Gibbs–Donnan effect

[2] For example, the large anionic proteins in blood plasma are not permeable to capillary walls.

The effect is named after the American Josiah Willard Gibbs who proposed it in 1878 and the British chemist Frederick G. Donnan who studied it experimentally in 1911.

[3] The Donnan equilibrium is prominent in the triphasic model for articular cartilage proposed by Mow and Lai,[4] as well as in electrochemical fuel cells and dialysis.

The Donnan effect is tactic pressure attributable to cations (Na+ and K+) attached to dissolved plasma proteins.

Note that Sides 1 and 2 are no longer in osmotic equilibrium (i.e. the total osmolytes on each side are not the same) In vivo, ion balance does equilibriate at the proportions that would be predicted by the Gibbs–Donnan model, because the cell cannot tolerate the attendant large influx of water.

This is balanced by instating a functionally impermeant cation, Na+, extracellularly to counter the anionic protein.

Na+ does cross the membrane via leak channels (the permeability is approximately 1/10 that of K+, the most permeant ion) but, as per the pump-leak model, it is extruded by the Na+/K+-ATPase.

In many instances, from ultrafiltration of proteins to ion exchange chromatography, the pH of the buffer adjacent to the charged groups of the membrane is different from the pH of the rest of the buffer solution.

[6] When the charged groups are negative (basic), then they will attract protons so that the pH will be lower than the surrounding buffer.

When the charged groups are positive (acidic), then they will repel protons so that the pH will be higher than the surrounding buffer.

The Donnan effect may explain why some red blood cells do not have active sodium pumps; the effect relieves the osmotic pressure of plasma proteins, which is why sodium pumping is less important for maintaining the cell volume .

[7] Brain tissue swelling, known as cerebral oedema, results from brain injury and other traumatic head injuries that can increase intracranial pressure (ICP).

ATP pumps maintain a negative membrane potential even though negative charges leak across the membrane; this action establishes a chemical and electrical gradient.

The increased osmotic pressure forces water to flow into the cell and tissue swelling occurs.