NCERT Solutions for Class 12 Biology Chapter 2 (Human Reproduction) – Free PDF Download

Q1. 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 a vascular connection between the foetus and uterus is called _____________

Answer:

(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 an 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 an ampulla of the oviduct (fallopian tube)
(j) The zygote divides to form a blastocyst, which is implanted in the uterus.
(k) The structure that provides a vascular connection between the foetus and the uterus is called the placenta

Q2. Draw a labeled diagram of the male reproductive system.

Answer:

The labelled diagram of the male reproductive system in the human reproduction class 12 is as follows:

Q3. Draw a labelled diagram of the female reproductive system.

Answer:

Female reproductive system

Q4. Write two major functions of each of the testis and the ovary.

Answer:

Functions of the testis

1. The testis produces spermatozoa, which are male gametes.
2. In the testis, the male sex hormone testosterone is developed.

Functions of the ovary

1. The ovary produces an ovum by the process of oogenesis.
2. Graafian follicles in the ovary produce a female sex hormone called estrogen.

Q5. Describe the structure of a seminiferous tubule.

Answer:

Seminiferous tubules are highly coiled structures present in testicular lobules. The seminiferous tubules are lined by two types of cells, i.e., spermatogonia and Sertoli cells, from the inside. Spermatogonia are the male germ cells that form sperm through meiotic divisions followed by mitotic division, while Sertoli cells provide nutrition to the germ cells. The region outside the seminiferous tubules is called the interstitial space, and it contains small blood vessels and Leydig cells. The Leydig cells secrete androgens.

Q6. What is spermatogenesis? Briefly describe the process of spermatogenesis.

Answer:

Spermatogenesis is the process of the formation of spermatozoa from spermatogonia in the testes at the time of puberty. In this process, the spermatogonia present at the inner side of the seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium contains 46 chromosomes. Spermatogonia form spermatocyte that undergoes meiotic division to produce secondary spermatocytes having 23 chromosomes. The secondary spermatocytes undergo 2nd meiotic division to form spermatids. The spermatids are transformed into spermatozoa by a process called spermiogenesis. The sperm heads remain embedded in the Sertoli cells and are released from the seminiferous tubules by the process of spermiation

Q7. Name the hormones involved in the regulation of spermatogenesis.

Answer:

Spermatogenesis is initiated due to an increase in the secretion of gonadotropin-releasing hormone by the hypothalamus. An increase in GnRH acts on the anterior pituitary and stimulates the secretion of two gonadotropins, LH (luteinizing hormone) and FSH (follicle-stimulating hormone). Among these, LH acts on Leydig cells and stimulates them to secrete androgens, whereas FSH acts on Sertoli cells and stimulates the secretion of some factors that help in spermiogenesis.

Q8. Define spermiogenesis and spermiation.

Answer:

Spermiogenesis- The spermatids are transformed into spermatozoa by the process called spermiogenesis.

Spermiation- The release of sperm from the seminiferous tubules occurs through a process called spermiation.

Q9. Draw a labelled diagram of sperm.

Answer:

The labelled diagram of human sperm is as follows:

Q10. What are the major components of seminal plasma?

Answer:

The major components of seminal plasma are mucous, fructose, calcium, ascorbic acid, certain enzymes, and some secretions of accessory glands.

Q11. What are the major functions of male accessory ducts and glands?

Answer:

Male accessory ducts include the vasa efferentia and epididymis. Vas deferens and rete testis. They transport and temporarily store spermatozoa. Male accessory glands include the seminal vesicle, bulbourethral gland, and prostate gland. These glands secrete fluids that lubricate the female reproductive system and sperm.

Q12. What is oogenesis? Give a brief account of oogenesis.

Answer:

The process of the formation of mature female gametes is called oogenesis. It starts during the embryonic development stage when millions of oogonia (gamete mother cells) are formed in each fetal ovary. The gamete mother cells start division and enter into prophase I of meiotic division and get temporarily arrested. At this stage, they are called primary oocytes. Each primary oocyte is surrounded by a layer of granulosa cells, which is called the primary follicle. The primary follicle gets surrounded by more layers of granulosa cells called secondary follicles that transform into the tertiary follicle that contains a fluid-filled cavity called an antrum. The tertiary follicles further change into the mature follicle called the Graafian follicle, which ruptures to release secondary oocytes (ovum) from the ovary by the process of ovulation.

Q13. Draw a labelled diagram of a section through the ovary.

Answer:

A labelled diagram of a section through the ovary is as follows:

Q14. Draw a labelled diagram of a Graafian follicle.

Answer:

Diagram of Graafian follicle

Q15. Name the functions of the following:

(a) Corpus luteum

Answer:

Corpus luteum-

It secretes progesterone. Progesterone is essential for the maintenance of the endometrium.

