twophotosystem

Two photosystems, namely photosystem II (PSII) and photosystem I (PSI), are the core components of oxygenic photosynthesis in cyanobacteria, algae, and plants. Located in the thylakoid membranes, they operate in sequence to convert light energy into chemical energy, producing ATP and NADPH that drive carbon fixation in the Calvin cycle. The two systems are coupled by an electron transport chain and linked by a mobile electron carrier pool, forming the Z-scheme of photosynthesis.

Photosystem II contains chlorophyll and accessory pigments organized into light-harvesting antenna complexes that funnel energy to the P680 reaction center. When excited, PSII receives electrons from water at the oxygen-evolving complex, splitting water to release oxygen, protons, and electrons. The electrons are transferred through pheophytin and plastoquinone to the plastoquinone pool, which is reduced to plastoquinol and passes electrons to the cytochrome b6f complex. This complex pumps protons across the membrane, contributing to the proton gradient used by ATP synthase.

Photosystem I contains the P700 reaction center and accepts electrons from plastocyanin after the cytochrome b6f step. Photoexcited PSI transfers electrons to ferredoxin and then to NADP+ reductase, forming NADPH. In non-cyclic flow, PSII and PSI cooperate to produce ATP and NADPH; PSI can also operate in cyclic electron flow, sending electrons back to the plastoquinone pool to increase ATP production without net NADPH formation.

Together, PSII and PSI implement the Z-scheme, capturing light twice and elevating electrons to higher redox potentials. They are found in cyanobacteria and chloroplasts of plants and algae, reflecting an evolutionary origin tied to endosymbiotic events and enabling oxygenic photosynthesis.