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In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II , cytochrome b6f complex, Photosystem I , and ATP synthase. These four complexes work together to ultimately produce ATP and NADPH.
The photosystems absorb light energy through pigments—primarily chlorophylls, which are responsible for the green color of leaves. The light-dependent reactions begin in photosystem II. The photosystems follows a counterintuitive order due to them following the order of discovery instead of order of participation in the process. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains an excited energy level. Because this state of an electron is relatively unstable, the electron is transferred to an electron acceptor molecule creating a chain of redox reactions, called an electron transport chain. Electrons flow from PSII to cytochrome b6f to PSI. In PSI, the electron receives energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a by-product. In anoxygenic photosynthesis various electron donors are used.
Cytochrome b6f and ATP synthase work together to create ATP. This process, called photophosphorylation, occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses electrons from PSII and energy from PSI to pump protons from the stroma to the lumen. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses electrons and energy from PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.
The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: