CAIE A Level Biology 9700 · Homeostasis
A gardener notices that during a prolonged dry spell, the leaves of a tomato plant wilt and the stomata are observed to be closed, even during daylight hours. Using your knowledge of homeostasis in plants, describe the role of abscisic acid (ABA) and calcium ions (Ca²⁺) in bringing about stomatal closure under water stress.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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