Describe how the structure of the phloem is adapted to transport sugars from the leaves to other parts of a plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants produce glucose during photosynthesis in the leaves. This glucose must be transported to other parts of the plant, such as the roots and growing shoots, where it is needed for metabolic reactions.
Model answer (5 marks)
Phloem consists of living sieve‑tube elements that lack nuclei and have plasmodesmata connecting their cytoplasm, allowing rapid bulk flow of sugars.
Companion cells, which are also living, supply the energy (ATP) required for loading sugars into the sieve tubes.
Sugars produced in the leaves (source) are loaded into the sieve tubes and move by pressure‑flow (translocation) to sinks such as roots and growing shoots.
When sugars are not immediately required, they are converted to starch and stored in sink tissues.
Companion cells, which are also living, supply the energy (ATP) required for loading sugars into the sieve tubes.
Sugars produced in the leaves (source) are loaded into the sieve tubes and move by pressure‑flow (translocation) to sinks such as roots and growing shoots.
When sugars are not immediately required, they are converted to starch and stored in sink tissues.
Examiner tips
- Mention living sieve tubes and companion cells; note lack of nuclei and plasmodesmata; explain source‑sink translocation; include storage as starch
Common mistakes
- Forgetting that sieve tubes are living cells; confusing phloem with xylem; omitting the role of companion cells or the source‑sink concept
Mark scheme (5 marks)
- Phloem is made of living cells (sieve tubes)
- Companion cells provide energy for transport
- Sieve tubes have no nucleus / cytoplasms are connected
- Sugars are transported from the source (leaves) to the sink (roots/growing regions) — this is called translocation
- When sugar is not needed it is converted to starch for storage
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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