HSC Exam Practice

Sample response. Transpiration is the evaporation of water vapour from leaf surfaces and its loss to the atmosphere, occurring primarily through stomata — microscopic pores in the leaf epidermis. A small proportion (~5%) also occurs through the cuticle (cuticular transpiration).

Marking notes. 1 mark for defining transpiration as evaporation of water vapour from leaf surfaces (or equivalent); 1 mark for identifying stomata as the primary structure through which it occurs.

Sample response. K⁺ ions are actively transported into guard cells (via ATP-dependent ion pumps). Water then follows K⁺ by osmosis — entering the guard cells from surrounding cells and the apoplast. This increases turgor pressure inside the guard cells. Because the inner cell wall (facing the pore) is thicker than the outer wall, increased turgor causes the guard cells to bow outward asymmetrically, pulling the pore open.

Marking notes. 1 mark for K⁺ actively transported into guard cells; 1 mark for water following by osmosis (not directly pumped); 1 mark for turgor increase → asymmetric cell wall → guard cells bow outward → pore opens.

Sample response. (1) Dense silvery trichomes in silver saltbush (Atriplex vesicaria): fine hairs trap a layer of still, humid air adjacent to the leaf surface, reducing the water vapour concentration gradient between the leaf interior and the air surrounding the stomata, slowing outward diffusion of water vapour. Trichomes also reflect solar radiation, reducing leaf temperature and further lowering the gradient. (2) Sunken stomata in needle bush (Hakea leucoptera): stomata positioned in pits below the leaf surface trap a pocket of humid air, reducing the concentration gradient between the leaf interior and the atmosphere and slowing diffusion of water vapour out through the pore.

Marking notes. 2 marks per adaptation: 1 mark for naming the adaptation with a correct Australian species example; 1 mark for the mechanism (must explain the physical process, not just restate the feature name). Max 4 marks for 2 adaptations. Accept: waxy cuticle (blocks cuticular transpiration), small/reduced leaves or phyllodes (reduced surface area and total stomata number), rolled leaves/spinifex (sunken-equivalent humid microenvironment), vertical leaf orientation (reduced solar radiation absorption → lower leaf temperature → lower vapour pressure gradient), light-coloured/silvery leaves (reflects radiation → cooler leaf). Named Australian species must accompany each example.

Sample response. An osmoconformer allows its internal body fluid osmolarity to change with the external environment — its blood or body fluids remain approximately iso-osmotic with the surroundings. This requires little active energy expenditure for osmoregulation because no osmotic gradient is being maintained. An osmoregulator actively maintains a constant internal osmolarity regardless of external conditions, using active transport (e.g. in gills and kidneys) to move ions against concentration gradients. This requires a continuous energy expenditure.

Marking notes. 1 mark for osmoconformer: allows internal osmolarity to match environment; 1 mark for osmoregulator: actively maintains constant internal osmolarity; 1 mark for energy distinction (osmoconformer requires little energy for osmoregulation; osmoregulator requires active transport / ongoing energy expenditure).

Sample response. Stomatal closure during drought is a homeostatic response because it satisfies the key requirements of homeostasis: there is a stimulus (reduction in leaf water potential as water is lost), a chemical signal acting as a receptor/coordinator (ABA released by stressed mesophyll cells), and an effector (guard cells — K⁺ is actively exported, water follows osmotically, turgor falls, stomata close). The closure reduces transpiration and opposes the original stimulus (opposing water loss), constituting a negative feedback loop that maintains internal water balance within the tolerance range needed for cell function. It is not passive drying because it requires active K⁺ transport driven by ATP and a regulated hormonal signal (ABA).

Marking notes. 1 mark for identifying the stimulus and the regulatory signal (ABA as a chemical hormone signalling water stress); 1 mark for identifying the effector response (active K⁺ export → osmosis → loss of turgor → stomatal closure); 1 mark for explicitly linking this to the negative feedback concept (response opposes the stimulus, maintaining an internal variable within a range — not passive).

Sample response. Stomatal closure conserves water by blocking the main pathway for transpiration, which addresses the homeostatic need during drought. However, it simultaneously blocks CO₂ entry into the leaf. Without CO₂, photosynthesis cannot proceed, so the plant cannot produce glucose for energy or growth. The plant therefore trades water conservation against the loss of photosynthetic capacity — a homeostatic solution that prevents dehydration but halts carbon fixation.

Marking notes. 1 mark for identifying that stomatal closure conserves water (reduces transpiration); 1 mark for identifying the trade-off (blocks CO₂ entry → photosynthesis cannot proceed / growth and energy production sacrificed).

Sample response (a). The blood osmolarity of Species 3 (marine bony fish) remains approximately constant at around 350 mOsm kg⁻¹ across the entire range of environmental salinities from 0 to 1200 mOsm kg⁻¹. There is minimal change as environmental salinity increases — a nearly flat horizontal line, very different from the iso-osmotic diagonal followed by Species 1.

Marking notes (a). 1 mark for correctly identifying the flat/nearly constant relationship; 1 mark for an approximate value read from the graph (approximately 350 mOsm kg⁻¹) cited to support the trend description.

Sample response (b). Species 3 is an osmoregulator. Its blood osmolarity remains at approximately 350 mOsm kg⁻¹ regardless of whether the external environment is very dilute (<50 mOsm kg⁻¹) or concentrated (~1000 mOsm kg⁻¹ seawater). An osmoconformer (as shown by Species 1) would track the iso-osmotic line — Species 3 does not: its blood osmolarity does not change with external salinity.

