For example, with O2, there would be two menaquinone molecules that would transfer both their electrons to O2 simultaneously in order to transfer protons against the charge gradient. Likewise NAD would have to accept two electrons simultaneously from menaquinone to undergo reduction. Thus we can formulate the number of protons moved across the gradient per electron by moving electrons to a lower energy acceptor (pup) and the number of protons required to moved down the gradient per electron (pdown) to reduce NAD respectively.