Samacheer Kalvi 12th Biology Chapter 12 Exercise Solutions

Spermatogenesis occurs in the seminiferous tubules of testes, starting at puberty, and produces millions of sperms continuously. Each primary spermatocyte produces 4 functional sperms. Oogenesis occurs in the ovaries, begins before birth (oogonia formed in foetal life), and produces one ovum per month at puberty. Each primary oocyte produces only 1 functional ovum and polar bodies.

The corpus luteum is a yellowish body formed from the remnants of the Graafian follicle after ovulation. It secretes progesterone, which maintains the thickened endometrium for potential implantation. If fertilisation does not occur, the corpus luteum degenerates, progesterone levels fall, and the endometrium is shed (menstruation).

Implantation is the embedding of the blastocyst (fertilised egg after several cell divisions) into the thickened endometrial lining of the uterus. It occurs approximately 6–7 days after fertilisation. After implantation, the embryo derives nutrition from the endometrium until the placenta forms.

The seminal vesicles secrete a fructose-rich alkaline fluid that provides energy for sperms and constitutes about 60% of semen volume. The prostate gland secretes a milky, alkaline fluid that neutralises the acidity of the vagina, helping sperms survive. Both fluids together with sperms form semen.

Menopause is the permanent cessation of menstrual cycles, typically occurring between ages 45–55. It marks the end of a woman's reproductive life. It occurs because the ovaries no longer produce responsive follicles, leading to a decline in oestrogen and progesterone levels. Symptoms may include hot flashes, mood changes, and bone density loss.

The male reproductive system consists of the following organs:

• Testes: A pair of oval organs located outside the abdominal cavity in the scrotum. The scrotum provides a temperature 2–3 degrees lower than body temperature, which is essential for spermatogenesis. Each testis contains about 250 compartments with seminiferous tubules where sperms are produced. Leydig cells (interstitial cells) between tubules secrete testosterone.

• Epididymis: A coiled tube on the posterior surface of each testis. Sperms produced in the seminiferous tubules are transported here for maturation and temporary storage. Sperms gain motility and the ability to fertilise during their passage through the epididymis (about 18–24 hours).

• Vas deferens: A muscular tube that carries sperms from the epididymis to the ejaculatory duct. It ascends through the inguinal canal into the abdominal cavity, loops over the ureter, and joins the duct of the seminal vesicle.

• Seminal vesicles: A pair of glands that secrete a thick, alkaline, fructose-rich fluid. This fluid provides energy (fructose) for sperm motility and constitutes about 60% of semen volume. The secretion also contains prostaglandins.

• Prostate gland: A single gland below the bladder that secretes a thin, milky, alkaline fluid. This neutralises the acidity of the vaginal secretions, protecting sperms and enhancing their viability.

• Cowper's glands (Bulbo-urethral glands): A pair of small glands below the prostate that secrete a clear, mucus-like pre-ejaculate fluid. This lubricates the urethra and neutralises any residual acidity from urine.

• Penis: The copulatory organ that delivers semen into the female reproductive tract during mating. It contains erectile tissue that engorges with blood during sexual arousal (erection). The urethra runs through the penis and serves as a common passage for both urine and semen.

The menstrual cycle is a 28-day cycle of changes in the ovaries and uterus, regulated by hormones.

Phase 1 — Menstrual phase (Day 1–5):

• The endometrial lining, along with blood vessels and unfertilised ovum, is shed through the vagina as menstrual flow.
• Low levels of oestrogen and progesterone trigger this shedding.

Phase 2 — Follicular/Proliferative phase (Day 6–13):

• FSH (Follicle Stimulating Hormone) from the anterior pituitary stimulates the growth of a primary follicle into a Graafian follicle in the ovary.
• The growing follicle secretes oestrogen, which causes the endometrium to thicken and become vascularised (proliferation).
• Rising oestrogen levels also stimulate the cervix to produce thin, alkaline mucus favourable for sperm passage.

