Phloem and xylem are the two main transport tissues in plants. Explain how the structure of phloem is adapted for the transport of sugars through a plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants produce sugars by photosynthesis in their leaves. These sugars must be transported to other parts of the plant, such as the roots and growing regions, where they are needed for respiration and growth.
Model answer (5 marks)
Phloem consists of living cells – mainly sieve‑tube elements and companion cells. The sieve‑tube elements are elongated, have no nuclei or organelles, giving more space for the sugar solution to move. Their ends are joined by sieve plates, which are perforated so that the solution can flow directly from one cell to the next. Companion cells are attached to the sieve tubes and supply ATP for the active loading of sugars into the sieve‑tube lumen. This arrangement allows sugars produced in the source leaves to be translocated through the phloem to sink tissues such as roots and growing regions.
Examiner tips
- Mention that phloem cells are living and that sieve tubes lack nuclei
- Explain the role of sieve plates in allowing continuous flow
- State that companion cells provide ATP for active loading
- Include the source–sink concept and the term translocation
Common mistakes
- Confusing xylem with phloem structure
- Forgetting that sieve‑tube elements are living cells
- Not mentioning the active loading of sugars by companion cells
Mark scheme (5 marks)
- Phloem is made of living cells / sieve tubes are living cells
- Sieve tubes have no nuclei, allowing more space for sugar solution to flow through
- Sieve tubes are connected via their cytoplasms / sieve plates allow sugar solution to pass between cells
- Companion cells provide energy (ATP) for the active loading of sugars into the phloem
- Sugars are transported from source (leaves where photosynthesis occurs) to sink (roots / growing regions) — this is called translocation
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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