CAIE A Level Biology 9700 · Homeostasis
Plants experiencing water shortage close their stomata to reduce water loss. This closure is regulated by the hormone abscisic acid (ABA) and involves calcium ions (Ca²⁺) as a second messenger. Describe the sequence of events from the production of ABA to the closure of the stomatal pore, including the specific roles of ABA and Ca²⁺ ions.
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.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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