HSC Exam Practice

Sample response. Fick’s law: Rate of diffusion ∝ (Surface area × Concentration gradient) / Membrane thickness. Surface area: larger SA increases the total number of molecules that can cross simultaneously, increasing SA increases rate. Concentration gradient: a steeper gradient means a greater difference in particle density across the membrane, increasing gradient increases rate. Membrane thickness: a thicker membrane means molecules must travel further by random motion, increasing thickness decreases rate.

Marking notes. 1 mark for the correct formula or correct verbal statement including all three variables. 1 mark for correctly stating the direction of effect of at least two variables. 1 mark for correctly stating the direction of effect of the third variable AND providing at least one mechanistic explanation (e.g. “greater gradient means greater driving force” or “thicker membrane means longer diffusion path”).

Sample response. The cohesion-tension hypothesis explains water transport in xylem. Energy comes from solar energy: transpiration (evaporation) from leaf mesophyll cells creates a water deficit, generating tension (negative pressure) that is transmitted through a cohesive water column from leaf to root, pulling water upward. The mechanism is entirely passive, no ATP is used at the xylem vessel. The pressure-flow hypothesis explains sugar transport in phloem. Energy comes from ATP: companion cells actively load sucrose into sieve tubes at a source (e.g. a photosynthesising leaf) by active transport, raising osmotic pressure. Water enters by osmosis, creating a high turgor pressure. This pressure gradient drives bulk flow of phloem sap to a lower-pressure sink (e.g. a growing root or fruit), where sucrose is unloaded.

Marking notes. 1 mark for correctly naming xylem as the tissue for cohesion-tension and identifying transpiration (solar energy / passive) as the energy source. 1 mark for correctly describing how tension in the xylem is generated and transmitted (transpiration creates water deficit → tension → cohesive column pulled upward). 1 mark for correctly naming phloem as the tissue for pressure-flow and identifying active transport / ATP as the energy source. 1 mark for correctly describing the mechanism: active sucrose loading at source → osmotic water entry → turgor pressure → bulk flow to sink.

Sample response. As a multicellular organism grows larger, its SA:V ratio decreases (volume increases as the cube of a linear dimension while surface area increases as only the square). This means the cell surface area available for diffusion becomes proportionally smaller relative to the metabolic demand of the internal volume, so diffusion alone cannot supply O₂, nutrients, and remove waste products fast enough to sustain metabolism [1]. The structural hierarchy (cell → tissue → organ → system) solves this by providing specialised cells that perform one function very efficiently (division of labour) [1], organised into organs with internal exchange surfaces that maintain large SA, thin membranes, and concentration gradients for rapid diffusion at the cellular level, and transport systems that carry substances in bulk between these exchange surfaces and the rest of the body [1].

Marking notes. 1 mark for explaining why decreasing SA:V ratio in large organisms creates a diffusion problem (diffusion alone insufficient). 1 mark for defining or applying division of labour (specialised cells or tissues performing one function more efficiently). 1 mark for connecting the hierarchy explicitly to at least one solution, either specialised exchange surfaces or transport systems that extend the effective reach of diffusion.

Sample response. (Any four of the following, one mark each.) Temperature (increases transpiration): higher temperature increases the kinetic energy of water molecules, accelerating evaporation from mesophyll cells and increasing the water vapour concentration inside the leaf, steepening the gradient for outward diffusion. Light intensity (increases transpiration): light triggers K⁺ pumping into guard cells → osmotic water entry → increased turgor → wider stomatal aperture → larger pathway for water vapour diffusion out. Wind speed / air movement (increases transpiration): moving air removes water vapour from the boundary layer adjacent to the leaf, maintaining or increasing the water vapour concentration gradient between the leaf interior and the atmosphere. Humidity (decreases transpiration): high atmospheric humidity reduces the water vapour concentration gradient between the leaf air spaces and the surrounding air, decreasing the driving force for diffusion out of the leaf.

Marking notes. 1 mark per correctly named factor (direction of effect stated) with one sentence of mechanistic justification linking the factor to either stomatal aperture, the water vapour gradient, or evaporation rate. Accept any four from: temperature, light intensity, wind speed, humidity, leaf area, soil water availability. Do not award marks for stating a factor without a mechanistic explanation.

Sample response (a), four universal features (4 marks). 1. Large surface area: optimises the SA variable in Fick’s law, a greater SA means more molecules can cross per unit time. Achieved in different ways: spongy mesophyll air spaces (plant), gill lamellae (fish), 500 million alveoli (mammal), branching tracheoles (insect), whole body surface (earthworm). 2. Thin membrane (small diffusion distance): optimises the membrane thickness variable in Fick’s law, shorter diffusion path means faster random walk to the other side. Achieved by single cell layers, flattened cells, or direct delivery to cells (tracheoles). 3. Moist surface: gases must dissolve in water before crossing lipid membranes; a moist surface ensures O₂ and CO₂ can dissolve and enter solution, enabling diffusion across the membrane. This does not directly optimise a Fick variable but is a prerequisite for the diffusion pathway. 4. Maintained concentration gradient: optimises the concentration gradient variable in Fick’s law, if products accumulate on one side and reactants are not replenished on the other, the gradient collapses and diffusion stops. Achieved by ventilation + circulation (mammal), countercurrent flow (fish), transpiration stream (plant), metabolic demand + pumping (insect), closed circulatory system (earthworm).

