Gibbs-Donnan relationship

In the presence of a non-diffusible ion (e.g. protein),

--> The diffusible ions (e.g. K+ and Cl-) will redistribute such that...

--> Ratio of [K+] across the membrane = ratio of [Cl-] across the membrane

At such equilibrium
--> There is unequal distribution of diffusible ions and electric charge

Thus,

1. The osmosis of water is balanced out by Na-K ATPase pump
2. The electric charge is determined by Nernst equation

Double Donnan effect

[KB2:p7]

Portein is present in higher level intracellularly
* Non-diffusible anion
--> Gibbs-Donnan equilibrium
--> Unequal distribution of diffusible ions and electric charge
--> Water move into cells

Sodium is the main extracellular cation, due to
* Low membrane permeability
* Active extrusion by Na-K pump
--> second Gibbs-Donnan equilibrium
--> Water moves out of cells

In equilibrium, the two effects balance out and cell volume remains stable

If Na-K pump is inhibited
--> Cell will swell and rupture due to the first Gibbs-Donnan equilibrium