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Learning path

Full curriculum

Full curriculum
  1. NAD, NADP and FAD as reversible biological electron carriers
  2. Membrane electron-transport chains and redox-driven proton pumping
  3. Photosystem II and light-driven oxidation of water
  4. Linear photosynthetic electron flow from water to NADPH
  5. ATP synthase and chemiosmotic ATP formation
  6. Photophosphorylation driven by the thylakoid proton gradient
  7. Cyclic photosynthetic electron flow and ATP-to-NADPH balancing
  8. Triose-phosphate output and carbohydrate synthesis from photosynthetic carbon

Unit content

Linear photosynthetic electron flow from water to NADPH

In oxygenic photosynthesis, linear electron flow moves electrons from water through two photosystems and ultimately into NADPH.

The sequence is often summarized by a Z-scheme because electron free energy is raised twice by light:

H2O → PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin → NADP+
       ↑ light                                      ↑ light

PSII injects electrons from water

Photosystem II uses a photon to generate charge separation. Water supplies replacement electrons to the oxidized PSII reaction center, releasing $O_2$ and protons into the thylakoid lumen.

The electron leaving PSII passes to plastoquinone, a mobile lipid-soluble carrier, and then through the cytochrome b6f complex. Electron transfer through this region contributes to proton accumulation in the lumen.

Plastocyanin, a small copper-containing protein carrier, then transfers the electron to photosystem I.

PSI raises the electron's free energy again

By the time the electron reaches photosystem I (PSI), part of the energy supplied at PSII has been used to support proton-gradient formation.

A second photon excites the PSI reaction center, commonly called P700. The energized electron is transferred to ferredoxin, an iron-sulfur electron carrier.

Ferredoxin-NADP$^+$ reductase then transfers reducing equivalents to NADP$^+$, producing NADPH. For a two-electron reduction,

$$\mathrm{NADP^+ +2e^-+H^+\rightarrow NADPH}.$$

The two photons solve two different redox problems

Water is a poor electron donor: extracting electrons from it requires strong oxidizing power. NADP$^+$ reduction requires high-energy reducing power at the other end.

Using two photosystems in series allows oxygenic photosynthesis to span this large redox gap:

water oxidation
   ↓ PSII photon
electron transport + proton-gradient formation
   ↓ PSI photon
NADP+ reduction to NADPH

Linear flow therefore produces three important outcomes together: $O_2$ from water oxidation, a proton-motive force for ATP synthesis, and NADPH as reducing power for carbon fixation.