Mitchell has explained the concept based on the fact that the reducing equivalents are transferred as H atoms by electron carriers (ubiquinone) and others (such as Fe-S centre and cytochrome). He explained that the hydrogen- and electron-carrying proteins are present in alternate in the respiratory chain to form three functional loops called the oxidation–reduction loops (= o/r loops). Each loop corresponds functionally to the coupling sites I, II, and III of chemical hypothesis.