What is the significance of Osmosis ?

Osmosis (Greek word for ‘push’) is the movement of water between two solutions separated by a membrane which is permeable to water and less permeable or impermeable to the substance (solute) in solution.

When a diluted solution and a concentrated solution are separated by such a membrane, there is a net transfer of the solvent from the diluted solution to the concentrated one. Entry of water into root hairs and movement of water within the plant body are good examples of osmosis.

There is a slight difference between osmosis and simple diffusion. In osmosis, the movement of water is more rapid. Osmosis involves a bulk flow of water through pores in the semi-permeable membrane. Osmosis is associated with living as well as nonliving membranes.

Significance of Osmosis

1. Entry of water in to the roots from the soil takes place by this process.
2. Cell to cell diffusion of water is controlled through this process.
3. Young cells require turgid condition for their growth which is fulfilled by osmosis.
4. Turgidity of cells is maintained by the process of osmosis.

Experimental to demonstrate osmosis

Take a wide-mouthed small thistle funnel. Tie its mouth with a natural membrane taken from the bladder of pig, sheep or goat which acts as a semi-permeable membrane. Fill the partially with a concentrated sugar solution giving a drop of safranine stain. Immerse the bulb portion of the funnel in a beaker containing water. Mark the level of sugar solution on the stem of the funnel. After sometime observe the level of the solution in the stem of the funnel. You will note a rise in the level which is due to movement of water in the funnel from the beaker.

Explanation: In the above experiment the sugar molecules are larger than the water molecules. The natural membrane is semi-permeable containing tiny holes which allow small water molecules to pass through but prevent the sugar molecules to enter. The water molecules from the beaker enter the funnel because the concentration of water molecules inside the funnel is less than that in the beaker.

A pressure is developed inside the funnel due to the inward movement of water. It is known as osmotic pressure. Osmotic pressure is the pressure developed when the solution is separated from the put solvent by a semi-permeable membrane.

Experiment to demonstrate osmosis by potato osmometer

Take a big potato and out a flat slice at once and so that the potato can stand at this end. With the help of a knife make a deep groove on the other side. Partially till this groove with concentrated solution of sugar and mark its level. Now put this potato in a beaker having enough water to partially submerge the potato. You will observe a rise in the level of sugar solution in the groove of the potato. This is due to osmosis of water from the beaker.

Osmosis and Stomatal Activity

You know that the stomata are the pores found on leaves mainly on their lower side, and these are responsible for transpiration. The process of osmosis actually regulates the opening and closing of stomata. The pores (stomata) are surrounded by two guard cells. Due to internal osmotic pressure, the guard cells can reduce the size of the stoma and close it completely. Guard cells bulge out, when full of water, and the stomatal pore is wide open. Water and light intensity are the two important factors which control this opening and closing of stomata. Through these pores, exchange of respiratory gases (oxygen and carbon dioxide) takes place.

The concentration of the cell sap (cytoplasm) of the guard cells increases during the daytime because of photosynthetic activity. Photosynthesis leads to the accumulation of sugars. Under such circumstances, water starts moving from the outside to the inside of guard cells, causing them to distend or swell. The firm cell wall keeps the cell from expanding indefinitely. In this way, a pressure is produced within the cell by the osmotic entry of water, and this is called turgor. In the guard cell, the cell walls towards the stoma, is thickened and, therefore, less stretchable. The cell wall of the guard cell which is farthest from the stoma is more stretchable. When water enters the guard cells by osmosis, they become turgid and the thickened part of the cell wall is pulled apart causing a wider gap of stomata.

During night, when photosynthesis does not take place, carbon dioxide is accumulated in the guard cells. Carbon dioxide combines with water to form carbonic acid which dissociates into H+ and HCO-3.

In this process, hydrogen ions are generated. Increase in H+ means greater acidity. In the acidic medium of the cytoplasm of guard cell, sugars are polymerized into starch. The latter does not need much water. Therefore, some water molecules have to leave the guard cells, making them less turgid. The result is that stomatal openings are narrowed.