Oxygenic photosynthesis (in plants, algae, and cyanobacteria) uses two distinct photosystems working in series: Photosystem II (PS II) and Photosystem I (PS I). These are large protein-pigment complexes embedded in the thylakoid membrane. Each photosystem contains: an antenna complex (light-harvesting complex, LHC) with hundreds of accessory pigments (chlorophyll a, chlorophyll b, carotenoids) that absorb light and funnel energy to the reaction centre; and a reaction centre containing a special pair of chlorophyll a molecules (P680 in PS II, P700 in PS I) that undergo photoinduced charge separation — the actual photochemical event of photosynthesis. The two photosystems operate in sequence in the Z-scheme of non-cyclic photophosphorylation.

Photosystem I (PS I) is a large multiprotein complex containing approximately 12-14 protein subunits, about 100 chlorophyll molecules (chlorophyll a and some chlorophyll b), 12-16 carotenoid molecules, and several iron-sulfur clusters. The reaction centre of PS I is the P700 special pair — two chlorophyll a molecules with an absorption maximum at 700 nm. When P700 absorbs a photon (or receives excited energy from the antenna complex), one electron is promoted to a higher energy state and transferred to the primary electron acceptor A0 (a chlorophyll a monomer), then to A1 (a phylloquinone/vitamin K1), then through a series of iron-sulfur (Fe-S) clusters (Fx, FA, FB), and finally to ferredoxin (Fd) — a small soluble iron-sulfur protein in the stroma. From ferredoxin, electrons are transferred to NADP+ by the enzyme ferredoxin-NADP+ reductase (FNR), producing NADPH. The P700 "hole" (oxidised P700, P700+) is filled by electrons arriving from plastocyanin, which carries electrons from the cytochrome b6f complex.

The Z-scheme (named for the Z-shaped pattern when electron energy is plotted against redox potential) describes the complete path of electrons from water to NADPH in non-cyclic photophosphorylation. Water splitting: PS II reaction centre P680 absorbs light → P680 becomes excited → donates electron to pheophytin → the electron hole in P680 is filled by oxidation of water: 2H2O → O2 + 4H+ + 4e- (catalysed by the oxygen-evolving complex, OEC, containing a Mn4CaO5 cluster). Electron transport chain: electrons flow from PS II through plastoquinone (PQ) → cytochrome b6f complex (pumps H+ into thylakoid lumen, contributing to the proton gradient for ATP synthesis) → plastocyanin (PC, small copper-containing protein) → PS I reaction centre P700. PS I: P700 absorbs light → excited electron passed to ferredoxin → NADP+ reduced to NADPH by FNR. Energy coupling: the proton gradient generated across the thylakoid membrane drives ATP synthase (CF0-CF1 complex) to produce ATP from ADP + Pi (photophosphorylation).

Two types of photophosphorylation occur in plants: Non-cyclic photophosphorylation: involves BOTH PS I and PS II in series; produces ATP, NADPH, and O2; is the primary pathway for driving the Calvin cycle. Electrons flow from water → PS II → electron transport chain → PS I → NADPH (electrons are not recycled). Cyclic photophosphorylation: involves ONLY PS I; produces ONLY ATP (no NADPH, no O2 released); electrons from ferredoxin cycle back to the cytochrome b6f complex instead of reducing NADP+, generating additional proton gradient and ATP. This is used when the ATP/NADPH ratio needs adjustment — the Calvin cycle requires ATP and NADPH in an approximately 3:2 ratio, and cyclic photophosphorylation helps maintain this balance.