CAIE A Level Biology 9700 · Homeostasis
The table shows the mean stomatal aperture of leaves from well-watered and water-stressed plants of a shrub species, measured at midday.
| Condition | Mean stomatal aperture / µm |
|---|---|
| Well-watered | 8.2 |
| Water-stressed (soil water potential −1.5 MPa) | 2.1 |
Outline the mechanism by which water stress leads to stomatal closure.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Outline means: Give the main points briefly. Less detail than describe, but every key step has to be there. One short line per step, in order. The marks are for coverage, not depth.
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