Plasmolysis

It occurs when a cell is placed in a solution having a higher concentration of solutes than inside the cell (hypertonic solution), causing water to move out into the surrounding extracellular fluid.

The cell membrane is semipermeable. It is responsible for maintaining cell turgidity by allowing the selective entry of water and ions while blocking others. The turgor pressure generated by water molecules against the cell wall is necessary to the vitality of the plant structure. A turgid plant cell prevents the further intake of water. The plant vacuole also plays a vital role in this regulation process.  

When a cell is exposed to a hypertonic solution, water starts to flow down its concentration gradient. It thus moves from a region of low solute concentration inside the cell to the high solute concentration in the extracellular fluid. The net water movement across the plasma membrane through osmosis causes the plant cell wall to lose its turgor pressure. Finally, it causes the central vacuole and the cytoplasm (together known as protoplast) to detach from the cell wall, and the cell is said to be in plasmolyzed state.

After plasmolysis, the size of the protoplast gets reduced. Simultaneously the pressure on the cell wall decreases, causing it to contract and reduce in size. At times, if water is lost severely, the cell walls may collapse, which results in the death of the cell.

When a cell is exposed to an isotonic solution, it loses its turgor pressure. At this stage, although the cell does not get plasmolyzed, it is not turgid either. As a result, the green parts of the plant droop and cannot hold the leaves up in the sunlight.

Plasmolysis hardly happens in nature. Instead, it is induced in the laboratory by immersing living cells in strong saline or sugar solutions to undergo exosmosis. Generally, the cells of Tradescantia or Rheo plant, Elodea plants, or onion epidermal cells are used for this experiment. The reason is that they have colored sap, making them easily observable and identifiable under the microscope.

The whole process of plasmolysis can be described in three stages:

1. Incipient Plasmolysis: It is the first plasmolysis stage during which no morphological changes appear in the cell. The first sign of shrinkage of cell contents from the cell wall is evident at this stage as water starts flowing out of the cell. During this stage, the cell wall exerts no pressure on the cell contents, but the protoplast moves away from the corners of the cell wall. This stage is reversible, and the cell is in flaccid condition.
2. Evident Plasmolysis: It is the next stage in which the protoplasm reaches the limit of contraction. The cytoplasm and plasma membrane get detached from the cell wall (except one or a few points) and turns spherical due to exosmosis. During this stage the cell wall is not in touch with the protoplasm and the leaves appear wilted. This stage is also reversible.
3. Final Plasmolysis: It is the final stage of plasmolysis when the cytoplasm completely detaches itself from the cell wall and moves to the cell center.

Plasmolysis can be classified into two types depending on the appearance of shrunken protoplasm.

1. Concave Plasmolysis: In this case, both the plasma membrane and protoplasm shrinks and get detached from the cell wall due to water efflux. A ‘half-moon shape’ is formed in the cell due to detachment of cytoplasm, thus giving its name concave. Concave plasmolysis can be reversed by keeping the cell in a hypotonic solution, helping them regain the lost water.
2. Convex Plasmolysis: Excessive water loss from the cell causes the cell membrane and protoplasm to shrink away from the cell wall. As convex plasmolysis is irreversible, it eventually leads to cytorrhysis, i.e., the complete collapse of the cell.

As stated above, plasmolysis is generally a reversible decrease in the volume of a plant cell. When exposed to a hypotonic solution or water, a plasmolyzed cell swells up and becomes turgid due to endosmosis. This phenomenon is called deplasmolysis or cytolysis. It is only possible if the cell is placed in a hypotonic solution immediately after plasmolysis; otherwise, the cell protoplast gets damaged permanently.

During deplasmolysis, water again passively diffuses into the protoplast, i.e., the central vacuole and cytoplasm. As a result, the protoplast swells up. Initially, it comes in contact with the cell wall and then develops the turgor pressure.

• Helping to prove that the cell membrane is semipermeable in nature.
• Helping to prove that the cell wall is elastic as well as permeable.
• Measuring the osmotic pressure (OP) of a cell. This is because the OP of a cell is roughly equivalent to the osmotic pressure of the extracellular solution causing incipient plasmolysis.
• Determining if a cell is dead or alive, as only living cells exhibit plasmolysis.
• Making tennis lawns free of weeds by salting, which kills the weeds by permanent plasmolysis.
• Inhibiting plant growth in the cracks of the walls by salting.
• Preventing the microbial decay of foods. For example, salting pickles, meat, and fish and sweetening of the jams and jellies.

• Shrinking of vegetables when kept in a hypertonic environment.
• Contraction of blood cells, like RBCs, when placed in a hypertonic environment.
• Depositing salts by ocean water during extreme coastal flooding.
• Killing weeds in lawns, orchards, and agricultural fields by spraying weedicides.  
• Salting of foods like jams, jellies, and pickles, which act as preservatives. As microorganism cells lose water due to a higher concentration of the solute outside the cell by plasmolysis, the foods become less susceptible to their growth.