A series of membrane-linked oxidation-reduction reactions in which electrons are transferred from reduced cofactors, formed in TCA cycle and other *dehydrogenase
reactions , to oxygen, which then combines with hydrogen ions to form water. At present seven different electron-transferring enzymes or cofactors are known intermediates between NAD, the starting point of the chain, and oxygen. The *redox potential
is most negative at the start of the chain, and rises as the electrons flow towards oxygen. For every two electrons travelling down the respiratory chain, three molecules of ATP are generated by the process of *oxidative phosphorylation
, which is linked to respiratory electron transfer.