Most often, cell walls are associated with plants. Plant cell walls have up to three layers: the middle lamella, the primary cell wall and the secondary cell wall, each differing somewhat from the others in composition. The outermost layer is the middle lamella, which is rich in pectins and glues adjacent plant cells together. Under that is the thin, flexible and extendable primary cell wall, which is made up of cellulose, hemicellulose and pectin. The innermost layer is the thick secondary cell wall, which is composed of cellulose, xylan and lignan.
In fungi, the cell wall is the outermost layer. Three substances combine to form fungal cell walls: chitin, glucans and proteins.
The cell walls of bacteria are made up of peptidoglycan. Also called murein, peptidoglycan is comprised of polysaccharide chains cross-linked by unusual peptides containing D-amino acids.
With the exception of one group of methanogens, the archaea have four types of cell walls, and they do not have peptidoglycan. The first type of cell wall is made of pseudopeptidoglycan, the second type is composed of polysaccharides, the third type is made of glycoprotein, and the fourth type is comprised of proteins.Learn more about Cells