Guy C Brown wrote:
> Personaly I do not think that "work" is a useful concept in
> biochemical thermodynamics: does a reaction really do "work"? I do not
> see how the concept can be used to tell us anything useful or
> important about an ATP driven reaction or any reaction.
Marc Roussel wrote:
Obviously, this last statement might mean different things to different<
people, but I disagree with it. ATP is used to perform or drive a <
number of tasks that we would recognize as work: Muscle movement and <
nervous function are obvious examples. On the other hand, if we're <
talking about the use of ATP for biosynthesis, I agree that the concept<
of work is applied with greater difficulty there. <
Yes, but we could have an oversimplified but still quite useful presentation
of work done by ATP, relevant to all cases. If work is always "force times
distance", we can imagine that a force originating in the ATP molecule "pushes"
something belonging to some other system. If this something is missing, the
"push" is entirely dissipated as heat, i.e. this is the enthalpy of hydrolysis
of ATP. However even when work is done, the "push" is not entirely efficient -
part of it is still dissipated as heat. This last heat is the enthalpy of the
coupled reaction, i.e. we have
Wu = -delta H(ATP hydrolysis) + delta H(coupled reaction) /1/
Of course, the process cannot be so pure - the energy received by the second
system may not come solely from the ATP "push" - there may be other sources,
e.g. hydration or dehydration mechanisms. It is at this stage that the
concept of work becomes vague, but we can again clarify it by the following
definition: The work done in the coupled reaction is defined as the energy
the second system receives - this leads to
Wu = delta H(2nd system) = delta H(coupled reaction) - delta H(ATP hydrolysis)
/2/
In my opinion, this is the only natural definition of chemical work.
Pentcho
