CAIE A Level Biology 9700 · Control and coordination
A researcher treated isolated stem segments with auxin (IAA). She measured the pH of the cell wall space and the length of the segments over time. The results showed that the pH dropped from 5.5 to 4.5 within 15 minutes of auxin application, and the segments began to elongate after 20 minutes. Explain how auxin causes cell elongation, referring to the observed pH change and your knowledge of the acid growth hypothesis.
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.
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