The electron transport chain (ETC) is located in the inner membrane of the mitochondria. NADH is the electron donor for the ETC. Electron donation to the first complex (blue) and hydrogen ion pumping are coupled. The donated electrons are then passed to the first mobile carrier protein (pink). The electrons are then passed to the second complex (red). This transfer is again coupled with the pumping of hydrogen ions. Electrons are then passed to the second mobile carrier protein (purple). Next, the electrons are transferred to the third complex (orange). While contained by the third complex, the electrons interact with oxygen and hydrogen ions. The reaction ends with hydrogen ions, or protons, being pumped across the membrane and the release of water molecules (H2O). The pumping of protons creates a proton gradient. The hydrogen ion pumping actions of the ETC are coupled with ATP synthesis.