CAIE A Level Biology 9700 · Homeostasis
A tomato plant in a garden is observed on a hot, dry afternoon. The leaves appear wilted, but by the evening they have fully recovered. A biologist explains that the stomata closed during the day to reduce water loss and reopened later.
Outline the mechanism by which guard cells regulate stomatal aperture. In your answer, include the role of abscisic acid (ABA) and calcium ions (Ca²⁺) in closing stomata during 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.
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.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.