Explain how differences in water potential between the soil, root hair cells, and the xylem drive the movement of water into a plant root.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants absorb water from the soil through root hair cells. Water must travel from the soil solution, across the root cortex, and into the xylem vessels before it can be transported to the shoot.
Model answer (4 marks)
1. The water potential of the soil solution is higher (less negative) than that of the root hair cells.
2. Water therefore moves by osmosis from the soil into the root hair cells down the water‑potential gradient.
3. Uptake of water lowers the water potential of the adjacent cortex cells, so water continues to move inward across the cortex.
4. The xylem has the lowest (most negative) water potential in the root, so water moves into the xylem by osmosis/down the gradient.
2. Water therefore moves by osmosis from the soil into the root hair cells down the water‑potential gradient.
3. Uptake of water lowers the water potential of the adjacent cortex cells, so water continues to move inward across the cortex.
4. The xylem has the lowest (most negative) water potential in the root, so water moves into the xylem by osmosis/down the gradient.
Examiner tips
- Use the term ‘water potential’ and ‘osmosis’ explicitly.
- Show the direction of movement (soil → root hair → cortex → xylem).
- Mention that the gradient is from higher to lower water potential.
Common mistakes
- Confusing ‘higher’ with ‘more negative’ for water potential.
- Forgetting to state that the xylem has the lowest water potential.
- Using ‘diffusion’ instead of ‘osmosis’ for water movement.
Mark scheme (4 marks)
- Water potential of soil solution is higher (less negative) than the water potential of root hair cells
- Water therefore moves by osmosis from the soil into root hair cells down a water potential gradient
- Water uptake lowers the water potential of adjacent cortex cells relative to the root hair cells, so water continues to move inward across the cortex
- Xylem has the lowest (most negative) water potential in the root, so water moves into the xylem by osmosis / down the water potential gradient
Key terms in this question
water potential · root hair cells · xylem
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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