CAIE A Level Biology 9700 · Homeostasis
Broad bean (Vicia faba) plants typically have stomata that open during the day and close at night. When the soil becomes dry, the roots produce abscisic acid (ABA), which is transported to the leaves and causes stomatal closure. Outline the mechanism by which guard cells control stomatal aperture, including the changes that occur during normal daily opening and during ABA-induced closure.
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.
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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