What determines how much energy is available in a molecule?

Answer 1

This is related to the Gibbs free energy: G = H —TS (where, H is the enthalpy, T is the absolute temperature, and S is the entropy), that is the required indicator of spontaneity for constant temperature and pressure processes. For systems that can only do pressure - volume work (w' = 0), Gibbs free energy equation can be expressed as: DG = DH — TDS = qp — TDS (where, qp is the heat transferred at constant pressure). Now, a spontaneous process is that one with negative DG value and is said to be "exergonic" and it can be utilized to do work. A process that is not spontaneous, that one with positive DG value is called "endergonic" and it must be driven by the input of free energy. Those processes that are at equilibrium (when the forward and the backward reactions are exactly balanced) are characterized by a DG = 0. From above considerations, the endergonic processes that maintain the living state are driven by the exergonic reactions of nutrient oxidation. Living organisms are not at equilibrium. Rather, they require a continuous influx of free energy to maintain order in a universe bent of maximizing disorder. They do so by coupling the exergonic processes required to maintain the living state such as the performance of mechanical work, the active transport of molecules against concentration gradients, and the biosynthesis of complex molecules. The key is to know "how much free energy carry a particular molecule" in order to carry out a work. This can be achieved measuring the "free energy" of a given intermediate molecule whose exergonic consumption drives endergonic processes. In other words, to determine how much "energy" carries a particular molecule, we have to measure its DG value.