Plant Transport Systems: Xylem and Phloem

Q1, Sample Band 6 response (7 marks), annotated

Xylem transports water and dissolved minerals upward 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 the xylem, generating tension (negative pressure). [1, transpiration creates tension] Cohesion between water molecules (hydrogen bonds) transmits this tension down the continuous water column without it breaking; adhesion to xylem vessel walls provides further support against gravity. [1, cohesion/adhesion mechanism] At the root, the reduced water potential in xylem (under tension) drives osmotic uptake of water from root hair cells, which in turn draw water from soil by osmosis, completing the passive pathway from soil to atmosphere. No ATP is required at the xylem itself; solar energy drives the process via transpiration. [1, osmosis at root, passive / no ATP]

Phloem transports sugars (mainly sucrose) and amino acids bidirectionally from source to sink by the pressure-flow hypothesis. At the source (e.g. photosynthesising leaf), companion cells use ATP to actively load sucrose into sieve tubes via carrier proteins, raising the solute concentration and lowering the water potential in sieve tubes. Water enters by osmosis, raising turgor pressure at the source end. [1, active loading, osmosis, turgor pressure] The turgor pressure gradient between high-pressure source and low-pressure sink drives bulk flow of phloem sap through sieve tube elements toward the sink. At the sink (e.g. growing root tip), sucrose is unloaded, water exits, and turgor falls, maintaining the pressure gradient. [1, bulk flow, unloading]

Comparing the two systems: xylem transport is passive (no metabolic energy at the vessel), whereas phloem requires ATP at the source for active loading. [1, energy criterion with mechanism] Xylem operates under negative pressure (tension), whereas phloem operates under positive turgor pressure; xylem transport is unidirectional (roots to leaves only), whereas phloem transport is bidirectional (source to sink, any direction). Xylem vessel elements are dead at maturity (death creates the hollow lumen and removes osmotic resistance), whereas phloem sieve tubes are living (active loading requires membrane transport proteins). [0.5 each, two more criteria identified]

The key structural reason xylem is suited to passive bulk-flow of water is that dead, lignified, hollow vessels with no end walls or cytoplasmic contents offer minimal resistance to flow under tension. Phloem is suited to active, pressure-driven sugar transport because living sieve tubes connected by porous sieve plates, supported by companion cells providing ATP, allow controlled loading and unloading at any point along the pathway. [included in the criteria marks above]

Marking criteria.

• 1 markDescribes xylem mechanism: transpiration creates tension, transmitted via cohesion, osmosis at root.
• 1 markNames cohesion (H-bonds) and correctly links it to transmitting tension down the water column.
• 1 markStates xylem transport is passive, no ATP at xylem vessel; solar energy drives the process.
• 1 markDescribes phloem mechanism: active loading at source, osmosis, turgor pressure rise.
• 1 markDescribes bulk flow from source to sink and sucrose unloading at sink; turgor falls maintaining gradient.
• 1 markCompares energy requirement (passive xylem vs ATP-requiring phloem) explicitly.
• 1 markIdentifies two further comparison criteria correctly (e.g. pressure type, direction, cell state, contents) with correct detail.

Q2, Sample Band 6 response (8 marks), annotated

Observation (1), Branch drop. During drought, soil water potential falls as water is depleted. The xylem water potential must become more negative to maintain the gradient for uptake. Eventually the tension in the xylem exceeds the critical limit and cavitation (embolism) occurs, air bubbles form in xylem vessels, breaking the water column. [1, links branch drop to cavitation/embolism in xylem] Branches experience the most extreme tension because they are furthest from the soil water source; embolised branches can no longer receive water, so the tree sheds them to reduce total transpiring leaf area and preserve the water supply to the remaining crown. [1, mechanism of why branches drop first]

Observation (2), Clicking sounds. The acoustic clicking is the sound of xylem vessels cavitating, as the water column breaks under extreme tension, the sudden collapse of the water column produces a sound wave detectable by sensors. [1, cavitation / snapping water column] This confirms that cohesion-tension theory is operating at its physical limit under drought stress.

Observation (3), Falling root starch despite normal photosynthesis. Photosynthesis continues in leaves (sucrose is produced), but phloem transport delivers sucrose to sinks including roots. Drought does not directly block phloem, it can continue as long as companion cells have ATP and sieve tubes remain functional. However, roots actively hydrolyse stored starch to sucrose to maintain metabolic activity and to maintain a low enough water potential to continue drawing soil water into root cells by osmosis, even as soil dries. The falling starch is not evidence of blocked phloem; it reflects the root sink increasing its demand for carbon to survive water stress. [1, starch decline linked to increased root metabolic demand under drought, not blocked phloem]

