photosystem II

GO:0009523 · Energy complex · Multiprotein complex · Proposed

A photosystem that contains a pheophytin-quinone reaction center with associated accessory pigments and electron carriers. In cyanobacteria and chloroplasts, in the presence of light, PSII functions as a water-plastoquinone oxidoreductase, transferring electrons from water to plastoquinone, whereas other photosynthetic bacteria carry out anoxygenic photosynthesis and oxidize other compounds to re-reduce the photoreaction center. [GO:0009523]

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Essential	Role
pheophytin-quinone reaction center core	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Central PSII reaction-center proteins that couple P680 charge separation to pheophytin and plastoquinone electron transfer. `GO:0009523` GO:0009523 defines PSII as a pheophytin-quinone reaction center photosystem with associated pigments and electron carriers. `PMID:21499260` Umena et al. 2011 resolved the oxygen-evolving PSII structure, including protein subunits and cofactors.
oxygen-evolving manganese-calcium cluster	Small molecule	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Catalytic metal-oxo cofactor cluster that accumulates oxidizing equivalents for water splitting in oxygenic PSII. `PMID:21499260` Umena et al. 2011 located all metal atoms in the oxygen-evolving Mn4CaO5 cluster in high-resolution PSII.
PSII chlorophyll and carotenoid cofactors	Small molecule	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Reaction-center and accessory pigments that absorb excitation energy and enable P680 charge separation. `GO:0009523` GO:0009523 defines PSII as a photosystem with associated accessory pigments and electron carriers. `PMID:21499260` Umena et al. 2011 modeled cofactors bound by the oxygen-evolving PSII complex.

Taxonomic distribution

NCBITaxon:1117 Cyanobacteriota — COMMON: Cyanobacteria use PSII as a thylakoid water-plastoquinone oxidoreductase that supplies electrons from water into the photosynthetic electron-transport chain. [GO:0009523]

Canonical examples

NCBITaxon:32053 Thermostichus vulcanus — Thermophilic cyanobacterial source used by Umena et al. to solve the 1.9 A oxygen-evolving PSII crystal structure. [PMID:21499260]

Functions

photosynthetic electron transport chain GO:0009767 — PSII captures light energy, extracts electrons from water through its oxygen-evolving metal cluster, and reduces plastoquinone to start oxygenic photosynthetic electron transport.
- GO:0009523 GO:0009523 defines PSII as a water-plastoquinone oxidoreductase in cyanobacteria and chloroplasts.
- PMID:21499260 Umena et al. 2011 resolved the catalytic metal cluster of oxygen-evolving PSII.

Mechanism graphs

PSII couples light capture to water oxidation (FUNCTION)

In cyanobacterial PSII, bound pigments feed excitation into the reaction center, the manganese-calcium cluster extracts electrons from water, and plastoquinone accepts electrons on the reducing side.

Subject	Predicate	Object	Evidence
PSII chlorophyll and carotenoid cofactors	feed excitation to	pheophytin-quinone reaction center core	`GO:0009523` GO:0009523 defines PSII as a photosystem with associated accessory pigments.
oxygen-evolving manganese-calcium cluster	extracts electrons from	water	`PMID:21499260` Umena et al. 2011 resolved the Mn4CaO5 cluster that catalyzes water splitting in PSII.
oxygen-evolving manganese-calcium cluster	feeds electrons to	pheophytin-quinone reaction center core	`GO:0009523` GO:0009523 defines PSII by electron transfer from water to plastoquinone in cyanobacteria and chloroplasts.
pheophytin-quinone reaction center core	reduces	plastoquinone pool	`GO:0009523` GO:0009523 defines PSII as a water-plastoquinone oxidoreductase.
pheophytin-quinone reaction center core	initiates	photosynthetic electron transport chain	`GO:0009523` GO:0009523 places PSII at the water-to-plastoquinone step of light-dependent photosynthesis.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral component groundings for D1/D2 core, cytochrome b559, antenna, oxygen-evolving and extrinsic PSII families before replacing reviewed-label-only PSII components. GO exactly grounds the complete PSII complex, but the first component boundary should not flatten D1, D2, cytochrome b559, antenna, and oxygen-evolving subfamilies into unsupported broad protein-family rows.

Evidence

GO:0009523 Gene Ontology cellular-component term for photosystem II.
GO:0009521 Gene Ontology cellular-component term for photosystems.
GO:0009767 Gene Ontology biological-process term for the photosynthetic electron transport chain.
PMID:21499260 Umena et al. 2011, Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 A, Nature.

Curation history

2026-09-24T02:08:00Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T02:08:46Z codex · EXPANDED_RECORD — Added GO photosystem parentage, reviewed-label-only PSII component classes, cyanobacterial distribution and Thermosynechococcus example, photosynthetic electron-transfer function, GO/PubMed evidence, a PSII water-oxidation graph, and a component-grounding TODO.
2026-09-24T02:29:24Z codex · ADDRESS_REVIEW — #1442: updated the NCBITaxon:32053 canonical-example label from the accepted Thermosynechococcus vulcanus synonym to the current NCBI label Thermostichus vulcanus.

Source: data/structures/energy_complex/photosystem_ii.yaml