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

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

## Mark scheme (4 marks)

1. The double membrane / envelope of the chloroplast separates the internal environment from the cytoplasm, maintaining distinct conditions (e.g. pH, ion concentrations) needed for photosynthesis.
2. The thylakoid membranes contain photosynthetic pigments (e.g. chlorophyll) and electron carriers, providing a large surface area for the light-dependent reactions / absorption of light energy.
3. Stacking of thylakoids into grana greatly increases the total membrane surface area within a small volume, allowing more light energy to be captured / more light-dependent reactions to occur simultaneously.
4. The stroma provides the aqueous medium containing enzymes (e.g. RuBisCO) and substrates for the light-independent reactions (Calvin cycle), where carbon dioxide is fixed / organic molecules are synthesised.

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