Oxidative phosphorylation is a mechanism for ATP synthesis in both plant and animal cells. It involves the chemiosmotic coupling of electron transport and ATP synthesis. Oxidative phosphorylation occurs in the mitochondria. The mitochondrion has two membranes: an inner membrane and an outer membrane. The space defined by the inner membrane is the matrix, and the space between the two membranes is the intermembrane space. NADH and FADH 2 , generated in glycolysis and the citric acid cycle, are oxidized in the mitochondria. They donate their electrons to protein complexes embedded in the mitochondrial inner membrane, composed of many polypeptides with a variety of prosthetic groups capable of accepting and donating electrons. These complexes are components of the respiratory chain. Electrons donated from NADH and FADH 2 are transported along the respiratory chain and they will eventually be donated to oxygen, thus generating water. The direction of electron transport is determined by the redox potential of each potential electron carrier. There are three major protein complexes that participate in the respiratory chain that transports electrons and that pump hydrogen ions across the inner membrane, hence resulting in a hydrogen electrochemical gradient. This electrochemical gradient generates a proton motive force (PMF) that drives the hydrogen ions back across the inner membrane through the ATP synthase. The ATP synthase is composed of two subunts: the F0 subunit, which provides a channel for the flow of hydrogen ions back across the inner membrane; and the F1 subunit, which catalyzes the synthesis of ATP from ADP + Pi. As hydrogen ions flow through the F0 subunit, a portion of the subunit rotates in the membrane. As it rotates, it induces conformational changes in the F1 subunit that activate the ATP synthesis activity, thereby converting the free energy of the hydrogen electrochemical gradient (generated by the proteins of the electron transport chain) into the energy of a chemical bond.
Q-cytochrome c oxidoreductase is also known as cytochrome c reductase, cytochrome bc1 complex, or simply complex III  . In mammals, this enzyme is a dimer, (Figure 3) with each complex monomer containing 11 protein subunits, a [2Fe-2S] iron–sulfur cluster and three cytochromes (one cytochrome c1 and two b cytochromes  ). A cytochrome is a kind of electron-transferring protein that contains at least one heme group  . The iron atoms inside the heme groups of complex III, alternate between a reduced ferrous (+2) and oxidized ferric (+3) state as the electrons are transferred through the protein  .