NADH and FADH₂ carry protons (H⁺) and electrons (e^-) to the electron transport chain located in the membrane. The energy from the transfer of electrons along the chain transports protons across the membrane and creates an electrochemical gradient or proton motive force. At the end of the electron transport system, protons, electrons, and oxygen molecules combine to form water. As the accumulating protons follow the electrochemical gradient back across the membrane through an ATP synthase complex, the movement of the protons (proton motive force) provides energy for synthesizing ATP from ADP and phosphate.