Explain ABA-mediated stomatal closure
Trace how abscisic acid and calcium ions contribute to guard-cell responses during water stress.
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
Which signal increases during water stress to promote stomatal closure?
What you will learn
- Describe the role of abscisic acid during water stress
- Explain how calcium ions act as a second messenger in guard cells
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
During water stress, abscisic acid (ABA) contributes to stomatal closure. ABA signalling in guard cells increases cytosolic calcium ions, which act as a second messenger. The signalling response promotes ion loss from guard cells; water follows by osmosis, turgor falls and the pore closes. This reduces transpiration and conserves water, although carbon-dioxide uptake may also decrease.
Apply the limits
ABA is a signal and calcium ions are a second messenger inside guard cells. The net response is reduced turgor and closure. Do not present ABA as a pore plug or claim closure eliminates all water loss.
Pause and explain
Explain one benefit and one possible cost of ABA-related stomatal closure.
Worked example
What is the role of calcium ions in ABA-related stomatal closure?
Show the worked solution
- ABA signalling changes calcium-ion levels in guard cells.
- Calcium acts as an intracellular second messenger that helps trigger the response.
Answer Calcium ions relay ABA signalling inside guard cells and contribute to ion loss and reduced turgor.
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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.
Trace the role of ABA and calcium ions in closure during water stress.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- ABA signalling increases cytosolic calcium in guard cells.
- Ion and water loss reduces turgor, closing the pore.
During water stress, ABA signalling increases cytosolic calcium ions in guard cells. Calcium acts as a second messenger, promoting ion loss; water follows, turgor falls and the pore closes.
Look for these in your work
- Correct mechanism
- Direction of response explained
A student calls calcium ions the hormone that travels from the root to close stomata. Correct the description.
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- ABA is the signal associated with water stress.
- Calcium acts as a second messenger within guard cells.
Abscisic acid is the signalling molecule associated with water stress. Calcium ions act as a second messenger inside guard cells, helping relay the signal that leads to closure.
Look for these in your work
- Evidence interpreted
- Control step identified
Explain one benefit and one possible cost of ABA-related stomatal closure.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Reduced aperture limits transpiration and conserves water.
- Less CO₂ may diffuse into the leaf, constraining photosynthesis.
Closure reduces water-vapour loss and helps conserve water during stress. It may also reduce carbon-dioxide uptake, which can constrain photosynthesis.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Calling calcium the external hormone
- Saying ABA closes the pore without guard-cell signalling
What can you explain now?
Which signal increases during water stress to promote stomatal closure?
Compare with the explanation
Abscisic acid
ABA signalling involves calcium ions as a second messenger.
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Make it stick
How do ABA and calcium signalling change stomatal aperture?
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