In an electron transport system, energy from electron transfer during oxidation-reduction reactions enables certain carriers to pump protons (H+) across a membrane. As the H+ concentration increases on one side of the membrane, an electrochemical gradient called proton motive force develops. As the accumulating protons follow the electrochemical gradient back across the membrane through an ATP synthase complex, the movement of the protons provides energy for synthesizing ATP from ADP and phosphate.