A plant transports water and sugars through two separate transport systems. Describe how the structure of xylem vessels and phloem are each adapted to carry out their transport functions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants are multicellular organisms that require specialised transport tissues to move substances between different parts of the plant body.
Model answer (4 marks)
Xylem vessels are lined with lignin, giving the vessels strength and support so that water can be transported under tension without collapse. The vessels are hollow and contain no living cell contents, allowing water to flow freely through them.
Phloem sieve tubes are connected by plasmodesmata, so that the cytoplasm of adjacent sieve tube elements is continuous and sugars/sucrose can move between cells. Companion cells are attached to sieve tubes and provide the energy (via active transport) needed to load sugars into the sieve tubes and to support translocation.
Phloem sieve tubes are connected by plasmodesmata, so that the cytoplasm of adjacent sieve tube elements is continuous and sugars/sucrose can move between cells. Companion cells are attached to sieve tubes and provide the energy (via active transport) needed to load sugars into the sieve tubes and to support translocation.
Examiner tips
- Use the exact terms ‘lignin’, ‘hollow’, ‘plasmodesmata’, ‘companion cells’ and ‘active transport’.
- Show the two functions of each tissue: support and water flow for xylem; continuous cytoplasm and energy supply for phloem.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing xylem with phloem or vice versa.
- Omitting the role of lignin or companion cells.
- Using vague language such as ‘support’ without specifying lignin or ‘energy’ without mentioning active transport.
Mark scheme (4 marks)
- Xylem vessels are lined with lignin, which provides strength and support (allowing water to be transported under tension without the vessel collapsing).
- Xylem vessels are hollow/have no living cell contents, allowing water to flow freely through them.
- Phloem sieve tubes are connected via their cytoplasms, allowing sugars/sucrose to move between cells.
- Companion cells provide energy (via active transport) to support the movement of sugars through the phloem / for translocation.
Key terms in this question
Related
- Revision notes: The challenges of size →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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