The variation of A,G with composition

The reactants and products in a biological cell are rarely at equilibrium, so we need to know how the reaction Gibbs energy depends on their concentrations. 

Our starting point is the general expression for the composition dependence of the chemical potential derived in Section 3.8  

For perfect gases, aj = pj/p, the partial pressure of J relative to the standard pressure p = 1 bar. 

As in Chapter 3, to simplify the appearance of expressions in what follows, we shall not write c andp explicitly. 

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