lightdependent

Light-dependent reactions, or photophosphorylation, are the stages of photosynthesis that convert light energy into chemical energy. They take place in the thylakoid membranes of chloroplasts in plants and algae, and in the photosynthetic membranes of cyanobacteria. The core components are two photosystems, photosystem II (PSII) and photosystem I (PSI), connected by an electron transport chain and the ATP synthase enzyme. Chlorophyll and other pigments absorb light, exciting their electrons. In PSII, absorbed light energy drives the oxidation of water by the oxygen-evolving complex, releasing molecular oxygen, protons, and electrons. The released electrons are transferred to the plastoquinone pool, then through the cytochrome b6f complex, creating a proton gradient across the thylakoid membrane. Electrons arrive at PSI and are re-energized by light. They are transferred to ferredoxin and finally to NADP+, reducing it to NADPH via ferredoxin-NADP+ reductase. The proton motive force across the thylakoid membrane powers ATP synthase to convert ADP and inorganic phosphate into ATP. Overall, light-dependent reactions produce ATP and NADPH, and release O2 as a byproduct. The ATP and NADPH are used in the Calvin cycle (light-independent reactions) to convert carbon dioxide into glucose and other carbohydrates. Approximately eight photons are required to drive the transfer of four electrons from water to NADP+, resulting in the evolution of one molecule of O2.