Experiment to demonstrate Osmosis

Experiment: To demonstrate the phenomenon of osmosis through plant membrane with the help of potato osmoscope.

Requirements: A large potato tuber, 10 Equation sugar solution, beaker, water scalpel, pin.

Method: Take a large sized potato. Remove its skin and cut its base to make it flat. Make a hollow cavity in the centre with the help of a scalpel leaving a thin wall at the base. Fill this cavity with a strong salt or sugar solution. Mark the initial level of the solution with the help of a pin. Hang this tuber within a beaker filled with coloured water. After a few hours the solution within the cavity is found to be at a higher level.

Observation and Conclusion: After some time the level of liquid rises in the osmoscope. This experiment shows the phenomenon of osmosis.

Explanation: The living cells of potato tuber collectively act as differentially permeable membrane (membrane which permits movement of certain molecules only through it). The movement of water will not occur if the skin of potato is not removed because the skin acts as impermeable layer.

Difference between Diffusion and Osmosis

Table showing difference between Diffusion and Osmosis.



Diffusion is a movement of a given substance from the place of its higher concentration to an area of its lesser concentration. Presence of semipermeable membrane is not required.

Osmosis is a special type of diffusion of solvent molecules such as water from lower concentration of solution when the two are separated by a semi permeable membrane.

The diffusion may occur in any medium. The moving particles may be solid, liquid or gas.

It occurs in liquid medium and only the solvent molecules such as water move from one place to another.

It is a physical process.

It is a physiological process.

Diffusion ceases when entire system becomes homogenous.

Osmosis ceases by generation of osmotic pressure.

Semipermeable membrane involvement is not essential.

Semipermeable membrane involvement is essential.

If you place a cell in a solution, it may shrink, swell or remain unchanged on the basis of relative concentration of water and solutes with respect to their concentration in the cell:

If a solution in which a cell is placed has osmotic pressure equal to that of the cell sap, the outer solution is called isotonic solution. Cell remains stable in isotonic solution or there is no entry or exit of water from the cell.

If the osmotic pressure of outer solution is more than that of the cell sap, the outer solution is known as hypertonic solution. Water from cell moves out so, the protoplasm of the cell shrinks and collects in the centre of the cell.

In case osmotic pressure of the outer solution is less than that of the cell sap, the outer solution is less than that of the cell sap; the outer solution is called hypotonic solution.

Osmotic Pressure and Osmotic Potential

Osmotic Pressure: may be defined as the actual pressure which develops in solution when it is separated from pure water by means of a semipermeable membrane. Osmotic pressure is measured as the minimum force required to prevent the osmotic entry of water into a system, when it is separated by a semipermeable membrane.

Osmotic Potential: it is observed that the free energy of water molecules or solvent changes on adding a solute to the solvent. This change in the free energy of water molecules is called the osmotic potential of the solution.


Imbibition is the process of adsorption of water by the particles of the solid substances without forming a solution. The substance that imbibes water is called imbibant or adsorbent and the liquid which is imbibed is called imbibate. Substances like proteins and cellulose are imbibants. Imbibition produces a large pressure, so much so that dry wood can even break a piece of rock in the presence of water. Because of imbibition, the wooden doors, during rainy season, swell up and it becomes difficult to close the door. During imbibition, water molecules get tightly held between or over the surface of imbibant molecules through the process of capillarity or by the force of adsorption and immobilized.

Importance of Imbibition:

Imbibition is the initial step in the germination of seeds and It causes swelling of seeds and breaking of seed coat.

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