Cyclic Photophosphorylation Diagram


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The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H+ + 1/2 O2 + 2e− by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H+ and 1/2O2 are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin. The deficit of electrons is replenished by taking electrons from another molecule of water. The electrons transfer from pheophytin to plastoquinone, which takes the 2e− from Pheophytin, and two H+ Ions from the stroma and forms PQH2, which later is broken into PQ, the 2e− is released to Cytochrome b6f complex and the two H+ ions are released into thylakoid lumen. The electrons then pass through the Cyt b6 and Cyt f. Then they are passed to plastocyanin, providing the energy for hydrogen ions (H+) to be pumped into the thylakoid space. This creates a gradient, making H+ ions flow back into the stroma of the chloroplast, providing the energy for the regeneration of ATP.