The electron transfer chain (also called the electron transport chain, ETC, or simply electron transport), is any series of protein complexes and lipid-soluable messengers that convert the reductive potential of energized electrons into a cross-membrane proton gradient. This proton motive force created by the ETC is used to power membrane transporters and adenosine triphosphate synthesis by ATP synthase (aka the F0F1 particle). They are used in photophosphorylation and respiration.
In eukaryotes, including humans, an ETC is found spanning the inner mitochondrial membrane and accepts electrons from electron donors such as NADH or succinate, shuttles these electrons from within the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space, where the ultimately reduce oxygen.
There are five complexes normally associated with the mitochondrial electron transfer chain.
All of these are proteolipid complexes, with the first four containing either flavins, iron-sulfur clusters, copper centers, or heme moieties. Complexes I, III, and IV are proton pumps. Complex II is part of the Krebs cycle and does not pump protons, and Complex V uses the electrochemical potential generated to create ATP. Complex IV is the terminus of the electron transfer chain, where oxygen from the lungs is reduced by electrons and hydrogen protons (provided by NADH and/or FADH2) to make water.
Cytochrome c is also an essential part of the electron transfer chain. It is a soluble protein loosely associated with the inner mitochondrial membrane, and transfers electrons between Complexes III and IV.