Explain how the structure of phloem tissue is adapted to allow the transport of sucrose throughout a plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants produce sucrose by photosynthesis in their leaves. This sucrose must be transported to all living cells in the plant, including the roots.
Model answer (5 marks)
1. Phloem contains sieve tube elements that are living cells, lacking a nucleus at maturity, which allows their cytoplasm to be continuous with neighbouring cells.
2. The end walls of sieve tube elements have sieve plates with large pores, permitting the free flow of sucrose solution between cells.
3. Companion cells are attached to sieve tube elements and provide the ATP required for the active loading of sucrose into the phloem.
4. Sucrose is moved by translocation from source tissues (leaves) to sink tissues (roots, growing regions, storage organs).
5. Because phloem can transport sucrose in both directions, it can deliver the carbohydrate to any part of the plant, whether above or below the source.
2. The end walls of sieve tube elements have sieve plates with large pores, permitting the free flow of sucrose solution between cells.
3. Companion cells are attached to sieve tube elements and provide the ATP required for the active loading of sucrose into the phloem.
4. Sucrose is moved by translocation from source tissues (leaves) to sink tissues (roots, growing regions, storage organs).
5. Because phloem can transport sucrose in both directions, it can deliver the carbohydrate to any part of the plant, whether above or below the source.
Examiner tips
- Use the exact terms: sieve tube elements, sieve plates, companion cells, source, sink, translocation.
- Show the flow of sucrose from source to sink and emphasise bidirectional transport for full marks.
Common mistakes
- Confusing sieve tube elements with xylem vessels.
- Forgetting that sieve tube elements are living cells or that companion cells supply ATP.
- Not mentioning the bidirectional nature of phloem transport.
Mark scheme (5 marks)
- Phloem contains sieve tube elements / sieve cells that are living cells (with no nucleus at maturity) allowing cytoplasm to be continuous between cells
- Sieve plates / perforated end walls between sieve tube elements allow sucrose solution to flow through
- Companion cells provide the energy (ATP) needed to actively load sucrose into the phloem / sieve tube elements
- Sucrose is transported by translocation from a source (leaves) to a sink (roots / growing regions / storage organs)
- Phloem can transport sucrose in both directions / up and down the plant, allowing it to reach all living cells
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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