The free energy of a reaction predicts the probability of its occurring spontaneously but tells nothing of the rate at which it will occur. For a given reaction to occur, its must be accompanied by a decrease in the free energy of the system, i.e. ?F must be negative; if ?F is positive, work must be done on the system to make it react. In the living organism many reactions occur which would never occur in an isolated system. The fact that in the living cell many interrelated reactions occur simultaneously permits the necessary free to be supplied by other simultaneously occurring chemical reactions. Such reactions are designated as coupled reactions.

The concept of entropy is related to the probability of a given reaction’s occurring. It represents the capacity of a system to utilize the energy inherent in the random vibration and movements of the atoms and is therefore proportional to the absolute temperature. The entropy of a reaction must increase in any spontaneously occurring reaction i.e. the reaction must proceeds to a more probable state. At equilibrium, accordingly, the entropy equals zero.

The living organism, as will be shown in the next section, is never in a state of equilibrium, in the thermodynamic sense. Only at death and after autolysis dissolution of the tissues has occurred is equilibrium reached and does the entropy is at a maximum.

This relation of the heat of a reaction and the change in free energy may be expressed by the equation

?H =?F + T?S

Where ?H = heat liberated by reaction occurring at constant temperature and pressure

?F= change in free energy

T = absolute temperature

?S= reversible heat of reaction or change in entropy

?S, in the case of the oxidation of 1 mole of glucose at 25o C, would equal 15,160/ (273+25) or 50,8 kcal.