Human Reproduction
Class 12 | Biology — Zoology | Chapter 2
Class 12 Unit 2 Biology
Book-back Exercises — Questions & Answers
Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.
I. Choose the Correct Answer
1. The mature sperms are stored in the
a) Seminiferous tubules b) Vas deferens c) Epididymis d) Seminal vesicle
Answer: c) Epididymis
Explanation: The epididymis is a single highly coiled tube that temporarily stores the spermatozoa, where they undergo physiological maturation and acquire increased motility and fertilizing capacity.
2. The male sex hormone testosterone is secreted from
a) Sertoli cells b) Leydig cell c) Epididymis d) Prostate gland
Answer: b) Leydig cell
Explanation: Interstitial cells or Leydig cells are endocrine in nature and secrete androgens, namely the testosterone hormone, which initiates the process of spermatogenesis.
3. The glandular accessory organ which produces the largest proportion of semen is
a) Seminal vesicle b) Bulbourethral gland c) Prostate gland d) Mucous gland
Answer: a) Seminal vesicle
Explanation: The seminal vesicles secrete an alkaline fluid called seminal plasma containing fructose sugar, ascorbic acid, prostaglandins and a coagulating enzyme called vesiculase which enhances sperm motility.
4. The male homologue of the female clitoris is
a) Scrotum b) Penis c) Urethra d) Testis
Answer: b) Penis
Explanation: The penis is the male external genitalia functioning as a copulatory organ, and is the male homologue of the female clitoris.
5. The site of embryo implantation is the
a) Uterus b) Peritoneal cavity c) Vagina d) Fallopian tube
Answer: a) Uterus
Explanation: The inner cell mass of the blastocyst becomes embedded in the endometrium of the uterus. This process is called implantation and it results in pregnancy.
6. The foetal membrane that forms the basis of the umbilical cord is
a) Allantois b) Amnion c) Chorion d) Yolk sac
Answer: a) Allantois
Explanation: The allantois forms a small outpocketing of embryonic tissue at the caudal end of the yolk sac. It is the structural base for the umbilical cord that links the embryo to the placenta.
7. The most important hormone in initiating and maintaining lactation after birth is
a) Oestrogen b) FSH c) Prolactin d) Oxytocin
Answer: c) Prolactin
Explanation: The anterior pituitary secretes prolactin which plays a major role in lactogenesis (production of milk by mammary glands).
8. Mammalian egg is
a) Mesolecithal and non cleidoic b) Microlecithal and non cleidoic c) Alecithal and non cleidoic d) Alecithal and cleidoic
Answer: c) Alecithal and non cleidoic
Explanation: Human ovum is non-cleidoic, alecithal and microscopic in nature, as stated in the description of the structure of the ovum.
9. The process which the sperm undergoes before penetrating the ovum is
a) Spermiation b) Cortical reaction c) Spermiogenesis d) Capacitation
Answer: d) Capacitation
Explanation: The sperms deposited in the female reproductive tract undergo capacitation, which is a biochemical event that enables the sperm to penetrate and fertilise the egg.
10. The milk secreted by the mammary glands soon after child birth is called
a) Mucous b) Colostrum c) Lactose d) Sucrose
Answer: b) Colostrum
Explanation: The mammary glands secrete a yellowish fluid called colostrum during the initial few days after parturition.
11. Colostrum is rich in
a) Immunoglobulin E b) Immunoglobulin A c) Immunoglobulin D d) Immunoglobulin M
Answer: b) Immunoglobulin A
Explanation: Colostrum is rich in IgA antibodies, which helps to protect the infant's digestive tract against bacterial infection.
12. The Androgen Binding Protein (ABP) is produced by
a) Leydig cells b) Hypothalamus c) Sertoli cells d) Pituitary gland
Answer: c) Sertoli cells
Explanation: FSH stimulates testicular growth and enhances the production of Androgen Binding Protein (ABP) by the sertoli cells and helps in the process of spermiogenesis.
13. Find the wrongly matched pair
a) Bleeding phase - fall in oestrogen and progesterone b) Follicular phase - rise in oestrogen c) Luteal phase - rise in FSH level d) Ovulatory phase - LH surge
Answer: c) Luteal phase - rise in FSH level
Explanation: During the luteal phase, the corpus luteum secretes large amounts of progesterone (not FSH). FSH rises during the follicular phase, not the luteal phase.
Assertion and Reason Questions
Choose the correct answer using the codes below:
a) A and R are true, R is the correct explanation of A
b) A and R are true, R is not the correct explanation of A
c) A is true, R is false
d) Both A and R are false
14. A – In human male, testes are extra abdominal and lie in scrotal sacs.
R – Scrotum acts as thermoregulator and keeps temperature lower by 2°C for normal sperm production.
Answer: a) A and R are true, R is the correct explanation of A
Explanation: The scrotum is placed outside the abdominal cavity to provide a temperature 2 to 3°C lower than the normal internal body temperature. Thus the scrotum acts as a thermoregulator for spermatogenesis.