Marking notes part (a). 1 mark per correct feature including the Fick variable it optimises and a brief mechanism (max 4 marks). If a student names moist surface correctly but cannot identify a specific Fick variable, allow 1 mark for the feature alone with a valid explanation of why moisture is required.

Sample response (b), earthworm vs mammal SA:V ratio (3 marks). An earthworm is a relatively small, elongated organism with a high SA:V ratio compared with a large mammal. Because its body surface area is proportionally large relative to its internal volume, and its metabolic rate is relatively low, simple diffusion across the moist skin surface can deliver enough O₂ to all cells within a short diffusion distance. A mammal’s SA:V ratio is much lower, most internal cells are far from the body surface, and mammals have a high metabolic rate that demands rapid O₂ delivery. A body-surface-alone gas exchange strategy would be wholly inadequate: diffusion distances to deep tissues would be measured in centimetres, far exceeding the effective range of simple diffusion (micrometres to millimetres). Internalising the exchange surface as folded alveoli inside the thorax produces an enormous SA (~250 m²) in a compact volume, and pairing this with a circulatory system and ventilation overcomes the SA:V constraint.

Marking notes part (b). 1 mark for correctly identifying that earthworms have a higher SA:V ratio and/or smaller body and lower metabolic demand. 1 mark for explaining why this makes skin-surface gas exchange sufficient for earthworms. 1 mark for explaining why mammals need a specialised internal organ, low SA:V ratio, large body size, high metabolic rate, long diffusion distances to internal cells.

Sample response. The small intestine is the primary site of nutrient absorption, and its structure systematically maximises every relevant Fick variable to achieve the highest possible absorption rate.

Feature 1, Villi: Millions of finger-like projections (villi) line the intestinal wall, dramatically increasing the total surface area available for absorption. According to Fick’s law, greater surface area directly increases absorption rate per unit time. A flat surface would have only a fraction of this area. [1 mark: feature named + structure described + Fick SA variable linked]

Feature 2, Microvilli (brush border): Each epithelial cell on the villus surface bears microscopic projections called microvilli, further multiplying surface area by an estimated 20-fold over the villus alone. Additionally, microvilli carry membrane-bound digestive enzymes (e.g. maltase, peptidase) at the exact point of absorption, ensuring final digestion and absorption are coupled in the same location. [1 mark: feature named + structure described + function including SA multiplication]

Feature 3, Single epithelial cell layer: The villus epithelium is only one cell thick (~10 μm), minimising diffusion distance between the intestinal lumen and the capillaries within the villus. According to Fick’s law, thinner membranes increase diffusion rate; absorbed nutrients therefore enter the blood rapidly. [1 mark: feature named + thickness variable in Fick’s law explicitly applied]

Feature 4, Dense capillary network and lacteals: Each villus contains a network of blood capillaries (for glucose and amino acids) and a central lacteal (lymph vessel, for fatty acids and glycerol reassembled as chylomicrons). The capillary blood flow continuously removes absorbed nutrients, maintaining a low concentration on the blood side of the epithelium and thus preserving the steep concentration gradient across the membrane (Fick gradient variable). Without this removal, the lumen-to-blood gradient would collapse. [1 mark: feature named + gradient maintenance mechanism + Fick gradient variable named]

Feature 5, Intestinal folds (plicae circulares): Large circular folds of the entire intestinal wall (not just the mucosal layer) further multiply the total absorptive area approximately 3-fold before even accounting for villi and microvilli. The combined SA-amplification from folds, villi, and microvilli is estimated at ~600-fold over a smooth tube of the same length. [1 mark: feature named + SA variable correctly applied; accept any fifth valid feature, e.g. rich blood supply maintaining concentration gradient, enzyme activity at brush border, length of small intestine at ~6–7 m]

Conclusion: Each of these structural features addresses one or more variables in Fick’s law: SA is maximised by folds, villi, and microvilli; diffusion distance is minimised by the single-cell epithelium; and the concentration gradient is maintained by continuous blood and lymph flow removing absorbed products. The combined effect is an absorption surface of approximately 250–300 m² in a tube ~6–7 m long, meeting the entire nutritional demand of a large, metabolically active mammal. [2 marks: for a synthesising conclusion that explicitly integrates all three Fick variables and reaches an evaluative statement about the overall design]

Marking criteria.

• 1 markFeature 1: villi, with SA as the Fick variable correctly linked.
• 1 markFeature 2: microvilli / brush border, with SA multiplication (and optionally enzyme coupling) correctly described.
• 1 markFeature 3: single cell layer / thin epithelium, with membrane thickness as the Fick variable correctly linked.
• 1 markFeature 4: capillary / lacteal network, with gradient maintenance explicitly described as the Fick mechanism.
• 1 markFeature 5 or any additional valid structural feature, correctly linked to a Fick variable or to a specific absorption mechanism (enzyme activity, hepatic portal route, chylomicron formation, plicae circulares).
• 1 markResponse integrates at least two of the three Fick variables explicitly (SA, thickness, gradient) across the full answer.
• 1 markReaches an evaluative conclusion that contextualises the structural adaptations against the mammal’s nutritional demands (e.g. states approximate total SA or explains why the combination of adaptations is necessary).