Photophosphorylation is the light dependent process by which a proton gradient generates ATP from ADP and Pi.

ADP + Pi →ATP synthase→ ATP

The process of generating a proton gradient resembles that of the electron transport chain of respiration.

Photophosphorylation may be cyclic or noncyclic:

In cyclic photophosphorylation, excited electrons resulting from the absorption of light in Photosystem I are received by the primary electron acceptor and then transferred to the cytb6-f complex, which acts as an electron transport chain. The electrons are returned back to the reaction center of Photosystem I. The excited electrons of cyclic photophosphorylation generate the proton gradient that the ATPase employs to synthesize ATP. No reduction of NADP+ occurs in cyclic photophosphorylation.

In noncyclic photophosphorylation, ATP is generated by the protons gradient created across the thylakoid membranes during the Z-scheme (diagram). The Cytochrome b6-f complex acts as an electron transfer chain. As the electrons release energy during a series of redox reactions, protons are pumped into the thylakoid space. This proton gradient is used for chemiosmotic generation of ATP. The excited electrons are passed on to Photosystem II, where an extra photon of light is harnessed for the reduction of NADP+.

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