I am sorry for disturbing the group again, but Paul's comments are
very stimulating (at leat to me). Let me only answer him.
Paul Schlosser wrote:>>>>>>>>>>>
Without going into an extensive treatment, it seems that this analysis
ignores osmotic pressure, and assumes that the rate of A moving upward
is exactly equal to the rate of B moving downward. The rate of movement
will be proportional to the chemical activity difference across the two
compartments. Finally, if the concentration of A is greater (than
equilibrium) below, then some of that A will react to B in the lower
compartment.
In short, if you wrote down the set of differntial equations for this
system, accounting for changes in osmotic pressure and the effect of
that on chemical activities (not concentration), then you'd find that
the whole will approach equilibrium with time, independent of the
actual values of the rate constants.<<<<<<<<<<<<<<
You are right. So, in order to maintain the ATP system far from equilibrium,
and if the chemiosmotic mechanism is used for this purpose, the energy
required per mole is -(delta G), where delta G refers to ATP hydrolysis.
This is universally accepted.
However this is only the "restoring" part of the cycle that the ATP system
undergoes. The "working" part is that in which ATP transfers energy to
another system, e.g. drives the reaction B -> C (which could be some
biopolymer synthesis):
ATP + B -> ADP + P + C /1/
Thermodynamics says almost nothing about the work done by the ATP system
in the non-equilibrium reaction /1/, but what could be rigorously proved
is that this work is independent of the concentrations, e.g. of the
ATP concentration.
On the other hand, the delta G work does depend on the concentrations and
approaches zero as their values approach the equilibrium ones. Therefore
a cycle is possible in which the ATP system does some concentration-
independent work Wi in /1/, then its initial (non-equilibrium) state is
restored by a chemiosmotic mechanism and concentration-dependent (delta G)
work Wd is spent, and the final balance is Wi > Wd.
It seems to me that Wi - Wd is an extremely important quantity - it
describes the net work done by the ATP system in biosynthetic processes
for a given STEADY STATE of the ATP system. So I decided to call it
"thermodynamic potential" and to offer it for discussion in this group
(I do not see a more suitable one), but.......
Best regards,
Pentcho
