When a plant or animal cell is kept in a hypertonic solution, water from cell sap flows out due to a reverse concentration gradient (concentration gradient pointing out of the cell). As the solution in the external medium is more concentrated, it has more OP and hence water moves out of the cell resulting in shrinkage of the cell volume. If the conditions persist for a long time the cell will die.
This phenomenon of water loss from cells which is akin to exosmosis is called plasmolysis. Wilting of plants noticed under conditions of water scarcity is an indication of plasmolysis of cells. There are three stages in plasmolysis – viz, incipient plasmolysis, evident plasmolysis and final plasmolysis.
Incipient plasmolysis:
In a turgid cell, with adequate amount of water, the plasma membrane closely appresses the cell wall and is in contact with it alround. When this cell is kept in a hypertonic solution, water starts moving out of the cell. Initially there will be no effect on the cell wall. But as water continues to go out, the cell shrinks in volume.
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But the plasma membrane maintains its contact with the cell wall alround due to its elastic capacity. As the exit of water continues, the plasma membrane reaches its limit of elasticity and gets torn from the cell wall at the comers, while maintaining contact at other regions. This is the first stage of plasmolysis called incipient plasmolysis.
Evident plasmolysis:
The cell under hypertonic conditions continues to lose water to the external medium and shrinks further in volume. The plasma membrane gets torn completely from the cell wall and goes on contracting. This is the second stage of plasmolysis called evident plasmolysis.
Final plasmolysis:
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As exosmosis continues, the shrinkage of the cell and cytoplasm reaches the minimum limit and no further shrinkage in volume is possible. Cytoplasm will be completely free from the cell wall and remains in the centre of the cell. Based on the final shape of the cytoplasm the final plasmolysis is divided into two types’ concave plasmolysis and convex plasmolysis. In the former, cytoplasm assumes a concave shape and in the latter it assumes a convex shape. A cell which has undergone plasmolysis is called plasmolysed.