Guard cells control gas exchange in plant leaves. Explain how the structure of guard cells is suited to their function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Guard cells are found in pairs surrounding stomata on the surface of plant leaves. They control the opening and closing of stomata, regulating gas exchange and water loss.
Model answer (5 marks)
Guard cells contain chloroplasts that produce glucose and ATP via photosynthesis. The ATP powers active transport of K⁺ ions into the cells, lowering their water potential. Water then enters osmotically, making the guard cells turgid. Their cell walls are unevenly thickened – the inner wall is thicker – so turgor causes the cells to bend outward, opening the stoma. When open, CO₂ enters for photosynthesis and O₂ and water vapour exit, allowing gas exchange.
Examiner tips
- Mention chloroplasts and ATP production first, then ion transport, water potential, turgor, wall asymmetry, and final gas exchange. Use the exact terms "chloroplasts", "active transport", "turgor", "unevenly thickened cell wall". Show the sequence of events clearly.
- Keep the answer concise – 5 marks, so one sentence per point. Avoid unnecessary detail.
Mark scheme (5 marks)
- Guard cells contain chloroplasts, which carry out photosynthesis to produce glucose/ATP
- The glucose/ATP produced is used for active transport of ions (e.g. potassium ions) into the guard cells
- This lowers the water potential inside the guard cells, so water enters by osmosis
- Guard cells have an unevenly thickened cell wall — the inner wall is thicker — so as the cells become turgid they curve/bend outwards, opening the stoma
- When the stoma is open, carbon dioxide can enter for photosynthesis and oxygen / water vapour can leave, enabling gas exchange
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
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- Decode the mark scheme abbreviations →
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