osmosis

Osmosis

If two solutions of different concentration are separated by a semi permeable membrane , the solvent moves across the membrane from less less concentrated into the more concentrated solution. This process is termed osmosis. Water is the universal solvent in biological systems.

The total solute concentration of a solution is known as its osmolarity. It is indirectly a measure of the water concentration in the solution; the higher the osmolarity of a solution, the lower is the water concentration. An osmole is a gram molecular weight of osmotically active particles in 1000 gm of solvent. Osmolal concentration are some time expressed in milliosmoles.

Osmotic pressure: The pressure exerted by the osmotic process is called the osmotic pressure. It can be regarded as a form of suction drawing water from a weak to a strong solution. The osmotic pressure of the solution depends upon the number and size of particles in a solution, E.g. in NaCl in a salt solution of a given strength is in the form of a large number of tiny particles which exert a stronger osmotic pressure that would a solution of albumin of the same concentration, but containing a small number of large particles. Similarly, globulin, which has a much larger molecule than a albumin, would exert a corresponding lower osmotic pressure than would a solution of albumin of equal strength. At the end of the last century, Hamburger demonstrated that the living cells behaves like an osmotic system. It is surrounded by a semi permeable membrane highly permeable to water, and its cytoplasm contains many molecules which are unable to cross the plasma membrane. Osmosis, therefore, is of great importance in the activity of body.

When a cell is placed in a solution, water will pass in or out of it according to the osmotic pressure of the cell contents and the surrounding medium. From the biological viewpoint, solutions can be grouped into three classes:

1. Isotonic solutions, which have the same osmotic pressure as that of the cells. In an isotonic solution, the cell volume remains the same as when surrounded by body fluids.
2. Hypotonic solutions, which have a lower osmotic pressure than that of the cells, Thus, if a cell is placed in a hypotonic solution, it will gradually swell and finally burst, beacuse of the inward movement of water.
3. Hypertonic solutions, which exert higher osmotic pressures make more water, leave the cell than enters, and so the cell shrinks.

 It is found that the osmotic pressure of blood plasma is equivalent to the osmotic pressure exerted by a solution of 0.9% sodium chloride(NaCl), and this strength is taken as the isotonic level in physiological calculations. The red blood corpuscles, which are surrounded by a semi permeable membrane, are thus in balance with an 0.9% NaCl. If red corpuscles are placed in stronger solutions they will some of their contained water to the stronger solution outside and become wrinkled up. If they are placed in hypotonic sodium chloride solution, they swell up and burst. This is known as haemolysis.

Biological importance of osmosis:

1. In living cells water is distributed and redistributed across membranes by osmosis. Continuous osmotic exchange goes on between blood, tissue fluid, tissue cell and lymph.

2. Osmosis plays a significant role in the regulation of urine formation.