cell wall

cell wall
1. (of eukaryote) A strong, often rigid, extraprotoplasmic layer in plant cells, whose growth is directed from within the cell. In addition to water, which may comprise by weight up to 70 per cent of the wall, it consists of a variety of polysaccharides, notably celluloses, hemicelluloses, and pectins, as well as variable (but smaller) amounts of proteins, lipids, lignins, tannins, and even mineral salts. Cell walls provide a skeletal support to the whole plant and also a barrier against injury and infection.

2. (of prokaryote) A rigid wall structure that lies inside the capsular layer, but outside the plasma membrane of cells. In cyanobacteria it is composed primarily of cellulose, but in bacteria it is composed of a mixture of materials not found elsewhere. Gram-positive (see GRAM-REACTION) bacteria have relatively thick walls (15–80 nm) composed principally of peptidoglycans (40–90 per cent), techoic acids, and other complex polysaccharides, with little sign of differentiation into separate layers. Gram-negative bacteria have much thinner walls (10 nm) composed of several distinct layers which are chemically different from those of Gram-positive species. Peptidoglycans represent only 1–10 per cent of the wall material by weight; techoic acids are absent, but complex lipopolysaccharides are present.