Plasmolysis in plant cell

 Plasmolysis

Plasmolysis is the shrinkage of the protoplast of a cell from its cell wall under the influence of a hypertonic solution.

If a plant is placed in highly concentrated sugar or salt solution (hypertonic solution), water from cell sap flows out due to exosmosis through plasma membrane.  Loss of water from the cell sap causes contraction or shrinkage of protoplasm.

Since the cell wall is firm, it cannot keep pace with the contraction of the plasma membrane.  Ultimately, protoplasm is separated from the cell wall and assumes a spherical shape.  This condition is known as plasmolysis and the cell is said to be plasmolysed.

Different types of solution

Hypertonic solution:

Hypertonic  solution is a solution whose concentration is more than that of the cell sap.  A cell placed in such a solution leads to diffusion of water and shrinkage of protoplasm.

Hypotonic solution:

Hypotonic solution is a solution whose concentration  is less than that of the cell sap.  Immersion of cell in hypotonic solution leads to diffusion of water, increasing the size of the cell.

Isotonic solution:

Isotonic is a solution with concentration equal to that of the cell sap.   A cell placed in isotonic solution prevents diffusion of water.  As a result, there would be no change in volume and weight of the cell.

Demonstration of Plasmolysis:

In a plasmolysed plant cell, the space between the contracted protoplasm and cell wall remains filled with the external solution.  Plasmolysis can be demonstrated in the epidermal peel of Rhoeo discolor leaf.  The cell contents are purple in colour due to the presence of anthocyanin pigments.

The epidermal peels of Rhoeo discolor leaves are placed in hypertonic solution, which results in exosmosis. The purple cell contents start pulling away from the cell wall and the cell wall begins to lose turgidity.  A colourless space is seen between the cell wall and the purple contracted protoplasm.  This initial stage of plasmolysis is called incipient plasmolysis.

When a cell showing incipient plasmolysis is immersed in water or in a solution whose concentration is less than that of the cell sap (hypotonic solution),  then the cell regains its turgidity due to endosmosis.  However, if the exosmosis continues, it causes permanent plasmolysis.

Incipient Plasmolysis:  Incipient Plasmolysis is the point at which the protoplast starts to pull itself away from the cell wall.

Evident Plasmolysis:  It is the stage where the cell wall has reached its limit of contraction and cytoplasm has detached from cell wall attaining spherical shape.

Types of Plasmolysis:

Concave Plasmolysis:

Concave plasmolysis is usually an irreversible process.  During concave plasmolysis, the protoplasm and the plasma membrane shrink away from the cell wall in places due to loss of water.  Half moon shaped pockets form in the cell as the protoplast peels from the surface of the cell wall.

Placement of cell in hypotonic solution reverses the process of concave plasmolysis, which will cause water to rush back into the cell.

Convex Plasmolysis:

It is a process where a cell undergoes complex plasmolysis, the plasma membrane and the protoplast lose so much water that they completely detach from the cell wall.  The cell wall collapses in a process called Ctyorrhysis.

Convex Plasmolysis is irreversible and results in the destruction of the cell.

Deplasmolysis:

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

Difference between Plasmolysis and Deplasmolysis 

Plasmolysis	Deplasmolysis
1.  A tissue when placed in hypertonic solution leads to plasmolysis.	1.  Freshly plasmolysed cells when placed in hypotonic solution or pure solvent promotes plasmolysis.
2.  It involves shrinkage of protoplast from the cell wall.	2.  It is swelling of shrunken protoplast so as to come in contact with cell wall.
3.  Plasmolysis is a result of exosmosis.	3.  Deplasmolysis is a result of endosmosis.
4.  Prolonged plasmolysisis not reversible.	4.  Deplasmolysis is reversible even after an interval.

Importance of plasmolysis

1. Living cells show plasmolysis . It can, therefore, determine whether a cell is living or dead.
2.  The method of salting prevents the plants to grow in the cracks of the walls. This treatment causes plasmolysis in the root cells leading to the death of the plant.
3. High concentration of sugar in jams and jelly prevents the growth of molds, bacteria etc.
4. High-salted pickle kills bacteria due to plasmolysis.
5. Salting of meat and fish kill the spores of fungi and bacteria.
6. Plasmolysis facilitates measurement of osmotic pressure of a cell. It will be roughly equivalent to the osmotic pressure of a solution, which will be strong enough to cause only incipient plasmolysis.
7. It shows that the cell wall is elastic as well as permeable.
8. Salting tennis lawns promotes death of weeds and consequent death of their cells due to permanent plasmolysis.