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The Donnan equilibrium effect can be correlated to living cells. Cell membranes are selectively permeable, which means that they allow some molecules to pass through while keeping others out.
The flow of molecules and ions between a cell and its environment is regulated by the Donnan effect. Living cells contain impermeable anionic colloids, which are mostly made up of proteins and organic phosphates; and these colloidal anions cannot cross the cell membrane.
As a result of this, there is a high concentration of non-diffusible anions across the cell membrane, thus creating the Donnan Equilibrium. This means that there are more ions inside the cell than outside. For ease of explanation let's call this Donnan Equilibrium 1.
What does this do to cells? Water will continuously move into the cell by the process of osmosis. If this process continues, the cells will inevitably rupture.
What is the mechanism that prevents cells from swelling and rupturing?The answer is the sodium pump (Na⁺- K⁺ ATPase) in the cell membrane. It is the most ubiquitous system in animal cells. The presence of the ATP-driven Na⁺ and K⁺ pump is nature's way of preventing cells from rupturing by continuously pushing out excess ions.The pump together with the membrane's low permeability to sodium effectively prevents sodium from entering the cell. The sodium pump renders the membrane impermeable to sodium, setting up a second Donnan Equilibrium (let's call this Donnan Equilibrium 2).
The earlier Donnan effect (Donnan Equilibrium 1) with respect to impermeable anionic colloids balances the latter Donnan effect (Donnan Equilibrium 2) of impermeable extracellular sodium. The balancing act between these two effects is by way of allowing cells to maintain and regulate normal cell volume in living functions.
Hope this answers the question.
