HSC Exam Practice

Sample response. Water moves through xylem by transpiration pull (cohesion-tension mechanism). The driving force is evaporation of water from leaf mesophyll cells, which creates tension (negative pressure) that pulls the water column upward. The energy source is solar radiation, no metabolic ATP is consumed at the xylem vessel itself.

Marking notes. 1 mark for identifying transpiration pull / cohesion-tension / evaporation from leaves as the driving force. 1 mark for identifying solar energy / sunlight as the energy source (not ATP / not metabolic energy).

Sample response. Phloem transport is bidirectional, it moves photosynthate (sucrose and amino acids) from source regions (e.g. mature leaves) to sink regions (e.g. roots, growing shoot tips) in any direction depending on where sources and sinks are located. Phloem does not have valves because flow direction is regulated by the source-to-sink turgor pressure gradient. In contrast, veins carry blood in one direction only, always toward the heart, and contain pocket valves that prevent backflow of blood at low venous pressure.

Marking notes. 1 mark for phloem bidirectional vs veins always toward heart. 1 mark for phloem no valves (with reason: turgor gradient sets direction) vs veins have pocket valves. 1 mark for identifying that venous valves prevent backflow / are needed because of low venous pressure.

Sample response. Xylem vessel elements die at maturity, their cytoplasm, nucleus and organelles are broken down and removed. This is advantageous because: (1) the empty hollow lumen offers no resistance to the bulk flow of water, allowing rapid low-resistance transport; (2) the absence of living cytoplasm prevents osmotic barriers that would impede water movement through the continuous water column; and (3) the dead cell wall is reinforced with lignin during development and then maintained indefinitely without ongoing metabolic cost.

Marking notes. 1 mark for stating that the cells die (cytoplasm removed / hollow at maturity). 1 mark for first functional advantage (hollow lumen = unobstructed low-resistance bulk flow OR no osmotic barrier). 1 mark for either a second functional advantage or for explaining why living cytoplasm would be a disadvantage (obstruction / osmotic resistance / ATP cost).

Sample response. Both leaf mesophyll air spaces and pulmonary alveoli share: (1) large total surface area achieved through internal subdivision (lobed mesophyll cells / ~500 million alveoli); (2) thin membrane separating the exchange surface from the transport medium (~0.1–0.5 µm); (3) continuously moist surface (water film on mesophyll cell walls / alveolar lining fluid) allowing gases to dissolve before diffusing across the membrane.

Marking notes. 1 mark each for any three of the four shared features: large surface area, thin membrane, moist surface, maintained concentration gradient. Accept brief description of each feature, explicit Fick's law reference not required here (required in the extended response).

Sample response. The pattern is that vessels operating under negative pressure (xylem, tension) use rigid inorganic reinforcement (lignin) to resist inward collapse, while vessels operating under positive pressure (phloem, artery, vein) use flexible organic materials (elastic fibres, collagen, smooth muscle, or unlignified walls) that accommodate outward pressure. The sign of pressure determines the type of structural reinforcement required.

Marking notes. 1 mark for identifying the pattern: negative pressure (xylem) = lignin / rigid reinforcement; positive pressure = elastic/flexible materials. 1 mark for justifying with at least one specific data comparison from the table (e.g. xylem, negative, lignin vs artery, high positive, elastic fibres + collagen).

Sample response. Xylem vessel elements are dead so that all living cytoplasm is removed from the lumen. If living cells remained, their cytoplasm would physically obstruct the water column, increasing resistance to bulk flow [1]. Living cells also possess membranes that would impose osmotic barriers, water molecules would have to cross membrane proteins rather than flowing freely through a continuous aqueous column, greatly slowing transport [1]. Additionally, living cells would consume water and metabolic energy, undermining the plant's passive, solar-powered transport strategy [1].

Marking notes. 1 mark for obstruction of lumen / increased resistance to flow. 1 mark for osmotic barrier imposed by living membranes. 1 mark for any valid third point (ATP cost / water consumption by living cells / inconsistency with passive transport strategy).

