Donnan potential - how far from the membrane? (Pentcho Valev)

William Tivol tivol at
Wed Oct 15 16:21:11 EST 1997

Pentcho Valev wrote:

> Please consider the following EQUILIBRIUM system:
>                              low pH
>membrane-permeable-to-H+ only // impermeable-membrane-------------X-------
>                              high pH
>At equilibrium, a capacitor-like double layer is established where the
>membrane is permeable to H+ and, accordingly, a transmembrane electrical
>potential opposes the H+ concentration gradient. This is a common and
>universally accepted picture. The essential problem is: Does this
>electrical potential reach the point X situated on the impermeable
>membrane, far enough from the permeable one?

	That may well depend on the nature of the membrane.  If the mem-
brane is permeable *only* to H+, then there will be an excess of positive
ions on the low-pH side and an excess of anions on the high-pH side.  These
can induce an electrical double layer, which, *at equilibrium*, can result
in uniform [H+] and electrical potential on each side of the membrane.
(Of course, these []'s and potentials will be different for the 2 sides.)
	If, however, the impermeable part of the membrane is composed of 
neutral, non-ionizable lipids, there could be both [] & potential gradients.

> In other words, is there, at
>the point X, a transmembrane electrical force which would oppose eventual
>movement of H+ down the concentration gradient? If not (as I believe),
>an ATPsynthase placed at the point X could use this movement of H+ down
>the concentration gradient for ATP synthesis. As the "working" system
>is at equilibrium, this would be a violation of the second law.

	In any event, the 2nd law predicts that the chemical potential
for H+ should be the same throughout the system, so the combination of
potential & [] gradients at X would result in the ATP synthase not pro-
ducing ATP spontaniously.  I realize that it is just this prediction
which you are calling into question, and the only real test is to do
the experiment.  I have to believe, however, that some life-form would
have evolved this "free lunch" system in the last 3 billion years if
ATP could be synthesized this way.
				Bill Tivol

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