Richard Kondo wrote:>>>>>>>>>>>>>>>
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.<<<<<<<<<
Thank you, Richard. My claim (as well as your agreement) has rather serious
consequences - it implies the possibility of ATP synthesis at equilibrium
and, in general, leads to an alternative bioenergetics. Another set of
interesting implications stem from the fact that biomembranes are
heterogeneous with respect to their capacitance - large hydrophilic
structures such as ATPsynthases can be expected to have an enormous
capacitance so the potential difference across them is probably close to
zero. The implications are extremely interesting, but perhaps only to me -
my several attempts to discuss them brought me only accusations of
ignorance.
Still I hope that the discussion on action potential and its influence on
mitochondria has shown how subtle the problem is and how wrong firmly
established beliefs could be. So, although I do not expect any constructive
development on this forum, I very much hope that scientists will find time
at the coming Bioenergetics and BTK meetings to revise the conventional
paradigm of energized membrane - the potential is simply not uniform on each
side of the membrane, and the sooner the conclusions are drawn, the better.
Best regards,
Pentcho
