13.1.7—Light-dependent stage

Photophosphorylation converts light energy into chemical energy in the thylakoid membrane system. Light excites electrons, electron transfer helps build a proton gradient, and ATP synthase uses proton flow to form ATP; in non-cyclic flow, the same stage also produces reduced NADP and oxygen from water.

• Excitation and electron transfer: pigments in a photosystem absorb light and pass energy to a reaction-centre electron. The excited electron enters an electron-carrier chain in the thylakoid membrane, releasing energy as it moves between carriers.
• Proton gradient: carrier-chain energy is used to move H+ from the stroma into the thylakoid lumen. Photolysis of water also adds H+ to the lumen in non-cyclic flow, giving a higher H+ concentration there than in the stroma.
• ATP formation: H+ returns down its gradient through ATP synthase by chemiosmosis. The released energy drives ADP + Pi → ATP, so the proton gradient couples electron movement to ATP synthesis.
• Reduced NADP and oxygen boundary: in non-cyclic photophosphorylation, water supplies replacement electrons and produces oxygen; electrons and H+ combine with NADP to form reduced NADP. ATP and reduced NADP then pass to the light-independent stage.
• Cyclic versus non-cyclic: cyclic flow uses photosystem I only and returns excited electrons to that photosystem, producing ATP but not reduced NADP or oxygen from photolysis. Non-cyclic flow uses photosystems II and I, passes electrons onward, and produces ATP, reduced NADP and oxygen.

Do not treat ATP synthase as the source of the proton gradient, confuse photophosphorylation with the Calvin cycle, or assume cyclic flow produces reduced NADP. Detailed wavelength spectra, chromatography and carbon-fixation steps belong to neighbouring cards.