Photophosphorylation
Photophosphorylation is the process by which ATP is synthesized using the energy derived from light during the Light-Dependent Reactions of Photosynthesis. It occurs within the Thylakoid membranes of Chloroplast and is driven by the transfer of electrons through the Electron Transport Chain (ETC). There are two types of photophosphorylation:
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Non-cyclic Photophosphorylation:
- This is the primary pathway of photophosphorylation.
- It involves both Photosystem II (PSII) and Photosystem I (PSI).
- Light excites electrons in PSII, which are passed through the ETC, producing a Proton Gradient that powers ATP synthesis via ATP Synthase. The electrons are then transferred to PSI and ultimately reduce NADP+ to NADPH.
- Water is split (photolysis) to replenish the electrons in PSII, releasing Oxygen as a Byproduct.
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Cyclic Photophosphorylation:
- This occurs only in PSI and recycles electrons within the ETC.
- Instead of reducing NADP+, the electrons return to PSI, generating additional ATP without producing NADPH or releasing oxygen.
- This pathway is used to balance the ATP and NADPH needs of the Calvin Cycle.
Photophosphorylation is essential for producing the ATP and NADPH required to drive the light-independent reactions, where Carbon Fixation occurs to synthesize G3P and eventually Glucose.