photosystem I

GO:0009522 · Energy complex · Multiprotein complex · Proposed

A photosystem that contains an iron-sulfur reaction center associated with accessory pigments and electron carriers. In cyanobacteria and chloroplasts, photosystem I functions as a light-dependent plastocyanin-ferredoxin oxidoreductase, transferring electrons from plastocyanin to ferredoxin; in photosynthetic bacteria that have only a single type I photosystem, such as the green sulfur bacteria, electrons can go either to ferredoxin (Fd) -> NAD+ or to menaquinone (MK) -> Cytb/FeS -> Cytc555 -> photosystem I (cyclic photophosphorylation). [GO:0009522]

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

Components

Component	Type	Part of / acts on	Grounding	Essential	Role
type I reaction center core	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Large transmembrane core polypeptides that bind the primary donor and early electron-transfer cofactors of the type I photosystem. `GO:0009522` GO:0009522 defines PSI as an iron-sulfur reaction-center photosystem associated with accessory pigments and electron carriers. `PMID:11418848` Jordan et al. 2001 resolved the cyanobacterial PSI structure including the PsaA/PsaB reaction-center core.
terminal iron-sulfur electron acceptors	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Iron-sulfur proteins or domains that receive electrons from the PSI reaction-center core for transfer toward ferredoxin. `GO:0009522` GO:0009522 defines PSI as an iron-sulfur reaction-center photosystem that transfers electrons toward ferredoxin. `PMID:11418848` Jordan et al. 2001 located the Fe4S4 clusters in the cyanobacterial PSI structural model.
PSI chlorophyll and carotenoid cofactors	Small molecule	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Accessory chlorophyll and carotenoid cofactors that capture excitation energy and couple it to reaction-center charge separation. `GO:0009522` GO:0009522 defines PSI as a photosystem with accessory pigments. `PMID:11418848` Jordan et al. 2001 modeled the chlorophylls and carotenoids bound by the cyanobacterial PSI complex.

Taxonomic distribution

NCBITaxon:1117 Cyanobacteriota — COMMON: Cyanobacteria use PSI as a major photosynthetic-membrane complex that receives electrons from plastocyanin and passes them toward ferredoxin. [GO:0009522]

Canonical examples

NCBITaxon:32046 Synechococcus elongatus — Thermophilic cyanobacterial source used by Jordan et al. to solve the 2.5 A crystal structure of PSI. [PMID:11418848]

Functions

photosynthetic electron transport chain GO:0009767 — PSI accepts electrons from an upstream soluble carrier, oxidizes the P700 special pair after light absorption, and transfers electrons through iron-sulfur centers toward ferredoxin.
- GO:0009522 GO:0009522 defines PSI as a light-dependent plastocyanin-ferredoxin oxidoreductase in cyanobacteria and chloroplasts.
- PMID:11418848 Jordan et al. 2001 connected the cyanobacterial PSI structure to light capture and electron transfer.

Mechanism graphs

PSI drives light-dependent electron transfer toward ferredoxin (FUNCTION)

In cyanobacterial PSI, bound pigments feed excitation to the P700 reaction center, and electrons move through internal iron-sulfur acceptors toward soluble ferredoxin.

Subject	Predicate	Object	Evidence
PSI chlorophyll and carotenoid cofactors	feed excitation to	type I reaction center core	`PMID:11418848` Jordan et al. 2001 modeled PSI pigments and discussed how their arrangement supports efficient light capture and electron transfer.
type I reaction center core	passes electrons through	terminal iron-sulfur electron acceptors	`GO:0009522` GO:0009522 defines PSI as an iron-sulfur reaction center with electron carriers.
terminal iron-sulfur electron acceptors	reduce	ferredoxin	`GO:0009522` GO:0009522 describes cyanobacterial PSI as transferring electrons from plastocyanin to ferredoxin.
type I reaction center core	contributes to	photosynthetic electron transport chain	`PMID:11418848` Jordan et al. 2001 present PSI as one of the large protein-cofactor complexes driving photosynthesis in cyanobacteria.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral component groundings for Psa, Psc, iron-sulfur, and species-specific accessory subfamilies before replacing reviewed-label-only PSI components. GO exactly grounds the generic PSI complex, but cyanobacterial PsaA/PsaB, chloroplast accessory subunits, and bacterial Psc subfamilies should not be conflated in source-neutral protein family rows.

Evidence

GO:0009522 Gene Ontology cellular-component term for photosystem I.
GO:0009521 Gene Ontology cellular-component term for photosystems.
GO:0009767 Gene Ontology biological-process term for the photosynthetic electron transport chain.
PMID:11418848 Jordan et al. 2001, Three-dimensional structure of cyanobacterial photosystem I at 2.5 A resolution, Nature.

Curation history

2026-09-24T01:37:28Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T01:39:06Z codex · EXPANDED_RECORD — Added GO photosystem parentage, reviewed-label-only PSI component classes, cyanobacterial distribution and Synechococcus example, photosynthetic electron-transfer function, GO/PubMed evidence, a PSI electron-transfer graph, and a component-grounding TODO.
2026-09-24T01:48:37Z codex · ADDRESS_REVIEW — #1440: removed an uncited cytochrome c6 alternate-donor claim from the Cyanobacteriota distribution note.

Source: data/structures/energy_complex/photosystem_i.yaml