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A cell wall is a fairly rigid layer surrounding a cell, located external to the cell membrane, that provides the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. They are found in plants, bacteria, archaea, fungi, and algae. Animals and most protists do not have cell walls.
The cell wall is constructed from different materials dependent upon the species. In plants, the cell wall is constructed primarily from a carbohydrate polymer called cellulose, and the cell wall can therefore also functions as a carbohydrate store for the cell. In bacteria, peptidoglycan forms the cell wall. Archaea have various chemical compositions, including glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls of chitin, and algae typically possess walls constructed of glycoproteins and polysaccharides, however certain algal species may have a cell wall composed of silicic acid. Often, other accessory molecules are found anchored to the cell wall.
 Plant cell walls
The major carbohydrates making up the primary cell wall are cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.
The three primary polymers that make up plant cell walls consist of about 35], 20 to 35 % hemicellulose and 10 to 25% lignin. Lignin fills the spaces in the cell wall between cellulose, hemicellulose and pectin components.
Plant cells walls also incorporate a number of proteins; the most abundant include hydroxyproline-rich glycoproteins (HRGP), also called the extensins, the arabinogalactan proteins (AGP), the glycine-rich proteins (GRPs), and the proline-rich proteins (PRPs). With the exception of glycine-rich proteins, all the previously mentioned proteins are glycosylated and contain hydroxyproline (Hyp). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Chimeric proteins contain two or more different domains, each with a sequence from a different class of glycoprotein. Most cell wall proteins are cross-linked to the cell wall and may have structural functions.
The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.
The middle lamella is laid first, formed from the cell plate during cytokinesis, and the primary cell wall is then expanded inside the middle lamella. The actual structure of the cell wall is not clearly defined and several models exist - the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall, can be defined as composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cells interact though plasmodesma(ta), which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.
In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plant cell and primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible through pits in the secondary cell wall that allow plasmodesma to connect cells through the secondary cell walls.
 Algal cell walls
Like plants, algae have cell walls. Algal cell walls contain cellulose and a variety of glycoproteins. The inclusion of additional polysaccharides in algal cells walls is used as a feature for algal taxonomy.
- Manosyl form microfibrils in the cell walls of a number of marine green algae including those from the genera, Codium, Dasycladus, and Acetabularia as well as in the walls of some red algae, like Porphyra and Bangia.
- Alginic acid is a common polysaccharide in the cell walls of brown algae
- Sulfonated polysaccharides occur in the cell walls of most algae; those common in red algae include agarose, carrageenan, porphyran, furcelleran and funoran.
The group of algae known as the diatoms synthesis their cell walls (also known as frustules or valves) from silicic acid (specifically orthosilicic acid, H4SiO4). The acid is polymerised intra-cellularly, then the wall is extruded to protect the cell. Significantly, relative to the organic cell walls produced by other groups, silica frustules require less energy to synthesize (approximately 8%), potentially a major saving on the overall cell energy budget, and possibly an explanation for higher growth rates in diatoms.
 Bacterial cell walls
- Further information: Cell envelope
Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D-amino acids. Bacterial cell walls are different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria and the antibiotic penicillin is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.
There are broadly speaking two different types of cell wall in bacteria, called Gram-positive and Gram-negative. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species.
Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the Gram-negative cell wall and only the Firmicutes and Actinobacteria (previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility, for instance vancomycin can kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa.
 Fungal cell walls
Not all species of fungi have cell walls but in those that do, the cell walls are composed of glucosamine and chitin, the same carbohydrate that gives strength to the exoskeletons of insects. They serve a similar purpose to those of plant cells, giving fungal cells rigidity and strength to hold their shape and preventing osmotic lysis. They also limit the entry of molecules that may be toxic to the fungus, such as plant-produced and synthetic fungicides. The composition, properties, and form of the fungal cell wall change during the cell cycle and depend on growth conditions.
The group Oomycetes (also known as water molds) are saprotrophic plant pathogens like fungi, but anomalously possess cellulose cell walls. Until recently they were widely believed to be fungi, but structural and molecular evidence has led to their reclassification as heterokonts, related to autotrophic brown algae and diatoms.
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