HSC Exam Practice

Sample response. Thermoregulation is the homeostatic process by which an organism maintains its core body temperature within a narrow, functional range despite changes in external temperature.

Marking notes. 1 mark for identifying thermoregulation as a homeostatic process (or process of maintaining/controlling temperature); 1 mark for specifying that temperature is maintained within a range or set point despite external variation. Accept equivalent phrasings.

Sample response. An endotherm generates body heat internally through metabolic processes (cellular respiration, particularly in skeletal muscle and the liver) and maintains a relatively stable core temperature independent of the environment. Its primary thermoregulation strategy is physiological — sweating, shivering, vasodilation/vasoconstriction. An ectotherm relies on external heat sources (solar radiation, warm surfaces) and its body temperature varies with environmental temperature. Its primary thermoregulation strategy is behavioural — basking, shuttling between microenvironments, burrowing.

Marking notes. 1 mark for correctly identifying heat source for each (internal metabolic vs external environment). 1 mark for describing primary strategy for each (physiological vs behavioural). 1 mark for linking strategy to a specific mechanism or example for both (e.g. shivering/sweating for endotherm; basking/burrowing for ectotherm). Do not accept “warm-blooded/cold-blooded.”

Sample response. (1) Physiological adaptations: automatic body responses to temperature change; e.g. sweating (evaporative cooling) or shivering (heat generation). (2) Behavioural adaptations: conscious or instinctive movements to control heat gain/loss; e.g. basking, seeking shade, huddling. (3) Structural adaptations: permanent physical features reducing the thermal challenge; e.g. blubber, fur, countercurrent heat exchange.

Marking notes. 1 mark per category correctly named with a valid temperature-regulation example. All three required for full marks. Accept any lesson example for each category.

Sample response. When core temperature rises above the set point (~37.5°C), the hypothalamus signals smooth muscle in peripheral arterioles to relax. This widens the vessels (vasodilation), increasing blood flow to capillary beds near the skin surface. The skin becomes flushed; heat from the warm blood is conducted through the skin and radiated to the cooler environment, removing heat from the body.

Marking notes. 1 mark for identifying that arterioles widen, increasing blood flow to the skin surface. 1 mark for explaining that this transfers heat from warm blood to the skin. 1 mark for specifying that heat is then lost to the environment (conduction/radiation), constituting a cooling mechanism. Must include the direction of heat flow (core/blood → skin → environment) for full marks.

Sample response. The thorny devil is an ectotherm that relies entirely on behavioural thermoregulation. In the morning, it basks on sun-warmed dark rock surfaces, orienting its body perpendicular to the sun’s rays to maximise solar radiation absorbed and rapidly raise body temperature to its preferred range (~28–35°C). As midday temperatures rise above this range, it shuttles between sun and shade to moderate heat gain. When ambient surface temperature exceeds the thermal tolerance limit, it retreats underground: soil below ~20 cm depth remains much cooler and more stable than the surface (10–15°C cooler during heatwaves), preventing lethal overheating. It may also orient its body parallel to the sun’s rays (minimising cross-sectional exposure) as a transitional strategy before retreating.

Marking notes. 1 mark each for any four of: morning basking on warm rock; body orientation to maximise solar radiation absorption; shuttling between sun and shade; burrowing/retreating underground when ambient exceeds tolerance; body orientation to minimise radiation at high temperatures. Must identify these as behavioural strategies. Must name the thorny devil explicitly or it is implied by the question stem.

Sample response. Torpor in echidnas is a controlled, reversible reduction in the homeostatic set point for body temperature, not an uncontrolled collapse of temperature regulation. During torpor, body temperature may fall to 5–10°C, but this is a deliberate physiological adjustment made when maintaining normal temperature (~35°C) would consume more energy than the animal can acquire (food is scarce in winter). The echidna is still maintaining temperature within a reduced tolerance range; when conditions improve, it rearms fully. This represents a strategic modification of the homeostatic set point to maximise survival, not a failure to maintain homeostasis.

Marking notes. 1 mark for identifying torpor as controlled and reversible (not a failure). 1 mark for explaining that the set point is deliberately lowered as an energy-conservation strategy when food is scarce. 1 mark for noting that the animal still maintains temperature within a (lowered) range and returns to normal set point when conditions allow.

Sample response (a). Skin temperature drops rapidly and continuously over the 30-minute immersion, falling from approximately 35°C at 0 minutes to approximately 20°C at 30 minutes. The total change is approximately −15°C. The rate of fall is greatest in the first 10 minutes and slows slightly thereafter.

Marking notes (a). 1 mark for correctly describing the downward trend in skin temperature (must include direction). 1 mark for correctly calculating (or reading) the magnitude of change: 35 − 20 = 15°C, or equivalent from data. Accept values within the range shown on the graph (±1°C).

