A photosystem is a membrane-associated complex of proteins, pigments, and other cofactors that converts absorbed light into chemical activity during photosynthesis. It combines light-harvesting components with a reaction centre, where excitation initiates the transfer of an electron to an acceptor. In oxygen-producing organisms, two distinct complexes—photosystem II and photosystem I—work in sequence to extract electrons from water and provide reducing power for cellular reactions. Photosystems are therefore energy-converting molecular assemblies, rather than simply collections of light-absorbing pigments. (elifesciences.org)
Location and organization
In plants and algae, photosystems occupy the thylakoid membranes inside chloroplasts. Cyanobacteria also contain photosystems in their photosynthetic membranes, but do not possess chloroplasts. The membrane provides an organized framework for electron transfer and separates compartments between which proton concentrations and electrical potentials can differ. Photosystem activity is coordinated with other membrane complexes rather than operating independently. (nature.com)
Photosystems contain a conserved core and associated antenna components whose organization varies among organisms. In green plants, photosystem II associates with light-harvesting complex II, or LHCII, and smaller antenna proteins; photosystem I associates with LHCI. Together these components form larger supercomplexes. In plant chloroplasts, photosystem II is concentrated in stacked membrane regions called grana, while photosystem I is enriched in unstacked regions. Changes in membrane organization can influence electron transport between them. (nature.com)
Light harvesting and charge separation
Antenna pigments increase the probability that a photosystem will capture a photon. They include chlorophylls and carotenoids, positioned by proteins at distances and orientations that enable excitation energy to move through the assembly. Plant LHCII contains chlorophylls a and b, whereas reaction-centre photochemistry principally involves chlorophyll a. Antenna pigments broaden the usable absorption range and deliver excitation to the reaction centre. (nature.com)
Energy transfer within the antenna must be distinguished from electron transfer. During light harvesting, excitation passes between pigments without requiring an electron to travel along the entire antenna. In the reaction centre, excitation instead produces charge separation: an electron moves to an acceptor, leaving an oxidized donor. Subsequent transfers stabilize this separation and connect it to a sequence of oxidation–reduction reactions. Pigments and cofactors are held in precise arrangements that make these processes possible. (nature.com)
Photosystem II
Photosystem II, abbreviated PSII, supplies electrons to the oxygenic photosynthetic pathway by oxidizing water. Its reaction-centre core contains the D1 and D2 proteins, with the core antenna proteins CP43 and CP47 nearby. Its primary donor is conventionally designated P680, referring to characteristic absorption near 680 nanometres. Electron transfer proceeds through pheophytin and bound plastoquinone acceptors; reduced plastoquinone then carries electrons onward through the membrane. (nature.com)
On the membrane’s lumen-facing side, PSII contains an oxygen-evolving complex. Its catalytic centre includes four manganese atoms and one calcium atom connected by oxygen bridges. Successive photochemical events accumulate the oxidizing equivalents needed to remove four electrons from two water molecules. The net water-oxidation reaction is:
2 H₂O → O₂ + 4 H⁺ + 4 e⁻
The oxygen released by oxygenic photosynthesis consequently originates from water, not carbon dioxide. Water oxidation also releases protons into the thylakoid lumen. (nature.com)
Photosystem I
Photosystem I, abbreviated PSI, provides a second light-driven excitation step. Its core is organized around the PsaA and PsaB proteins. The primary donor, P700, is named for characteristic absorption near 700 nanometres. Electrons pass through chlorophyll and phylloquinone acceptors and then iron–sulfur centres before reaching ferredoxin, a soluble electron-transfer protein. (elifesciences.org)
In linear electron flow, plastocyanin supplies replacement electrons to oxidized P700. Reduced ferredoxin transfers electrons to ferredoxin–NADP⁺ reductase, which produces NADPH. This reducing agent supports biosynthetic reactions, including carbon fixation. The two photosystems have complementary functions: PSII couples photochemistry to water oxidation, while PSI generates strongly reducing electron carriers. Their different donor and acceptor systems allow electrons to move from water toward NADP⁺ through two successive light-dependent steps. (elifesciences.org)
Electron flow and ATP formation
The pathway connecting the two photosystems forms part of the photosynthetic electron transport chain. Electrons move from PSII through plastoquinone, the cytochrome b₆f complex, and plastocyanin to PSI. Water oxidation and cytochrome b₆f activity contribute to an electrochemical gradient across the thylakoid membrane. Through chemiosmosis, ATP synthase uses this gradient to produce ATP. Photosystems thus support ATP formation indirectly; they do not themselves synthesize ATP. (elifesciences.org)
Linear flow produces NADPH alongside the gradient supporting ATP synthesis. In cyclic electron flow around PSI, electrons return to the membrane transport pathway instead of producing net NADPH. This contributes to proton-gradient formation and helps regulate the balance between energy supply and downstream demand. ATP and NADPH support the Calvin cycle, whose carbon-assimilation reactions are distinct from photosystem photochemistry. (nature.com)
Protection, repair, and diversity
Absorbed excitation can exceed the capacity of downstream reactions. Non-photochemical quenching dissipates excess excitation as heat, reducing the burden on photochemical reactions. Photosystems nevertheless sustain light-induced damage, especially to PSII’s D1 protein. Repair involves partial disassembly, removal of damaged D1, insertion of a replacement, and reassembly of an active complex. Photosynthetic performance reflects both photochemical activity and ongoing maintenance. (nature.com)
Related reaction centres occur in anoxygenic photosynthetic bacteria. These are classified as type I, with iron–sulfur acceptors, or type II, with quinone acceptors; PSI and PSII belong to the corresponding families. Many anoxygenic phototrophs use one reaction-centre type and do not oxidize water or release oxygen. Pigment composition also varies: some cyanobacteria incorporate chlorophyll f into PSI antenna sites, extending light harvesting into the far-red region. (nature.com)