Sample response. Xylem sap velocity closely follows transpiration rate over the 24-hour period, both are near zero overnight (0–6 h), rise during the day with transpiration, peak around solar noon to early afternoon, then decline in the evening. Sap velocity lags slightly behind transpiration (peak at ~14 h versus ~13 h for transpiration) and reaches a slightly lower peak value.

Marking notes. 1 mark for describing the positive correlation / that both follow the same daily pattern. 1 mark for noting the slight lag of xylem velocity behind transpiration rate (peak later / slightly lower maximum). Accept reference to approximate time/value readings from the graph.

Sample response. According to the cohesion-tension mechanism, transpiration from leaf mesophyll cells creates tension (negative pressure) at the top of the xylem column. This tension is transmitted downward through the cohesive water column (water molecules held together by hydrogen bonds), pulling water up from the roots. When transpiration rate increases (e.g. midday when sunlight is intense and stomata are open), greater tension is generated, pulling water more rapidly through the xylem and increasing sap velocity. When transpiration decreases (e.g. at night when stomata close), tension falls and sap velocity decreases accordingly.

Marking notes. 1 mark for linking increased transpiration to increased tension (negative pressure) in the xylem column via the cohesion-tension mechanism. 1 mark for explaining that greater tension pulls water more rapidly through xylem, increasing velocity, and that reduced transpiration reduces this pulling force.

Sample response. Plants have two specialised vascular tissues: xylem and phloem. Xylem transports water and dissolved inorganic minerals from roots to leaves by the cohesion-tension mechanism, transpiration (evaporation from leaf mesophyll cells) creates tension (negative pressure) transmitted through the cohesive water column, pulling water upward. This is entirely passive; no metabolic ATP is consumed at the xylem vessel. Phloem transports photosynthate (sucrose, amino acids) bidirectionally from photosynthetic source leaves to non-photosynthetic sink tissues (roots, meristems, developing fruit) via the pressure-flow hypothesis. Companion cells actively load sucrose into sieve tubes using ATP, creating osmotic turgor pressure that drives bulk flow toward the lower-pressure sink.

Animals use a closed circulatory system of arteries, capillaries and veins. Arteries carry oxygenated blood away from the heart at high positive pressure (~120 mmHg), driven by left ventricular contraction, an active process requiring continuous ATP. Veins return deoxygenated blood to the heart at low positive pressure, assisted by skeletal muscle compression and pocket valves that prevent backflow. Exchange of O2, CO2, glucose and waste occurs only at thin-walled capillaries where the single-cell-thick wall minimises diffusion distance.

Similarity: In both plants and animals, the composition of the transport medium changes as it passes through metabolically active tissue, substances are removed by cells. In animals, O2 and glucose fall and CO2 rises across muscle capillary beds. In plants, mineral ions are progressively absorbed by cells along the xylem pathway, reducing concentration from root to leaf.

Difference: Animal blood changes composition in both organic and inorganic components (O2, glucose, CO2, hormones), while xylem sap changes only in inorganic mineral content, xylem carries no organic compounds such as glucose or sucrose (these are exclusively in phloem).

Marking criteria.

• 1 markNames both plant vascular tissues (xylem and phloem) and describes each correctly (water + minerals; photosynthate / sugars).
• 1 markCorrectly describes the mechanism driving xylem flow (cohesion-tension / transpiration pull) and identifies it as passive / solar-powered / no ATP at xylem.
• 1 markCorrectly describes the mechanism driving phloem flow (pressure-flow / active sucrose loading by companion cells / turgor pressure gradient) and identifies it as active / requires ATP.
• 1 markNames the three animal vessel types (arteries, capillaries, veins) with at least one structural or functional feature of each, and identifies the driving force as cardiac contraction.
• 1 markIdentifies one valid similarity in how composition changes in transit in plants and animals (both show composition changes as active tissues take up substances from the transport fluid).
• 1 markIdentifies one valid difference in how composition changes in transit (e.g. animal blood changes in organic AND inorganic components; xylem changes only in inorganic minerals / xylem has no organic substances while blood has both).