CELL WALL

In plants the plasma membrane is surrounded by a rigid cellulose cell wall which determine the shape of the cell, that protect  and support the plant cell. The cell wall have pits. These pits enable
cytoplasmic strands to pass through the coneccting cytoplasm of one cell and its adjacent cell. The primary cell wall is laid down during cell division. In most plant cell, extra layer are later secreted on the inside surface of the primary wall and this become the secondary cell wall. The primary cell wall contains cellulose loosely arranged. Each cellulose microfibril is formed from about 2000 cellulose molecules. The microfibril are embedded in an amorphous matrix of hemicellulose, calcium and magnesium pectate. This arrangement produces space between microfibrils. The cell wall is freely permeable to water and solute. The cellulose fibril have high tensile strength and prevent the plant cell from bursting when water enters by osmosis. In the secondary cell wall, the cellulose microfibril are more compact and succesive layer are arranged at different angles. Additional thickening such a lignin in tracheid and xylem vessels give mechanical strength and support, suberin and cutin on epidermal surface reduce evaporation of water and entry of pathogens. Pits in the lignified wall of tracheids and xylem vessel allow lateral transport to take place. Cell walls of bacteria contain murein. Fungal cell wall contain chitin.