An example of a biochemical reaction

Based on our exploration of a biochemical reactant in the previous section, we now recognize that the familiar biochemical reaction 

Equation (2.12) is the convenient form for biochemists who are interested in tracking reactant concentrations and apparent equilibrium constants and free energies Equation (2.13) is the physicochemically rigorous equation from which we can build relationships for the apparent thermodynamics of Equation (2.12). 

3 The ionic strength is a function of the total concentrations of all ionic species in a solution. It is defined / = (1 /2) J2i zfci where Zi and c/ are the charge number and concentration of each dissolved species. 

Next we explore the question of how the apparent thermodynamic variables change as a reaction progresses from a non-equilibrium initial condition toward equilibrium in a closed system. Imagine a closed system initially contains ATP, ADP, and PI, at concentrations of 10 mM, 1 mM, and 1 mM, respectively, at neutral pH of 7. To see how pH and related thermodynamic properties change as the reaction progresses toward equilibrium, imagine that an ATP hydrolysis enzyme is present in our system, and the reference reaction of Equation (2.13) moves in 

4 Equations (2.16) and (2.17) describe equilibria in terms of concentrations rather than activities. The concept of activity versus concentration of a species is discussed in Section 2.5. 

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