What are the similarities and differences between photophosphorylation and oxidative phosphorylation?
Both processes operate on the same basic principle; passing electrons down a chain in order to create a proton (H+) gradient, allowing the formation of ATP. The biggest difference is where they occur. Photophosphorylation occurs in the thylakoid membrane of chloroplasts during the light-dependent stage of photosynthesis. Light in the form of photons supplies the energy needed to excite two e- s in PSII (photosystem II), which are then passed along the transport chain. Oxidative phosphorylation occurs in the membrane of mitochondrial christae during cellular respiration. Here, the e-s are supplied by NAD and FAD, with oxygen acting as the last electron acceptor, leading to the formation of H2O. During photophosphorylation, NADP acts as the last e- acceptor , leading to the formation of NADPH.
In both processes e-s are passed down a chain of electron transfer agents in a series of redox reactions. In both reactions, as e-s are passed along the cytochrome complex, H+ ions are pumped from an area of low to high concentration, creating a proton gradient. During photophosphorylation e-s are pumped from the stroma into the thylakoid, while in oxidative phosphorylation e-s are pumped from the matrix into the intermembranal space. The chemiosmosis of H+ ions down the concentration gradient through the pores of ATPsynthase then supplies the energy needed to phosphorylate ADP into ATP, which is the primary "energy carrier" in cells.
