c07 JWBS043-Rogers September 13, 2010 11:25 Printer Name: Yet to Come
FREE ENERGY AND EQUILIBRIA IN BIOCHEMICAL SYSTEMS 1035 ′-triphosphate would be carried out at a specified and constant temperature, pres-
sure, pH, pMg, and ionic strength where pH and pMg refer to the ion concentrations
pH=−log[
H+
]
and pMg=−log[
Mg^2 +]
, and the ionic strength is constant for all
dissolved salts in the reaction solution. Ionic strength is essentially the ionic charge
concentration in solution calculated as the sum^12∑
ciz^2 iover all salt concentrations
cithat dissolve to give ions of chargezi.
These conditions specify a unique standard state which is not the thermodynamic
standard state but which is adhered to throughout the experiment and experiments
with which results will be combined or compared. Under these controls, it is proper
to writeGA=G◦A+RTln[A]forG◦Ain the specified standard state with a concentration of reactant A, with similar
expressions for B, C,.... The change in free energy for a reaction quotientQ,now
in terms of initial concentrations, isG=G◦+RTlnQ=G◦+RTln[C]ξ[D]ξ...
[A]ξ[B]ξ...Different stoichiometric coefficients are all expressed using the same symbolξfor
simplicity. It is understood thatQis a concentration quotient, which might fortuitously
be equal to the equilibrium constant but in general will not. In general,Gwill be
different from zero but when the reaction has arrived at equilibrium, the concentrations
will have arrived atKeqandGwill have arrived at zero so thatG◦=−RTlnKeqThus theformof the free energy relation to the equilibrium constant is reproduced but
only under rigorously controlled background conditions.^1 Change the ionic strength
or the pH, for example, and you can expect to find a differentG◦andKeq.
How canG◦change when we usually think of it as a sum of rock-firmG◦
values? By changing our background conditions, we have changed the standard state,
the benchmark to which we refer allG◦values.7.8.1 Making ATP, the Cell’s Power Supply
The difference betweenrGandG◦in the metabolic degradation of glucose,
a physiological energy source, to lactate and ATP can be broken down into two
individual steps (Hammes, 2007), one taking up 2 mol of ATP and producing 2 mol
of the diphosphate ADPGlucose+2ATP→2 ADP+2 glyceraldehyde-3-phosphate(^1) See Treptow (1996).