HSC Exam Practice

Sample response. An autotroph synthesises its own organic compounds from inorganic sources (e.g. CO&sub2;, H&sub2;O, minerals) using light or chemical energy; plants are the principal autotrophs, using light energy via photosynthesis. A heterotroph obtains organic compounds by consuming other organisms or organic matter; it relies on organic carbon (e.g. glucose, amino acids) from its diet and releases energy by cellular respiration.

Marking notes. 1 mark for autotroph: inorganic sources + light/chemical energy (photosynthesis). 1 mark for heterotroph: organic carbon from food + cellular respiration. A mark is not awarded if only “makes own food” / “eats other things” without reference to the source type.

Sample response. The term “net producer” is required because a plant performs both photosynthesis and cellular respiration simultaneously. Photosynthesis produces O&sub2; (in the light reactions when water is split); cellular respiration consumes O&sub2; continuously in all living cells. In bright sunlight, the rate of photosynthesis exceeds the rate of respiration, so the plant releases more O&sub2; than it consumes, this surplus is the “net” amount. The plant is still consuming O&sub2; via respiration; it just produces more than it consumes in bright light.

Marking notes. 1 mark for identifying that both photosynthesis and cellular respiration are occurring simultaneously. 1 mark for explaining that photosynthesis rate exceeds respiration rate in bright sunlight, producing a surplus O&sub2;. 1 mark for explicitly stating the plant still consumes O&sub2; via respiration (i.e. gross production > consumption = net positive).

Sample response. Xylem transports water and inorganic minerals passively by cohesion-tension (transpiration pull); the driving force is solar energy evaporating water at the leaf surface. Phloem transports organic solutes (primarily sucrose and amino acids) by active loading at source cells, requiring ATP; the driving force is a pressure gradient generated by osmosis after active solute loading.

Marking notes. 1 mark for a correct difference in what is transported (water + minerals vs organic solutes/sucrose). 1 mark for a correct difference in how transport is driven (passive cohesion-tension/solar energy vs active loading/ATP). Accept equivalent phrasings.

Sample response. The liver produces urea by deaminating excess amino acids. When amino acids are broken down, the amino group is removed as toxic ammonia (NH&sub3;); the liver converts this ammonia to the less toxic urea via the urea cycle. Urea then enters the blood and circulates to the kidneys. The kidneys remove urea from the blood by filtering blood through the glomerulus into the Bowman’s capsule; urea is not reabsorbed and is excreted in urine, reducing blood urea concentration.

Marking notes. 1 mark for liver producing urea via deamination of amino acids. 1 mark for the conversion of toxic ammonia to less toxic urea (urea cycle or equivalent). 1 mark for kidneys filtering / excreting urea in urine, reducing blood concentration.

Sample response (a). In Pond A, dissolved O&sub2; rises steeply from 4.5 mg/L at 08:00 to a maximum of 9.8 mg/L at 14:00 (a rise of 5.3 mg/L). From 20:00 onwards, DO falls sharply, reaching its minimum of 1.6 mg/L at 02:00 (a fall of 2.6 mg/L from 20:00). The pattern is clearly cyclic, rising during daylight and falling during darkness.

Marking notes (a). 1 mark for correctly describing the rise from 08:00 to 14:00 with at least one value quoted. 1 mark for correctly describing the fall from 20:00 to 02:00 with at least one value quoted.

Sample response (b). Between 08:00 and 14:00, the aquatic plants photosynthesise at an increasing rate as sunlight intensifies; photosynthesis splits water and produces O&sub2; as a by-product. As sunlight is bright and CO&sub2; is available, the rate of O&sub2; production by photosynthesis greatly exceeds the rate of O&sub2; consumption by cellular respiration, so dissolved O&sub2; accumulates in the water, rising to 9.8 mg/L. By 02:00 (complete darkness), photosynthesis has ceased entirely because no light energy is available. The plants now perform only cellular respiration, continuously consuming dissolved O&sub2; and releasing CO&sub2; into the water. Over several hours of darkness, this respiratory O&sub2; consumption without any photosynthetic replenishment drives dissolved O&sub2; to its lowest point (1.6 mg/L). This demonstrates that plants are net O&sub2; consumers at night.

Marking notes (b). 1 mark for explaining the O&sub2; rise: photosynthesis rate exceeds respiration rate in bright sunlight, O&sub2; produced faster than consumed. 1 mark for explaining the fall: photosynthesis ceases in darkness, only respiration continues, net O&sub2; consumption lowers DO. 1 mark for explicitly linking the night-time fall to continuous cellular respiration in the absence of photosynthesis (must state that both processes occur).

Sample response (c). Accept any one of: (i) O&sub2; diffuses into the pond from the atmosphere across the water surface, partially replenishing what the fish consume. (ii) The ponds are described as open, so atmospheric O&sub2; dissolution prevents complete depletion. (iii) The fish may reduce activity at night, lowering their O&sub2; consumption rate.

Marking notes (c). 1 mark for any plausible mechanism that prevents complete O&sub2; depletion. Most expected answer is atmospheric diffusion into the open water.

