Some eukaryotic cells have cell walls, although these walls are generally much simpler than those of prokaryotic cells. Most algae have cell walls consisting of the polysaccharide cellulose (as do all plants). Cell walls of some fungi also contain cellulose, but in most fungi the principal structural component of the cell wall is the polysaccharide chitin, a polymer of N-acetylglucosamine (NAG) units. (Chitin is also the main structural component of the exoskeleton of crustaceans and insects). The cell walls of yeasts contain the polysaccharides glucan and mannan.
In eukaryotes that lack a cell wall, the plasma membrane may be the outer covering (Costerton, 2001); however, cells that have direct contact with the environment may have coatings outside the plasma membrane. Protozoans do not have a typical cell wall; instead, they have a flexible outer covering called pellicle. In other eukaryotic cells, including animal cells, the plasma membrane is covered by a glycocalyx (“sugar coat”), a layer of material containing substantial amounts of sticky carbohydrates.
Some of these carbohydrates are covalently bonded to proteins and lipids in the plasma membrane, forming glycoproteins and glycolipids that anchor the glycocalyx to the cell. The glycocalyx strengthens the cell surface; helps attach cells together, and may contribute to cell-to-cell recognition (Ferris & Beveridge, 2005). An important clinical consideration is that eukaryotic cells do not contain peptidoglycan, the framework of the prokaryotic cell wall. This is significant medically because antibiotics, such as penicillins and cephalosporins, act against peptidoglycan and therefore do not affect human eukaryotic cells.