Understanding redox reactions in biochemistry requires a clear grasp of three definitions: electron exchange, oxidation number, and oxygen atom insertion. Here, the electrochemical cell is explained for redox-active ions, introducing a new comparison between the measurement of midpoint potential and the measurement of initial velocity for enzymes. Oxygen atom insertion is shown to be of use in readily classifying oxidation states. Finally, the use of several of these ideas is combined into an explanation of proton motive force development in the mitochondrial respiratory chain.