Explain how the structure of xylem and phloem tissues allows each one to carry out its transport function in a plant.

AQA GCSE Biology (8461) — 4.2.3 Plant tissues, organs and systems · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Xylem transports water and dissolved minerals from the roots to the leaves. The vessels and tracheids that make up xylem are long, hollow tubes with no living cell contents, so water can move through them by cohesion‑tension and capillary action.
Phloem transports sugars and other food molecules from source tissues (e.g. leaves) to sink tissues (e.g. roots, fruits). Phloem sieve‑tube elements are living cells that contain cytoplasm and companion cells, which are required for the active loading and unloading of sugars during translocation.

Examiner tips

  • Use the terms ‘xylem vessels/tracheids’ and ‘phloem sieve‑tube elements’. Explain the physical property (hollow, no cytoplasm) that allows water flow. Mention that phloem cells are living and need cytoplasm for active transport of sugars.
  • Include the direction of flow (roots to leaves for xylem, source to sink for phloem).

Common mistakes

  • Confusing the direction of flow in phloem. Forgetting to state that xylem vessels are hollow. Using non‑examiner terminology such as ‘tubes’ instead of ‘vessels’ or ‘sieve‑tube elements’.

Mark scheme (4 marks)

  1. Xylem transports water (and dissolved minerals) from roots to leaves
  2. Xylem vessels are hollow / have no living cell contents, allowing water to flow through
  3. Phloem transports sugars from source to sink / carries food molecules (translocation)
  4. Phloem cells are living / contain cytoplasm, which is needed to actively move sugars

Key terms in this question

xylem · phloem

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