Cyclic Photophosphorylation

Cyclic photophosphorylation involves Photosystem I only, and gets its name because the electrons ejected from its reaction centre eventually cycle all the way back to that same reaction centre rather than being handed off elsewhere. When photons activate the P700 reaction centre, an electron is raised to a high energy level and captured by the primary electron acceptor, ferredoxin-reducing substance (FRS); from there it passes sequentially to ferredoxin (Fd), then to plastoquinone (PQ), then through the cytochrome b6-f complex, then to plastocyanin (PC), and finally back to chlorophyll P700 itself, completing a closed loop. As the electron moves through this loop, ADP is phosphorylated - using the energy released at each transfer step - to form ATP; because the electron simply returns to where it started rather than reducing any external acceptor, only ATP is generated by this route, with no NADPH+H+ formed at all. At every individual transfer step, the electron loses some of its potential energy, and the electron transport chain uses that released energy to pump protons (H+ ions) across the thylakoid membrane; it is the resulting proton gradient that ultimately triggers ATP synthesis at ATP synthase. Photosystem I responds preferentially to longer wavelengths of light (above roughly 700 nm), and functions well under conditions of …