CAIE A Level Biology 9700 · Homeostasis
A botanist observes that a plant growing in a drying soil closes its stomata during the day, even though sunlight is available for photosynthesis. Explain how the plant's response to water stress reduces water loss but also limits CO2 uptake, and describe the cellular mechanism by which guard cells bring about stomatal closure.
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.