Which unit in the chloroplast is responsible for capturing light?

• A. Photosystem I
• B. Photosystem II
• C. Chlorophyll
• D. Grana

Answer: (A)

Light capture in chloroplasts happens in photosystems, the pigment–protein complexes embedded in the thylakoid membranes. Each photosystem contains chlorophyll and other pigments that absorb photons and transfer that energy to drive the excited-electron reactions of photosynthesis. There are two photosystems working in sequence: Photosystem II, which is typically the first to absorb light and initiate the electron transport chain, and Photosystem I, which re-energizes electrons to help form NADPH. The grana are simply stacks of thylakoid membranes where these light-dependent processes occur, while chlorophyll is the pigment that actually absorbs the light within the photosystems. So the unit responsible for capturing light is the photosystem itself—there isn’t just one in practice, both Photosystem II and Photosystem I perform that role.