The external covering of a plant cell composed mainly of cellulose molecules organized into *microfibrils
. Small quantities of proteins have also been identified, e.g. enzymes such as invertase, phosphatase, and ATPase, whose functions are related to the uptake of nutrients and their passage from cell to cell. Enzymes responsible for cellulose synthesis are produced in the *golgi apparatus
and reach the cell surface, where they become functional, within the membranes of vesicles, *Microtubules
beneath the plasma membrane are thought to have a role in the organization of cellulose into microfibrils. Following cell division the primary wall is laid down by the deposition of microfibrils on the *middle lamella
. The orientation of the microfibrils differs depending on whether the cells are destined to be parenchymatous or more specialized tissue. The secondary wall normally consists of three layers laid down after cell extension is complete. The microfibrils are closely packed and aligned in the same direction in each layer but in different directions in successive layers. In maturing fibres, the microfibrils are mainly parallel to the long axis, while in developing xylem vessels they are laid down in rings or in helical strands. The hydrated nature of the microfibrils renders the wall elastic and permeable to water and solutes, including the soluble respiratory gases. The pressure of cell contents against the walls causes stretching and confers turgidity to plant tissues. This is a major factor in the provision of mechanical support to the nonwoody tissues of plants. In the cells of sclerenchyma and xylem tissues, lignin is deposited within the layers of the secondary walls and the protoplasts of these cells eventually disintegrate. Lignin confers considerable strength and these tissues form the wood of plants. Other substances, including suberin and callose, may also be deposited. The walls of spores are often impregnated with sporopollenin. The cell walls of fungi differ in that they usually contain chitin, while those of bacteria consist mainly of mucopeptide substances. Silica is an important constituent of certain algal cell walls (e.g. diatoms) and is also found in some grasses and sedges.