How does ATP act as an energy source?

Two misleading conclusions drawn from the classical treatment of thermodynamics are, first, that single molecules cannot be treated thermodynamically and, second, that all real processes must be irreversible. The first conclusion has meant that the disruptive effect of thermal motions has been ignored, the second has meant that dynamic forces which cannot exist without stored energy, have been left out of account. The effect of these beliefs is demonstrated by analysis in detail of a simple electrostatic model of muscle. It is shown that the necessity of protecting each step against decay of the stored energy, and the requirement that each should be specific, has the effect of stopping the model actually working. Moreover, the character of static forces, which are strongest after their stored energy has been used up, precludes their use in cyclic processes because as much stored energy is required to undo them later in the cycle. Covalent intermediates are also examined from a quantum mechanical viewpoint and it is shown how these store energy which can be transferred by resonant transitions in the ground state when groups like phosphate are transferred chemically between molecules. But this analysis shows that, necessarily, the energy remains stored and therefore not available for muscular contraction. Then it follows naturally both that bond vibrations are produced in exothermic chemical reactions and that, because the force disappears when the energy has been used up, resonant exchange of stored energy in the excited state should solve the difficulties of previous approaches to these problems.