Osmosis And Significance Of Osmosis In Plants

Osmosis

The movement or diffusion of water molecules from a region of higher concentration to lower concentration through a semi-permeable membrane is called as osmosis. It is also referred as Osmotic diffusion.
In case there are two solutions of different concentrations separated by the semi permeable membrane, the diffusion of solvent will take place from the less concentrated solution into the more concentrated solution till both the solutions attain equal concentration.

Osmotic pressure

As a result of the separation of two solutions by the semi permeable membrane, a pressure is developed in solutions due to the presence of dissolved solutes in it. This is called as osmotic pressure (OP). OP is measured in terms of atmospheres and is directly proportional to the concentration of dissolved solutes in the solution. More concentrated solution has higher OP. The OP of a solution is always higher than its pure solvent.
During osmosis, the movement of solvent molecules takes place from the solution whose osmotic pressure is lower i.e., less concentrated or hypotonic into the solution whose osmotic pressure is higher i.e., more concentrated or hypertonic.
Osmotic diffusion of solvent molecules will not take place if the two solutions separated by the semi permeable membrane are of equal concentration having equal osmotic pressures (i.e., they are isotonic). In plant cells, plasma membrane and tonoplast act as selectively permeable or differentially permeable membrane. Depending on this, the solution have been classified as;
- Hypertonic solution: A solution having a concentration such that it gains water or solvent by osmosis across a semi-permeable membrane from some other solution is termed as hypertonic solution (more concentrated solution)
- Hypotonic solution: A solution having a concentration such that it loses water or solvent by osmosis across a semi-permeable membrane to some other solution is termed as hypotonic solution (less concentrated solution)
- Isotonic solution: A solution having a concentration such that it neither gains nor loses water or solvent by osmosis when separated by a semi permeable membrane from another solution is termed as isotonic solution

Endosmosis

If a living plant cell is placed in water or hypotonic solution whose OP is lower than cell sap, water enters into the cell sap through osmosis and the process is called endosmosis.
Due to the entry of water within the cell sap, a pressure is developed which press the protoplasm against the cell wall and the cell becomes turgid. This pressure is called as turgor pressure.
Consequence of the turgor pressure is the wall pressure which is exerted by the elastic cell wall against the expanding protoplasm. At a given time, turgor pressure (TP) equals the wall pressure (WP) i.e. TP=WP.

Exosmosis

If the plant cell is placed in hypertonic solution (whose OP is higher than cell sap) the water comes out of the cell sap into the outer solution and the cell becomes flaccid. This process is known as exosmosis.
During the exosmosis, the cell membrane will be separated from the cell wall and this condition is called as plasmolysis. In the case of isotonic solution, cell (or) tissue will remain as such as there is no movement of water molecules.

Significance of diffusion

Large quantities of water are absorbed by roots from the soil by osmosis
Cell to cell movement of water and other dissolved substances involves osmosis
Opening and closing of stomata depend upon the turgor pressure of guard cells.
Due to osmosis, the turgidity of the cells and hence the shape or form of the organs is maintained.
The resistance of plants to drought and frost increases with increase in osmotic pressure.
Turgidity of the cells of the young seedling allows them to come out of the soil.