Non-cyclic Photophosphorylation

Non-cyclic photophosphorylation, by contrast, uses both photosystems together and produces both ATP and NADPH2. The electron-transport chain begins with electrons released from PS-II: these high-energy electrons pass through a chain of carriers to PS-I, but unlike the cyclic pathway they do not return to PS-II — instead, PS-I passes them onward to reduce NADP+ to NADPH2, which is then used to reduce CO2 in the dark reaction. Because PS-II is left electron-deficient, it oxidises water to replace the electrons it lost; this releases protons, electrons and oxygen, with the electrons taken back up by PS-II itself, the protons accepted by NADP+, and oxygen released as gas. Because the high-energy electrons that leave PS-II never return to it, and because the process is accompanied by ATP synthesis as well, it is called non-cyclic photophosphorylation. Together, the photochemical reactions — cyclic and non-cy …