Structural models of bovine complex I (PDB 5LNK [4]), porcine complex II (PDB 1ZOY;[5]), human complex III (PDB 5XTE [2]), bovine cytochrome c (PDB 2B4Z [6], bovine complex IV (PDB 1V54;[7]), and Saccharomyces cerevisiae ATP synthase (PDB 6B8H;[3]) are colored blue, magenta, cyan, violet, green, and light brown, respectively. The inner and outer mitochondrial membranes (IMM, OMM) are shown in yellow, and the inter membrane space (IMS) shown in grey. Ubiquinone (UQ, Coenzyme Q10)/ubiquinol (UQH2) are the oxidized and reduced forms of the membrane soluble small molecule carrier of protons and electrons between complex I, complex II and complex III. These are stylistically shown as green and pink boxes. Complexes in this figure are shown side-by-side for clarity, but may exist in a megacomplex in the mitochondria [2].