With very few exceptions, all cells are enveloped by an extracellular matrix composed of proteins , carbohydrates , and other substances. Owing to its exceptional strength and its ability to control cell shape, the extracellular matrix of eubacteria, algae, fungi, and plants is called the cell wall. The composition of cell walls varies widely among these kingdoms and the species within them, but the central functions are similar for most organisms.
Cell walls provide rigidity and protection. For multicellular organisms, the cell wall also binds different cells together. Plants use their cell wall as part of their system for maintaining their shape and stiffness. The plant concentrates ions and other substances within the cell, which pulls in water by osmosis . The cell swells, pressing tightly against the cell wall. The swelling increases rigidity, or turgor , while the wall keeps the cell from bursting.
Eukaryotic Cell Walls
Eukaryotic organisms, such as algae, fungi, and higher plants, have multilayered cell walls composed in large part of either cellulose or chitin . Cellulose and chitin are polysaccharides , meaning they are composed of many linked sugar molecules. Cellulose is a polymer of glucose , which contains only carbon, hydrogen, and oxygen, while chitin is a polymer of N-acetylglucosamine, a sugar that contains nitrogen as well. Both cellulose and chitin are linear, unbranched polymers of their respective sugars, and several dozen of these polymers are assembled into large crystal-like cables, called microfibrils, that spool around the cells.
Cellulose microfibrils form the scaffold of all plant cell walls. At least two types of primary walls are found among the species of flowering plants (angiosperms). In the Type I walls of eudicots and some monocots, the microfibrils are tethered together by sugars called xyloglucans, and this framework is embedded in a gel of pectins , another type of polysaccharide. The pectins establish several of the wall's physical characters, such as electrical charge, density, porosity, enzyme and protein distribution, and cell-to-cell adhesions . Pectins are used commercially to thicken jellies and jams. The Type II walls of cereal grains and other monocot relatives tether the microfibrils with different sugars, and is relatively pectin-poor. The hardness of wood comes from lignin , which is impregnated between the cellulose microfibrils. Lignin is a phenolic compound, chemically related to benzene.
The cell walls of fungi are diverse among the taxonomic groups, but most contain chitin microfibrils embedded in a polysaccharide matrix and covered with a loose coating of additional molecules combining sugars and peptides (amino acid chains). However, the cell walls of the Oomycetes contain cellulose instead of chitin. Different groups of fungi can be distinguished partly by the composition of their cell wall components.
Cellulose forms a substantial part of the microfibrillar framework of most algae, although some contain other polysaccharides instead. These microfibrillar networks are embedded in a thick gel of polysaccharides of immense diversity. Three important classes of algae, the Chlorophyceae (green), Rhodophyceae (red), and Phaeophyceae (brown), can be distinguished to a certain extent based on their polysaccharide constituents. Alginic acid and fucans are found in brown algae, whereas agarose and carrageenan are found predominately in red algae. Several of these polysaccharides are used as thickening and stabilizing agents in a variety of foods.
Bacterial Cell Walls
In eubacteria, the cell wall is composed of one or more layers of a peptidoglycan, called murein. A peptidoglycan is a combination of peptides and sugars. Murein is composed of the sugars N-acetylglucosamine and N-acetylmuramic acid. To murein are linked peptide extensions that are crosslinked to form the netlike wall. The antibiotic penicillin shuts down the enzyme that creates these cross-links, thus preventing bacterial growth.
Many bacteria produce a capsule to the exterior of the murein wall, composed of a diverse selection of molecules, including polypeptides and several complex carbohydrates, which may include cellulose. Bacteria with this outer capsule do not absorb a particular dye, called Gram stain, and therefore known as Gram-negative bacteria. Bacteria lacking the outer capsule do absorb the dye and are called Gram-positive bacteria. The Gram stain is a basic tool for identifying bacteria. Escherichia coli bacteria in the human large intestine are Gram-negative bacteria.
In contrast to eubacteria, archaea possess a pseudomurein wall, with a different set of sugars, no D-amino acids, and exterior layers of proteins, glycoproteins, and polysaccharides similar to those found in higher organisms.
Nicholas C. Carpita
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.