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.

7. Thank you