Living organisms and thermodynamics (Pentcho Valev)

Wed Oct 22 03:27:14 EST 1997

In a private message  Bryant Fujimoto presented the thermodynamic
problem much better than I did, so I am taking the liberty to ask the
group to comment on his version. As one can see, the final solution is
almost reached.

Bryant Fujimoto wrote:
The problem is that you have forgotten to include the electrodes as       <
part of your system, and maintain electroneutrality.                      <

Hg/K(electrode)|  10 mM KCl  |membrane|  1 mM KCl  |Hg/K(electrode)       <

(I believe you said somewhere that you were using a potassium amalgam     <
electrode,  in any event, these are supposed to be your K electrodes.)    <
Consider the following process:                                           <

1. a potassium atom at the right electrode is oxidized, releasing a       <
K+ ion to the solution on the right and an electron in the electrode.     <
2. The electron flows to the left electrode where it reduces a K+, thus   <
consuming a K+ from the left solution.                                    <

So far, it looks like we've done something, one K+ ion disappears on the  <
left and one K+ appears on the right, which looks like we transfered a K+ <
ion from the left solution the right solution. However, we've also made a <
net change in the charge in the two solutions, increasing the charge in   <
the right by one and decreasing the charge in the left by one.  In order  <
to maintain electro-neutrality, you need                                  <

3. One K+ ion flows across the membrane from right to left.               <

Net final result, nothing.  Therefore, you do not expect to find a        <
potential using K electrodes.  There is no mysterious force, when K       <
electrodes are used, the system starts at equilbrium.  Its different with <
different electrodes.                                                     <

I would only add that the final result is not nothing. A current has passed
from one electrode to the other, and this current CAN DO WORK. Yes the system
starts at equilibrium, the same equilibrium is reached at the end, but
meanwhile work has been done. This is possible because the electrodes are
distant from the membrane and the two processes - generating the current and
restoring electroneutrality - are separated IN TIME. If the electrodes are
close to the membrane, the two processes are not separated in time and this
is expressed experimentally as zero voltage.
   The first part of the process is in fact work production driven by
translocation of a cation from a high to a low concentration. This is fully
analogous to what in my initial scheme was ATP synthesis driven by the
pH gradient. I think the theoretical cycle is closed now - we have the
bioenergetic model plus a simple analogous electrochemical system which is
very easy to verify experimentally. I hope the group will appreciate this.

Best regards,
Pentcho Valev

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