13.1.8—Cyclic photophosphorylation

Cyclic photophosphorylation is a light-dependent process in the thylakoid membrane in which excited electrons from photosystem I pass through carriers and return to photosystem I. Their energy builds a proton gradient, which drives ATP synthesis, but the cycle does not produce reduced NADP or oxygen.

• Excite: light is absorbed by photosystem I and raises the energy of an electron in its primary pigment.
• Cycle the electron: the excited electron is accepted and passed along an electron-carrier chain before returning to photosystem I. The electron is therefore recycled rather than replaced by photolysis of water.
• Build the gradient: as the electron moves through the carriers, released energy powers proton pumping from the stroma into the thylakoid lumen.
• Make ATP: protons move back down their gradient through ATP synthase by chemiosmosis, driving ADP + Pi → ATP. The ATP can then supply the light-independent stage.
• Product boundary: cyclic flow uses photosystem I only and makes ATP, but it does not split water; therefore it produces neither oxygen from photolysis nor reduced NADP.
• Contrast: non-cyclic photophosphorylation uses photosystems II and I, replaces electrons from water and transfers electrons onward to reduce NADP, so it produces ATP together with reduced NADP and oxygen.

Do not describe cyclic photophosphorylation as carbon fixation or as a source of reduced NADP. The defining loop is photosystem I → electron carriers → photosystem I; detailed Calvin-cycle reactions belong to a separate card.