photophosphorylation

Photophosphorylation is the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) driven by light-induced electron transport. It occurs in the thylakoid membranes of chloroplasts in plants and algae and in the photosynthetic membranes of cyanobacteria and some bacteria. As part of the light-dependent reactions of photosynthesis, photophosphorylation uses a proton motive force generated by electron transport to power ATP synthase.

There are two major modes: non-cyclic (linear) photophosphorylation and cyclic photophosphorylation. In non-cyclic photophosphorylation, electrons travel from water through photosystem II (PSII) and the cytochrome b6f complex to photosystem I (PSI), and finally to ferredoxin-NADP+ reductase that reduces NADP+ to NADPH. The oxidation of water at PSII releases oxygen. The electron flow pumps protons across the membrane, creating a proton motive force that drives ATP synthase to produce ATP in addition to NADPH.

In cyclic photophosphorylation, electrons excited in PSI are transferred back to the plastoquinone pool and the cytochrome b6f complex, cycling without involvement of PSII and without NADP+ reduction. This mode generates additional ATP without producing NADPH or oxygen, helping balance the cellular energy budget according to demand.

Photophosphorylation thus provides the ATP and reducing power required for carbon fixation in the Calvin cycle and other cellular processes, and its relative contribution of ATP and NADPH is adjusted in response to light conditions and metabolic needs.