The controlled stepwise oxidation of sugar that occurs in the cell preserves useful energy, unlike the simple burning of the same fuel molecule. In the cell, enzymes catalyze oxidation via a series of small steps in which free energy is transferred in conveniently sized packets to carrier molecules — most often ATP and NADH. At each step, an enzyme controls the reaction by reducing the activation energy barrier that has to be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (A) and (B).  Fair Use and Copyright info