Richard Kondo wrote:>>>>>>>>>>>>>>>>>>
I am afraid that I am not following your line. Could you state it again?
Since I view the membrane potential as the consequence of the separation of
charges across a 'flat' capacitor, the potential is zero everywhere because the
electric field is zero outside the capacitor. In reality, the assumptions used
to calculate the field generated by an infinite flat capacitor are only
approximated
by a membrane. So that there is some 'measurable' falloff of charge density and
potential.<<<<<<<<<<<<<<<<<<<<
Yes my line may be confused - I have tried to discuss this problem at least
10 times in this group and in the biophysics group and at least 100 times
privately. The objections I have received are so absurd and contradictory
that confusion in my own line of argument is unavoidable.
Let us start with a simple model:
10 mM KCl M 1 mM KCl
where M is a membrane permeable only to K+. At equilibrium, a capacitor-like
double layer, positive on the right and negative on the left, is established
at the membrane, and the transmembrane potential difference is about 60 mV.
Also, both the charge density and the field quickly get zero away from the
membrane, in both directions.
So far everybody (except for Guy Brown and, if I am not mistaken, Lukacz
Salwinski) agree. However the crucial problem is: What is the potential
difference between points symmetrical with respect to but distant from the
membrane?
The following evidence suggests that it is the same - 60 mV:
1. Measurements show 60 mV. If, for instance, the electrodes are reversible
for K+, then the measured voltage would be zero.
2. The infinite capacitor analogy. Unlike your interpretation above, experts
say that, as the field outside an infinite capacitor is zero, the potential
does not change, i.e. remains the same as in the plate - different from zero.
3. Second law prediction. According to the second law, the K+ electrochemical
potential must be uniform throughout the system. As the K+ concentration
differ in the two bulk phases, this is compensated by 60 mV potential
difference.
This seems to be a strong evidence, and yet it is successfully (in my
opinion) counteracted by a single argument. Potential at a point is defined,
most basically, as the work needed to move a positive test charge from
infinity to the point. According to this definition, the potential at a
wire is zero, althout the voltage of the wire may be great in the other
sense. The analogy is straightforward: the work needed to move a positive
test charge from infinity to a point far from the membrane is zero, although
a current is produced in the electrode and a non-zero potential is measured.
Just two different interpretations of "potential", but the concequences of
this difference are incredible. The measured potential is galvanic-reaction-
dependent, so whenever there is no galvanic reaction involved, the potential
should be regarded as zero. An example:
pH 2
membrane-permeable-only-to-H+//////impermeable-membrane----ATPsynthase------
pH 7
At equilibrium, will there be a transmembrane potential difference at the
impermeable sector, where the ATPsynthase is? YES if it is measured, but
NO for H+ ions - if the enzyme is initially "closed" and if it suddenly
starts letting H+ to pass, some portion of H+ will cross the membrane driven
by the pH gradient alone, until the created electrical gradient stops it.
I suspect that the first bioenergetic mechanisms worked similarly.
Best regards,
Pentcho