Marking notes (b). 1 mark for correct identification (osmoregulator); 1 mark for justification using values from the graph (blood osmolarity approximately constant at ~350 mOsm kg⁻¹ while external salinity varies from 0 to 1200).

Sample response (c). At ~1000 mOsm kg⁻¹ (seawater), the fish blood osmolarity (~350 mOsm kg⁻¹) is far lower than the surrounding water. Because seawater has a higher solute concentration, water moves out of the fish by osmosis through permeable gill surfaces — the fish is continuously dehydrated [1]. To compensate, the marine fish must drink large volumes of seawater to replace the water lost [1]. This introduces excess salt, which must be actively excreted — chloride cells in the gills use active transport (energy-dependent) to excrete Na⁺ and Cl⁻ against the concentration gradient. The kidneys produce small volumes of concentrated urine to conserve water [1].

Marking notes (c). 1 mark for explaining the osmotic problem (seawater more concentrated than blood → water leaves by osmosis → constant dehydration risk); 1 mark for drinking strategy (drinks seawater to replace lost water); 1 mark for salt excretion strategy (gill chloride cells actively excrete Na⁺/Cl⁻) AND/OR concentrated urine (kidneys produce small volume concentrated urine to conserve water).

Sample response. Xerophytes maintain water balance homeostasis through two complementary systems: permanent structural adaptations that passively reduce water loss at all times, and active physiological responses (principally ABA-driven stomatal control) that dynamically respond to changing water status. Neither system alone is sufficient under extreme Australian arid conditions; it is their interaction that allows survival.

Structural adaptations in Australian xerophytes reduce the rate of water loss passively. In silver saltbush (Atriplex vesicaria), dense silvery trichomes trap a still, humid boundary layer adjacent to leaf surfaces, reducing the water vapour concentration gradient between the leaf interior and the atmosphere and slowing diffusion outward. The silvery coating also reflects solar radiation, keeping leaf temperature 5–10°C lower than a dark mesophyte leaf under the same sun — a lower leaf temperature means lower internal vapour pressure and therefore a smaller driving gradient for evaporation. In spinifex (Triodia pungens), leaves roll longitudinally to enclose the stomata on the inner surface within a humid cylindrical cavity, functionally equivalent to sunken stomata. Both species also have thick waxy cuticles preventing cuticular transpiration through non-stomatal surfaces. These structural features are permanent and energy-free; they reduce the baseline rate of water loss without requiring any physiological response from the plant.

The active homeostatic mechanism is ABA-mediated stomatal control. As water stress increases (soil water potential falls, leaf cells lose turgor), mesophyll cells synthesise and release abscisic acid. ABA binds to receptors in guard cell membranes, activating ion channels that allow K⁺ to leave the guard cells. Water follows K⁺ out by osmosis; guard cells lose turgor pressure and straighten (become flaccid); the stomatal pore closes. This is a classic negative feedback loop: the stimulus is a fall in leaf water potential; the effector response (stomatal closure) reduces transpiration and opposes the further loss of water. The variable being maintained is leaf water potential / cellular hydration within the tolerance range for metabolic function.

The two systems are complementary, not alternatives. Structural features reduce the passive rate of water loss so that the active ABA mechanism has less work to do. In a xerophyte, even when stomata are fully open, the rate of transpiration is far lower than in a mesophyte because trichomes, cuticle, and sunken or rolled stomata all reduce the effective concentration gradient. When water stress triggers ABA and stomata close, the structural features ensure that cuticular and boundary-layer water loss remain near zero as well. The result is that a xerophyte can maintain cellular hydration homeostasis under conditions where a mesophyte’s active mechanism alone is overwhelmed — as shown by the severe wilting of wheat versus the maintained hydration of Acacia dealbata in the same 45°C Australian paddock. Evaluating ABA specifically: ABA is the critical homeostatic signal that converts the information “water potential is falling” into the effector response “reduce stomatal aperture.” Without ABA, the plant would have no mechanism to actively regulate pore size in response to water stress — it would rely entirely on structural features, which are fixed and cannot increase their effectiveness under acute stress. ABA is therefore the dynamic, adjustable component of the homeostatic system, while structural adaptations provide the stable baseline that reduces the physiological burden placed on ABA-mediated responses.

Marking criteria.

• 1 mark — Names at least two structural adaptations with correct mechanisms (must link each feature to a physical process: gradient reduction, radiation reflection, surface area reduction, or cuticular impermeability), with named Australian species.
• 1 mark — Correctly explains that structural adaptations are passive and permanent (energy-free, always active) — contrasting with active physiological responses.
• 1 mark — Correctly traces the ABA pathway from drought stimulus through K⁺ efflux and osmosis to turgor loss and stomatal closure.
• 1 mark — Applies the homeostasis framework to stomatal control: identifies the variable, stimulus, effector, and negative feedback nature of the response.
• 1 mark — Evaluates ABA specifically: identifies its role as the dynamic signal that converts water-stress information into a stomatal effector response; explains why ABA is necessary (structural features alone cannot dynamically adjust to acute stress).
• 1 mark — Analyses the complementary relationship: explains why structural adaptations reduce the demand on the active ABA system, and why this interaction is what makes the overall homeostatic system effective under extreme conditions.
• 1 mark — Reaches an evaluative judgement using at least one named Australian example and evidence-based reasoning (e.g. spinifex or saltbush surviving where mesophytes wilt, or wheat vs wattle comparison), showing higher-order synthesis of the two systems.