HSC Exam Practice

Sample response. The apoplast pathway is the route water takes through cell walls and intercellular spaces without crossing any membranes; it is rapid but non-selective. The symplast pathway is the route water takes through the cytoplasm and plasmodesmata of connected cells, crossing membranes; it is slower but subject to cellular regulation.

Marking notes. 1 mark for apoplast: cell walls/intercellular spaces, no membranes crossed. 1 mark for symplast: cytoplasm/plasmodesmata, membrane crossing/regulation.

Sample response. The Casparian strip is a waxy suberin band impregnated in the cell walls of root endodermal cells [1]. It blocks the apoplast pathway at the endodermis, so all water and dissolved minerals must cross the plasma membrane of endodermal cells to reach the xylem [1]. The membrane contains specific ion transport proteins that determine which mineral ions can pass, ions without matching transporters are excluded, giving the plant selective control over which minerals enter the vascular tissue [1].

Marking notes. 1 mark for identifying Casparian strip as suberin/waxy band in endodermal cell walls. 1 mark for blocking apoplast pathway / forcing membrane crossing. 1 mark for explaining membrane transport proteins enable selective mineral uptake (exclusion of ions without transporters).

Sample response. Companion cells use ATP to actively pump sucrose from mesophyll cells into sieve tubes at the source, raising the solute concentration in source sieve tubes [1]. This lowers the water potential of sieve tubes at the source; water enters by osmosis from adjacent cells and xylem, raising turgor pressure at the source end [1]. At the sink, sucrose is unloaded and turgor is lower; the turgor pressure difference (high at source, low at sink) drives bulk flow of phloem sap toward the sink [1].

Marking notes. 1 mark for ATP used for active sucrose loading at source / raises solute concentration. 1 mark for osmosis of water into sieve tubes / turgor pressure rises at source. 1 mark for pressure gradient (source to sink) drives bulk flow.

Sample response. When xylem vessel elements die, their cell contents (cytoplasm, nucleus, organelles) are removed, creating a hollow lumen with no obstruction to water flow [1]. Death also dissolves the end walls between adjacent vessel elements, producing a continuous unobstructed tube, cross-walls would create resistance to bulk flow [1]. Living cell contents would also impose osmotic resistance to water movement across membranes; an empty tube allows rapid bulk flow of water under tension without any membrane crossings [1].

Marking notes. 1 mark for removal of cell contents creating hollow lumen (no obstruction to flow). 1 mark for dissolution of end walls creating continuous tube / no cross-wall resistance. 1 mark for absence of osmotic resistance / living cytoplasm would obstruct or slow water movement through membranes.

Sample response. There is an inverse relationship between transpiration rate and xylem water potential: as transpiration rate increases from 6 am to 1 pm, xylem water potential decreases (becomes more negative), reaching its minimum around 1 pm when transpiration is highest [1]. After 1 pm, as transpiration rate declines, xylem water potential recovers (becomes less negative) [1]. The curves are mirror images of each other across the day.

Marking notes. 1 mark for describing inverse relationship (as transpiration rises, water potential falls). 1 mark for describing recovery (as transpiration falls in afternoon, water potential recovers). Accept references to specific approximate values.

Sample response. When transpiration rate is highest (around 1 pm), water evaporates from leaf mesophyll cells at the greatest rate, creating a large water deficit at the leaf end of the xylem [1]. This water deficit generates the most negative water potential (greatest tension) at the leaf, which is transmitted down the cohesive water column to the stem base [1]. Cohesion, hydrogen bonding between water molecules, holds the water column intact and transmits the tension along the entire column from leaf to root; the greater the transpiration rate, the more tension accumulates in the column and the more negative the xylem water potential measured at the stem [1].

Marking notes. 1 mark for highest transpiration = greatest water deficit at leaf = most negative water potential at leaf end. 1 mark for tension transmitted through cohesive column to stem base. 1 mark for cohesion (H-bonds) holds column intact and transmits tension; more transpiration = more tension = more negative stem water potential.

