CAIE A Level Biology 9700 · Homeostasis
In an experiment, leaves of the plant Commelina communis were treated with a solution containing abscisic acid (ABA). Within minutes, the stomatal apertures decreased significantly. Outline the mechanism by which ABA brings about stomatal closure in guard cells during water stress. Include the roles of calcium ions and changes in ion and water movement.
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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