production of new individuals. Living organisms die due to ageing, diseases, and environmental changes. If reproduction does not occur, the number of individuals in a species will gradually decrease and may finally become extinct. Through reproduction, new organisms are produced to replace the dead ones, which helps in maintaining the population of the species. Thus, reproduction ensures the continuity and stability of a species generation after generation.

42. How are general growth and sexual maturation different from each other?

Answer: The difference between General Growth and Sexual Maturation

General Growth	Sexual Maturation
It refers to the increase in height, weight, and overall body size of an organism.	It refers to the development of reproductive organs and the ability to reproduce.
It occurs from childhood onwards.	It mainly occurs during puberty.
It involves growth of all body parts.	It involves changes related to reproduction and secondary sexual characters.
Example: increase in height and muscle growth.	Example: development of breasts in girls and facial hair in boys.

43. Trace the path of sperm during ejaculation and mention the gland and their functions associated with the male reproductive system.

Answer: During ejaculation, the sperm travel through the following path:

Testes → Epididymis → Vas deferens → Urethra → Penis

The sperm are produced in the testes and stored in the epididymis. During ejaculation, they pass through the vas deferens and enter the urethra, from where they are released outside the body through the penis.

Glands Associated with the Male Reproductive System and Their Functions :

(a) Seminal vesicles

(i) They secrete a fluid rich in nutrients like fructose.

(ii) This fluid provides energy to the sperm.

(b) Prostate gland

(i) It secretes a thin alkaline fluid.

(ii) This fluid helps the sperm to move easily and protects them from acidity.

(i) They secrete mucus-like fluid.

(ii) This fluid lubricates the passage and helps in smooth movement of sperm.

44. What changes are observed in the uterus if fertilisation does not occur?

Answer: If fertilisation does not occur, the thick lining of the uterus is no longer needed. The uterine lining breaks down along with blood vessels and is removed from the body through the vagina in the form of menstrual flow. This process is called menstruation.

45. What changes are observed in the uterus subsequent to implantation of young embryo?

Answer: After the implantation of the young embryo, the lining of the uterus becomes thick, soft, and rich in blood vessels. This helps in providing nourishment and oxygen to the developing embryo. The uterus also supports the growth and development of the baby during pregnancy.

46. What are the benefits of using mechanical barriers during sexual act?

Answer: Mechanical barriers such as condoms help in preventing unwanted pregnancy by stopping the sperm from entering the female reproductive tract. They also protect both partners from sexually transmitted diseases (STDs) such as AIDS, gonorrhoea, and syphilis. Thus, mechanical barriers are a safe and effective method of contraception.

47. In the given Figure 8.3 label the parts and mention their functions

Fig. 8.3

(a) Production of egg

(b) Site of fertilisation

(c) Site of implantation

(d) Entry of the sperms

Answer: The diagram of parts of the female reproductive system:

(a) Production of egg → Ovary.

Ovary : It produces female gametes called eggs (ova).

(b) Site of fertilisation → Fallopian tube (Oviduct):

Fallopian tube (Oviduct): Fertilisation of egg and sperm takes place here.

Uterus: The fertilised egg gets implanted and develops into an embryo in the uterus.

(d) Entry of the sperms → Vagina

Vagina: Sperms enter the female reproductive system through the vagina.

48. What would be the ratio of chromosome number between an egg and its zygote? How is the sperm genetically different from the egg?

Answer: The chromosome number in an egg is half the chromosome number of the zygote because the egg is haploid (n) and the zygote is diploid (2n).

Therefore, the ratio of chromosome number between an egg and its zygote is 1 : 2

The sperm is genetically different from the egg because sperm may carry either an X chromosome or a Y chromosome, whereas the egg always carries only the X chromosome. This difference helps in determining the sex of the child.

Long Answer Questions

49. Why are budding, fragmentation and regeneration all considered as asexual types of reproduction? With neat diagrams explain the process of regeneration in Planaria.

Answer: Budding, fragmentation and regeneration are considered asexual modes of reproduction because only one parent is involved in these processes. There is no formation or fusion of gametes, and the new organisms are produced from the body parts or cells of the parent organism.

(a) In budding, a small outgrowth develops into a new individual.

(b) In fragmentation, the parent body breaks into pieces and each fragment grows into a new organism.

Planaria has a special ability to regenerate lost body parts. If the body of Planaria is cut into pieces, each piece can grow into a complete new organism through repeated cell division.

The diagram of regeneration in planaria :

50. Write two points of difference between asexual and sexual types of reproduction. Describe why variations are observed in the offspring formed by sexual reproduction.

Answer: The difference between asexual and sexual reproduction

Asexual Reproduction	Sexual Reproduction
Only one parent is involved.	Two parents are involved.
Gametes are not formed.	Male and female gametes are formed.
Offspring are genetically similar to the parent.	Offspring show variations.
It is a faster process.	It is comparatively slower.

