The electron transport chain is a series of electron carriers embedded in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of aerobic prokaryotes. Like NADH and FADH2, each carrier exists in an oxidized form or a reduced form. Electrons pass down the electron transport chain in a series of redox reactions. Each acceptor molecule becomes alternately reduced as it accepts electrons and oxidazed as it gives them up. The electrons entering the electron transport chain have a relatively high energy content. They lose some of their energy at each step as they pass along the chain of electron carriers.
Members of the electron transport chain include the flavoprotein flavin mononucleotide, the lipid ubiquinone (coenzyme Q), several iron-sulfur proteins, and a group of closely related iron-containing proteins called cytochromes. Each electron carrier has a different mechanism for accepting and passing electrons. For example, as cytochromes accept and donate electrons the charge on the iron atom, which is the electron carrier part of the cytochromes, alternates between Fe2+ (reduced) and Fe3+ (oxidized).
Electrons are conducted through the electron transport chain in a defined sequence from reduced nucleotide coenzymes to oxygen, and the free energy changes drive the transport of protons from the matrix into the intermembrane space via the three proton pumps.