CAIE A Level Biology 9700 · Homeostasis
A tomato plant is subjected to drought stress for several days. In these dry conditions, the plant synthesises abscisic acid (ABA). Explain how ABA brings about the closure of stomata, and why this closure is beneficial despite reducing the rate of CO2 uptake.
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.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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