Cell wall

A cell wall is a more or less solid layer surrounding a cell. They are found in bacteria, archaea, fungi, plants, and algae. Animals and most other protists have cell membranes without surrounding cell walls. When a cell wall is removed using cell wall degrading enzymes, what is left of the cell and its surrounding plasma membrane is called a protoplast. The cell wall's main purpose is to actually protect the interior from any physical movement that may damage the cell.

Plant cell walls

Plant cell walls have a number of functions: they provide rigidity to the cell for structural and mechanical support, maintaining cell shape, the direction of cell growth and ultimately the architecture of the plant. The cell wall also prevents expansion when water enters the cell. The term turgor is used to describe this pressure that is induced by excess water inside the plant cell. Cell walls protect against pathogens and the environment and are a store of carbohydrates for the plant. The cell wall is constructed primarily from a carbohydrate polymer called cellulose.

The middle lamella is laid first, formed from the cell plate during cytokenesis, 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 are to name a few. 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.

Composition of 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.

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.

Secondary cell walls may contain lignin and suberin, making the walls rigid.

The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.

Algal cell walls

Like plants, algae have cell walls (Sendbusch, 2003). 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.
Xylanes
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.

Other compounds that may accumulate in algal cell walls include sporopollenin and calcium.

Diatom cell walls

The group of algae known as the diatoms synthesise 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 (Raven, 1983).

Prokaryotic cell walls

Cell walls of bacteria are primarily used for protection against hostile environments or, in the case of pathogenic bacteria, against the immune system of the host. They contain peptidoglycan, which can be made visible in Gram-positive bacteria by Gram staining. The cell walls of bacteria are also vital for containing the high osmotic pressure inside bacterial cells caused by the high concentration of solutes in the cytoplasm. This pressure can often be as high as 15 atmospheres. Many antibiotics, including penicillin and its derivatives, target the cell wall of bacteria.

The cell walls of archaea are not made of peptidoglycan, but some archaea may contain pseudopeptidoglycan, which is composed of N-acetyltalosaminuronic acid, instead of N-acetyl muramic acid in peptidoglycan.

Fungal cell walls

Not all species of fungi have cell walls but in those that do, the cell walls are composed of cellulose, 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. It also limits the entry of molecules that may be toxic to the fungus, like 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.

Pictures

Cell wall ultrastructure

References

Raven, J. A. (1983). The transport and function of silicon in plants. Biol. Rev. 58, 179-207.
Sendbusch, P. S. (2003). Cell Walls of Algae. Botany Online.

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