NADH and FADH2 carry hydrogen ions (H+) and electrons (e-) to the electron transport system located in the membrane. The energy from the electron transfers pumps hydrogen ions across the membrane creating an electrochemical gradient. As the accumulating hydrogen ions follow the electrochemical gradient back across the membrane through an ATP synthase complex, the movement of the ions provides energy for synthesizing ATP from ADP and phosphate. At the end of the electron transport system, two hydrogen ions, two electrons, and half of an oxygen molecule combine to form water.