Cell Wall
Cell Wall
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.
see also Amino Acid; Angiosperms; Archaea; Eubacteria; Extracellular Matrix; Fungi, Plant; Homeostasis; Protista
Nicholas C. Carpita
Bibliography
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.
Cell Wall
Cell Wall
A cell wall is a tough, semirigid case that surrounds a cell. Both plants and some single-celled organisms have cell walls. Cell walls are outside the cell membrane and are not part of the living cell. They protect the cell and provide it with support.
Cell walls are found only in some single-celled organisms like fungi and bacteria, but they are found in all plants. They are one of the characteristics that separate plant cells from animal cells (which do not have cell walls). A cell wall is different from a cell membrane, since all cells have plasma membranes that are a part of the living cell. Membranes are also semipermeable and only allow substances of a certain size to pass in and out of the cell. A cell wall in a plant is a structure that is just outside the membrane and provides a plant with protection and rigidity. In plants, it is made up of a complex carbohydrate called cellulose that, although it is very tough, also allows water and solutions to reach the plasma membrane. Since cellulose is both light and strong, it provides the ideal material for a cell wall, acting as a kind of external skeleton that gives the cell (and therefore the plant) its shape and strength. The stem of a plant is able to hold itself up despite gravity by having thousands of cells lined up next to and on top of each other. As the cells take in water, they expand like a balloon and exert pressure against their own walls and against the stem walls. It is their pressure that holds the stem up. When a plant droops, it is because its cells lack water to push against the walls, and the cells begin to shrink.
The cell walls of a green plant are made of cellulose, making it the most abundant organic compound on Earth. The cellulose in a plant's cell walls is formed by fibers that are very strong because they are linked in a criss-cross mesh pattern. Herbivores or animals who eat nothing but green plants must have special digestive systems since the tough cell walls of a plant make it very difficult to digest. This is why herbivores have a much longer and more elaborate digestive tract than do carnivores (meateaters) who consume mostly easy-to-digest proteins. Herbivores must also consume enormous amounts of plant material since each mouthful of vegetation contains a relatively small amount of energy (compared to a protein diet).
A plant's cell wall helps protect the important membrane and gives the plant cell and the plant its shape and support. Fungi and bacteria also have cell walls, but they are not made of cellulose. Most fungi have a cell wall made of chitin, while yeast (a type of bacteria) cell walls are made of a different complex of carbohydrates.
cell wall
1. (of eukaryote) A strong, often rigid, extraprotoplasmic layer in plant cells, whose growth is directed from within the cell. In addition to water, which may comprise by weight up to 70 per cent of the wall, it consists of a variety of polysaccharides, notably celluloses, hemicelluloses, and pectins, as well as variable (but smaller) amounts of proteins, lipids, lignins, tannins, and even mineral salts. Cell walls provide a skeletal support to the whole plant and also a barrier against injury and infection.
2. (of prokaryote) A rigid wall structure that lies inside the capsular layer, but outside the plasma membrane of cells. In cyanobacteria it is composed primarily of cellulose, but in bacteria it is composed of a mixture of materials not found elsewhere. Gram-positive (see GRAM-REACTION) bacteria have relatively thick walls (15–80 nm) composed principally of peptidoglycans (40–90 per cent), techoic acids, and other complex polysaccharides, with little sign of differentiation into separate layers. Gram-negative bacteria have much thinner walls (10 nm) composed of several distinct layers which are chemically different from those of Gram-positive species. Peptidoglycans represent only 1–10 per cent of the wall material by weight; techoic acids are absent, but complex lipopolysaccharides are present.