CAIE A Level Biology 9700 · Photosynthesis
A researcher treats isolated chloroplasts with an inhibitor that prevents the flow of electrons through the electron transport chain in the thylakoid membrane. They then measure the pH of the thylakoid lumen and the stroma. They observe that the pH of the thylakoid lumen becomes less acidic (higher pH) compared to untreated chloroplasts, and ATP synthesis stops. Explain why the inhibition of electron transport leads to these observations.
4 marks, so the mark scheme is looking for about 4 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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