(a) Role of calcium in muscle contraction (10 marks) (b) How is excretion different from osmoregulation in mammals? (10…

(a) Role of calcium in muscle contraction (10 marks)

Calcium is the trigger that turns contraction on and off.

• Release: an action potential spreads along the sarcolemma and down the T tubules. The voltage sensor (dihydropyridine receptor) opens the ryanodine receptor of the sarcoplasmic reticulum (SR), and Ca²⁺ floods the sarcoplasm.
• Switching on: Ca²⁺ binds troponin C, changing the shape of the troponin–tropomyosin complex and moving tropomyosin off the myosin-binding sites on actin. Myosin heads attach, and ATP-driven cross-bridge cycling follows.
• Relaxation: Ca²⁺ is pumped back into the SR by SERCA (Ca²⁺-ATPase), troponin loses its calcium, and tropomyosin again blocks actin.
• Variations: in smooth muscle, Ca²⁺ binds calmodulin, which activates myosin light-chain kinase; in cardiac muscle, Ca²⁺ entering through L-type channels triggers more release from the SR.
• Clinical: malignant hyperthermia comes from a defective ryanodine receptor, and rigor mortis develops when ATP runs out while calcium leaks in.

(b) Excretion and osmoregulation in mammals (10 marks)

Aim. Excretion removes nitrogenous metabolic wastes (urea, creatinine, uric acid) and foreign substances such as drugs. Osmoregulation keeps the water content, solute concentration (about 300 mOsm/L) and volume of body fluids constant.

Substances handled. Excretion deals with urea, creatinine, drugs and excess ions. Osmoregulation deals with water and electrolytes (Na⁺, K⁺, Cl⁻).

Mechanisms in the nephron.

• Excretion: glomerular filtration of wastes, and tubular secretion of H⁺, drugs and organic acids in the proximal tubule.
• Osmoregulation: the loop of Henle and the medullary gradient (countercurrent multiplier) make it possible to concentrate urine, and the collecting duct adjusts water reabsorption under ADH (aquaporin-2). Aldosterone controls Na⁺ and K⁺, atrial natriuretic peptide opposes it, and the renin–angiotensin system regulates volume.

The kidney performs both, and urea links them, since it is excreted but also recycled in the medulla to maintain the gradient. Skin and lungs also help.

(c) Biosynthesis of peptide hormones (10 marks)

Peptide hormones are made as larger precursors, illustrated by insulin.

• Transcription: the hormone gene (INS) is transcribed to mRNA, which is translated on ribosomes.
• Translation on the rough ER: an N-terminal signal peptide directs the ribosome to the ER through the signal recognition particle, and the chain enters the ER lumen as a preprohormone (preproinsulin).
• Signal cleavage: signal peptidase removes the signal peptide to give the prohormone (proinsulin), which folds and forms disulphide bonds.
• Golgi processing: vesicles carry it to the Golgi, and in the trans-Golgi network it is packaged into immature secretory granules.
• Maturation: prohormone convertases (PC1/3, PC2) and carboxypeptidase E cut out the C-peptide, giving mature insulin (A and B chains) stored with zinc in granules.
• Secretion: regulated exocytosis is triggered by Ca²⁺ entry after the stimulus (glucose).

POMC is processed differently in different tissues, to ACTH, MSH and β-endorphin.

(d) Significance of the placenta in maintaining pregnancy in human females (10 marks)

The placenta is formed from the chorion (trophoblast) and the decidua basalis, and is the endocrine, transport and immune organ of pregnancy.

• hCG: secreted by the syncytiotrophoblast from about day 8, it rescues the corpus luteum, so progesterone continues until about weeks 8–10. It is the basis of pregnancy tests.
• Luteo-placental shift: by weeks 10–12 the placenta itself makes progesterone and oestrogens.
• Progesterone maintains the endometrium and keeps the myometrium quiet, and it helps immune tolerance.
• Oestrogens (estriol) come from the fetoplacental unit, in which fetal adrenal precursors are converted by the placenta, and they grow the uterus and breasts.
• hPL changes the mother's metabolism to supply glucose to the foetus, and CRH helps to time birth.
• Transport and protection: the placenta exchanges O₂, nutrients, wastes and IgG, and protects the foetus, a semi-allograft, from rejection.

(e) Sperm capacitation during fertilisation (10 marks)

Capacitation is the final physiological maturation of ejaculated sperm in the female genital tract, by which they become able to fertilise an egg (Austin and Chang, 1951).

Site and time. It occurs in the uterus and the oviduct, over several hours in humans.

Changes.

• Seminal decapacitating factors are removed, and cholesterol is lost from the sperm membrane (to albumin), which increases its fluidity.
• Bicarbonate and calcium enter, the latter through CatSper channels. Bicarbonate activates soluble adenylyl cyclase, and the raised cAMP activates PKA, leading to protein tyrosine phosphorylation.
• Sperm motility becomes hyperactivated, with a stronger, asymmetrical whip-like beat.

Significance. Capacitated sperm can bind the zona pellucida, undergo the acrosome reaction (triggered by ZP3) and fuse with the egg. In IVF, sperm are capacitated in a medium with albumin, bicarbonate and calcium before insemination.