In eukaryotes, the electron transport and oxidative phosphorylation occurs in the inner membrane of mitochondria. These processes re-oxidize the FADH2 and NADH which comes from the citric acid cycle (situated in the mitochondrial matrix), glycolysis (situated in the cytoplasm) and fatty acid oxidation (situated in the mitochondrial matrix and trap the energy out as ATP. The Oxidative phosphorylation is by far the main source of ATP in the cell. In prokaryotes, the elements of electron transport and oxidative phosphorylation are situated in the plasma membrane.