Describe the role of xylem vessels in the transport of water through a flowering plant, including the features of xylem that make them well suited to this function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Xylem vessels transport water and mineral ions from the roots to the leaves and shoots, moving upwards through the plant.
The vessels are hollow, lacking cytoplasm, which creates a continuous tube that allows water to flow without obstruction.
Their walls are reinforced with lignin, giving the vessels strength and preventing collapse under the tension created by the water column.
Water is drawn up the xylem by transpiration: water evaporates from the leaf surface through stomata, creating a negative pressure (tension) that pulls the water column upward.
The vessels are hollow, lacking cytoplasm, which creates a continuous tube that allows water to flow without obstruction.
Their walls are reinforced with lignin, giving the vessels strength and preventing collapse under the tension created by the water column.
Water is drawn up the xylem by transpiration: water evaporates from the leaf surface through stomata, creating a negative pressure (tension) that pulls the water column upward.
Examiner tips
- Use the term "xylem vessels" and mention they are hollow and lignified.
- Explain the pull from transpiration and link it to upward movement.
- Show the continuous water column and the role of lignin in preventing collapse.
- Keep the answer concise and directly address each point in the mark scheme.
Common mistakes
- Confusing xylem with phloem or describing only roots.
- Omitting the role of lignin or the pull from transpiration.
- Using vague terms like "water moves" without explaining the mechanism.
Mark scheme (4 marks)
- Xylem transports water (and mineral ions) from the roots to the leaves / shoots (upwards through the plant).
- Xylem vessels are hollow / have no cytoplasm, forming a continuous tube that allows unobstructed flow of water.
- Xylem vessel walls are strengthened with lignin, which supports the plant and prevents the vessels from collapsing under the tension of the water column.
- Water moves up the xylem because of transpiration / water is lost from leaves by evaporation through stomata, creating a 'pull' / tension that draws water up.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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