AQA A-Level Biology 2025/6

Biology • A-Level

Question 1 of 40

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Explain how photolysis, the electron transport chain, and photophosphorylation are linked in the thylakoid membrane during the light‑dependent reactions of photosynthesis.

Hint: Think about the order of processes: water splitting, electron flow, proton gradient formation, and ATP synthesis.

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Correct answer: Photolysis of water in photosystem II splits H₂O, releasing O₂, H⁺ ions and electrons; the electrons replace those lost by P680 and enter the electron transport chain. As electrons move down the chain, energy is used to pump H⁺ into the thylakoid lumen, creating a proton gradient that drives ATP synthase to convert ADP to ATP (photophosphorylation). The final electron acceptor, NADP⁺, is reduced to NADPH, completing the light‑dependent reactions.

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Photolysis of water in photosystem II splits H₂O, releasing O₂, H⁺ ions and electrons; the electrons replace those lost by P680 and enter the electron transport chain. As electrons move down the chain, energy is used to pump H⁺ into the thylakoid lumen, creating a proton gradient that drives ATP synthase to convert ADP to ATP (photophosphorylation). The final electron acceptor, NADP⁺, is reduced to NADPH, completing the light‑dependent reactions.

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