CAIE A Level Biology 9700 · Homeostasis
Guard cells are specialised cells that surround stomata. Their cell walls are thicker on the inner side (adjacent to the stoma) and thinner on the outer side. They also contain chloroplasts. Outline how guard cells control the opening and closing of stomata in response to environmental conditions, and explain the role of abscisic acid (ABA) and calcium ions in closing stomata during periods of water stress.
7 marks, so the mark scheme is looking for about 7 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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