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Chapter: Biochemistry: Cell Membrane

Osmosis and Biological Significance

If a protein solution is separated by a semipermeable membrane from pure water, water tends to flow from the latter to the former.


If a protein solution is separated by a semipermeable membrane from pure water, water tends to flow from the latter to the former. The property of the movement of solvent particles is called as osmosis. Osmosis is the net diffusion of water from the dilute solution to the concentrated solution. Osmosis is a colligative property of solution that depends on the number of molecules or ions of the solute in the solutions. Osmol units give the number of osmotically active particles per mole of a solute. Each mole of a non-ionized solute is equivalent to 1 osmol. Osmolarity of a solution is its solute concentration in osmols / litre. Osmolality of a solution is its solute concentrations in osmols/kg of the solvent.

Two solutions with identical osmotic pressures are called as isoosmotic solutions. A solution having lower or higher osmotic pressure with respect to the other is called as hypo-osmotic or hyperosmotic solutions respectively.

The plasma membrane is a semipermeable membrane and it allows only certain solutes to diffuse. The osmotic pressure exhibited by these impermeable solutes is called as the tonicity of the solution. Tonicity is an important physiological parameter.

Two solutions with identical tonicities are called as isotonic solutions. A solution having lower or higher tonicities with respect to the other is called as hypotonic or hypertonic solutions respectively.


Biological Significance


·              Hemolysis and Crenation. The physiological or isotonic saline is 0.9% NaCl. When red blood cells are suspended in 0.3% NaCl (hypotonic solution), water will enter into the cells and the cell will burst releasing all its contents. This kind of lysis is called as hemolysis. The resulting membranes are called as ghosts. On the other hand, when the cells are placed in 1.5% NaCl, water comes out of the cell, leading to shrinkage of cells. The process is called as crenation.

·              The erythrocyte fragility test is based upon the osmotic diffusion property. The ability of the membrane to withstand hypotonic solution depends upon the integrity of the membrane. Certain genetic disorders like sickle cell anemia and deficiency of vitamin E makes the erythrocyte membrane more fragile.

·              Osmotic pressure of blood is largely due to its mineral ions such as sodium, potassium, chloride, calcium and protein. The osmotic pressure exerted by proteins is of considerable biological significance owing to the impermeability of the plasma membrane to the colloidal particles.

·              Absorption of water in the intestine is due to osmosis. Formation of urine in the kidneys may be attributed to osmotic pressure. The net difference in the hydrostatic pressure and osmotic pressure is responsible for the filtration of water at the arterial end of the capillary and the reabsorption of the same at the venous end. At the arterial end, the hydrostatic pressure is 22 mmHg and the osmotic pressure is 15 mm Hg. The pressure to drive out the fluid is 7 mm Hg.


·              At the venous end, the hydrostatic pressure is 15 mm Hg and osmotic pressure is 7 mm Hg. The net absorption pressure to draw water back into the capillaries is 15 – 7 = 8 mm Hg. This is called as Starling's hypothesis.


·              The renal excretion of water is regulated partly by the osmotic pressure exerted by the colloids in the blood plasma. Increased urination (polyuria) occurring in diabetes patients is due to the increased water retention by the urinary glucose.


·              Donnan Membrane Equilibrium


Let us consider two compartments separated by a semi permeable membrane, which is permeable to water and crystalloids, but not to colloidal particles. One of the compartment (A) is filled with a moles of NaCl, and the other compartment (B) is filled with b moles of NaR, in which R happens to be a non diffusible ion.



NaCl diffuses from (A) to (B) and after some time, the system attains equilibrium. At equilibrium, let us consider that x moles of NaCl have diffused from (A) to (B). So,  the ionic concentration at equilibrium in both the compartments will be as follows,



At equilibrium, the number of ions that move from one compartment to other will be equal, and this will occur only, when the ionic products of the concerned ions are equal.

Therefore, [Na+][Cl-] in both the compartments at equilibrium should be equal.



On substituting numerical values for a and b as 2 and 1 moles respectively,

 Calculating the total moles present in compartment (A) and (B) at equilibrium.

From this we can derive that:


·              The concentration of solutes in the non-diffusible ion side (B) is greater than the other.


·              There will be accumulation of the oppositely charged ion (Na+) in the side containing the non-diffusible ion (R-).


In biological systems, Donnan membrane equilibrium prevails due to the non-diffusible proteins and is also significant for the functional aspects of the cell.

If the non-diffusible ion happens to be R- and one of the diffusible ion H+, then there will be a change in the pH. Due to imbalance in the electrolytes, swelling of proteins occur, which is called as Donnan osmotic effect.


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