Cell wall

Cell wall is outermost, rigid, protective and semi-transparent covering of plant cells and cell of fungi, bacteria and some protists. It was discovered by Robert Hooke in 1665.

• Fungal cell wall – of chitin
• Bacterial cell wall – of peptidoglycan or mucopeptide or murein
• Plant cell wall – of cellulose, hemicelluloses and protein.

Structure of cell wall:

Cell wall has the following different layers:

Middle Lamella

• It is outermost thin cementing layer between adjacent cells
• Formed during cytokinesis
• Composed of magnesium pectate and calcium.

Primary Wall

• More or less elastic layer formed after the formation of middle lamella.
• Composed of protein, hemicelluloses and loose network of microfibril of cellulose.
• Primary wall and middle lamella are found in all types of plant cells.

Secondary wall

• Much thicker, rigid and inelastic layer formed inner to primary wall.
• Is formed only when cells attain maximum size
• Is common in aclerenchyma, collenchymas, tracheid and vessels.
• Composed of cellulose, hemicellular, pectin and lignin.
• S1, S2, and S3 are its sublayers.
• May contain wax, cutin and suberin and silica.

Plasmodesmata:
Primary and secondary walls are not formed continuously. They form gaps, known as plasmodesmata. It is a cytoplasmic bridge between two cells.

Desmosomes:
It is the protoplasmic bridge between two cells of animal cells.

Functions of Cell Wall:

• It provides definite shape to cell due to its rigidity.
• It protects the protoplasm against mechanical injury.
• It prevents the cell from osmotic burstiong.
• Cell wall of root hairs absorbs water.
• It has some enzymatic activity.
• Cutin and suberin of cell wall of stem and leaves help to reduce the rate of transpiration.
• The permeable nature of cell wall allows the exchange of any substance through it.