Relate guard-cell structure to stomatal movement
Explain how guard-cell water content changes aperture and connect it with carbon dioxide and water exchange.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What change in guard cells usually opens a stoma?
What you will learn
- Describe guard-cell structure and their role around the pore
- Explain opening and closing through changes in guard-cell turgor
Guard cells around a stomatal pore
| Feature | Meaning |
|---|---|
| More turgid | Guard cells bow apart; aperture widens |
| Less turgid | Pore narrows or closes |
| Trade-off | Carbon-dioxide uptake and transpiration |
Build the explanation
A stoma is a pore bordered by two guard cells. Guard cells have unevenly thickened walls and can change shape as their turgor changes. When solute uptake lowers their water potential, water enters by osmosis; the cells become more turgid and bow apart, opening the pore. When they lose solutes and water, turgor falls and the pore closes. This changes gas exchange and transpiration.
Apply the limits
The pore is the opening between guard cells, not a cell itself. Explain movement through turgor and differential wall stretching; avoid relying on a single oversimplified 'water pressure pushes open' phrase without the wall geometry.
Pause and explain
Guard cells lose water and become less turgid. Predict the aperture change and one consequence.
Worked example
What happens to guard-cell turgor when water enters them?
Show the worked solution
- Water enters by osmosis.
- The guard cells become more turgid and the pore tends to open.
Answer Turgor rises and the guard cells bow apart, opening the stoma.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how guard cells open a stoma after solute uptake.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- Lower guard-cell water potential causes water entry by osmosis.
- Increased turgor makes the cells bow apart and widen the pore.
Solute uptake lowers guard-cell water potential, so water enters by osmosis. Turgor rises and the cells bow apart, opening the pore.
Look for these in your work
- Correct mechanism
- Direction of response explained
A micrograph labels the gap between paired guard cells. Name the gap and state one exchange it permits.
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- The gap is the stomatal pore.
- It permits carbon dioxide or water vapour diffusion.
The gap is the stomatal pore. It permits gas diffusion, including carbon dioxide uptake and water-vapour loss.
Look for these in your work
- Evidence interpreted
- Control step identified
Guard cells lose water and become less turgid. Predict the aperture change and one consequence.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Reduced turgor allows the pore to narrow or close.
- This reduces transpiration but can also restrict carbon-dioxide uptake.
The pore narrows or closes as guard-cell turgor falls. This can reduce water loss by transpiration, while also restricting carbon-dioxide diffusion into the leaf.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Calling the pore a guard cell
- Saying guard cells close because they become more turgid
What can you explain now?
What change in guard cells usually opens a stoma?
Compare with the explanation
Increased turgor
Turgid guard cells bow apart and widen the pore.
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Make it stick
How do changes in guard-cell turgor alter the pore?
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