Explain how water moves from the soil into a plant root and then through the plant to the leaves, where it is lost to the atmosphere.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants continuously absorb water from the soil and lose water vapour through their leaves. This movement of water through a plant is essential for many life processes.
Model answer (5 marks)
1. Water enters root hair cells by osmosis from the soil into the cell cytoplasm.
2. It then moves across the root cortex towards the xylem, passing from cell to cell by osmosis.
3. From the xylem vessels the water is carried upward through the stem and leaves.
4. In the leaf cells water evaporates into the air and diffuses out through the stomata (transpiration).
5. The loss of water from the leaves creates a transpiration stream that pulls more water up from the roots.
2. It then moves across the root cortex towards the xylem, passing from cell to cell by osmosis.
3. From the xylem vessels the water is carried upward through the stem and leaves.
4. In the leaf cells water evaporates into the air and diffuses out through the stomata (transpiration).
5. The loss of water from the leaves creates a transpiration stream that pulls more water up from the roots.
Examiner tips
- Use the word ‘osmosis’ for the root hair and cortex stages. Mention ‘xylem vessels’ for the upward transport. Explain that evaporation in the leaf cells and diffusion through stomata is transpiration. Show that transpiration creates a pull – the transpiration stream – to move water up.
Common mistakes
- Confusing water movement in the stem with diffusion rather than xylem transport. Forgetting to mention osmosis in the root hair cells. Saying water moves ‘through the leaf cells’ instead of ‘evaporates and diffuses out through the stomata’.
Mark scheme (5 marks)
- Water enters root hair cells by osmosis
- Water moves across the root cortex towards the xylem (by osmosis / from cell to cell)
- Water travels up the plant through xylem vessels
- Water evaporates from the leaf cells and diffuses out through the stomata (transpiration)
- The loss of water from the leaves (transpiration) creates a transpiration stream / pulls more water up from the roots
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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