HSC Exam Practice

Sample response. As a large multicellular animal grows, its volume increases faster than its surface area, so its SA:V ratio falls. The outer body surface therefore provides proportionally far less membrane area relative to the number of cells that need oxygen, and many internal cells lie too far from the surface for diffusion alone to supply them quickly enough. A flatworm is small and flat: its high SA:V ratio and minimal diffusion distances mean direct diffusion across its outer surface can supply all cells.

Marking notes. 1 mark for SA:V ratio falling as body size increases. 1 mark for relating low SA:V ratio (or long diffusion distances) to the inability of the outer surface to supply internal cells. 1 mark for explaining why a flatworm can use direct diffusion (small, flat, high SA:V ratio, short diffusion distances).

Sample response. (1) Large surface area, more membrane area allows more O₂ molecules to diffuse simultaneously, increasing total uptake per second. (2) Thin barrier/membrane, reduces diffusion distance, so gases cross quickly (diffusion rate inversely proportional to distance). (3) Moist surface, O₂ and CO₂ must dissolve before crossing cell membranes; moisture enables this. (4) Maintained concentration gradient, ventilation and blood flow continuously replenish the medium, keeping O₂ high on one side and low on the other so diffusion never stops.

Marking notes. 1 mark per feature correctly named with a correct biological reason. Must name the feature AND explain the reason; naming alone scores 0 for that feature.

Sample response. Oxygen enters through the spiracle (an opening on the insect's body wall), passes into the trachea (a larger branching tube), then moves into tracheoles (the finest branches that reach individual cells), and finally diffuses directly from the tracheole into the body cell. No transport by blood or haemolymph occurs.

Marking notes. 1 mark for spiracle correctly identified as entry point. 1 mark for tracheae and tracheoles named in correct order. 1 mark for stating that oxygen diffuses directly into cells from tracheoles (and/or that haemolymph is not involved).

Sample response. The tracheal system delivers oxygen by diffusion through tubes directly to cells; this is only effective if cells are within a few millimetres of a tracheole. A larger insect body would have internal cells too far from any tracheole for diffusion to supply them quickly enough, no transport system extends the reach. In mammals, alveoli are paired with a blood-based circulatory system: O₂ diffuses from alveoli into blood and haemoglobin carries it to cells anywhere in the body regardless of their distance from the lung. Larger body size simply requires a larger heart and more blood vessels, not a fundamentally different exchange mechanism.

Marking notes. 1 mark for identifying that the tracheal system relies on diffusion over short distances only, limiting body size. 1 mark for explaining that the mammalian system uses blood transport to extend O₂ delivery beyond the exchange surface. 1 mark for a clear comparative statement linking the presence/absence of a transport system to the presence/absence of a size limitation.

Sample response. In concurrent flow, blood and water move in the same direction across the gill lamella. The O₂ gradient between water and blood quickly equalises at the midpoint, after which no further diffusion occurs, limiting blood O₂ to about 50% of the water's O₂ concentration. In counter-current exchange, blood flows in the opposite direction to water. Blood that has already picked up some O₂ meets water that still carries a high O₂ concentration, so a small gradient is maintained along the entire length of the gill. This allows blood to exit with up to 90% of the water's O₂ concentration, far more efficient extraction.

Marking notes. 1 mark for correctly describing the direction of flow in concurrent vs counter-current. 1 mark for explaining that concurrent flow leads to equilibrium (gradient disappears), capping extraction at ~50%. 1 mark for explaining that counter-current maintains a gradient along the full gill length, allowing higher O₂ extraction.

Sample response (a). There is a positive relationship between gill surface area and O₂ extraction efficiency. As gill surface area increases from 1.6 cm² g⁻¹ (flounder, 28% extraction) to 8.5 cm² g⁻¹ (dace, 82% extraction), extraction efficiency increases. More active species (trout, dace) have both larger gill surface areas and higher extraction efficiencies than less active species (carp, flounder).

Marking notes (a). 1 mark for identifying the positive relationship. 1 mark for supporting with at least one correctly read data value.

