DONNAN EQUILIBRIUM-1.pptx

biochemistry an easy subject

1. DONNAN EQUILIBRIUM

2. When a saline solution & distilled water(purified water) are separated by a permeable membrane final equilibrium will be reached when the concentration of salt on both sides will be same. But if there be any non-diffusible ion on one side, a different phenomenon will be seen. Suppose NaP is an ionisable compound of which P is non-diffusible . If this compound is kept on one side & Nacl on the other, Na & Cl ions will freely pass but not P. when the final equilibrium will be reached, it will be seen that the product of Na & Cl on one side is equal to the product of same two ions on the other side.for.e.g

3. INITIAL STATE FINAL STATE Na;Na Na; Na P; Cl CL ; CL ; P; In the final state Naxcl (left)= Naxcl (right). It is obvious that total Na of the left side is greater than total Na of the right side & the total chloride of the right side is greater than total chloride of the left side. (Similarly, if NaP be on the left side & water on the right side, in the final state the reaction on the right side will be alkaline due to Na & OH ions. If a compound Cl -P be on the left & H20 on the right , in the final state the reaction on the right side will be acid due to H & Cl - ions.) This type of equilibrium in which the products of the same pair of ions on two sides of the permeable membrane become same, is called Donnan equilibrium.

4. It is obvious that in the final state there will be a great difference in the nature & quantity of diffusible ions on two sides of the membrane which will lead to difference of electric potential & chemical reaction on the two sides. Since, in our body there are many compounds of the NaP type, Donnan equilibrium is of great importance. Physiological Importance ; It explains how difference of electric potential can be established on two sides of a membrane, how stomach can secrete a strongly acid juice & pancreas can secrete an alkaline juice.The phenomenon of chloride shift can also be explained from this standpoint.

5. Correlation with Biochemistry When membrane is freely permeable to ions (Na+, Cl -) & if the concentration of ions on both the sides is different, the ions freely diffuse to attain equal concentration. Gibbs- Donnan observed that the presence of a non-diffusible ion (usually a protein) on one side of the membrane alters the diffusion of diffusible ions.

6. Application of Donnan membrane equilibrium 1] Difference in the ionic concentrations of biological fluids ; The lymph & interstitial fluids have lower concentration of inorganic cations (Na+, K+) & higher concentration of anions ( cl -) compared to plasma. This is attributed to the higher protein (Pr-) content in the plasma. 2 ] Lower pH in RBC ; The hemoglobin of RBC is negatively charged & therefore, causes the accumulation of positively charged ions including H+. Therefore, the pH of RBC is slightly lower (7.25) than that of plasma (7.4). 3 ] Osmotic imbalance ; Donnan membrane equilibrium- which results in the differential distribution of ions in different compartments of the body- partly explains the osmotic pressure differences.

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