Photosystem I in the context of "Chlorophyll b"

Light-dependent reactions are the chemical reactions involved in photosynthesis induced by light; all light-dependent reactions occur in thylakoids. There are two light-dependent reactions: the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b₆f and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP to NADPH. In oxygenic photosynthesis, the first electron donor is water, creating oxygen (O₂) as a by-product. In anoxygenic photosynthesis, various electron donors are used.Cytochrome b₆f and ATP synthase work together to produce ATP (photophosphorylation) in two distinct ways. In non-cyclic photophosphorylation, cytochrome b₆f uses electrons from PSII and energy from PSI to pump protons from the cytoplasm (or stroma in chloroplasts), to the lumen of the thylakoid. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b₆f uses electrons and energy from PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.