Explain how the structure of xylem vessels and phloem vessels are each adapted to carry out their specific transport roles in a plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants have two main types of transport vessel — xylem and phloem — which move different substances around the plant.
Model answer (5 marks)
Xylem vessels are made from hollowed‑out dead cells, so they contain no living cell contents. The end walls of the cells are removed, forming a continuous tube that allows water to flow through. The cell walls are impermeable to water, preventing leakage as the water rises.
Phloem consists of living cells – sieve tubes – which lack nuclei and are connected by plasmodesmata. This arrangement allows sugars to be transported. Companion cells supply ATP and other energy to the sieve tubes, supporting the active transport of sugars during translocation.
Phloem consists of living cells – sieve tubes – which lack nuclei and are connected by plasmodesmata. This arrangement allows sugars to be transported. Companion cells supply ATP and other energy to the sieve tubes, supporting the active transport of sugars during translocation.
Examiner tips
- Mention that xylem cells are dead and hollow, with end walls removed for a continuous tube; note impermeable walls. Include that phloem sieve tubes are living, lack nuclei, and are connected; companion cells provide energy for active transport.
Common mistakes
- Confusing xylem as living cells or phloem as dead cells. Forgetting to state that xylem walls are impermeable or that phloem sieve tubes lack nuclei. Omitting the role of companion cells in phloem transport.
Mark scheme (5 marks)
- Xylem vessels are made from hollowed-out dead cells / have no living cell contents
- The end walls of xylem cells are removed, forming a continuous tube that allows water to flow through
- Xylem cells are impermeable to water, preventing water from leaking out as it travels up the plant
- Phloem is made of living cells / sieve tubes with no nuclei connected via their cytoplasm, allowing sugars to be transported
- Companion cells supply energy to phloem sieve tubes to support the active transport of sugars in translocation
Key terms in this question
Related
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- Decode the mark scheme abbreviations →
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