Pentcho wrote:
> You are right that the two different concepts of work can be used
>independently, but only insofar as "work" in a standard chemical reaction
>is something not very clear. However I do not believe that the idea of W.
>Gibbs was something like "The negative of delta G will be named work, but this
>work should not be juxtaposed with any other work". Rather, delta G was
>originally meant to be traditional work, but just because it is incompatible
>with the physical definition "force times distance", people have been trying
>for many years to forget that any work is first "force times distance" and then
>anything else. Moreover, there are cases in which the work is the same in any
>sense, and these cases show the seriousness of the problem. Let me describe
>one such case.
I do not think that delta G was ever meant to be traditional work, only it could
predict the maximal amount of work extractable from a system in certain
circumstances.
> An ATPase transports H+ against a transmembrane electrical gradient, but
>(for simplicity) in the absence of a transmembrane pH gradient. You can use
>both concepts of work done by the ATP-ADP-Pi system, but here we will have to
>chose one and reject the other, since they provide contradictory predictions.
>I have dealt with the case in the bio-info group (the title is "The Second
>Law Against the First Law"), so let me state only the two options:
> 1. If the reaction is close to equilibrium and ATP >> ADP = Pi,
>free energy calculations predict that the reaction is STRONGLY ENDOTHERMIC.
> 2. Under the same conditions the enthalpy concept says that the reaction
>is EXOTHERMIC - part of the energy stored in ATP is converted into electrical
>potential, but as the process cannot be 100% efficient, the other part is
>dissipated as heat.
> As you can see, the problem has quite concrete dimensions. Let us solve it.
>The "enthaply concept" does not predict that the net reaction will be
exothermic,
this depends on how much electrical work is done. A reaction can be more than
100% efficient if we define efficiency as delta H (driven reaction)/-delta
H (driving
reaction), indeed it can be infinite. Of course it can not be greater than 100%
efficient if we define efficiency as delta G (driven reaction)/-delta G
(driving reaction).
We can see this more clearly if the driving reaction has no enthalpy
change, e.g.
an isomerisation reaction A > B where A and B have the same internal energy
(and the same standard molar entropy).
If this reaction is coupled to the pumping of an ion against an electrical
gradient,
then if [A] > [B], ions can be pumped up hill doing work. The enthaply
efficiency
of this net reaction is infinite as the delta H of the driving reaction is
zero, but
work is done on the driven reaction (transport of the ion). The reaction
is driven
forward by the concentration difference of A to B.
Dr Guy C. Brown
Department of Biochemistry,
University of Cambridge,
Tennis Court Road,
Cambridge CB2 1QW,
UK
Tel: +44 (0)1223 333342
Fax: +44 (0) 1223 333345