Sample response (b). Active fish have higher metabolic rates and therefore greater oxygen demands than sluggish fish. To meet this demand, active species have evolved larger gill surface areas (more lamellae, more folds) to increase the rate of O₂ diffusion across the gill. By Fick's law, a larger surface area directly increases diffusion rate, so the gill structures of active fish are adapted to extract a higher proportion of available O₂ from each litre of water passing over them. Sluggish fish can survive on lower O₂ extraction because their metabolic demand is lower.

Marking notes (b). 1 mark for linking higher activity level to greater O₂ demand. 1 mark for explaining that larger gill surface area increases diffusion rate (or references Fick's law). 1 mark for completing the structure-function logic: greater surface area → greater extraction efficiency to meet metabolic demand.

Sample response (c). In warm, oxygen-depleted water, the O₂ concentration on the water side of the gill is reduced, so the concentration gradient between water and blood is steeper than usual in the wrong direction, there is less O₂ available to diffuse. The flounder's already-small gill surface area (1.6 cm² g⁻¹) would not provide enough exchange capacity to compensate, so O₂ extraction efficiency would fall even further below 28%. The flounder would risk hypoxia.

Marking notes (c). 1 mark for predicting a decrease in extraction efficiency (or risk of hypoxia) with a stated reason. 1 mark for correctly invoking reduced O₂ concentration gradient on the water side as the mechanism.

Sample response. Insects and fish face the same fundamental gas exchange problem, supplying O₂ to all body cells and removing CO₂, but solve it with structurally distinct systems adapted to their different body sizes, activity levels, and environments.

In insects, gas exchange occurs via the tracheal system. Air enters through spiracles on the body wall, moves through branching tracheae and tracheoles, and O₂ diffuses directly from tracheole walls into cells. No blood-based O₂ transport occurs: haemolymph does not carry oxygen. Ventilation is passive or assisted by body movements that compress the tracheal tubes. This system is highly effective for small bodies because every cell lies within a few millimetres of a tracheole, so diffusion distances are short and gradients remain steep.

In fish, gas exchange occurs across gill lamellaethin, folded surfaces richly supplied with capillaries. Water is pumped over the gills by buccal and opercular movements (active ventilation). Blood flows through gill capillaries in the opposite direction to water flow (counter-current exchange), maintaining a O₂ concentration gradient along the full lamella length and allowing the blood to exit with up to 90% of the water's O₂. O₂ binds to haemoglobin and is carried by the circulatory system to all tissues.

Key comparisons: (1) Gas transport: insects deliver O₂ directly to cells via tracheoles; fish use blood + haemoglobin. (2) Ventilation: insects rely on passive diffusion or body compression; fish actively pump water over gills. (3) Size limitation: the tracheal system limits insect body size because diffusion along tracheoles is only effective over millimetres; fish gills paired with circulation scale to much larger bodies. (4) Environmental medium: fish must extract O₂ from water, which is far less oxygen-rich than air, counter-current exchange partially compensates for this; insects exchange directly with oxygen-rich air.

In evaluating effectiveness in their natural environments: the insect tracheal system is highly effective and energetically cheap for small-bodied, air-breathing animals with moderate metabolic rates. Fish gills with counter-current exchange are more complex but solve the harder problem of extracting oxygen from a dilute, dense medium. Neither system is universally superior, each is well-matched to its environmental challenge and body plan.

Marking criteria.

• 1 markNames and describes the insect gas exchange structures (spiracles, tracheae, tracheoles) and the path O₂ takes to cells.
• 1 markCorrectly states that haemolymph does not transport O₂ in insects and explains why this limits body size (diffusion distance).
• 1 markNames and describes the fish gill structure (lamellae, capillaries) and explains how ventilation (buccal-opercular pumping) maintains the gradient.
• 1 markExplains counter-current exchange in fish gills and why it increases efficiency over concurrent flow.
• 1 markCompares gas transport: insects direct via tracheoles; fish via blood and haemoglobin through circulation.
• 1 markCompares effectiveness in natural environment: tracheal system suited to small bodies in air; gills suited to extracting O₂ from O₂-poor water and larger bodies.
• 1 markReaches an explicit evaluative judgement, rejects a single "superior" system and frames effectiveness as context-dependent (body size, medium, metabolic demand). Uses precise terminology throughout.