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. As electrons move through the chain, they release energy, which is used to pump protons (H+) across the membrane, creating a gradient. This proton gradient is essential for producing ATP, the energy currency of the cell. When protons flow back into the mitochondrial matrix through an enzyme called ATP synthase, they drive the conversion of adenosine diphosphate (ADP) into ATP, providing energy for various cellular processes.