# Explain how the ultrastructure of the chloroplast is adapted for the two stages of photosynthesis.

> IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. Thylakoid membranes (in grana) provide a large surface area for photosynthetic pigments / light-harvesting complexes / photosystems to absorb light energy for the light-dependent reactions.
2. The thylakoid membrane is the site of the electron transport chain and ATP synthase, allowing chemiosmosis / the proton gradient to drive ATP synthesis during the light-dependent reactions.
3. The stroma contains enzymes (including RuBisCO) needed for the Calvin cycle / light-independent reactions, where carbon dioxide is fixed and reduced to produce organic molecules (triose phosphate / G3P).
4. The double membrane / envelope of the chloroplast maintains the internal environment (concentrations of CO2, ATP, NADPH) needed for both stages, and the intermembrane space / envelope controls entry and exit of substances such as triose phosphate.

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