13.1.10—During photophosphorylation

Photophosphorylation is ATP formation in the light-dependent stage. In the thylakoid membrane, light-excited electrons pass through carriers whose released energy pumps protons into the thylakoid lumen; protons then return through ATP synthase and drive ADP + Pi → ATP.

• Capture and transfer: photosynthetic pigments absorb light and transfer energy to electrons in a photosystem. The excited electrons enter an electron-carrier chain in the thylakoid membrane.
• Pump protons: as electrons move through the carriers, they release energy. That energy powers proton transport from the stroma into the thylakoid lumen, establishing a high H+ concentration in the lumen and a lower concentration in the stroma.
• Chemiosmosis: H+ moves back down its concentration gradient through transmembrane ATP synthase. The enzyme couples this proton flow to phosphorylation of ADP, forming ATP.
• Product connection: ATP produced in the light-dependent stage is supplied to the light-independent reactions. In non-cyclic flow, electron transfer also contributes to reduced NADP formation; cyclic flow returns electrons to photosystem I and does not produce reduced NADP.
• Interpretation boundary: light supplies the initial energy, electron carriers build the gradient, and ATP synthase converts the gradient into ATP. These are linked steps, not separate sources of ATP.

Do not reverse proton directions: pumping is from stroma to lumen, while chemiosmotic return is from lumen to stroma through ATP synthase. Do not confuse photophosphorylation with carbon fixation, or treat reduced NADP as an ATP product of cyclic flow.