Observation (4), No water flows from cut stems. In a normally hydrated plant, xylem is under tension (negative pressure); cutting a stem under air would cause air to enter and lock the column, but cutting under water would cause water to be drawn in rather than out. The absence of any flow indicates the xylem vessels are embolised, filled with air rather than water, so the continuous water column has been broken and cannot be restored by cutting alone. [1, xylem embolism, air-filled vessels, broken water column]

Recovery after rain. Rain raises soil water potential. Roots absorb water by osmosis as soil water potential exceeds root hair cell water potential [1]. New water enters root xylem, raising its water potential. Embolised vessels cannot be refilled directly, but new xylem vessels can be formed (secondary growth), and existing functional vessels in other parts of the plant route water around embolised regions via the pit connections between adjacent vessels [1, osmosis / soil water potential restores root uptake]. The plant resumes transpiration through new leaves produced from epicormic buds, restoring the cohesion-tension mechanism and re-establishing phloem transport as photosynthate is again available [1, restoration of water column / new leaf growth mechanism].

Marking criteria.

• 1 markObservation 1: links branch drop to cavitation/embolism in xylem under extreme tension.
• 1 markObservation 1: explains why branches shed (reduce transpiring area / protect remaining water supply).
• 1 markObservation 2: identifies clicking as cavitation sound, breaking of the xylem water column.
• 1 markObservation 3: correctly identifies starch depletion as increased root metabolic demand (not blocked phloem); phloem remains functional (or provides any plausible linked explanation).
• 1 markObservation 4: explains absence of flow as evidence of embolised (air-filled) xylem vessels, broken water column.
• 1 markRecovery: soil water potential rises after rain; osmosis drives water uptake at root hair cells.
• 1 markRecovery: explains how water column is restored (new xylem / re-routing through intact vessels) and transpiration / phloem resume.
• 1 markResponse integrates xylem and phloem mechanisms coherently across all four observations using precise terminology (tension, cohesion, embolism, water potential, osmosis, bulk flow or pressure-flow).

Q3, Evaluate the claim (6 marks)

The claim is substantially incorrect. [1, overall evaluative judgement]

What is partly defensible: Phloem transport does involve a passive component, bulk flow through sieve tubes from source to sink is driven by a turgor pressure gradient, not directly by ATP at every point in the tube. In this narrow sense, the bulk-flow phase is "passive" (no direct energy expenditure at the sieve tube lumen). [1, correctly identifies the one defensible element]

What is wrong:

• "No energy input from the plant." Phloem transport fundamentally depends on active loading at the source, companion cells use ATP to pump sucrose into sieve tubes against a concentration gradient via carrier proteins. Without ATP, no sucrose enters the sieve tubes, no osmotic water entry occurs, no turgor pressure builds, and no bulk flow takes place. The system cannot operate without metabolic energy. [1, refutes "no energy input" with active loading mechanism]
• "Just a passive process like xylem." Xylem transport requires no metabolic energy at the vessel, it is driven entirely by solar energy (transpiration) creating tension. Phloem requires ATP at the source for active loading. They are mechanically distinct: xylem uses negative pressure (tension), phloem uses positive pressure (turgor). Calling them both "passive" ignores the energy step that initiates phloem transport. [1, refutes the analogy, distinguishes mechanisms correctly]
• "Cell types irrelevant, any tube would do." Companion cells are essential: sieve tube elements lack a nucleus and cannot sustain their own metabolism; without companion cells supplying ATP and maintaining membrane integrity, active loading cannot occur and sieve tubes degenerate. The specialised sieve plates between elements allow bulk flow while maintaining structural integrity of the living cells. [1, refutes irrelevance of cell types, explains companion cell function]

Defensible reformulation: "Phloem transport is a pressure-flow process that requires ATP for active loading of sucrose into sieve tubes at the source. This active loading creates a turgor pressure gradient that drives passive bulk flow of phloem sap to the sink. Unlike xylem, which is entirely passive, phloem transport is initiated by metabolic energy; the specialised living cell types (sieve tube elements and companion cells) are essential to this mechanism." [1, defensible reformulation with precise terminology and correct active/passive distinction]

Marking criteria.

• 1 markStates an overall evaluative judgement (e.g. "substantially incorrect" or "largely wrong").
• 1 markIdentifies the one defensible element: bulk flow in phloem is passive (pressure-driven), not ATP-powered at every point.
• 1 markCorrectly refutes "no energy input" by explaining that active loading at the source requires ATP from companion cells.
• 1 markCorrectly refutes the analogy with xylem: xylem is passive (tension/cohesion), phloem requires active loading; they operate under opposite pressure types.
• 1 markCorrectly refutes "cell types irrelevant" by explaining the functional role of companion cells (providing ATP and maintaining sieve tubes).
• 1 markReformulates into a biologically defensible statement that distinguishes passive bulk flow from active loading, names the key cell types, and uses precise terminology (pressure-flow, active loading, turgor pressure).