Reproductive System & Development

Question 1 (12 marks)

(a) Luteinizing Hormone (LH) (1); secreted by the anterior pituitary gland (1).

(b) Rising oestrogen from the maturing (Graafian) follicle (1); when oestrogen reaches a high threshold it exerts positive feedback on the hypothalamus/anterior pituitary (1), stimulating a surge of LH (and FSH) that triggers ovulation (1). Why: below threshold oestrogen is inhibitory (negative feedback); above threshold the response flips to positive, producing the sharp mid-cycle spike.

(c) The corpus luteum (formed from the ruptured follicle) secretes progesterone (1). Progesterone maintains and thickens the endometrium (1), keeping it vascular and glandular/secretory ready for implantation (1).

(d) If pregnancy occurs, progesterone remains high (does not fall) after day 28 (1). Menstruation is the breakdown of the endometrium caused by progesterone withdrawal; since progesterone stays high, the endometrium is maintained (1). The embryo/trophoblast secretes hCG (human chorionic gonadotropin) (1) which "rescues" the corpus luteum, keeping it active so it continues progesterone secretion (1).

Question 2 (12 marks)

(a) Cell X = secondary oocyte / egg (ovum), produced by oogenesis (1); Cell Y = spermatozoon (sperm), produced by spermatogenesis (1).

(b) Any two, 2 marks each:

• Cell X has abundant cytoplasm → provides nutrient/yolk store and organelles for the early embryo (2).
• Cell Y has a flagellum → provides motility to swim to the egg (2).
• Cell Y is small with little cytoplasm → streamlined for movement / produced in huge numbers (2).
• (Sperm acrosome for enzyme penetration also acceptable.)

(c) Spermatogonium → 4 functional sperm (1). Primary oocyte → 1 functional ovum (plus 2–3 polar bodies that degenerate) (1). Reason: in oogenesis cytokinesis is unequal, concentrating cytoplasm/resources into one cell (1) to maximally provision the future embryo, whereas sperm require no such store (1).

(d) Arrested at metaphase II (1); division completed only upon fertilization / sperm penetration (1).

Question 3 (12 marks)

(a) Cleavage (1). Rapid mitotic divisions occur without cell growth between divisions (1); each division splits the existing cytoplasm into smaller cells, so cell number rises but total volume stays roughly constant (1).

(b) On sperm fusion the egg undergoes the cortical reaction: cortical granules release contents that alter the zona pellucida (zona/slow block) (1), and a rapid membrane depolarisation gives the fast block (1), together preventing further sperm entry (1). If it fails, polyspermy gives an abnormal chromosome number (triploid, 3n) (1) → non-viable embryo.

(c) Blastocyst / blastula (1); cavity = blastocoel (1). The inner cell mass / trophoblast side implants (trophoblast makes the contact and invades the endometrium; embryo proper from inner cell mass) (1).

(d) Gastrulation establishes the three germ layers (ectoderm, mesoderm, endoderm) (1), which determine the future body plan / which tissues each cell will form (1).

Question 4 (12 marks)

(a) Any three, 1 mark each: gas exchange (O₂ to fetus, CO₂ away); nutrient supply (glucose, amino acids); waste removal (urea); hormone secretion (progesterone/hCG); passive immunity (maternal antibodies); barrier to some pathogens.

(b) Prevents mixing of blood so that maternal and fetal blood pressures/volumes are kept separate (1) and blood-group/immune incompatibility is avoided (1). Example: mixing could trigger an immune reaction such as Rh incompatibility, where maternal antibodies attack fetal red cells (1).

(c) Oxytocin (1). Contractions stretch the cervix → stretch receptors signal the hypothalamus/posterior pituitary to release more oxytocin (1) → stronger contractions (1) → further cervical stretch — a positive-feedback cycle that escalates until birth (1).

(d) Higher fetal-Hb affinity means at the placenta fetal Hb binds O₂ that maternal Hb releases (1), creating a favourable gradient so oxygen moves from maternal to fetal blood (1).

Question 5 (12 marks)

(a) 1 mark each:

• A → combined oral contraceptive pill (hormonal pill) (1)
• B → vasectomy (male) / tubal ligation (female) — sterilisation (1)
• C → condom (1)
• D → IUD / intrauterine device (copper coil) (1)

(b) The pill supplies synthetic oestrogen and/or progesterone (1); high steady levels exert negative feedback on the hypothalamus and anterior pituitary (1), suppressing FSH and LH secretion (1); without the LH surge there is no ovulation (and without FSH no follicle matures) (1).

(c) Sterilisation only cuts/blocks the tubes (vas deferens or oviducts) (1); the gonads (testes/ovaries) are left intact and continue secreting hormones normally, unlike gonad removal which would eliminate hormone production (1).

(d) The condom is a physical barrier preventing direct contact/exchange of body fluids (1), so it blocks pathogen transfer; hormonal, IUD and surgical methods do not create such a barrier and so do not reduce STI risk (1).

[
  {"claim":"One spermatogonium yields 4 sperm via meiosis","code":"result = (2*2 == 4)"},
  {"claim":"One primary oocyte yields 1 functional ovum plus 3 polar bodies (4 total nuclei)","code":"ova=1; polar=3; result = (ova+polar == 4 and ova == 1)"},
  {"claim":"Polyspermy (2 sperm sets + 1 egg set) gives triploid 3n","code":"n=1; result = (n+n+n == 3)"},
  {"claim":"LH surge occurs around day 13-14 preceding ovulation in a 28-day cycle","code":"surge_day=13; ovulation=14; result = (ovulation - surge_day == 1 and ovulation < 28)"}
]