13.1.9—Non-cyclic photophosphorylation

Non-cyclic photophosphorylation is the light-dependent electron flow through photosystems II and I in the thylakoid membrane. Water replaces electrons lost from photosystem II, the flow builds a proton gradient for ATP synthesis, and electrons from photosystem I reduce NADP to reduced NADP; photolysis also releases oxygen.

• Photosystem II starts the flow: light excites an electron in the primary pigment of photosystem II. The electron enters an electron-carrier chain and is passed towards photosystem I.
• Replace the lost electron: water is split in photolysis, producing electrons, H+ and oxygen. The electrons replace those removed from photosystem II; oxygen is released as a product of water splitting.
• Make ATP: energy released as electrons move through the carriers powers proton movement from the stroma into the thylakoid lumen. Protons return through ATP synthase by chemiosmosis, driving ADP + Pi → ATP.
• Photosystem I and reduced NADP: light also excites an electron in photosystem I. Electrons from its carrier chain combine with H+ and NADP to form reduced NADP, which passes to the light-independent stage.
• Boundary with cyclic flow: non-cyclic flow uses both photosystems and sends electrons onward to NADP, producing ATP, reduced NADP and oxygen. It is not an electron loop returning to photosystem I.

Do not say that oxygen comes from carbon dioxide or that non-cyclic electrons return to their original photosystem. This card explains light-dependent electron flow and products; Calvin-cycle carbon fixation and limiting-factor investigations belong elsewhere.