15. A – Ovulation is the release of ovum from the Graafian follicle.
R – It occurs during the follicular phase of the menstrual cycle.
Answer: c) A is true, R is false
Explanation: Ovulation is indeed the release of the ovum (secondary oocyte) from the Graafian follicle, but it occurs during the ovulatory phase (around the 14th day), not the follicular phase.
16. A – Head of the sperm consists of acrosome and mitochondria.
R – Acrosome contains spiral rows of mitochondria.
Answer: d) Both A and R are false
Explanation: The head of the sperm consists of the acrosome and nucleus (not mitochondria). Mitochondria are present in the middle piece as a mitochondrial spiral (nebenkern), not in the acrosome.
Answer the Following Questions
17. Mention the differences between spermiogenesis and spermatogenesis.
Answer: Spermatogenesis is the entire sequence of events in the seminiferous tubules of the testes that produce the male gametes (sperms), starting from spermatogonia up to the formation of spermatids. Spermiogenesis is a specific phase within spermatogenesis in which the spermatids are transformed into mature spermatozoa (sperms). Thus, spermiogenesis is a part of the broader process of spermatogenesis.
18. At what stage of development are the gametes formed in new born male and female?
Answer: In a new born female, the oogonial cells enter Prophase I of meiotic division I to form primary oocytes, which are temporarily arrested at this stage at birth. In new born males, the primordial germ cells migrate into the testes and become immature germ cells called spermatogonia; spermatogenesis begins only at puberty when spermatogonia start mitotic division.
19. Expand the acronyms: a. FSH b. LH c. hCG d. hPL
Answer:
a. FSH – Follicle Stimulating Hormone
b. LH – Luteinizing Hormone
c. hCG – human Chorionic Gonadotropin
d. hPL – human Placental Lactogen (also called human Chorionic Somatomammotropin)
20. How is polyspermy avoided in humans?
Answer: Once fertilization is accomplished, cortical granules from the cytoplasm of the ovum form a barrier called the fertilization membrane around the ovum. This fertilization membrane prevents further penetration of other sperms into the ovum, thereby avoiding polyspermy.
21. What is colostrum? Write its significance.
Answer: Colostrum is a nutrient-rich yellowish fluid secreted by the mammary glands during the initial few days after parturition. It has less lactose and almost no fat, but contains more proteins, vitamin A and minerals. It is rich in IgA antibodies, which protect the infant's digestive tract against bacterial infection. It acts as a natural antimicrobial agent and actively stimulates the maturation of the infant's immune system.
22. Placenta is an endocrine tissue. Justify.
Answer: During pregnancy, the placenta acts as a temporary endocrine gland and produces large quantities of human Chorionic Gonadotropin (hCG), human Chorionic Somatomammotropin or human Placental Lactogen (hPL), oestrogens and progesterone, which are essential for a normal pregnancy. A hormone called relaxin is also secreted during the later phase of pregnancy which helps in relaxation of the pelvic ligaments at the time of parturition. Thus, because the placenta secretes several hormones vital to pregnancy, it functions as an endocrine tissue.
23. Draw a labeled sketch of a spermatozoan.
Answer: The human sperm is composed of a head (containing acrosome and nucleus), a short neck (with proximal and distal centrioles), a middle piece (with mitochondrial spiral/nebenkern around the axial filament), and a long tail (with central axial filament/axoneme and outer protoplasmic sheath). The entire sperm is enveloped by a plasma membrane. (Draw and label: Acrosome, Nucleus, Neck, Proximal centriole, Distal centriole, Mitochondrial spiral, Axial filament, Tail/protoplasmic sheath.)
24. What is inhibin? State its functions.
Answer: Inhibin is a hormone secreted by the Sertoli cells (nurse cells) of the seminiferous tubules. Its function is to act in the negative feedback control of sperm production, thereby regulating the process of spermatogenesis.
25. Mention the importance of the position of the testes in humans.
Answer: The testes are located in the scrotum, which hangs outside the abdominal cavity. Since viable sperms cannot be produced at normal body temperature, the scrotum provides a temperature 2 to 3°C lower than the normal internal body temperature. Thus, the scrotum acts as a thermoregulator for spermatogenesis, ensuring production of normal and viable sperms.
26. What is the composition of semen?
Answer: Semen or seminal fluid is a milky white fluid which contains sperms and the seminal plasma. The seminal plasma is secreted from the seminal vesicles (alkaline fluid with fructose sugar, ascorbic acid, prostaglandins and vesiculase), the prostate gland (slightly acidic fluid with citrate, enzymes and prostate specific antigens), and the bulbourethral glands (lubricating secretions). The seminal fluid acts as a transport medium, provides nutrients, protects and activates the sperms, and facilitates their movement.
