Cell wall

THE WALL

Nature and Occurrence:

There is an outer thick layer in cells of plants called cell wall.
Cell wall is an additional covering over the cell membrane in plant cells, bacterial and fungal cells but absent in animal cells.
It is non-living and permeable.
It is secreted by cells itself and provides protection to the underlying plasma membrane and other parts of the cell.
It gives shape and rigidity to the cells.

Structure:

It is made up of fibrous polysaccharides.
In plant cells, it is composed of polysaccharide known as cellulose.
It is comprised of many cellulose fibrils which are glued together with the help of a mixture of different polysaccharides like pectin and hemicelluloses.
The cell wall of adjacent cells is connected to each other through a jelly like substance made up of calcium and magnesium pectate.
This layer is known as middle lamella.
At some points the cell wall of adjacent cells are connected with each other. These points are known as plamodesmata.

In fungi, it is made up of chitin and in bacteria, it is made up of peptidoglycan which are also the complex polysaccharides.
It helps these organisms to tolerate the hypotonic conditions in surrounding.
Due to high concentration of water in surrounding, water molecules tend to enter inside the cells as a result of which cell may swell up. But presence of dead rigid cell wall will not allow the cell to burst. It will exert an equal and opposite pressure over the plasma membrane and thus control the amount of water entering inside the cell.
This is how cell wall provides protection to such living cells.

PLASMOLYSIS

If a living cell is placed in a hypertonic solution like concentrated sugar solution then the concentration of water will be higher inside the cell as compared to surrounding. Thus water from inside the cell tends to move out resulting in shrinkage of protoplasm.
This process in which living plant cell loses water through osmosis and result in shrinkage or contraction of protoplasm away from the cell wall is known as plasmolysis.

Functions of cell wall:

Turgidity:

It provides the turgidity to the plant cell. Water enters the vacuole by osmosis and swells it up but the presence of cell wall will resist this expansion hence the cell will not burst rather it will appear turgid.

Mechanical strength:

It is made up of cellulose fibres and is dead in nature thus it provides mechanical strength to the cell and plant structure.

Exchange of material:

Protection:

This additional layer surrounding the cell membrane is required by the plants for protection against variations in temperature, high wind speed, atmospheric moisture, etc.

Interaction between adjacent cells:

Middle lamella and plasmodesmata helps in interaction between the adjacent cells.