Donnan Effect

How charged proteins affect the ion concentrations in solutions separated by a semipermeable membrane is a question I had since high school days.

The main thing that you need to understand is there are only two ways for things to be transported passively in biological systems. The molecule of interest which can either be a charged particle like an ion or something that does not have a charge like a glucose molecule has to either follow a chemical or an electrical gradient. Electrical gradients only apply to charged particle. Donnan effect is all about the equilibrium between the Electrical and Chemical gradients in a system hence it only applies to ions.

Imagine a system that has three types of ions, Cl^-, Na⁺ and some charged protein, let's call it prot^-.

Now there is a membrane that divides the system which is only permeable to Chloride and Sodium ions. Prote^- are trapped in one side. This effects the net outflow of Sodium ions from that compartment because it is attracted to Prote^-. Therefore, when the system is in equilibrium.

[Na⁺]^{compartment with protein-} > [Na⁺]^{compartment without protein-}

and

[Na⁺]^x + [Cl^-]^x + [Prot^-] > [Na⁺]^y + [Cl^-]^y

This also brings up a really interesting connection that looks like this

[Na⁺]^x / [Na⁺]^y = [Cl^-]^x / [Cl^-]^y

but this is not what happens in cells. Cells have charged proteins in their plasma therefore the ion concentration ( according to Donnan ) has to be always higher than the surrounding extracellular fluid and the cells should rupture due to them being hypertonic but this is not the case. Cell plasma membrane has pumps called Na, K ATPase that actively (which means it uses energy) to pump ions back out of the cells.