Explain why stomata open and close
Relate stomatal aperture to carbon-dioxide uptake and water loss by transpiration.
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 trade-off does stomatal aperture regulate?
What you will learn
- Explain how stomatal aperture responds to environmental conditions
- Relate opening to CO₂ uptake and closing to reduced water loss
Build the explanation
Stomata respond to environmental conditions by opening or closing. Opening permits carbon dioxide to diffuse into the leaf for photosynthesis, but also allows water vapour to leave by transpiration. Closing reduces water loss but can restrict carbon dioxide uptake. Aperture therefore balances carbon supply with water conservation; the response depends on conditions rather than following one absolute rule.
Apply the limits
Stomatal opening has both benefits and costs. Do not say closure stops all transpiration or that open stomata guarantee a high photosynthesis rate under every condition.
Pause and explain
Why is maximum stomatal opening not always the best response for a plant?
Worked example
Why might a plant close stomata during water stress?
Show the worked solution
- Closing reduces water-vapour loss by transpiration.
- The trade-off is reduced carbon-dioxide uptake.
Answer Closure conserves water but can limit carbon-dioxide diffusion into the leaf.
Try a topic test
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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.
Explain one benefit and one cost of stomatal opening.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- Opening supports CO₂ diffusion into the leaf.
- It also increases water-vapour loss by transpiration.
Opening allows carbon dioxide to diffuse into the leaf for photosynthesis, but it also increases water loss by transpiration.
Look for these in your work
- Correct mechanism
- Direction of response explained
A plant in dry air closes some stomata. Explain the likely effect on water loss and carbon supply.
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- Reduced aperture lowers water-vapour diffusion.
- It can also lower carbon-dioxide uptake.
Closing stomata can reduce transpiration and conserve water, but it also restricts carbon-dioxide diffusion into the leaf.
Look for these in your work
- Evidence interpreted
- Control step identified
Why is maximum stomatal opening not always the best response for a plant?
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Water availability matters as well as carbon uptake.
- Wide opening can increase water loss in conditions where water is limited.
Although opening supports CO₂ uptake, it can cause substantial water loss. The plant's response balances photosynthetic carbon supply with conserving water under the prevailing conditions.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Saying closure improves carbon-dioxide uptake
- Claiming stomata are permanently open in daylight
What can you explain now?
What trade-off does stomatal aperture regulate?
Compare with the explanation
CO₂ uptake versus water loss
Opening supports diffusion of CO₂ but increases transpiration.
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Make it stick
What two processes are balanced when stomata change aperture?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.