Cyclic Phosphorylation

Under certain condition, photo excited electron takes an alternative path called cyclic electron flow.

This path used Photosystem I but not Photosystem II. The electron cycle back from primary electron acceptor to ferredoxin (Fd) then to the cytochromes complex and from there continues on the P700 chlorophyll. The coupling of ETC by Chemiosmosis generates ATP. There is no production of NADPH and no release of oxygen. This results due to the accumulation of NADPH in the chloroplasts when Calvin cycle shows down.