HSC Exam Practice

Sample response. Homeostasis is the maintenance of a relatively stable internal environment despite changes in the external environment [1]. It is essential for enzyme function because enzymes have an optimal temperature, pH, and substrate concentration; deviating outside these optima alters the enzyme’s tertiary structure (denatures it), reducing or eliminating catalytic activity [1]. For example, blood pH must be maintained within the tolerance range of 7.35–7.45; a pH below 7.35 (acidosis) disrupts ionic interactions that maintain enzyme tertiary structure, slowing metabolic reactions and threatening cell survival [1].

Marking notes. 1 mark — definition includes “relatively stable” or “tolerance range”; 1 mark — enzyme denaturation/optimal range linked to function; 1 mark — specific variable + tolerance range cited.

Sample response. (1) Stimulus: a change (internal or external) that moves a variable outside its tolerance range, triggering the pathway. (2) Receptor: a specialised cell or structure that detects the specific stimulus and sends a signal to the control centre (e.g. thermoreceptors in the hypothalamus detect temperature change). (3) Control centre: processes the receptor signal, compares it against the set point, and determines the appropriate response (e.g. the hypothalamus for temperature; the pancreatic islets for blood glucose). (4) Effector: the organ, muscle, or gland that carries out the corrective response as directed by the control centre (e.g. sweat glands, blood vessels, the liver). (5) Response: the physical change produced by the effector that opposes the stimulus and returns the variable toward the set point.

Marking notes. 1 mark per component (name + role accurately described). Full marks require all five named and described.

Sample response. Negative feedback produces a response that opposes the original stimulus, returning the variable toward the set point (e.g. insulin release when blood glucose rises above the tolerance range — insulin lowers glucose, opposing the rise) [2 marks: direction + example]. Positive feedback produces a response that amplifies the original stimulus, driving the variable further from the set point (e.g. uterine contractions during childbirth — contractions stretch the cervix, which triggers more oxytocin, which increases contractions) [2 marks: direction + example].

Marking notes. 1 mark per direction statement; 1 mark per correctly matched example. Example for negative feedback must be a homeostatic system. Example for positive feedback may be any physiologically normal amplifying loop (childbirth, blood clotting, action potential). No marks if direction is reversed.

Sample response. When a person becomes dehydrated, blood osmolarity rises above ~295 mOsm/kg. Receptor: osmoreceptors in the hypothalamus detect the elevated osmolarity [1]. Control centre: the hypothalamus processes the signal and sends instructions to the posterior pituitary gland [1]. Hormone: ADH (antidiuretic hormone) is released from the posterior pituitary into the bloodstream [1]. Effector: the collecting duct of the kidney becomes more permeable to water; water is reabsorbed from the filtrate into the blood, producing concentrated urine and reducing blood osmolarity [1].

Marking notes. 1 mark each: receptor (osmoreceptors + location); control centre (hypothalamus); hormone (ADH + correct gland); effector/response (collecting duct, water reabsorption, concentrated urine). Accept “antidiuretic hormone” or “vasopressin”.

Sample response. The normal blood pH tolerance range is 7.35–7.45. A pH of 7.2 represents acidosis, falling 0.15 units below the lower tolerance limit [1]. Enzymes rely on precise ionic interactions between charged amino acid side chains (e.g. –NH3+ and –COO– groups) to maintain their tertiary structure and active site geometry. When pH falls outside the tolerance range, H+ ions denature these interactions, altering the enzyme’s tertiary structure and therefore its active site shape [1]. As a result, enzyme activity decreases across all metabolic pathways — cellular respiration, muscle contraction, neural signalling, and cardiac function are all impaired, explaining the life-threatening nature of severe acidosis [1].

Marking notes. 1 mark — identifies pH 7.2 is below tolerance range; 1 mark — explains mechanism (H+ alters ionic bonds/charge, disrupts tertiary structure/active site); 1 mark — explains systemic consequence (multiple enzyme-driven processes impaired, threat to survival).

Sample response. In Type 1 diabetes, beta cells in the islets of Langerhans have been destroyed by the immune system, so insulin cannot be produced. After a meal, blood glucose rises above the upper limit of the tolerance range (approximately 6.0 mmol/L) [1]. The effector in the normal negative feedback loop — insulin-secreting beta cells — is absent [1]. Without insulin, the negative feedback mechanism that normally lowers blood glucose (signalling cells to increase glucose uptake, and stimulating glycogenesis in the liver) cannot operate. Blood glucose remains chronically above the tolerance range (hyperglycaemia), damaging blood vessels, kidneys, retinas, and peripheral nerves over time [1].

Marking notes. 1 mark — identifies blood glucose rising above tolerance range; 1 mark — names the effector (insulin/beta cells) as absent; 1 mark — explains how absence of effector breaks the negative feedback loop, causing persistent hyperglycaemia and disease consequences. Must use all three specified terms.

