# Explain how the structural features of the chloroplast enable it to carry out both the light-dependent and light-independent reactions of photosynthesis in separate compartments.

> IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · Explain · 4 marks

## Mark scheme (4 marks)

1. The thylakoid membranes (within the grana) provide a large surface area for the embedding of photosystems / electron carriers / ATP synthase, enabling the light-dependent reactions to occur there.
2. The thylakoid lumen becomes acidic (low pH) as protons are pumped into it during the light-dependent reactions, generating a proton gradient that drives ATP synthesis via ATP synthase (chemiosmosis).
3. The stroma is a separate aqueous compartment containing the enzymes of the Calvin cycle (e.g. RuBisCO), allowing the light-independent reactions to occur there using the ATP and NADPH produced in the thylakoids.
4. The double membrane (envelope) of the chloroplast isolates the organelle's internal environment, maintaining the specific ion concentrations, pH, and enzyme complement needed for both stages of photosynthesis.

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