Plasmolysis: Meaning and Process

Made up of specialised cellular organelles that differ from animal cells in several ways, plant cells are eukaryotes. Plant cells have a strong cell wall which keeps them erect and protects them from losing their form. The cytoplasm, other cell organelles and the plasma membrane, all work together to ensure the plant is healthy and active. The water in the plant cell is held in vacuoles, situated within the cytoplasm which is an organelle that is bound by a membrane and is filled with fluid. Plant cells may not receive enough water or experience a significant loss of water under certain circumstances. This causes the plant cell to shrink completely, a condition known as plasmolysis.

Plasmolysis:

Plasmolysis in a plant cell is defined as the process by which water leaves the cell and the cell membrane of a plant cell shrinks away from its cell wall. 

Plasmolysis occurs rarely in nature as a result of osmosis in plants.

Isotonic means that the outside solution balances the osmotic pressure of the cytoplasm. Hypotonic means the exterior solution is more dilute than the cytoplasm. Hypertonic means that the external solution is more concentrated than the cytoplasm.

There is no net flow of water towards the inside or outside of a cell when it is submerged in an isotonic solution. 

The cells are termed to be flaccid when water flows into and out of them in a balanced manner.

Water diffuses into the cell when it is placed in a hypotonic solution, causing the cytoplasm to build up a pressure against the cell wall, which is known as turgor pressure.

The process of plasmolysis:

The complete process of Plasmolysis is divided into three stages:

Incipient plasmolysis: This is the first stage of plasmolysis, when water begins to flow out of the cell; the cell shrinks in volume and the cell wall becomes visible.

Evident plasmolysis: This is the next step of plasmolysis, when the cell wall has reached its contraction limit and the cytoplasm has disengaged from the cell wall and thus forming a spherical shape.

Plasmolysis at the end: It’s the third and last stage of plasmolysis, when the cytoplasm is entirely detached from the cell wall and remains in the cell’s centre.

Plasmolysis is created by submerging cells in a concentrated sugar (sucrose) or saline solutions, which is commonly done with epidermal cells of an onion or Elodea plants, Tradescantia or Rheo plant cell, which have coloured cell sap which makes the process visible. Plant cells can be stained using methylene blue.

Plasmolysis is explained as the shrinking of a cell membrane in a hypertonic solution under high pressure.

Two types of plasmolysis:

1. Concave plasmolysis – It is almost always reversible. The plasma membrane and the contained protoplast shrink from the cell wall during concave plasmolysis as half-spherical, inwardly curving pockets emerge between the plasma membrane and the cell wall
2. Convex plasmolysis- It is invariably irreversible. The plasma membrane along with its protoplast shrinks completely from the cell wall during convex plasmolysis, with the plasma membrane’s ends forming a symmetrically, spherically curved pattern

Most animal cells have only a phospholipid bilayer of plasma membrane and no cell wall. So, they do not shrink when exposed to such conditions.

Deplasmolysis:

When the plasmolysed plant cell is placed in a hypotonic solution (solution in which the solute concentration is less than the concentration of the cell sap),  water enters the plasmolysed cell. This is due to the higher concentration of water outside the cell. The cell swells and becomes turgid as a result. This process of regaining the cell turgidity is termed as Deplasmolysis.

Some examples of plasmolysis are:

• Vegetables shrink in hypertonic situations
• Weedicides are sprayed on lawns, orchards, and agricultural areas to kill weeds which occur due to plasmolysis of weed cells
• When blood cells are exposed to hypertonic circumstances, they shrink
• When more salt is used as a preservative in foods such as jams, jellies, and pickles, because of the increased concentration outside, the cells lose water and become less favourable to microbe development. The same thing occurs when more sugar is used as a preservative in foods
• Ocean water deposits salt onto land during times of extreme coastal flooding. Too much salt causes any plants on the affected land to lose their water out of the cells, thus killing them

Defense system in plants against plasmolysis:

Plasmolysis is a rare occurrence in nature, occurring only under extreme situations of water loss. Plants have a couple of mechanisms to defend themselves against water loss. Stomata, which are the small holes on the underside of a plant’s leaves, close to help the plants retain water. Plants also produce wax naturally, which acts as another defense to water loss.