FIGURE 12-2 The proton-motive force powers ATP synthesis. Transmembrane proton concentration and electrical (voltage) gradients, collectively called the proton-motive force, are generated during aerobic oxidation and photosynthesis in both eukaryotes and prokaryotes (bacteria). High-energy electrons generated by light absorption by pigments (e.g., chlorophyll), or held in the reduced form of electron carriers (e.g., NADH, FADH₂) made during the catabolism of sugars and lipids, pass down an electron-transport chain (blue arrows), releasing energy throughout the process. The released energy is used to pump protons across the membrane (red arrows), generating the proton-motive force. In chemiosmotic coupling, the energy released when protons flow down the gradient through ATP synthase drives the synthesis of ATP. The proton-motive force can also power other processes, such as the transport of metabolites across the membrane against their concentration gradient and rotation of bacterial flagella.