The cell wall is a protective barrier around a cell. They are possessed by plants, fungi and bacteria. They are made up of repeating polymers of carbohydrates.

Eubacterial cell walls are made up of a substance called peptidoglycan. Peptidoglycan is a polysaccharide of N-Acetylmuramic acid (NAM) and N-acetyl glucosamine (NAG). These two alternating subunits are linked by a Beta (1-4) glycosidic bond which results from acetal formation. These polymers are linked by short peptide chains which give the cell wall much greater strength. These peptides contain D-Amino acids, which are not normally used in proteins.

The cell walls of fungi are made up of chitin. Chitin is is made up of repeating N-acetyl glucosamine subunits that are connected by Beta (1-4) glycosidic bonds. Chitin can also be found in the exoskeletons of insects.

The cell walls of plants are made up of cellulose, a repeating polymer of glucose subunits linked by Beta(1-4) glycosidic bonds. Cellulose forms an extensively hydrogen bonded system that gives it great strength. Vertebrates do not have an enzyme that will cleave the Beta(1-4) glycosidic bond and thus cannot digest cellulose.

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