The light reactions and chemiosmosis: the organization of the thylakoid membrane.

This diagram shows a current model for the organization of the thylakoid membrane. The gold arrows track the noncyclic electron flow outlined in Figure 10.13. As electrons pass from carrier to carrier in redox reactions, hydrogen ions removed from the stroma are deposited in the thylakoid space, storing energy as a proton–motive force (H+ gradient). At least three steps in the light reactions contribute to the proton gradient: (1) Water is split by photosystem II on the side of the membrane facing the thylakoid space; (2) as plastoquinone (Pq), a mobile carrier, transfers electrons to the cytochrome complex, protons are translocated across the membrane into the thylakoid space; and (3) a hydrogen ion is removed from the stroma when it is taken up by NADP+. Notice how, as in Figure 10.16, hydrogen ions are being pumped from the stroma into the thylakoid space. The diffusion of H+ from the thylakoid space back to the stroma (along the H+ concentration gradient) powers the ATP synthase. These light–driven reactions store chemical energy in NADPH and ATP, which shuttle the energy to the sugar–producing Calvin cycle.