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