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 the the central vacuole of the cell. The size of the cytoplasm as well as central vacuole and hence protoplast becomes reduced. The pressure on the wall is reduced. This is first stage of plasmolysis is called limiting plasmolysis. 
                                          The extra hypertonic solution continues to withdraw water from the central vacuole by exosmosis. Initially the protoplast withdraws itself from the corners. 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 few points. It is known as evident plasmolysis.

Deplasmolysis:- The swelling up of a plasmolysed protoplast under the influence of hypotonic solution or water is called deplasmolysis. It is due to endosmosis. Deplasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged. 

Importance of Plasmolysis;- i) Plasmolysis proves the cell membrane is semipermeable.

ii) It shows that cell wall is elastic as well as permeable.

iii) Osmotic pressure of a cell can be measured by plasmolysis. It will be roughly equivalent to the osmotic pressure of solution.

iv) Plasmolysis can be shown only by living cells.

v) By salting tennis lawns, the weeds can be killed due to permanent plasmolysis.

vi) Salting of pickles, meat and fish kill the spores of fungi and bacteria.