Sample response. Accept any one well-justified answer. Examples: (i) Species B has a larger root system with greater water-absorbing surface area, more rapid water uptake from soil replaces water lost by transpiration, so tension does not accumulate as severely in the xylem [1+1]. (ii) Species B has stomatal closure behaviour that partially closes stomata at midday (e.g. partial stomatal closure triggered by ABA or turgor-sensing), reducing maximum transpiration rate and preventing the xylem from experiencing high tension [1+1]. (iii) Species B has a higher vessel density or wider xylem vessels, reducing hydraulic resistance and allowing water to flow more readily up the stem so tension does not build as much [1+1]. (iv) Species B has a more tightly coupled hydraulic system where root pressure or capacitance (water stored in bark/wood) buffers rapid changes in xylem water potential [1+1].

Marking notes. 1 mark for identifying a plausible structural or physiological feature. 1 mark for correctly justifying how that feature would buffer xylem water potential (prevent it becoming more negative). Both marks require the link to xylem water potential to be explicit.

Sample Band 6 response.

Xylem transports water and dissolved minerals unidirectionally from roots to leaves by cohesion-tension theory. Transpiration, evaporation of water from leaf mesophyll cells and diffusion through stomata, creates a water deficit at the leaf end of xylem vessels, lowering water potential and generating tension (negative pressure). Cohesion between water molecules via hydrogen bonds transmits this tension as a continuous pulling force down the entire water column to root xylem, where the lower water potential drives osmotic uptake from soil. Adhesion to vessel walls assists in maintaining the column against gravity. Xylem transport is entirely passive, no metabolic energy is required at the vessel; solar energy drives transpiration and thus the whole process. [3 marks, transpiration/tension, cohesion transmission, passive/osmosis at root]

Phloem transports sugars (sucrose), amino acids, and hormones bidirectionally from source to sink by the pressure-flow hypothesis. Companion cells use ATP to actively load sucrose into sieve tube elements at the source, raising solute concentration and lowering water potential. Water enters by osmosis, raising turgor pressure at the source. The turgor pressure gradient between high-pressure source and low-pressure sink drives passive bulk flow through perforated sieve plates to the sink, where sucrose is unloaded and turgor falls. Phloem requires metabolic energy (ATP) for active loading, without it, no pressure gradient forms and transport fails. [2 marks, active loading/ATP, bulk flow by turgor gradient]

Comparing the two systems on three criteria: (1) Energy: xylem transport is passive (no ATP); phloem transport requires ATP for active loading at source. (2) Pressure type: xylem operates under negative pressure (tension, below atmospheric); phloem operates under positive turgor pressure (above atmospheric). (3) Cell state: xylem vessel elements are dead at maturity (hollow lumen, no cytoplasm, no end walls, essential for unobstructed bulk flow under tension); phloem sieve tubes are living (membrane transport proteins needed for active loading; companion cells essential). [2 marks, three criteria correctly compared with mechanism detail]

Evaluating vulnerability to disruption: xylem is more vulnerable to physical disruption because it operates under negative pressure (tension). If tension exceeds the cohesive strength of the water column, e.g. during drought or when a vessel is cut, cavitation occurs and air fills the vessel, permanently blocking that vessel until replaced by new xylem growth. Phloem operates under positive pressure so cutting phloem causes sap to flow out (no air entrainment) and the sieve tubes can be patched by callose; disruption is more easily reversed. However, phloem is metabolically vulnerable, anything that stops ATP production (e.g. metabolic poison, anoxia) halts active loading and ends phloem transport, whereas xylem would continue passively. The systems therefore have different vulnerabilities: xylem to physical tension-breaking events, phloem to metabolic disruption. [1 mark, evaluative judgement distinguishing vulnerabilities of each system]

Marking criteria.

• 1 markDescribes xylem mechanism: transpiration creates tension, transmitted via cohesion down the water column.
• 1 markStates xylem is passive (no ATP) and osmosis drives uptake at root.
• 1 markExplains role of dead xylem cell structure: hollow lumen, no end walls, allows unobstructed bulk flow under tension.
• 1 markDescribes phloem mechanism: active loading with ATP at source raises sucrose concentration / turgor pressure.
• 1 markBulk flow from high-pressure source to low-pressure sink; unloading at sink maintains gradient.
• 1 markCompares energy requirement correctly (passive xylem, ATP-requiring phloem) with mechanism detail.
• 1 markCompares pressure type (negative/tension in xylem, positive/turgor in phloem) AND cell state (dead vs living) correctly.
• 1 markEvaluative judgement: identifies xylem as more vulnerable to physical disruption (cavitation under negative pressure) and/or phloem as more vulnerable to metabolic disruption; provides mechanism-based justification for the comparison.