Sample response (b). Mechanism 1: Vasoconstriction. The hypothalamus detects falling peripheral and core temperature and signals smooth muscle in peripheral arterioles to contract. This narrows peripheral blood vessels, dramatically reducing blood flow to the skin surface. Less warm blood reaches the skin capillaries, reducing heat conduction from the core to the cold skin and environment — this is why skin temperature falls steeply while core temperature is mostly preserved. [2 marks] Mechanism 2: Shivering. As core temperature begins to fall (detected by hypothalamic thermoreceptors), the hypothalamus sends rapid, repetitive signals to skeletal muscles throughout the body, causing uncoordinated contractions. These contractions generate heat as a byproduct of increased metabolic activity in muscle cells (ATP hydrolysis). This supplementary heat production partially offsets the ongoing heat loss through the skin, limiting the drop in core temperature. [2 marks]

Marking notes (b). 2 marks per mechanism (max 2 mechanisms, 4 marks total): 1 mark for correctly naming and describing the mechanism; 1 mark for accurately explaining how it specifically preserves core temperature (heat conservation for vasoconstriction; heat generation for shivering). Accept piloerection as a third mechanism (2 marks if mechanism and explanation are correct) if the student names only two. Do not accept behavioural mechanisms (leaving the water) — the question specifies physiological.

Sample response. The statement contains a partially correct claim but overstates the case with “any environment” and implies that endothermy is universally superior — both of which are incorrect when the full ecological context is considered.

The claim that endotherms can maintain a stable core temperature is correct in many environments. The red kangaroo, for example, uses a suite of physiological adaptations (sweating from distributed sweat glands, vasodilation of peripheral arterioles to radiate heat) and behavioural adaptations (forearm licking for evaporative cooling, shade-seeking at peak heat) alongside structural adaptations (pale fur reflecting infrared radiation) to maintain a core temperature near 38°C across a wide range of ambient temperatures. This allows year-round activity regardless of temperature — an advantage ectotherms cannot match in cold conditions or at night.

However, the statement fails in “any environment.” The 2019 Queensland heatwave event showed that when ambient temperature reached 42–45°C, an estimated 23,000 spectacled flying foxes — endotherms — died because their evaporative cooling mechanisms could not remove heat fast enough when the temperature gradient between body and environment was eliminated. Homeostatic systems have physical limits, and those limits were exceeded. Additionally, endothermy is highly energy-costly (60–80% of resting human energy intake goes to temperature maintenance), making it unsustainable in food-scarce environments.

By contrast, the eastern blue-tongue lizard (an ectotherm) manages thermoregulation effectively through behavioural means: basking on warm bitumen or rock in the morning to raise T_b to its preferred range (30–35°C), shuttling between sun and shade during the day, and burrowing underground when ambient temperature exceeds thermal tolerance. This strategy has zero metabolic cost for heat generation, making it highly effective in thermally stable, food-scarce environments such as arid Australia. The blue-tongue lizard cannot be active in cold conditions, but in its ecological niche, this limitation is not a survival constraint because ectothermy is matched to the environment it occupies.

The statement should therefore be qualified: endotherms are more effective than ectotherms at maintaining core temperature stability across thermally variable environments or when year-round activity is essential. Ectotherms are more effective in thermally predictable, resource-limited environments where the energy cost of endothermy would be prohibitive. Neither strategy is universally superior — effectiveness is environment-dependent.

Marking criteria.

• 1 mark — Identifies and evaluates the partial correctness of the statement (stable temperature maintained in many environments; “any environment” is too strong) with an explicit evaluative opening.
• 1 mark — Describes at least two specific adaptations (physiological, behavioural, or structural) of a named Australian endotherm with correct mechanisms (e.g. kangaroo sweating + vasodilation, OR flying fox panting + forearm spreading).
• 1 mark — Describes at least two specific behavioural strategies of a named Australian ectotherm with correct mechanisms (e.g. blue-tongue lizard basking + burrowing; thorny devil shuttling + orientation).
• 1 mark — Identifies a condition where endothermy is insufficient (extreme heat exceeding physiological capacity; or food-scarce conditions making energy cost prohibitive). Must be grounded in lesson content or a named example.
• 1 mark — Identifies a condition where ectothermy is the more effective strategy (thermally stable/predictable environments; resource-limited environments) with justification.
• 1 mark — Reaches an explicit evaluative conclusion that rejects the claim of universal endotherm superiority and frames effectiveness as environment-dependent. Uses precise lesson vocabulary (thermoregulation, homeostasis, physiological/behavioural/structural adaptation, set point or preferred temperature, endotherm, ectotherm).