CAIE A Level Biology 9700 · Homeostasis
In an experiment, leaf epidermal peels from a species of tomato were placed in a solution containing 100 mmol dm⁻³ KCl and exposed to blue light. Within 10 minutes, stomatal apertures increased significantly. Explain how the uptake of potassium ions into guard cells causes the observed increase in stomatal aperture.
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.
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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