Plant Cell Wall: Structure, Functions, Diagram

A cell wall is an outer rigid semi-elastic supportive and protective layer.
It is present around the plasma membrane.
It provides mechanical support and helps in maintaining the shape of the plant cell.
The cell wall is present in the plant cell and absent in the animal cell which distinguishes them from each other.
The cell wall is formed by the protoplast. Any plant cell which is devoid of the cell wall is called the protoplast.
The plant cell is mostly made up of the following components:
- Cellulose
- Hemicellulose
- Pectin
- Protein

In both the primary and secondary cell walls of the plant, cellulose is present.
Cellulose is an insoluble carbohydrate.
The fibrous structure present in the cell wall maintains the integrity of the structure.
In the primary cell wall, Pectin is present predominantly.
It plays the important role in:
- Expansion
- Strength
- Porosity
- Adhesion
- Intercellular signaling

Other non-cellulosic polysaccharides include xyloglucan, glucan, xylan, mannan, and callose.
Based on the sugar substitutes and side chains, pectic and non-cellulosic polysaccharides can be distinguished further too.
During biosynthesis, these components are attached to the polysaccharides.
These substituents are important in determining the solubility and viscosity within the cell wall.
They are also responsible for determining the interaction between polysaccharides and proteins.
The cell wall of fungi is made of chitin.
The cell wall of bacteria is made of the protein, lipid, and polysaccharides complex.

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Structure of Plant Cell wall

It is derived from the living protoplast.
It consists of the middle lamella, primary cell wall, plasmodesmata, secondary cell wall, and pits.

It is the first formed cell wall.
It is present in the inner side of the middle lamella.
It is the thin and permeable layer that can be expanded.
Cutin and cutin waxes are present in some epidermal cells of the leaf and stem. It makes the primary cell wall impermeable.
It is formed before the growth and development of the cell.
It is made up of matrix and microfibrils.
Matrix is made up of water, hemicelluloses, pectin, lipids, and proteins.
Microfibrils are embedded in the gel-like matrix.
The primary cell wall of the plant is made of cellulose.
In the fungi, chitin makes the primary cell wall, and in bacteria murein makes it.
Primary cell wall forms the only cell wall in the immature meristematic and parenchymatous cells.

Plasmodesmata are cytoplasmic or protoplasmic bridges present in the primary cell wall of adjacent cells.
They form a protoplasmic continuum called symplast.
They transfer cytoplasmic materials among adjacent cells.

The secondary cell wall is situated inner to the primary cell wall.
This is the thick layer, permeable, and cannot be expanded.
It forms after the growth and development of the cell.
It is present in the cells of the thick-walled dead tissue of the plant. Eg: Cells of sclerenchyma, tracheids, and vessels.
It is differentiated into the outer layer (S₁), middle layer (S₂), and inner layer (S₃).
Each layer is made up of a matrix and microfibrils.
The chemical composition of the matrix is almost similar to the matrix of the primary cell.
Microfibrils of the secondary cell wall is made up of cellulose and lignin.
Some chemicals like suberin, silica, wax, resins, oils, etc. are also deposited in the secondary cell wall.

In some plant cells, there is the presence of another cell wall beneath the secondary cell wall. It is known as the tertiary cell wall.
The morphology, chemistry, and staining properties of the tertiary cell wall are different from the primary and secondary cell walls.
In the tertiary cell wall, xylan is also present in it.

It provides mechanical support as the skeletal framework in the plant.
It protects the inner components of the cell from mechanical injuries.
It is permeable to the water and solutes. It is the presence of the water-filled channels which allows the free diffusion of water and water-soluble substances. Eg: gas, salt, sugar, hormones.
It prevents entry of the pathogenic agents inside the cell acting as the first line of defense.
When the cell is kept in the hypotonic solution, it prevents the osmotic bursting of the cell.
In the cell wall, cutin, wax, silica, and suberin is present which reduces the rate of transpiration.
The cell wall of root hairs helps in the absorption of sap from the soil.
Walls of tracheids and vessels help in the conduction of sap.
Middle lamella helps to join the adjacent cells.
Plasmodesmata help in the transfer of cytoplasmic materials among adjacent cells.

During the infection, oligosaccharides elicitors can be released.
These substances can be released from the host plant’s cell wall i.e DAMPs (Damage-associated Molecular patterns) or they can be from the pathogen cell wall i.e PAMPs ( Pathogen-associated Molecular patterns).
It occurs during the process of degradation.
In the plasma membrane, immune receptors are present which receive these elicitors.
It then activates the defense responses of DAMP or PAMP-triggered immunity.

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