Cell Wall

The cell wall is the rigid, supportive and protective outer covering that lies external to the plasma membrane of plant cells, fungi, and some protists; it is absent in animal cells. Its exact composition varies with the organism: algae show cellulose along with galactans, mannans and minerals such as calcium carbonate in their walls, while in other plants the wall is made up of hemicelluloses, pectin, lipids and protein. Within the wall, cellulose microfibrils are chiefly responsible for its rigidity. Additional materials may be deposited in particular walls, such as silica in grass stems, cutin in the epidermal walls of land plants, suberin in the endodermal cells of roots, along with wax and lignin. Together, these give the cell its shape and protect it from mechanical injury and from infection.

Layers of the plant cell wall

In plants, the cell wall is built up of three layers. The middle lamella is thin and lies between two adjacent cells; it is the very first structure to form, laid down from the cell plate during cytokinesis, and is made mainly of pectin along with calcium and magnesium pectate -- the softening of ripe fruit happens because this pectin is solubilised. The primary wall lies just inside the middle lamella; it is capable of further growth and is the only wall present in young, actively growing tissue such as meristematic tissue, mesophyll, and pith. The secondary wall is laid down inside the primary wall once the primary wall has stopped growing; at certain spots this secondary thickening is absent, leaving gaps known as pits.

Plasmodesmata

Plasmodesmata are cytoplasmic bridges that connect neighbouring cells, running through pores in the primary cell wall and middle lamella. They allow the cytoplasm of adjoining cells to communicate directly, which is part of how plants -- lacking any circulatory system -- are able to move materials from cell to cell.

How the wall grows …