The cell wall is a feature of bacteria, plant cells, and fungal cells.
Cell walls of plant cells
Plant cell walls have two main functions: to provide rigidity for the cell, and to prevent expansion when water enters the cell. Made of insoluble cellulose fibres meshed in to a matrix of carbohydrates called pectates and hemicelluloses, it gives the plant strength and support. The cell wall is generally permeable to anything entering the cell in solution unless impregnated with lignin in wood or suberin in cork tissue to produce wood.
The primary cell wall, built by the plant first, is composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. After the maximum size necessary has been reached, a secondary wall is constructed below. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly.
Cell walls of neighbouring cells are held together with a jellyish substance called lamella, which contains magnesium and calcium pectates.
Bacterial cell walls
Cell walls of bacteria are primarily used for protection against hostile envorinments or against the immune system of the host of pathogenic bacteria. It contains peptidoglycan, which can be made visible in gram positive bacteria by gram staining. The cell walls of bacteria are also vital for containing the high osmotic pressure inside bacterial cells caused by the high concentration of solutes in the cytoplasm, which can often be as high as 15 atmospheres. Many antibiotics target the cell wall of bacteria, including penicillin and its derivatives.
Fungal cell walls
The cell walls of fungal cells are composed of chitin, the same carbohydrate that gives strength to the exoskeletons of insects. They serve a similar to those of plant cells, giving fungal cells rigidity and strength to hold their shapes.