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Light-dependent reactions
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<p>Light-dependent reactions are certain <a href="/facts/Photochemical_reaction/kDTt7nrT">photochemical reactions</a> involved in <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>, the main process by which plants acquire energy. There are two light dependent reactions: the first occurs at <a href="/facts/Photosystem_II/9tmDPU50">photosystem II (PSII)</a> and the second occurs at <a href="/facts/Photosystem_I/z9hRH0kt">photosystem I (PSI)</a>. </p><p>PSII absorbs a photon to produce a so-called high energy electron which transfers via an <a href="/facts/Electron_transport_chain/QjcIkgeR">electron transport chain</a> to cytochrome <i>b6f</i> and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP+ to NADPH. In <a href="/facts/Photosynthesis/TMV7w693">oxygenic photosynthesis</a>, the first electron donor is <a href="/facts/Water/0lkXnJPK">water</a>, creating <a href="/facts/Oxygen/acyn6o8r">oxygen</a> (O2) as a by-product. In <a href="/facts/Photosynthesis/TMV7w693">anoxygenic photosynthesis</a>, various electron donors are used. Cytochrome <i>b6f</i> and ATP synthase work together to produce ATP (<a href="/facts/Photophosphorylation/vMcaVqpy">photophosphorylation</a>) in two distinct ways. In non-cyclic photophosphorylation, cytochrome <i>b6f</i> uses electrons from PSII and energy from PSI to pump <a href="/facts/Protons/clCjepXP">protons</a> from the <a href="/facts/Stroma_(fluid)/AYwQHKSm">stroma</a> to the <a href="/facts/Thylakoid_lumen/d9PEe649">lumen</a>. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome <i>b6f</i> 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 <a href="/facts/Light-independent_reactions/M1bIgQb9">light-independent reactions</a>. </p><p>The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: </p> 2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2 H+ + 2NADPH + 3ATP <p>PSI and PSII are <a href="/facts/Light-harvesting_complex/jwAEMRUN">light-harvesting complexes</a>. If a special pigment molecule in a photosynthetic <a href="/facts/Reaction_center/SNdpK0Dp">reaction center</a> absorbs a photon, an <a href="/facts/Electron/L0KBo5cW">electron</a> in this pigment attains the <a href="/facts/Excited_state/czcLYNnr">excited state</a> and then is transferred to another <a href="/facts/Molecule/N9ljSfFq">molecule</a> in the reaction center. This reaction, called <a href="/facts/Photoinduced_charge_separation/GVqfqpWP">photoinduced charge separation</a>, is the start of the electron flow and transforms light energy into chemical forms. </p>