Phloem and xylem are the two transport tissues found in plants. Describe how the structure of phloem and xylem vessels is adapted to carry out their transport functions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Xylem is composed of hollow, dead cells whose end walls have been removed, forming a continuous tube that allows water to move efficiently.
The cell walls of xylem are thick, lignified and impermeable to water, preventing leakage and providing structural support.
Phloem consists of living cells; sieve tubes are living cells that are connected through sieve plates, allowing cytoplasm to flow between them.
Companion cells are attached to sieve tubes and supply ATP and sucrose, enabling the active transport of sugars through the phloem.
The cell walls of xylem are thick, lignified and impermeable to water, preventing leakage and providing structural support.
Phloem consists of living cells; sieve tubes are living cells that are connected through sieve plates, allowing cytoplasm to flow between them.
Companion cells are attached to sieve tubes and supply ATP and sucrose, enabling the active transport of sugars through the phloem.
Examiner tips
- Use the word ‘hollow’ for xylem cells and ‘living’ for phloem cells. Mention lignin and impermeability for xylem. State that sieve tubes are connected via sieve plates and that companion cells provide energy.
- common_mistakes
- :
- Confusing the presence of end walls in xylem with phloem. Forgetting to mention lignin or the impermeability of xylem walls. Saying phloem cells are dead or that companion cells are not involved in sugar transport.
Mark scheme (5 marks)
- Xylem is made from hollowed-out dead cells / cells have no contents
- End walls of xylem cells are removed to form a continuous tube, allowing water to flow through
- Xylem walls are impermeable to water / strengthened with lignin, preventing water from leaking out
- Phloem is made of living cells / sieve tubes are living cells connected via their cytoplasms
- Companion cells provide energy to aid transport of sugars through the phloem
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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