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In plants, the light reactions of photosynthesis occur in the thylakoid membrane of the chloroplast. They involve the photosynthetic reaction centers known as Photosystem I and Photosystem II. Both of them are transmembrane protein complexes that each contain a variety of redox-active prosthetic groups. Photosystem I is responsible for reducing NADP⁺ to NADPH; Photosystem II oxidizes water to diatomic oxygen.

The following animation shows the movement of electrons through the protein complexes that participate in photosynthesis. A cartoon of these molecules in the thylakoid membrane is given in the top panel. The bottom panel is a diagram of the reactive species placed according to their reduction potential. The lower a species is on the diagram, the higher its standard reduction potential, that is, the greater its affinity for electrons.

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P680, the photon-absorbing center of Photosystem II, consists of one or possibly two closely associated molecules of chlorophyll a. The absorption of a photon of light puts P680 into an excited state known as P680^*. This excitation induces P680^* to rapidly donate an electron to a nearby molecule of pheophytin a, a process that appears to be influenced by an intervening molecule of chlorophyll a. The resulting cation, P680⁺, eventually regains its electron by abstracting it from water through the intermediacy of a tyrosine radical named Z and the oxygen-evolving complex, which contains four manganese ions. It requires four consecutive light-induced electron transfers for PSII to convert two molecules of water to one molecule of diatomic oxygen.

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Photosystem II binds two molecules of plastoquinone, which chemically resembles ubiquinone in the mitochondrial electron transport chain. The electron received by pheophytin a is rapidly transferred to the plastoquinone molecule designated Q_A and then is transferred to the second plastoquinone molecule which is called Q_B, to yield the radical anion Q_B^-. A second photoexcitation of P680 subsequently induces the transfer of a second electron to Q_B^- to yield Q_B^2-, the plastoquinol dianion. Q_B^2- then picks up two protons at the stromal surface of the thylakoid membrane to form Q_BH₂, which is released into the thylakoid membrane and is replaced by a plastoquinone molecule from the pool of plastoquinones in the membrane. PSII is thereby returned to its initial state ready to mediate the photoreduction of the newly bound plastoquinone molecule.

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Plastoquinol molecules from the membrane-bound pool bind to the cytochrome b₆f complex. This complex contains one subunit each of cytochrome f, cytochrome b₆, and an iron-sulfur protein. Two electrons from a plastoquinol molecule pass through the complex one at a time, via a Q cycle, much as occurs in the reduction of the mitochondrial cytochrome bc₁ complex by ubiquinol. In the process, four protons are translocated across the thylakoid membrane from the stroma to the thylakoid lumen. The dissipation of the proton gradient that is generated in this manner drives the synthesis of ATP by the CF₁CF₀-ATPase, which closely resembles the mitochondrial F₁F₀-ATPase.

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Electrons are transmitted, one at a time, from the cytochrome b₆f complex to Photosystem I by the mobile electron carrier plastocyanin. Plastocyanin is a peripheral membrane protein whose bound copper ion is reversibly oxidized from its Cu(I) state to its Cu(II) state. Thus, plastocyanin functions similarly to cytochrome c in the mitochondrial electron transport chain.

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The photon absorbing center of Photosystem I is designated P700. In a manner analogous to P680 in Photosystem II, P700 absorbs a photon, resulting in excitation of an electron that it donates to a chain of electron carriers in the complex. Photo-oxidized P700⁺ then accepts an electron from plastocyanin to regenerate P700.

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Returning to the electron lost from the excited P700*, this electron can follow one of two pathways. The more frequently used route is the noncyclic pathway in which electrons are transferred from Photosystem I to the soluble, iron-sulfur-containing protein ferredoxin [Fd]. Two ferredoxin molecules, resulting from two consecutive excitations of Photosystem I, each transfer one electron to the enzyme ferredoxin-NADP⁺ reductase, which in turn uses the two electrons to reduce NADP⁺ to NADPH. NADPH is the noncyclic pathway's final product.

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The other, less common, pathway for electrons traveling from Photosystem I is the cyclic pathway. Electrons travel from Photosystem I through cytochrome b₆ to the plastoquinone pool. In this process, protons are translocated across the thylakoid membrane to increase the proton gradient whose dissipation drives ATP synthesis. The cyclic pathway does not generate NADP⁺. Moreover, because it does not require the action of Photosystem II, it does not result in the generation of diatomic oxygen.