The Gibbs- Donnan equilibrium.

Gibbs-Donnan membrane Equilibrium- relevance in Cell Physiology.
The Gibbs-Donnan effect describes the unequal distribution of permeant charged ions on either side of a semipermeable membrane which occurs in the presence of impermeant charged ions.
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1. The Gibbs-Donnan effect describes the unequal distribution of permeant charged ions on either
side of a semipermeable membrane which occurs in the presence of impermeant charged ions.
At Gibbs-Donnan equilibrium-
1. On each side of the membrane, each solution will be electrically neutral.
2. The product of diffusible ions on one side of the membrane will be equal to the product
of diffusible ions on the other side of the membrane.
3. The electrochemical gradients produced by unequal distribution of charged ions
produces a transmembrane potential difference which can be calculated using the
Nernst equation.

4. The presence of impermeant ions on one side of the membrane creates an osmotic
diffusion gradient attracting water into that compartment.
Gibbs-Donnan effect:
➢ The Donnan equilibrium is a completely passive process: i.e. no active transporters are
involved in maintaining this equilibrium.
➢ A Donnan equilibrium is an equilibrium, i.e. ion concentrations on either side of the
barrier are static.

➢ At a Donnan equilibrium, the resting membrane potential would be only about -20 mV.
This potential would exist even if the membrane permeability for all ions was the same.
➢ The resting membrane potential, in contrast, requires different permeabilities for
potassium and for sodium, and is maintained actively by constant Na+/K+ ATPase
activity.

2. Gibbs Donnan potential.
(A) Gibbs Donnan experiment.
Diagram depicts the experimental
arrangement for obtaining G-D
potential. A membrane freely
permeable to all small ions, but
impermeable to the large protein
molecules, is used to separate two
solutions, only one of which (side 1)
contains protein. Side containing the
protein becomes negative, with
respect to the other side, by a small
voltage (e18 mV in example). This
membrane potential (Em) does not
depend on active ion transport or on
selective permeability properties of
the membrane, as normal cell RP
does. The diffusible ions (Na and Cl
in example), however, become
unequally distributed across the membrane and it is their diffusion potentials (ENa ¼ ECl) that
produce the G-D potential. (B) Equivalent circuit for experiment depicted in panel A,
demonstrating that Em ¼ ENa ¼ ECl. The Naþ and Cl batteries are of equal magnitude and of
the same sign. Therefore, the relative conductance of the membrane to Naþ and Cl, whether
equal or not, are irrelevant to the potential (gCl and gNa are conductance for Cl and Naþ,
respectively).
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