Cyclic photophosphorylation refers to the electrons ejected from the pigment system I (Photosystem I) and again cycled back to the PS I. When the photons activate P700 reaction centre photosystem II is activated. Electrons are raised to a high energy level. The primary electron acceptor is Ferredoxin Reducing Substance (FRS) which transfers electrons to Ferredoxin (Fd), Plastoquinone (PQ), cytochrome b6-f complex, Plastocyanin (PC) and finally back to chlorophyll P700 (PS I). During this movement of electrons, Adenosine Di Phosphate (ADP) is phosphorylated, by the addition of inorganic phosphate and generates Adenosine Tri Phosphate (ATP). Cyclic electron transport produces only ATP and there is no NADPFI + H+ formation. At each step of electron, transport, the electron loses potential energy and is used by the transport chain to pump H+ ions across the thylakoid membrane. The proton gradient triggers ATP formation in the ATP synthase enzyme situated on the thylakoid membrane. Photosystem I need the light of a longer wavelength (> P700 nm). It operates under low light intensity, less CO2 and under anaerobic conditions which makes it considered as earlier in evolution.