Sample response. Plants are autotrophs and mammals are heterotrophs, meaning their fundamental strategies for acquiring energy and nutrients differ completely, which in turn drives entirely different transport and gas exchange systems. Despite these differences, both organisms share the common requirement of supplying every cell with O&sub2; and organic substrate for cellular respiration, and removing CO&sub2; and other metabolic waste products.

(i) Obtaining nutrients and gases: The plant acquires inorganic raw materials from its environment. Carbon dioxide from the atmosphere diffuses into leaf air spaces through open stomata down a concentration gradient. Water and dissolved minerals (NO&sub3;⁻, K⁺, Ca²⁺) are absorbed from soil by root hairs, with mineral uptake requiring active transport via the Casparian strip. The plant then synthesises its own organic molecules (glucose) via photosynthesis in leaf chloroplasts, using light energy to fix CO&sub2; into organic carbon in the Calvin cycle. The mammal, by contrast, must consume organic molecules already made by autotrophs. Food is ingested and broken down physically by chewing and mechanically by the stomach, and chemically by digestive enzymes (amylase, protease, lipase) along the alimentary canal. The resulting monomers (glucose, amino acids, fatty acids) and water and minerals are absorbed across the large surface area of villi and microvilli in the small intestine. O&sub2; is acquired simultaneously at the alveoli, where it diffuses from high partial pressure alveolar air into pulmonary blood.

(ii) Internal transport: The plant uses two separate vascular tissues. Xylem carries water and dissolved minerals passively by cohesion-tension (transpiration pull) from root to leaf; the driving force is solar energy evaporating water at the leaf surface. Phloem carries organic solutes (primarily sucrose and amino acids) from source tissues (photosynthesising leaves) to sink tissues (growing roots, fruits, developing seeds) by active loading at the source, which raises osmotic pressure and drives pressure flow to sinks; this requires ATP. There is no heart and no single transport medium. The mammal uses a closed cardiovascular system. The heart, a muscular pump driven by cardiac ATP, pumps oxygenated blood through arteries to capillary beds in every tissue, where O&sub2; and glucose diffuse out of blood into cells down concentration gradients, and CO&sub2; and urea diffuse from cells into blood. Blood returns via veins to the heart and is re-oxygenated at the lungs. Unlike the plant’s two separate systems, blood is a single transport medium carrying all substances simultaneously.

(iii) Gas exchange at the cellular level: In both organisms, the ultimate gas exchange at the cellular level relies on diffusion across thin membranes down concentration (or partial pressure) gradients, consistent with Fick’s law. In the plant, CO&sub2; diffuses from the atmosphere into leaf air spaces via stomata and then into mesophyll cells where it is consumed by the Calvin cycle, maintaining the inward gradient. O&sub2; produced by the light reactions in chloroplasts diffuses outward through the same route. In the mammal, O&sub2; diffuses from alveoli into pulmonary capillary blood (high to low partial pressure) and then from systemic capillaries into tissue cells where it is consumed by cellular respiration; CO&sub2; moves in reverse. Ventilation and blood flow maintain the gradients by continuously replenishing O&sub2; and removing CO&sub2; at the exchange surfaces.

Despite these structural differences, both organisms apply the same physical principles, diffusion, osmosis, active transport, and concentration gradients, to solve the same fundamental challenge: supplying every cell with the O&sub2; and organic substrate needed for cellular respiration, and removing CO&sub2;, urea, and other metabolic wastes.

Marking criteria.

• 1 markCorrectly distinguishes autotroph (plant; CO&sub2; via stomata; water + minerals via root hairs; synthesises glucose by photosynthesis) from heterotroph (mammal; ingestion, digestion, absorption via villi; O&sub2; via alveoli) in acquiring raw materials.
• 1 markNames xylem as the plant tissue for passive water + mineral transport and explains the cohesion-tension mechanism (or transpiration pull).
• 1 markNames phloem as the plant tissue for organic solute transport; explains active loading at source and pressure-flow to sinks; states ATP is required.
• 1 markDescribes the mammalian cardiovascular system: heart pumps blood through arteries to capillary beds; capillary exchange of O&sub2;, glucose, CO&sub2;, urea; veins return to heart.
• 1 markExplains cellular gas exchange in the plant: CO&sub2; diffuses into mesophyll via stomata (concentration gradient maintained by Calvin cycle); O&sub2; diffuses out.
• 1 markExplains cellular gas exchange in the mammal: O&sub2; diffuses from alveoli into blood and then into tissue cells (partial pressure gradients); CO&sub2; reverse; ventilation and blood flow maintain gradients.
• 1 markIdentifies Fick’s law / diffusion as the shared physical principle driving gas exchange in both organisms; mentions large surface area and thin membranes as shared structural features.
• 1 markReaches an explicit comparative conclusion: despite structural differences, both organisms apply the same physical principles (diffusion, osmosis, concentration gradients) to supply cells for cellular respiration and remove metabolic wastes.