Part (a) — 2 marks. Sample response. Core body temperature increased progressively across the marathon, from 37.1°C at the start (0 km) to 39.4°C at the finish (42.2 km) [1]. The starting value (37.1°C) is within the normal tolerance range (36.5–37.5°C), but the finishing value (39.4°C) is substantially above the upper tolerance limit of 37.5°C [1].

Marking notes (a). 1 mark — start temperature and finish temperature cited; 1 mark — both correctly classified relative to tolerance range.

Part (b) — 4 marks. Sample response. Stimulus: core temperature rising above 37.5°C is detected by thermoreceptors in the hypothalamus. The control centre (hypothalamus) processes the signal and activates cooling effectors. Effector 1: sweat glands secrete sweat onto the skin surface; evaporation of sweat removes heat from the skin [1]. Effector 2: peripheral skin arterioles vasodilate, increasing blood flow to the skin surface and radiating heat to the environment [1]. The response (cooling) opposes the original stimulus (rising temperature) — this is negative feedback [1]. For full marks: must name at least two effectors with their mechanism [1 per effector, max 2] and correctly identify negative feedback [1].

Marking notes (b). 1 mark — hypothalamus named as control centre + thermoreceptors as receptor; 1 mark — sweat glands + evaporative cooling; 1 mark — vasodilation + radiant/convective heat loss; 1 mark — identifies negative feedback with direction stated.

Part (c) — 2 marks. Sample response. Accept any two of: (i) The rate of metabolic heat generation during sustained exercise exceeds the capacity of the negative feedback mechanisms to dissipate heat — the effectors are operating at maximum capacity but cannot overcome the heat load [1]. (ii) In high humidity (70%), water vapour in the air reduces the rate of sweat evaporation, significantly impairing the effectiveness of the sweating effector at dissipating heat [1]. (iii) As exercise continues, dehydration reduces sweat rate, further limiting the evaporative cooling response [1]. (iv) Vasodilation diverts blood from working muscles to the skin, creating a competing demand that limits performance — the body faces a trade-off and cannot fully maximise both effectors simultaneously [1].

Marking notes (c). 1 mark per plausible mechanistic reason that explains why negative feedback is insufficient to restore temperature during sustained exercise in heat. Must go beyond stating “it is hot” — must link to a specific limitation of the effectors or the feedback system.

Sample response. Homeostasis maintains biological variables within tolerance ranges essential for enzyme and cellular function. When a component of the stimulus-response pathway fails, the negative feedback loop is broken and the variable drifts chronically outside its tolerance range, leading to disease. Two Module 8 examples illustrate this directly:

Example 1 — Type 1 diabetes (blood glucose homeostasis). The homeostatic variable disrupted is blood glucose concentration (normal range: 4.0–6.0 mmol/L). The component that fails is the effector: autoimmune destruction of beta cells in the islets of Langerhans eliminates insulin production. Without insulin (the effector signal), the body’s cells cannot increase glucose uptake and the liver cannot perform glycogenesis after meals. Blood glucose rises far above the tolerance range (hyperglycaemia). Chronically elevated glucose exerts osmotic pressure on blood vessel walls, causing microvascular damage to the kidneys (nephropathy), retinas (retinopathy), and peripheral nerves (neuropathy). [3 marks: variable + component failure + disease consequence]

Example 2 — Chronic kidney disease (water/osmolarity homeostasis, or blood pressure homeostasis). The homeostatic variable disrupted is blood osmolarity (and fluid balance; normal range ~285–295 mOsm/kg). When nephrons are damaged by hypertension or diabetes, the effector (the collecting duct, which is made permeable to water by ADH) loses function. Water reabsorption is impaired, so blood osmolarity cannot be correctly regulated. The control centre (hypothalamus) continues to release ADH, but if the kidney tubules are damaged they cannot respond appropriately — the negative feedback loop is broken at the effector level. Over time, failure to regulate fluid balance and blood pressure leads to further kidney damage, cardiovascular complications, and uraemia (accumulation of waste products normally filtered by the kidney). [3 marks: variable + component failure + disease consequence]

Marking criteria (6 marks):

• 1 mark — Names a first specific disease and correctly identifies the homeostatic variable disrupted + its tolerance range.
• 1 mark — Identifies which component of the stimulus-response model fails in the first disease (with mechanism).
• 1 mark — Explains the disease consequences that result from chronic disruption of homeostasis in the first example.
• 1 mark — Names a second specific disease and correctly identifies the homeostatic variable disrupted + its tolerance range.
• 1 mark — Identifies which component of the stimulus-response model fails in the second disease (with mechanism).
• 1 mark — Explains the disease consequences that result from chronic disruption of homeostasis in the second example.

Accept any two diseases studied in Module 8 where a homeostatic variable and a specific failing component can be identified (e.g. type 2 diabetes — effector resistance; hypertension — disrupted blood pressure control; respiratory disease / COPD — CO2 regulation; hearing loss — cochlear fluid homeostasis). Reject vague responses that do not name a specific component of the stimulus-response pathway.