An electron transport chain or system (ETS) is a series of coenzymes and cytochrome that take part in the passage of electrons from a chemical to its ultimate acceptor. Reduced coenzymes participate in the electron transport chain. Electron transport takes place on the cristae of mitochondria [oxysomes (F 0 -F 1 , particles) found on the inner surface of the membrane of mitochondria]. Ubiquinone act as electron carriers of the respiratory electron transport chain. Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for the transfer of electrons between complex III and IV.

The ratio of the volume of CO 2 evolved to the volume of O 2 consumed in respiration is called the Respiratory quotient (R) or respiratory ratio. The respiratory quotient depends upon the type of respiratory substrate used during respiration.