# Explain how the structure of a chloroplast is related to its function in photosynthesis.

> IB DP Biology Higher Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks

## Mark scheme (4 marks)

1. The thylakoid membranes (granum/grana) provide a large surface area for the attachment of photosystems and ATP synthase, maximising the rate of the light-dependent reactions.
2. The thylakoid lumen acts as a proton reservoir; protons are pumped into it during the light-dependent reactions, creating the electrochemical gradient that drives ATP synthesis via chemiosmosis.
3. The stroma contains the enzymes of the Calvin cycle (e.g. RuBisCO) and is where carbon fixation / reduction of GP / regeneration of RuBP occurs, using the ATP and NADPH produced in the thylakoids.
4. The double membrane of the chloroplast (envelope) maintains a distinct internal environment, and the proximity of the thylakoids to the stroma allows efficient transfer of ATP and NADPH between the light-dependent and light-independent reactions.

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