The synthesis of the complex constituents of the cell from simple metabolism involves reactions which are accompanied by an increase in the free energy of the system. These, as has already been stated, require the simultaneous occurrence of several coupled reactions by which the necessary free energy liberated in one reaction is utilized for the performance of another. The availability of energy-rich compounds is thus necessary for many reactions which occur in the organism. These are formed as by products in the utilization of foodstuffs from which all the energy of the body must ultimately be obtained. Read more

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. Read more