Explain how the structure of xylem vessels is adapted for the transport of water through a plant.

WJEC A-Level Biology (Wales) — 2.3 Adaptations for transport in plants and animals · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Plants need to move water from their roots up to their leaves, where it is used in photosynthesis. This transport occurs through xylem vessels.

Model answer (5 marks)

Xylem vessels are formed from dead cells that have been hollowed out, leaving no cell contents. The end walls between adjacent cells are removed, creating a continuous tube that allows water to flow freely from roots to leaves. The cell walls are reinforced with lignin, which strengthens the vessel and prevents collapse under the negative pressure generated by transpiration. Because the walls are impermeable to water, the flow is directed upwards rather than leaking out through the sides.

Examiner tips

  • Use the exact terms ‘dead cells’, ‘hollowed out’, ‘continuous tube’, ‘lignin’, and ‘impermeable walls’.
  • Explain the function of each feature: no contents → no resistance, continuous tube → free flow, lignin → structural support, impermeable walls → directed flow.
  • Keep the answer concise – 5 points, one sentence per point.

Mark scheme (5 marks)

  1. Xylem vessels are made from hollowed-out dead cells / have no cell contents
  2. The end walls of the cells are removed / broken down to form a continuous tube
  3. This allows water to flow freely / continuously from roots to leaves
  4. The walls are strengthened with lignin to prevent the vessel from collapsing under pressure
  5. The walls are impermeable to water so water is directed upwards rather than leaking out through the sides

Key terms in this question

xylem

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