AQA A-Level Biology 2025/6

Biology • A-Level

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[DIAGRAM: Diagram of the photosynthesis process]

With reference to the diagram, describe the flow of energy and matter through the light‑dependent reactions and the Calvin cycle, and explain how the diagram illustrates the specific roles of ATP and NADPH in the reduction of CO₂ to glucose.

Hint: Follow the arrows from the thylakoid membranes to the stroma; note where ATP and NADPH are produced and where they are consumed in the Calvin cycle.

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Correct answer: The diagram shows that light energy is absorbed by chlorophyll in the thylakoid membranes, driving the light‑dependent reactions. Water is split (photolysis) producing O₂, H⁺ ions and electrons; the electrons travel through photosystem II, the plastoquinone pool, photosystem I and finally to NADP⁺, forming NADPH. The proton gradient generated across the thylakoid membrane powers ATP synthase, synthesising ATP. Both ATP and NADPH are then exported to the stroma, where the Calvin cycle (light‑independent reactions) occurs. In the Calvin cycle CO₂ is fixed by ribulose‑1,5‑bisphosphate carboxylase/oxygenase (Rubisco) to form 3‑phosphoglycerate, which is phosphorylated by ATP and reduced by NADPH to produce glyceraldehyde‑3‑phosphate (G3P). G3P molecules are used to regenerate ribulose‑1,5‑bisphosphate and to synthesise glucose. Thus the diagram demonstrates the sequential flow: light → water → O₂ + ATP + NADPH (thylakoid) → ATP/NADPH transport → CO₂ fixation and glucose formation (stroma).

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Correct Answer

The diagram shows that light energy is absorbed by chlorophyll in the thylakoid membranes, driving the light‑dependent reactions. Water is split (photolysis) producing O₂, H⁺ ions and electrons; the electrons travel through photosystem II, the plastoquinone pool, photosystem I and finally to NADP⁺, forming NADPH. The proton gradient generated across the thylakoid membrane powers ATP synthase, synthesising ATP. Both ATP and NADPH are then exported to the stroma, where the Calvin cycle (light‑independent reactions) occurs. In the Calvin cycle CO₂ is fixed by ribulose‑1,5‑bisphosphate carboxylase/oxygenase (Rubisco) to form 3‑phosphoglycerate, which is phosphorylated by ATP and reduced by NADPH to produce glyceraldehyde‑3‑phosphate (G3P). G3P molecules are used to regenerate ribulose‑1,5‑bisphosphate and to synthesise glucose. Thus the diagram demonstrates the sequential flow: light → water → O₂ + ATP + NADPH (thylakoid) → ATP/NADPH transport → CO₂ fixation and glucose formation (stroma).

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AQA A-Level Biology 2025/6

[DIAGRAM: Diagram of the photosynthesis process] With reference to the diagram, describe the flow of energy and matter through the light‑dependent reactions and the Calvin cycle, and explain how the diagram illustrates the specific roles of ATP and NADPH in the reduction of CO₂ to glucose.

[DIAGRAM: Diagram of the photosynthesis process]

With reference to the diagram, describe the flow of energy and matter through the light‑dependent reactions and the Calvin cycle, and explain how the diagram illustrates the specific roles of ATP and NADPH in the reduction of CO₂ to glucose.