27. Explain the process of fertilization and implantation of the fertilized ovum.
Answer: Fertilization occurs when a haploid sperm fuses with a haploid ovum to form a diploid zygote at the ampullary-isthmic junction of the fallopian tube. The sperms undergo capacitation in the female tract. The sperm penetrates the corona radiata by releasing hyaluronidase (acrosomal reaction), then passes through the zona pellucida. Once fertilization occurs, cortical granules form a fertilization membrane preventing polyspermy. The zygote undergoes cleavage to form 16-cell morula and then a blastocyst (about 100 cells). Under the influence of progesterone, the embryo travels through the fallopian tube in 4 to 5 days into the uterine cavity. The inner cell mass of the blastocyst becomes embedded in the endometrium of the uterus — this is called implantation — resulting in pregnancy.
28. Define gametogenesis.
Answer: Gametogenesis is the process of formation of gametes — that is, sperms and ova — from the primary sex organs in all sexually reproducing organisms. Meiosis plays the most significant role in the process of gametogenesis.
29. Describe the structure of the human ovum with a neat labelled diagram.
Answer: Human ovum is non-cleidoic, alecithal and microscopic in nature. Its cytoplasm, called ooplasm, contains a large nucleus called the germinal vesicle. The ovum is surrounded by three coverings: (i) an inner thin transparent vitelline membrane, (ii) a middle thick zona pellucida, and (iii) an outer thick coat of follicular cells called corona radiata. Between the vitelline membrane and zona pellucida is a narrow perivitelline space. (Draw and label: Corona radiata, Zona pellucida, Perivitelline space, Vitelline membrane, Ooplasm, Germinal vesicle.)
30. Give a schematic representation of spermatogenesis and oogenesis in humans.
Answer:
Spermatogenesis:
Spermatogonia (2n) → [Mitosis] → Primary spermatocytes (2n, 46 chromosomes) → [Meiosis I] → 2 Secondary spermatocytes (n, 23 chromosomes each) → [Meiosis II] → 4 Spermatids (n) → [Spermiogenesis] → 4 Mature spermatozoa (n)
Oogenesis:
Oogonia (2n) → [Mitosis] → Primary oocyte (2n) → [Meiosis I] → Secondary oocyte (n) + 1st Polar body → [Meiosis II, completed only at fertilization] → Ovum (n) + 2nd Polar body
31. Explain the various phases of the menstrual cycle.
Answer: The menstrual cycle occurs approximately once every 28/29 days and has four phases:
1. Menstrual phase (Days 1–5): Menstrual flow occurs due to the breakdown of the endometrial lining and blood vessels caused by declining levels of progesterone and oestrogen. This occurs only if the ovum is not fertilized.
2. Follicular/Proliferative phase (Days 5–14): The primary follicle grows into a mature Graafian follicle and the endometrium regenerates through proliferation. FSH and LH levels gradually increase, stimulating follicular development and oestrogen secretion.
3. Ovulatory phase (around Day 14): LH surge causes rupture of the Graafian follicle and release of the secondary oocyte (ovulation) into the peritoneal cavity.
4. Luteal/Secretory phase (Days 15–28): The Graafian follicle transforms into the corpus luteum, which secretes progesterone to maintain the endometrium. If fertilization does not occur, the corpus luteum degenerates into corpus albicans, leading to the next menstruation.
32. Explain the role of oxytocin and relaxin in parturition and lactation.
Answer: Oxytocin: The descent of the foetus causes dilation of the cervix and vaginal canal, resulting in the foetal ejection reflex (Ferguson reflex), which initiates the secretion of oxytocin from the neurohypophysis. Oxytocin brings about powerful contractions of the uterine muscles leading to expulsion of the baby through the birth canal (parturition). During lactation, oxytocin causes the "Let-Down" reflex — the actual ejection of milk from the alveoli of the mammary glands. It also stimulates the recently emptied uterus to contract, helping it return to pre-pregnancy size.
Relaxin: Relaxin is secreted by the placenta and corpus luteum. It promotes parturition by relaxing the pelvic joints and by bringing about dilatation of the cervix, facilitating the passage of the baby through the birth canal.
📝 Summary
- The human reproductive system has four main functions: producing gametes (sperms and ova), transporting and sustaining gametes, nurturing the developing offspring, and producing hormones.
- Major reproductive events include gametogenesis, insemination, fertilization, cleavage, implantation, placentation, gastrulation, organogenesis, and parturition — forming a complete sequence from gamete formation to birth.
- The primary reproductive organs are the ovary (producing ova) and testis (producing sperms), while accessory organs help transport, sustain gametes, and nurture the developing offspring.
- The testes are located in the scrotum outside the abdominal cavity, providing a temperature 2 to 3 degrees Celsius lower than normal body temperature, which is essential for viable sperm production (spermatogenesis).
- Each testis contains 200 to 250 lobules with 2 to 4 seminiferous tubules each; these highly coiled tubules form 80 percent of the testicular substance and are the sites for sperm production.