Explain how the structures of xylem and phloem are each suited to their transport functions 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 cells are long, narrow, hollow tracheids or vessels with thick lignified walls and no cytoplasm, so water and dissolved minerals can move unimpeded from roots to leaves.
Phloem consists of living sieve‑tube elements and companion cells; the sieve plates between cells have pores that allow sugars to pass, enabling translocation from source to sink.

Examiner tips

Use the word ‘hollow’ for xylem and ‘pore/pores’ for phloem; mention lignin and lack of cytoplasm for xylem, living cells for phloem.
Show the direction of flow (roots to leaves for xylem, source to sink for phloem).

Common mistakes

Confusing xylem with phloem structure, e.g. saying xylem has pores; describing phloem as hollow like xylem; omitting the role of lignin or living cells.

Mark scheme (4 marks)

Xylem transports water (and dissolved minerals) from the roots to the leaves
Xylem cells are hollow / have no cell contents, allowing water to flow through without obstruction
Phloem transports sugars (food molecules) from source to sink / carries out translocation
Phloem is made of living cells / has pores (sieve plates) between cells to allow sugars to move between them

Key terms in this question

xylem · phloem

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