NCERT Solutions for Class 12 Biology Chapter 3 - Human Reproduction

Aakash NCERT Solutions for Chapter 3 of Class 12 Biology teaches students about how gametes (gametogenesis) are formed. The solutions also talk about puberty and the male and female reproductive systems in humans.

Q1. Fill in the blanks:
(a)  Humans reproduce    (asexually/sexually)        
(b)Humans   are _____________(oviparous,viviparous,ovoviviparous)
(c)Fertilisation is _____________inhumans(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 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  ____________.
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 the 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 fertilization.
(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 a vascular connection between the foetus and uterus is called the placenta.

Q2. Draw a labeled diagram of the male reproductive system.
Answer: The labelled diagram of the male reproductive system is as follows:
 

Q3. Draw Labelled Diagram Of Female Reproductive system.
Answer: Female reproductive system:
 

Q4. Write two major functions of each testis and ovary.
Answer: Functions of testis-
1. Testis produces Spermatozoa which are male gametes.
2. In testis, the male sex hormone testosterone is developed.
Functions of ovary-
1.The ovary produces 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 inside. Spermatogonia are the male germ cells that form sperms through meiotic divisions followed by mitotic division, while Sertoli cells provide nutrition to the germ cells. The regions outside the seminiferous tubules are called the interstitial space and it contains small blood vessels and Leydig cells. Leydig's cells secrete androgen.

Fig: Structure of seminiferous tubule

Q6. What is spermatogenesis? Briefly describe the process of spermatogenesis.
Answer: Spermatogenesis -Spermatogenesis is the process of formation of spermatozoa from spermatogonia in testes at the time of puberty. In this process, the spermatogonia present at the inner side of seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium contains 46 chromosomes. Spermatogonia forms a spermatocyte that undergoes meiotic division to produce secondary spermatocytes having 23 chromosomes. The secondary spermatocyte undergoes 2nd meiotic division to form spermatids. The spermatids are transformed into spermatozoa by the process called spermiogenesis. The sperm heads remain embedded in the Sertoli cells and are released into 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 (follicular 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 which 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 sperms from Sertoli cells into the seminiferous tubules occurs through a process called spermiation.

Q9. Draw a labeled diagram of sperm.
Answer: The labeled 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 vasa efferentia, epididymis. vas deferens and rete testis. They transport and temporarily store spermatozoa.
Male accessory glands include seminal vesicles, bulbourethral gland, and prostate gland. These glands secrete fluids that lubricate the female reproductive system and sperms.

Q12. What is oogenesis? Give a brief account of oogenesis.
Answer: The process of 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 dividing and enter into prophase-I of meiotic division and get temporally arrested. At this stage, they are called primary oocytes. Each primary oocyte gets surrounded by a layer of granulosa cells then it is called the primary follicle. The primary follicle gets surrounded by more layers of granulosa cells called secondary follicles that transform into tertiary follicles that contain a fluid-filled cavity called the antrum. The tertiary follicles further change into the mature follicle called Graafian follicle, which rapture 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 labelled diagram of Graafian Follicle.
Answer: Diagram of Graffian 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 hydrolyze egg membrane thus, helping the sperm in penetration of ovum during fertilization of the ovum.
(d) Sperm tail
Answer: Sperm tail-  It provides motility to sperm.
(e) Fimbriae
Answer: Fimbriae- After ovulation, fimbriae helps 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: False. Androgens are produced by Leydig's cells
(b) Spermatozoa get nutrition from Sertoli cells. (True/False)
Answer: True.
(c) Leydig cells are found in the ovary. (True/False)
Answer: False. Leydig cells are found in seminiferous tubules of the testis.
(d) Leydig cells synthesize androgens. (True/False)
Answer: True.
(e) Oogenesis takes place in the corpus luteum. (True/False)
Answer: False. Oogenesis takes place in the ovary.
(f) Menstrual cycle ceases during pregnancy.
Answer: True.
(g) Presence or absence of a hymen is not a reliable indicator of virginity or sexual
experience. (True/False)
Answer: 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 days menstrual cycle, the menses take place on cycle days 3-5. The menstrual cycle consists of phases like the menstrual phase, follicular phase, ovulatory phase, and 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. Production of estrogen is also reduced in this phase.
4. The endometrium of the uterus breaks down & menstruation begins.
5. The cells of endometrium secretions, blood & unfertilized ovum constitute the menstrual flow.
Follicular phase-
1. This phase usually includes cycle days 6-13 or 14 in a 28 days cycle.
2. The follicle-stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete oestrogens.
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 blood increases.
3. Rapid secretion of LH induces rupturing 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 corpus luteum to secrete progesterone.
3. 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 the 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 uterine. Contraction of the uterine becomes stronger and stronger by the stimulatory reflex between uterine contraction and oxytocin secretion. This leads to the expulsion of the baby out of the uterus through the birth canal.
Hormones that are responsible for the induction of parturition are oxytocin and relaxin.
- Oxytocin: It is secreted by the posterior pituitary gland. It facilitates uterine contractions to push the fetus downward which in turn stretches the cervix. The amount of oxytocin is increased just before and during labor pain.
- Relaxin- It is secreted by the corpus luteum after the completion of pregnancy. Its main function is to relax the pelvic ligament and cervix to facilitate the birth of a child.

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 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 the sperm whether it contains  X chromosomes or  Y chromosome which will further fertilize the ovum having 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 gave 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 which gave birth to 6 puppies?
Answer: There are six eggs released by the female dog to give birth to six puppies.

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