Class 12 Biology NCERT Solutions Chapter 3 Human Reproduction

NCERT Solutions for Biology Chapter 3 Human Reproduction

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NCERT Biology Chapter 3 Human Reproduction Solved Textbook Questions

1.Fill in the blanks:
(a) Humans reproduce _ (asexually/sexually)
(b) Humans are _ (oviparous, viviparous, ovoviviparous)
(c) Fertilisation is _ in humans (external/internal)
(d) Male and female gametes are _ (diploid/haploid)
(e) Zygote is _ (diploid/haploid).
(f) The process of release of ovum from a mature follicle is called _
(g) Ovulation is induced by a hormone called _
(h) The fusion of male and female gametes is called _
(i) Fertilisation takes place in _
(j) Zygote divides to form _______which is implanted in uterus.
(k) The structure which provides vascular connection between foetus and uterus is called _______

Ans- (a) Humans reproduce sexually
(b) Humans are viviparous
(c) Fertilisation is internal in humans
(d) Male and female gametes are haploid
(e) Zygote is diploid
(f) The process of release of ovum from a mature follicle is called ovulation
(g) Ovulation is induced by a hormone called luteinizing hormone
(h) The fusion of male and female gametes is called fertilisation
(i) Fertilisation takes place in a fallopian tube
(j) Zygote divides to form a blastocyst which is implanted in the uterus.
(k) The structure which provides vascular connection between foetus and uterus is called placenta

2. Draw a labelled diagram of male reproductive system.


male reproductive system

3. Draw a labelled diagram of female reproductive system.


female reproductive system

4. Write two major functions each of testis and ovary.

Ans- Testes are the primary male reproductive structure. The two main functions of the testes are: Leydig cells of the testes produce the male sex hormone testosterone. It produces male gametes “sperms” in the seminiferous tubules. Ovaries are the primary female reproductive structure. The two main functions of the ovaries are: It produces female gametes which is egg or ovum. It secretes female sex hormones which are oestrogen and progesterone.

5. Describe the structure of a seminiferous tubule.

Ans- The production of sperms in the testes takes place in a highly coiled structure called the seminiferous tubules. These tubules are located in the testicular lobules. Each seminiferous tubule is lined by germinal epithelium. It is lined on its inner side by two types of cells namely spermatogonia and sertoli cells respectively. Spermatogonia are male germ cells which produce primary spermatocytes by meiotic divisions. Primary spermatocytes undergo further meiotic division to form secondary spermatocytes and finally, spermatids. Spermatids later metamorphoses into male gametes called spermatozoa. Sertoli cells are known as nurse cells of the testes as they provide nourishment to the germ cells. There are large polygonal cells known as interstitial cells or leydig cells just adjacent to seminiferous tubules. These cells secrete the male hormone called testosterone.

6. What is spermatogenesis? Briefly describe the process of spermatogenesis.

Ans- Spermatogenesis is the process in which the formation of sperms takes place within the seminiferous tubules. The process occurs in the seminiferous tubules located inside the testes. The process of spermatogenesis is as follows:- Production of sperms starts from diploid spermatogonia present in the inner wall of the seminiferous tubules. The spermatogonia undergo mitotic divisions to form primary spermatocytes. The primary spermatocytes undergoes meiosis I and gives rise to two haploid secondary spermatocytes which further it undergoes meiotic division-2 to form four haploid spermatids. These spermatids undergo differentiation to form male gamete “sperm” or spermatozoa through the process of spermiogenesis, after which the sperm heads becomes embedded in the sertoli cells. These sperms are released out of seminiferous tubules by spermiation

7. Name the hormones involved in regulation of spermatogenesis.

Ans- Follicle-stimulating hormones (FSH) and luteinizing hormones (LH) are secreted by gonadotropin releasing hormones from the hypothalamus .These hormones are involved in the regulation of the process of spermatogenesis. FSH acts on sertoli cells, whereas LH acts on leydig cells of the testis and stimulates the process of spermatogenesis.

8. Define spermiogenesis and spermiation.

Ans-  Spermiogenesis: It is the process of transforming spermatids into matured spermatozoa or sperms. Spermiation: It is the process when mature spermatozoa are released from the sertoli cells into the lumen of seminiferous tubules.

9. Draw a labelled diagram of sperm.


diagram of sperm

10. What are the major components of seminal plasma?

Ans- Semen (produced in males) is composed of sperms and seminal plasma. The major components of the seminal plasma in the male reproductive system are mucus, spermatozoa, and various secretions of accessory glands. The seminal plasma is rich in fructose, calcium, ascorbic acid, and certain enzymes. It provides nourishment and protection to sperms

11. What are the major functions of male accessory ducts and glands?

Ans- The male accessory ducts are vasa efferentia, epididymis, vas deferens, and rete testis. They play an important role in the transport and temporary storage of sperms. On the contrary, male accessory glands are seminal vesicles, prostate glands, and bulbourethral glands. These glands secrete fluids that lubricate the reproductive system and sperms. The sperms get dispersed in the fluid which makes their transportation into the female body easier. The fluid is rich in fructose, ascorbic acid, and certain enzymes. They also provide nutrients and activate the sperm.

