Explain how the structure of a sieve tube element is adapted for its function in the phloem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Phloem tissue transports assimilates such as sucrose through the plant. Sieve tube elements are the main conducting cells of the phloem and are closely associated with companion cells.
Model answer (4 marks)
Sieve tube elements are highly specialised for bulk transport of assimilates.
1. They lack a nucleus and most organelles, so the cytoplasm is very thin – this gives the cell a large lumen and reduces resistance to flow.
2. The cell walls contain sieve plates with many pores that allow sucrose to move rapidly between adjacent sieve tube elements.
3. Because the cells are essentially empty, the space available for mass flow is maximised.
4. Companion cells, which are closely associated, supply ATP and carry out the energy‑intensive loading of sucrose into the sieve tube elements, compensating for the lack of metabolic machinery in the sieve tube cells themselves.
1. They lack a nucleus and most organelles, so the cytoplasm is very thin – this gives the cell a large lumen and reduces resistance to flow.
2. The cell walls contain sieve plates with many pores that allow sucrose to move rapidly between adjacent sieve tube elements.
3. Because the cells are essentially empty, the space available for mass flow is maximised.
4. Companion cells, which are closely associated, supply ATP and carry out the energy‑intensive loading of sucrose into the sieve tube elements, compensating for the lack of metabolic machinery in the sieve tube cells themselves.
Examiner tips
- Use the word ‘lack’ or ‘absence’ for the nucleus and organelles. Mention ‘sieve plates with pores’ as the conduit between cells. Explain that companion cells supply ATP for active loading. Keep the answer concise – 4 points, 1–2 sentences each.
Common mistakes
- Forgetting to mention the pores in sieve plates. Describing sieve tube elements as ‘large’ rather than ‘nucleus‑free’. Omitting the role of companion cells in ATP supply.
Mark scheme (4 marks)
- Sieve tube elements have no nucleus, allowing more space for mass flow of assimilates
- Sieve plates have pores that allow mass flow of assimilates between adjacent sieve tube elements
- Sieve tube elements have little or no cytoplasm / few organelles, reducing resistance to flow
- Companion cells provide ATP / carry out metabolic functions to support active loading of assimilates into the sieve tube element
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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