Overview of oxidative phosphorylation. Oxidation and ATP synthesis are coupled by transmembrane proton fluxes. Electrons flow from NADH and FADH2 through four protein complexes to reduce oxygen to water (yellow tube). Three of the complexes pump protons from the mitochondrial matrix to the intermembrane space. The protons return to the matrix by flowing through another protein complex, ATP synthase (red structure), powering the synthesis of ATP.