Phase 3 — Ovulatory phase (Day 14):

• A sharp surge in LH (Luteinising Hormone) triggers ovulation — the release of the secondary oocyte from the Graafian follicle.
• The oocyte is swept into the fallopian tube where fertilisation may occur.

Phase 4 — Luteal/Secretory phase (Day 15–28):

• After ovulation, the ruptured follicle transforms into the corpus luteum.
• The corpus luteum secretes progesterone (and some oestrogen), which maintains the thickened endometrium and makes it suitable for implantation.
• If fertilisation occurs: the corpus luteum persists, progesterone continues, and pregnancy is maintained.
• If fertilisation does NOT occur: the corpus luteum degenerates, progesterone and oestrogen levels fall, the endometrium breaks down, and menstruation begins — starting a new cycle.

Hormonal summary: FSH → follicle growth; Oestrogen → endometrium thickening; LH surge → ovulation; Progesterone → endometrium maintenance; Fall in progesterone → menstruation.

Spermatogenesis is the process of sperm formation from spermatogonia in the seminiferous tubules of the testes. It begins at puberty and continues throughout life.

Flow chart:

• Spermatogonia (2n) — diploid stem cells in the seminiferous tubules. They divide by mitosis to maintain their population.
• Primary spermatocytes (2n) — formed when spermatogonia grow and differentiate. Each undergoes Meiosis I.
• Secondary spermatocytes (n) — haploid cells produced after Meiosis I. Each undergoes Meiosis II.
• Spermatids (n) — haploid cells produced after Meiosis II. They are round, non-motile cells.
• Spermatozoa (n) — formed from spermatids by spermiogenesis (a process of morphological transformation). The spermatid develops a head (with acrosome containing enzymes), a middle piece (with mitochondria for energy), and a tail (flagellum for motility).

Key points:

• One primary spermatocyte produces 4 functional sperms.
• Sertoli cells (nurse cells) in the seminiferous tubules provide nutrition and support to developing sperms.
• The entire process takes about 64–72 days.
• Sperms are then transported to the epididymis for maturation and storage.
• Testosterone from Leydig cells is essential for spermatogenesis.

Fertilisation:

• During intercourse, millions of sperms are deposited in the vagina. Only a few hundred reach the fallopian tube.
• The sperm undergoes capacitation (becomes capable of fertilising) in the female reproductive tract.
• Fertilisation occurs in the ampulla region of the fallopian tube when a sperm penetrates the secondary oocyte.
• The sperm releases enzymes from the acrosome (acrosomal reaction) that digest the outer layers (corona radiata and zona pellucida) of the oocyte.
• Upon penetration, the oocyte undergoes the cortical reaction, which blocks polyspermy (prevents additional sperms from entering).
• The sperm nucleus (n) fuses with the oocyte nucleus (n) to form the diploid zygote (2n). This fusion is called syngamy.
• Meiosis II of the oocyte is completed at this point, producing the ovum and the second polar body.

Cleavage and blastocyst formation:

• The zygote undergoes mitotic divisions (cleavage) as it moves through the fallopian tube toward the uterus.
• At the 16-cell stage, it is called a morula.
• The morula develops into a blastocyst — a hollow ball of cells with an inner cell mass (embryo) and an outer trophoblast layer.

Implantation:

• The blastocyst reaches the uterus about 4–5 days after fertilisation.
• Implantation occurs on about day 6–7 when the blastocyst embeds into the thickened endometrium.
• The trophoblast cells invade the endometrium and begin forming the placenta.
• After implantation, the embryo derives nutrition from the endometrium until the placenta is fully functional.

Placenta formation:

• The placenta is a disc-shaped organ formed from both maternal (endometrium) and foetal (trophoblast) tissue.
• It serves as the interface for exchange of nutrients, gases (O2 and CO2), and wastes between mother and foetus, without their blood mixing directly.
• The placenta also produces hormones (hCG, progesterone, oestrogen) to maintain pregnancy.