Light reactions of photosynthesis

Two photons per electron: water in at PSII, NADPH out at PSI.

Per 2 H₂O split: 1 O₂ · 2 NADPH · ~3 ATP

Which photosystem does what

PSII splits H₂O (the 2s go together); PSI makes NADPH. And 680 runs before 700.

An electron transport chain run in reverse of the mitochondrial one: instead of burning NADH down to water, light pushes electrons uphill from water to NADPH. Each electron is boosted twice, at 680 nm and then 700 nm, which is why the energy diagram is a Z. The protons piled into the lumen along the way pay for the ATP.

2 H₂O P680 (PSII)

Oxygen-evolving complex O₂ + 4 H⁺ (lumen)

Water is split here at PSII; PSI carries out no water-splitting. The Mn₄CaO₅ cluster pulls four electrons off two waters, one photon at a time; the O₂ is a by-product. ¹⁸O-labeling proved the oxygen you breathe comes from water; none of it comes from CO₂.

P680* (PSII) Plastoquinone (PQH₂)

Photosystem II photon (680 nm)

Light kicks P680 to P680*, and the P680⁺ left behind is the strongest oxidant known in biology, which is what makes stealing electrons from water possible at all. PQ is the chloroplast’s ubiquinone: lipid-soluble, carrying 2 e⁻ and 2 H⁺. The herbicide DCMU blocks this handoff.

Plastoquinone (PQH₂) Plastocyanin

Cytochrome b₆f 4 H⁺ pumped → lumen

The proton-pumping step, and the chloroplast twin of mitochondrial Complex III, Q cycle included. Cyclic electron flow rejoins the chain here, which is why the cyclic route still makes ATP.

Plastocyanin Ferredoxin

Photosystem I photon (700 nm)

Plastocyanin is a small copper protein in the lumen, playing the role cytochrome c plays in mitochondria. The second photon boosts the electron up the second limb of the Z. The herbicide paraquat hijacks electrons on the far side of PSI into superoxide.

Ferredoxin NADPH

Ferredoxin-NADP⁺ reductase NADP⁺ → NADPH

Made on the stromal face, exactly where the Calvin cycle will spend it. The fork in the road: ferredoxin can instead hand electrons back to cytochrome b₆f (cyclic flow), which buys extra ATP but no NADPH and no O₂. The Calvin cycle needs ATP and NADPH in a 3:2 ratio, and cyclic flow is how the books get balanced.

H⁺ (lumen) H⁺ (stroma)

ATP synthase (CF₀CF₁) ADP → ATP

The lumen sits near pH 5 and the stroma near pH 8: a thousandfold proton gradient. Jagendorf made thylakoids synthesize ATP in the dark with nothing but an acid bath, the experiment that sold chemiosmosis. Same rotary machine as mitochondria, aimed so the ATP lands in the stroma.