During oxidative phosphorylation, the electrons from NADH and FADH₂ are combined with O₂ and the energy released in the process is used to synthesize ATP from ADP.

The components of the electron transport chain are located in the internal part of the mitochondrial membrane in eukaryotic cells, and in the cell membrane in bacteria. The transporters in the electron transport chain are organized into four complexes in the inner mitochondrial membrane. A fifth complex then couples these reactions to the ATP synthesis.

Complex II receives the electrons from the succinate, which is an intermediary in the Krebs cycle. These electrons are transferred to the FADH₂ and then to the Q coenzyme. This liposoluble molecule will transport the electrons from Complex II to Complex III. In this complex, the electrons are transferred from the b cytochrome to the c cytochrome. This c cytochrome, which is a peripheric membrane protein located in the external part of the inner membrane, then transports the electrons to Complex IV where finally they are transferred to the oxygen.

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