The Electron Transport Chain (ETC) is a series of protein complexes located in the inner membrane of the mitochondria. It plays a crucial role in cellular respiration by transferring electrons from NADH and FADH2 to oxygen, the final electron acceptor. This process generates a proton gradient across the membrane, which is essential for ATP production. As electrons move through the ETC, energy is released and used to pump protons into the intermembrane space, creating a concentration gradient. This gradient drives protons back into the mitochondrial matrix through ATP synthase, ultimately producing ATP, the energy currency of the cell.