CAIE A Level Biology 9700 · Homeostasis
Researchers studying the stomatal behaviour of a drought-tolerant shrub observe that during the early morning, guard cells accumulate potassium ions and the stomata open. By midday, if the soil is dry, abscisic acid (ABA) levels rise and the stomata close. Explain the cellular mechanisms that link potassium ion uptake to stomatal opening, and how ABA signalling leads to stomatal closure during 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.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.