12. What is oogenesis? Give a brief account of oogenesis.

Ans- The process of formation of a mature female gamete is called oogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. The corpus luteum is a temporary endocrine structure involved in ovulation and early pregnancy

13. Draw a labelled diagram of a section through ovary.


section through ovary

14. Draw a labelled diagram of a Graafian follicle?


diagram of Graafian follicle

15. Name the functions of the following:
(a) Corpus luteum (b) Endometrium
(c) Acrosome (d) Sperm tail (e) Fimbriae

Ans- (a) Corpus luteum − Corpus luteum is formed from the ruptured Grafiaan follicle. It secretes progesterone hormone during the luteal phase of the menstrual cycle. A high level of progesterone inhibits the secretions of FSH and LH, thereby preventing ovulation. It also allows the endometrium of the uterus to proliferate and to prepare itself for implantation.

(b) Endometrium − It is the innermost lining of the uterus. It is rich in glands and undergoes cyclic changes during various phases of the menstrual cycle to prepare itself for the implantation of the embryo.

(c) Acrosome − It is a cap-like structure present in the anterior part of the head of the sperm. It contains hyaluronidase enzyme, which hydrolyses the outer membrane of the egg, thereby helping the sperm to penetrate the egg at the time of fertilization.

(d) Sperm tail − It is the longest region of the sperm that facilitates the movement of the sperm inside the female reproductive tract.

(e) Fimbriae − They are finger-like projections at the ovarian end of the fallopian tube. They help in the collection of the ovum (after ovulation), which is facilitated by the beating of the cilia.

16. Identify True/False statements. Correct each false statement to make it true.
(a) Androgens are produced by Sertoli cells. (True/False)
(b) Spermatozoa get nutrition from Sertoli cells. (True/False)
(c) Leydig cells are found in ovary. (True/False)
(d) Leydig cells synthesise androgens. (True/False)
(e) Oogenesis takes place in corpus luteum. (True/False)
(f) Menstrual cycle ceases during pregnancy. (True/False)
(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)

Ans-  (a) (False) Androgens are produced by Leydig cells found in seminiferous tubules of the testis.

(b) Spermatozoa get nutrition from Sertoli cells. (True)

(c) (False) Leydig cells are found in the seminiferous tubules of the testis.

(d) Leydig cells synthesise androgens. (True)

(e) (False) Oogenesis takes place in the ovary.

(f) Menstrual cycle ceases during pregnancy. (True)

(g) Presence or absence of the hymen is not a reliable indicator of virginity or sexual experience. (True)

17. What is menstrual cycle? Which hormones regulate menstrual cycle?

Ans- The menstrual cycle is a series of cyclic physiologic changes that take place inside the female reproductive tract in primates. The whole cycle takes around 28 days to complete. The end of the cycle is accompanied by the breakdown of uterine endothelium, which gets released in the form of blood and mucous through the vagina. This is known as menses. The follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone are the various hormones that regulate the menstrual cycle. The level of FSH and LH secreted from the anterior pituitary gland increases during the follicular phase. FSH secreted under the influence of RH (releasing hormone) from the hypothalamus stimulates the conversion of a primary follicle into a graafian follicle. The level of LH increases gradually leading to the growth of follicle and secretion of estrogen. Estrogen inhibits the secretion of FSH and stimulates the secretion of luteinizing hormone. It also causes the thickening of the uterine endometrium. The increased level of LH causes the rupturing of the graafian follicle and release the ovum into the fallopian tube. The ruptured graafian follicle changes to corpus luteum and starts secreting progesterone hormone during the luteal phase. Progesterone hormone helps in the maintenance and preparation of endometrium for the implantation of the embryo. High levels of progesterone hormone in the blood decrease the secretion of LH and FSH, therefore inhibiting further ovulation.

18. What is parturition? Which hormones are involved in induction of parturition?

Ans- When the embryo is fully developed into a baby, vigorous contraction of the uterus results in the release or delivery of the baby from the mother’s womb. This process is referred to as parturition. The hormone which is involved in parturition is “oxytocin”. This hormone is synthesised in the hypothalamus and is stored in the pituitary gland. When the foetal development is completed, it is released by the pituitary gland and acts on the smooth muscles of the uterus. The uterine muscles contract and pushes the baby out from the mother’s body. The hormone relaxin is involved in widening and softening of the cervix and after the baby is being expelled out, the hormone relaxin relaxes the pelvic ligaments.

19. In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?

Ans- The sex chromosome in human female is XX while in a male it is XY. Thus, female produces haploid gametes having all X chromosomes but male produces two types of gametes with either X or Y. The sex of the child will be dependent on which sperm whether the one with X chromosomes or the one with Y chromosome fertilises the ovum with X chromosome. Therefore, the sex of the baby is determined by the father’s chromosomes, not the mother’s.

20. How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?

Ans- An ovary releases an egg every month. When two babies are produced in succession, they are called twins. Generally, twins are produced from a single egg by the separation of early blastomeres resulting from the first zygotic cleavage. As a result, the young ones formed will have the same genetic make- up and are thus, called identical twins. If the twins born are fraternal, then they would have developed from two separate eggs. This happens when two eggs (one from each ovary) are released at the same time and get fertilized by two separate sperms. Hence, the young ones developed will have separate genes and are therefore, called non-identical or fraternal twins.

21. How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?

Ans- Dogs and rodents are polyovulatory species. In these species, more than one ovum is released from the ovary at the time of ovulation. Hence, six eggs were released by the ovary of a female dog to produce six puppies.

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