Relative Osmotic Pressure - QS Study
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Relative Osmotic Pressure

In 1884 De Vries, a botanist developed an approximate method for comparing the osmotic pressures of solutions by using plant cells. The primary plant cell generally has a relatively rigid cellulosic wall which is permeable to water only. The cellulosic wall is not normally permeable to the solutes in the cell sap (liquid). If such a cell is placed in a solution of osmotic pressure lower than the solution in the sap, there will be no change in

the cell shape, because water from the more dilute solution outside the cell will not be able to cause any deformation due to the rigid cell wall. But if such a cell is immersed in a solution of higher osmotic pressure, water will pass out through the cell and consequently there will be partial collapse of the cell membrane causing a change in the shape of cytoplasm (Figure).

Figure: (a) Normal plant cell, (b) Plasmolyzed plant cell

 The cell is then said to be plasmolyzed and the phenomenon is known as plasmolysis. The osmotic pressure of cell saps may be determined by immersing the cell in a series of solutions of varying concentrations and observing the cell under the microscope. The concentration of the solution which just causes shrinkage of the membrane is noted. The osmotic pressure of this solution is appropriately equal to the osmotic pressure of the sap solution. (Solutions having the same osmotic pressure are said to be isotonic or iso-osmotic). Consequently, plasmolysis may be used for comparing osmotic pressure of solutions.

Hamburger in 1890 suggested that red blood corpuscles (the cell wall is permeable to water) may be used conveniently for comparing osmotic pressures of solutions. If a red blood corpuscle is immersed in a solution of lower osmotic pressure, water will pass into the cell, make it swell and finally cause it to burst – a phenomenon known as haemolysis.

On the other hand, if the red blood corpuscle is put in a solution of higher osmotic pressure, water will flow out of the cell and it will shrink. Blood plasma contains 0.85% of NaCl in addition to trace quantities of other salts. If a stronger solution of NaCl is injected in large dose the red blood corpuscles will shrink, whereas if only distilled water is injected in massive dose the red blood corpuscles will first swell and finally burst causing fatal effects. Saline injection is a solution of 0.85 % NaCl in distilled water. This solution is isotonic with the contents of the red blood corpuscles. Balance of osmotic pressure in living beings is of extreme importance. The osmotic pressure is vital importance to life.