The electron transport chain (ETC) is a group of protein complexes and mobile electron carriers in the inner mitochondrial membrane. Each can accept electrons from reduced NAD or reduced FAD and be reduced. The electron carrier is then oxidised as the electrons are passed to the next electron carrier in the chain then finally to oxygen which forms water. Each redox step releases energy, because the electrons pass to carriers at lower and lower energy levels, and this energy is coupled with the energy-requiring active transport of hydrogen ions into the inter-membrane space.The result of the redox reactions of the ETC is a proton gradient between the inter-membrane space and the matrix. This is used to generate ATP from ADP and inorganic phosphate by ATP synthase.