Pentcho Valev wrote:
>>> 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.
I agree that the electric field is zero, and the potential is the work integral;
therefore, can be varied by an arbitrary constant.
> 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:
With microelectrodes, we are measuring voltage differences not voltageitself
(which can be varied arbitrarily). I would agree with your contention
that in the absence of permeability, there is no potential difference.
> 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
