The inner mitochondrial membrane has groups of several proton (H+) and electron (e-) acceptors. These groups are arranged in a specific series called electron transport chain (ETC) or electron transport system (ETS). The electron transport system is arranged in a manner that electrons are carried downhill as the electron carriers are located along increasing redox potential or increasing affinity for electrons. Each carrier of the electron transport chain can receive electrons from an electron donor and can subsequently donate electrons to the next carrier in the chain, which ultimately react with oxygen and protons to form water.
2H+ + O2- -> H2 O