(b) Endometrium

Answer:

Endometrium- Implantation of the fertilized ovum and many events of pregnancy take place in the endometrium.

(c) Acrosome

Answer:

Acrosome- It is filled with enzymes that hydrolyse the egg membrane, thus helping the sperm in penetration of the ovum during fertilisation.

(d) Sperm tail

Answer:

Sperm tail- It provides motility to sperm.

(e) Fimbriae

Answer:

Fimbriae- After ovulation, they help in the collection of the ovum.

Q16. Identify True/False statements. Correct each false statement to make it true.

(a) Androgens are produced by Sertoli cells. (True/False)

Answer:

Androgens are produced by Sertoli cells. (False).

Androgens are produced by Leydig's cells.

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

Answer:

Spermatozoa get nutrition from Sertoli cells. (True)

(c) Leydig cells are found in the testis (True/False)

Answer:

Leydig cells are found in the ovary. (False)

Leydig cells are found in the seminiferous tubules of the testis

(d) Leydig cells synthesise androgens. (True/False)

Answer:

Leydig cells synthesise androgens. (True)

(e) Oogenesis takes place in the corpus luteum. (True/False)

Answer:

Oogenesis takes place in the corpus luteum. (False)

Oogenesis takes place in the ovary

(f) Menstrual cycle ceases during pregnancy

Answer:

The menstrual cycle ceases during pregnancy. (True)

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

Answer:

The presence or absence of a hymen is not a reliable indicator of virginity or sexual experience. (True)

Q17. What is the menstrual cycle? Which hormones regulate the menstrual cycle?

Answer:

Menstrual cycle: The reproductive cycle in female primates is called the menstrual cycle. In a 28-day menstrual cycle, the menses take place on cycle days 3-5. The menstrual cycle consists of phases like the menstrual phase, the follicular phase, the ovulatory phase, and the luteal phase.

Menstrual phase-

1. The production of LH from the anterior lobe of the pituitary gland is reduced.

2. The withdrawal of this hormone causes degeneration of the corpus luteum, and, therefore, progesterone production is reduced.

3. The production of oestrogen is also reduced in this phase.

4. The endometrium of the uterus breaks down & menstruation begins.

5. The cells of endometrium secretions, blood & unfertilised ovum constitute the menstrual flow.

Follicular phase-

1. This phase usually includes cycle days 6-13 or 14 in a 28-day cycle.

2. The follicle-stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete estrogens.

3. Oestrogen stimulates the proliferation of the endometrium of the uterine wall.

4. The endometrium becomes thicker by rapid cell multiplication, and this is accompanied by an increase in uterine glands & blood vessels.

Ovulatory phase-

1. Both LH & FSH attain a peak level in the middle of the cycle (about the 14th day).

2. Oestrogen concentration in the blood increases.

3. Rapid secretion of LH induces the rupture of the Graafian follicle and thereby the release of the ovum. LH causes ovulation.

Luteal Phase-

1. Includes cycle days 15 to 28.

2. Corpus luteum secretes progesterone.

3. Endometrium thickens.

4. Uterine glands become secretory.

Hormonal control of the menstrual cycle.

1. FSH stimulates the ovarian follicles to produce oestrogens.

2. LH stimulates the corpus luteum to secrete progesterone.

3. The menstrual phase is caused by the increased production of oestrogens.

4. LH causes ovulation

5. The proliferative phase is caused by the increased production of oestrogens.

6. The secretory phase is caused by increased production of progesterone.

Q18. What is parturition? Which hormones are involved in the induction of parturition?

Answer:

The process of delivery of the foetus is called parturition. The Oxytocin hormone causes a strong contraction of the uterus. The contraction of the uterus becomes stronger and stronger through the stimulatory reflex between uterine contraction and oxytocin secretion. This leads to the expulsion of the baby from the uterus through the birth canal.

Q19. In our society, women are often blamed for giving birth to daughters. Can you explain why this is not correct?

Answer:

The sex chromosome in a human female is XX, while in a male it is XY. Thus, female produces haploid gametes with 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 chromosomes, fertilises the ovum with an X chromosome. Therefore, the sex of the baby is determined by the father's chromosomes, not the mother's.

Q20. 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 had given birth to identical twins? Would your answer change if the twins born were fraternal?

Answer:

One ovum is released by a human ovary in a month. Identical twins are developed from a single egg. Hence, only one egg would have been released when the mother gave birth to identical twins. If the twins are fraternal, then they must have been developed from two eggs.

Q21. How many eggs do you think were released by the ovary of a female dog that gave birth to 6 puppies?

Answer:

Dogs are polyovulatory animals, and they release more than one egg at a time. So, a female dog with 6 puppies in one generation releases 6 eggs.