Oxygenic photosynthesis

Oxygenic photosynthesis utilizes light energy to generate ‘excited’ electrons from water:
2H2O → 4e^- + 4H⁺ + O2

A photon is absorbed by Photosystem II and the two resultant excited electrons are passed to Photosystem I, which employs a second photon to further boost their energy for the overall reaction: NADP⁺ + H⁺ + 2e^- → NADPH.

Thus, the excited electron is subsequently passed to an electron acceptor rather than cycling as in nonoxygenic photophosphorylation. Photophosphorylation is the process of creating ATP using a proton gradient created by the energy gathered from sunlight

Oxygenic photosynthesis by photosystem II (P680-PSII) incorporates photobiochemical capacity specific to cyanobacteria and chloroplasts. Protons (H+) are pumped into the thylakoid lumen, generating a concentration gradient that induces thylakoid ATP synthase to phosphorylate ADP to the energy-storage moiety ATP.

ADP + Pi →ATP synthase→ ATP

Utilization of proton movement to join ADP and Pi is termed chemiosmosis. This is accomplished by enzymes called ATP synthases or ATPases.

Diagram • Z-scheme of noncyclic photophosphorylation :

Oxygenic photosynthesis apparently developed several billion years ago in an ancestor of present cyanobacteria. The oxygenic photosynthetic machinery is located within the specialized internal thylakoid membrane system. The ability to construct and modify this thylakoid membrane system appears to be an important feature of oxygenic photosynthesis.

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