CAIE A Level Biology 9700 · Control and coordination
A student investigates the effect of auxin (IAA) on the pH of the cell wall (apoplast) of stem segments from a plant. The student measures the pH of the apoplast and the elongation of the segments over time after applying auxin. The results show that within 10 minutes, the apoplast pH drops from 6.5 to 4.5, and elongation begins shortly after this pH drop.
Explain how auxin causes cell elongation through the acid growth mechanism.
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.