In this article we will discuss about the Meaning and Importance of Plasmolysis.
Meaning of Plasmolysis:
Shrinkage of the protoplast of a cell from its cell wall under the influence of a hypertonic solution is called plasmolysis. Hypertonic solution causes exosmosis or withdrawal of water from cytoplasm and then the central vacuole of cell.
The size of cytoplasm as well as central vacuole and hence protoplast becomes reduced. The pressure on the wall is simultaneously reduced and the elastic wall contracts causing a reduction in cell size.
This is first stage of plasmolysis called limiting plasmolysis. At limiting plasmolysis, the pressure potential (Ѱp) is zero and the osmotic concentration of cell interior is just equivalent to that of external solution (isotonic). The cell is called flaccid.
The extra hypertonic external solution continues to withdraw water from the central vacuole by exosmosis. Central vacuole shrinks further causing a similar shrinkage of protoplast from the cell wall. Pressure potential becomes negative.
Initially the protoplast withdraws itself from the comers. This stage is known as incipient plasmolysis. The hypertonic solution now enters the cell in between the protoplast and the cell wall. Due to continued exosmosis, protoplast shrinks further and withdraws from the cell wall except one or a few points. It is known as evident plasmolysis.
De-Plasmolysis:
The swelling up of a plasmolysed protoplast under the influence of hypotonic solution or water is called de-plasmolysis. It is due to endosmosis. De-plasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged.
During de-plasmolysis water diffuses into protoplast. It enters the central vacuole and cytoplasm. Consequently, the protoplast swells up. It first comes in contact with cell wall and then starts building a pressure on cell wall. This pressure is called turgor pressure. It makes the cell turgid.
Importance of Plasmolysis:
1. Plasmolysis proves that the cell membrane is semipermeable.
2. It shows that the cell wall is elastic as well as permeable.
3. Osmotic pressure of a cell can be measured by plasmolysis. It will be roughly equivalent to the osmotic pressure of a solution which will be strong enough to cause only incipient plasmolysis.
4. Plasmolysis can be shown only by living cells. It can, therefore, determine whether a cell is living or dead.
5. By salting tennis lawns, the weeds can be killed due to permanent plasmolysis and consequent death of their cells.
6. Plants are not allowed to grow in the cracks of the walls by the method of salting.
7. Salting of pickles, meat and fish and sweetening of the jams and jellies with sugar, kill the spores of fungi and bacteria.
8. Excessive concentration of chemical fertilizers at one place in the soil should be avoided otherwise the plants will die down.
Dead cells are fully permeable. It can be observed by cutting beet root into thin slices and washing them thoroughly under tap water till no more colour diffuses out. The slices are placed in water. No coloured sap comes out of them. Heat them. A reddish sap begins to come out of the slices. Heating has killed the cell membranes and made them permeable so that the sap diffuses out.