Variations are observed in the offspring formed by sexual reproduction because the offspring receive genetic material from both parents. During fertilisation, the male and female gametes fuse and combine different characteristics from the parents. This mixing of genes produces new combinations of traits, leading to variations in the offspring. These variations help organisms to adapt better to changing environmental conditions.

51. 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.

Answer: The difference between pollination and fertilisation :

Pollination	Fertilisation
It is the transfer of pollen grains from the anther to the stigma.	It is the fusion of male and female gametes.
It takes place before fertilisation.	It takes place after pollination.
No zygote is formed.	A zygote is formed.
It is an external process in the flower.	It occurs inside the ovule.

Fertilisation takes place inside the ovule present in the ovary of the flower. The product of fertilisation is the zygote.

52. Distinguish between a gamete and zygote. Explain their roles in sexual reproduction.

Answer: The difference between gamete and zygote :

Gamete	Zygote
Gametes are reproductive cells such as sperm and egg.	Zygote is the cell formed after fusion of male and female gametes.
They are haploid and contain half the number of chromosomes.	It is diploid and contains the full number of chromosomes.
Gametes are produced in reproductive organs.	Zygote is formed during fertilisation.
They take part in fertilisation.	It develops into an embryo and later into a new organism.

Roles in sexual reproduction

(a) Gametes carry hereditary information from parents and fuse during fertilisation.

(b) The zygote formed after fertilisation undergoes repeated cell divisions and develops into a new individual.

53. Draw the diagram of a flower and label the four whorls. Write the names of gamete producing organs in the flower.

Answer: The diagram of a flower:

In flowering plants, the gamete-producing organs are:

(i) Stamens: The male reproductive organs, which produce pollen grains containing male gametes (sperm cells).

(ii) Pistils (or Carpels): The female reproductive organs, which include the ovary, where ovules containing female gametes (egg cells) are produced.

54. What is placenta? Mention its role during pregnancy?

Answer: The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta.

Role of placenta during pregnancy :

(a) It supplies oxygen and nutrients from the mother to the developing baby.

(b) It removes carbon dioxide and waste materials from the baby’s blood.

(d) It also secretes hormones that help in maintaining pregnancy.

55. What are various ways to avoid pregnancy? Elaborate any one method.

Answer: Various ways to avoid pregnancy are called contraceptive methods. These methods help in preventing fertilisation or implantation.

The different methods are: (a) Natural methods , (b) Barrier methods , (c) Chemical methods , (d) Surgical methods

Barrier Method: In this method, physical barriers such as condoms are used to prevent the sperm from reaching the egg. Condoms are used by males during sexual intercourse. They not only help in avoiding pregnancy but also protect against sexually transmitted diseases (STDs) like AIDS and gonorrhoea. This method is simple, safe, and widely used.

56. How does fertilisation take place? Fertilisation occurs once in a month. Comment.

Answer: Fertilisation is the process of fusion of the male gamete (sperm) and female gamete (egg) to form a zygote. During sexual intercourse, sperms are released into the female reproductive tract. One sperm fuses with the egg in the fallopian tube, resulting in fertilisation. The zygote formed then starts developing into an embryo.

The statement “Fertilisation occurs once in a month” is related to the release of an egg from the ovary during ovulation, which generally occurs once in every menstrual cycle. Fertilisation can take place only if the sperm meets the egg during this period. Therefore, the chances of fertilisation are usually once in a month.

57. Reproduction is essentially a phenomenon that is not for survival of an individual but for the stability of a species. Justify.

Answer: Reproduction is not necessary for the survival of an individual because an organism can live its life without reproducing. However, reproduction is essential for the continuity and stability of a species. Living organisms die due to ageing, diseases, or accidents. If new individuals are not produced, the species may gradually disappear from the Earth. Reproduction helps in maintaining the population by replacing dead individuals with new ones. It also transfers hereditary characters from parents to offspring and introduces variations, which help organisms adapt to changing environmental conditions. Thus, reproduction ensures the existence and stability of a species generation after generation.

58. Describe sexually transmitted diseases and mention the ways to prevent them.

Answer: Sexually transmitted diseases (STDs) are diseases that spread from one person to another mainly through sexual contact with an infected person. Some common STDs are AIDS, gonorrhoea, and syphilis. These diseases can affect the reproductive organs and may also damage overall health if not treated properly.

Ways to Prevent Sexually transmitted diseases :

(a) Use condoms during sexual intercourse.

(b) Maintain personal hygiene and cleanliness.

(d) Use only sterilised needles and syringes.

(e) Take proper medical advice and treatment if symptoms appear.

(f) Ensure safe transfusion of blood.