CAIE A Level Biology 9700 · Homeostasis
In an experiment, plants of a drought-tolerant species were subjected to soil drying. Within 30 minutes, the concentration of abscisic acid (ABA) in the leaves increased significantly. Soon after, the stomata closed. Explain the mechanism by which ABA causes stomatal closure in response to water stress.
6 marks, so the mark scheme is looking for about 6 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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