Cell Wall | Definition, Structure, Components & its Function
The cells of bacteria, fungi, algae and plants have an additional non-living. rigid structure called the cel wa all that surrounds the plasma membrane. The composition of cell wall varies in different groups.
1. Fungal cell wall : The fungal cell wall is generally composed of chitin, a polymer of N-acetylglucosamine (NAG) units.
2. Algal cell wall : The algal cell wall is made up of cellulose, galactans, mannans and minerals like calcium carbonate.
3. Plant cell wall : The plant cell wall is chiefly composed of the insoluble polysaccharides (cellulose). Certain other compounds, such as hemicellulose, pectin and proteins are also present in the cell wall.
The cell wall of plants consists of two regions : primary wall and secondary wall.
Primary wall : A young plant cell forms a sinale laver of wall material. This layer ís known as the primary cell wall. The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by addition of more wall material within the existing one. Meristematic and parenchymatous cells have primary cell wall only.
Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall. These layers are called the secondary cell wall. Addition of secondary wall brings about thickening of the cell wall. Thickening of cell wall occurs particularly in cells that form the harder wOody parts of plants such as lignified and suberised cell wall.
Middle lamella : Adjacent cells in a plant tissue are held together by a thin, sticky, amorphous layer of cementing material. This layer is called the middle lamella. Middile lamella is chiefly made up of calcium and magnesium pectate. In ripening fruits, the pectate compounds solubilize to a jelly-like material, making the fruits soft. The cell wall and middle lamella may be traversed by plasmodesmata which connect the cytoplasm of neighbouring plant cells.
Functions of Cell Wall:
The cell wall serves many functions
- It maintains shape of the cells.
- It protects the cells from mechanical injury. b
- It wards off the attacks of pathogens like viruses, bacteria, fungi, etc.
- It allows the materials to pass in and out of the cell.
- It helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.
Pits : The cell wall is not uniform in thickness throughout; because at certain places secondary wall is not laid down. Such unthickened areas are called pits. Pits of the adjacent cells generally lie opposite to each other and form pit pairs.
Pits are of two types:
- Simple pit : Pit cavity is unjform in diameter.
- Bordered pit : Pit cavity is flask-shaped, as in tracheids.
Plasmodesmata : It forms the living component in the dead wall. A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with other. These are lined by plasma membrane and contains a fine tubule called desmotubule. Endoplasmic reticulum plays a role in origin of plasmodesmata. These form the symplastic system between two cells.