Keyword - Cell wall (KW-0134)
Protein found in or associated with a complex and rigid layer surrounding the cell. Cell walls are found in bacteria, archaea, fungi, plants, and algae. The cell wall is surrounded by the outer membrane in gram-negative bacteria, and envelopes the inner or plasma membrane in gram-negative, gram-positive and acid-fast bacteria. Cell walls of bacteria contain peptidoglycan whereas those of archaea do not. Some archaea may contain pseudopeptidoglycan, which is composed of N-acetyltalosaminuronic acid, instead of N-acetyl muramic acid in peptidoglycan. The plant cell wall is made of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as pectin and lignin. Algal cell walls are usually composed of cellulose, glycoproteins, sporopollenin, calcium and various polysaccharides such as manosyl, xylanes, alginic acid. Diatom cell walls (or frustules) contain silica. The cell wall plays a role in cell shape, cell stability and development, and protection against environmental dangers.