Oxidative phosphorylation is a metabolic pathway in which the energy produced by the oxidation of nutrients is stored in the mitochondria of cells in the form of ATP. When oxidative phosphorylation occurs, electrons are transferred from donor compounds to acceptor compounds through redox reactions. The process is crucial for most of the organisms containing mitochondria. First, during these reactions, energy is released, which is then stored in the form of ATP. Compared with fermentation, oxidative phosphorylation gives a large energy yield. In glycolysis, the total yield of ATP is only 2 molecules, but during oxidative phosphorylation up to 36 ATP molecules are synthesized while β-oxidation of fatty acids produces about 14 ATP molecules. Secondly, the free energy released during redox reactions cannot be fully utilized by the body. About 50% of it is dispersed in the body in the form of heat, which is especially important as a way to maintain body temperature in dormant animals or animals inhabiting the environment with low temperatures.