Non-cyclic photophosphorylation is the ATP synthesis in the presence of light following a non- cyclic pathway. It requires both PSI and PSII+ work in sequence and are linked by electron carriers of the electron transport system. The path of electrons is from H2O to PSII, PSII to PSI and PSI to NADP.
When a photon of light is absorbed by PSII its electron is boosted to a high energy level P680* and transferred to pheophytin which acts as the primary electron acceptor. The electron is then transferred to quinone on the stromal surface of the thylokoid membrane which by accepting a second electron and 2H+ from stroma forms H2. The H2 dissociates releasing 2e– on the luminal side to cyt b complex and 2H+ to the lumen to add to the proton pool. Electrons released by cyt b/f complex move to PC which diffuses through the lumen to P700 of PSI. The PSI is now activated and the electron from PSII is boosted to P700* and is passed along A0, A1, Fx,Fab and is transferred to Fd on the stromal surface and gets reduced. Reduced ferredoxin transfers its electrons to NADP+ which absorbs 2e- and H+ from stroma to produce NADPH and 1. excellent reducing agent by photoreduction. The electron gap formed in P680* is filled by electrons produced by photolysis through OEC.
The electron flow is non-cyclic, H2 is used to produce NADPH, ATP is formed from ADP, O2 from H2O and water.
A schematic diagram of the Non cyclic photophosphorylation and Non cyclic ETS. Abbrevations : Ph = Pheophytin, Q = Quinone, PQ = Plastoquinone, PQH2 = Plastohydro quinone, cytb6 = Cytochrome b6, cyt f = Cytochrome f, PC = Plastocyanin, A1, A2, A3 = electron acceptors of PSI, Fd = Ferridoxin, NADP = Nicotinamide adenine dinucleotide phosphate.
