Plasmolysis

Introduction

In plants the movement of water takes place from one cell to another and it depends on a number of factors but the chief factor is concentration of solution. Movement of water is called osmosis.By this process nutrients in dissolved state are transported from one part to another.Plasmolysis is the process with the help of which water moves out of the cell which is termed exosmosis, resulting in shrinkage of the cell membrane. This process usually occurs in plant cells. In animal cells a similar process called crenation is observed.The plant cells contain an extracellular barrier: the cell wall (which is not present in animal cells). This cell wall helps in maintaining the structure and shape of the cell, as it is very rigid in nature.

Components of the Cell

• A plant cell is an eukaryotic cell that contains all the complex organelles–a well-defined nucleus, containing a nucleolus along with the genetic material DNA (DeoxyriboNucleic Acid), and endoplasmic reticulum, both rough and smooth. The rough endoplasmic reticulum contains ribosomes on the surface, while the smooth endoplasmic reticulum does not. Golgi apparatus, lysosomes, mitochondria, chloroplast, a big vacuole, all these organelles make up the cytoplasm of the cell
• Cytoplasm along with the nucleus is called Protoplasm. This term was coined by Czech Scientist J.E. Purkinje. The protoplasm is covered with a lipid bilayer that is hydrophobic in nature. This membrane is called the cell membrane
• The cell membrane is further protected in plant cells with the help of a structure built up by polymers of ‘cellulose’ known as the cell wall.
• The cell wall is a classic characteristic of plant cells. Animal cells are devoid of cell walls. 
• The cell membrane is semi-permeable in nature. That is, it allows the movement of only selected particles, molecules, or minerals in and out of the cell, depending upon the requirement of the cell. The movement of water occurs through special proteins, which protrude through the cell membrane. These are known as Aquaporins. 
• These proteins regulate the movement of water in and out of the cell. 

Method in the Movement of Water

• Water moves in and out of the cell due to transpiration as it creates suction called transpiration pull in the xylem
• The movement of water in plants usually takes place with the help of a process known as osmosis
• Osmosis is the passive transport of water
• In osmosis, no energy input is required, that is, no energy is required by the cell to move the water in and out
• It is a process in which water moves according to the gradient concentration, that is, it moves from a higher concentration towards the lower concentration
• This movement of water happens till the time equilibrium is reached (when the concentration of water becomes equal at both places)
• The process of movement of water across the semipermeable membrane is  in accordance with concentration gradient during osmosis

Movement of Water When the Cell is Placed in Different Types of Solutions

Now, when we know that water moves around in the cell by osmosis, we need to know the fate of cells when put in different solutions with different concentrations.

Cells have different types of minerals and molecules in their cytoplasm and vacuole. These minerals and molecules make up the “Solute” in a cell. At the same time, the cytoplasm and vacuole contain water as well.

  1. When the cell is put in isotonic solution

When the cell is kept in this solution, no water movement takes place in and out of the cell as the concentration of water in the cytoplasm and the surrounding membrane is the same. And so, there is no specific change in the shape of the cell. 

  2. When  the cell is put in a hypotonic solution

• Hypotonic solution is a type of solution in which the amount of water in the solution is more when compared to the water present inside the cell cytoplasm
• Due to this difference in water concentration, water moves into the cell through the semipermeable membrane (so water moves from the solution and enters into the cell), this process is also called endosmosis
• Now, because water enters into the cell, it becomes turgid (it swells)
• This situation is reversible, meaning when we put the same cell in a slightly concentrated solution, the cell will lose water

  3. When the cell is put in hypertonic solution

• Hyper means more
• Hypertonic solution means a solution in which solutes are present in more amounts as compared to water
• When the cell is put in this type of solution, it will lose a lot of water, starting from its cytoplasm, then through the vacuole. This loss of water is because here water is present in lower concentration in the outer solution, and inside the cell, it is in higher concentration, so water moves out of the cell
• This movement of water results in a process known as Plasmolysis
• The cell membrane, due to water loss, will lose its shape and shrink. That is, it will pull itself apart from the cell wall. Because the cell wall is a dead part of the cell, it will remain rigid and will not lose its shape
• This cell will be called a plasmolysed cell
• This process is also known as exosmosis, as the water goes outside the cell

Consequences

• When water is lost by any cell, many of the processes like enzyme functioning, metabolic processes, protein formation, glucose, ATP formation, nucleic acid formation, etc., are going to stop
• Hence, plasmolysis is considered a very dangerous process. Usually, the process results in the death of the cell during prolonged effects, that is, when the cell remains in a plasmolysed condition for a long time
• Plasmolysis occurs in animal cells, as well as in plant cells. Hence, it is important that we maintain the environment of the cell properly so that the death of cells due to plasmolysis does not occur

Conclusion

Plasmolysis is a process that rarely occurs in nature because the shape of the cell restores right away when water is present in the surrounding environment. Such a process is helpful during water scarcity such as drought for survival temporarily. Hence, this process is a reversible one till the plasmolysis is initial and temporary. It is induced inside labs by putting cells in extreme saline conditions for understanding this process.