A scientist is studying how plants transport substances around their bodies. Explain how the structure of phloem tissue is adapted to carry out translocation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants have two main transport tissues: xylem and phloem. Phloem transports sucrose from the source to the sink via translocation. The phloem tissue contains sieve tubes and companion cells.
Model answer (5 marks)
Sieve tubes lack nuclei, leaving more cytoplasmic space for sucrose transport.
Sieve tubes are connected by sieve plates, allowing continuous cytoplasmic flow of sucrose.
Companion cells supply ATP and other energy‑rich molecules to power the active loading of sucrose into sieve tubes.
Translocation moves sucrose from source tissues (e.g. leaves) to sink tissues (e.g. roots, growing buds).
The absence of a nucleus in sieve tubes increases the volume available for sucrose, enhancing transport efficiency.
Sieve tubes are connected by sieve plates, allowing continuous cytoplasmic flow of sucrose.
Companion cells supply ATP and other energy‑rich molecules to power the active loading of sucrose into sieve tubes.
Translocation moves sucrose from source tissues (e.g. leaves) to sink tissues (e.g. roots, growing buds).
The absence of a nucleus in sieve tubes increases the volume available for sucrose, enhancing transport efficiency.
Examiner tips
- Mention the lack of nuclei and continuous cytoplasm for space and flow. Explain the role of companion cells in energy supply. State source and sink relationship. Highlight the functional advantage of the structure.
Common mistakes
- Forgetting to mention the absence of nuclei. Confusing phloem with xylem. Not linking companion cells to energy supply. Over‑generalising without specifying source/sink.
Mark scheme (5 marks)
- Sieve tubes have no nuclei
- Sieve tubes are connected via their cytoplasms, allowing continuous flow of sucrose
- Companion cells provide energy for translocation
- Translocation moves sucrose from source (e.g. leaves) to sink (e.g. roots / growing regions)
- The absence of a nucleus in sieve tubes leaves more space / room for the transport of sucrose
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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