Photosynthesis: Light Reactions

Photosystems I & II, electron transport chain, and ATP synthesis

The light reactions of photosynthesis occur in the thylakoid membrane. Photosystem II (P680) absorbs photons and splits water, releasing O₂ and electrons. These electrons pass through the electron transport chain (plastoquinone → cytochrome b6f → plastocyanin), pumping H⁺ into the thylakoid lumen. Photosystem I (P700) re-energizes the electrons, which reduce NADP⁺ to NADPH via ferredoxin. The H⁺ gradient drives ATP synthase (chemiosmosis), producing ATP. Red light (680 nm) is most efficiently absorbed by chlorophyll a; action spectra show peaks at ~430 nm (blue) and ~680 nm (red).