General Veterinary Biochemistry

DONNAN'S MEMBRANE EQUILIBRIUM

• Semi permeable membrane: A membrane that permits the passage of some solutes usually small molecules and solvent but not colloids.
• When two different concentrations of electrolyte solutions are separated from each other by a semipermeable membrane, if a non-diffusible ion is present in one of the sides, then there is an unequal distribution of the diffusible ions between the compartments.

Consider the following experiment

• The two compartments A and B are separated by a semi permeable membrane, which is freely permeable to K⁺ and Cl^- and impermeable to protein.
• Assume that at the start of the experiment the concentrations of the anion and cation on the two sides are equal.

A	B
K+ Cl- Protein	K+ Cl-

• Due to Donnan effect, the –ve charge of the non-diffusible anion hinders the diffusion of cation from ‘A’ side and favours the diffusion of anion from ‘B’ side. Hence, Cl^- diffuses from ‘B’ side to ‘A’ side down its concentration gradient along with some K⁺ due to its opposite charge.
• At equilibrium more osmotically active particles are present on ‘A’ side than ‘B’ side. Therefore the osmotic pressure on the ‘A’ side is greater than ‘B’ side (there is more ions on the ‘A’ side) because osmotic pressure depends on number of ions.

Donnan’s equillibrium has the following effects in the body

• Proteins are present inside the cells. Due to Donnan effect more osmotically active particles are present in cells than in interstitial fluid. Na⁺-K⁺ pump counter balances the effect.
• Due to differences in the concentration of diffusible ions, an electrical difference is formed across the membrane (side ‘A’ is more –ve than side’B’), which is balanced by opposite charges on the membrane.
• There are more proteins in the blood, due to which ionic movement occurs across the capillary walls.