Bioenergetics the application of thermodynamic principles to biological systems

2 Bioenergetics the application of thermodynamic principles to biological systems 

In practice, the actual values for the free energy of a given reaction are difficult to measure experimentally. However, during a chemical reaction when one compound 

The account given above applies to any chemical reaction, but as our attention is on reactions inside cells, the symbol AG is adopted to indicate actual free energy change under physiological conditions . 

The main significance of being able to calculate or to measure AG values is that we are then able to make predictions about reactions and in particular identify which reactions are likely to be control points in pathways. The absolute numerical value of the actual change in free energy (AG ) is dependant upon the actual concentrations of the reactant(s) and product(s) involved in the reaction. Comparisons of values for different reactions are meaningless unless they have been determined under identical and standardized experimental conditions. The term standard free energy (symbol AG°) is used to specify just such conditions. 

The standard free energy change is the value obtained when the reactants and products (including H+) are at molar concentration and gasses (if present) are at 1 atmosphere of pressure. Such conditions are quite unphysiological, especially the proton concentration, as 1 molar H+ concentration gives a pH 0 biochemical reactions occur at a pH of between 5 and 8, mostly around pH 7. So a third term, AG°, is introduced to indicate that the reaction is occurring at pH 7. 

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