Cell Wall Definition
A cell wall is a structural layer that surrounds some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. The cell wall provides protection and defines the shape of the cell. All cells have cell membranes, but generally, only certain cells have a cell wall.
The primary wall is the cellulose-containing layer laid down by cells that are dividing and growing. To allow for cell wall expansion during growth, plants also produce secondary walls composed of lignin and cellulose.
Cell Wall Functions
The cell wall is a rigid structure that surrounds the plasma membrane of plant cells, fungi, bacteria, algae, and some archaea. It provides structural strength and support to the cell, which helps protect it against mechanical and osmotic stress. The cell wall also allows cells to develop turgor pressure, which is the pressure of the cell contents against the cell wall.
In addition to providing support and protection, the cell wall also has other functions such as preventing loss of water, filtering molecules that enter or exit the cell, and protecting against insects and pathogens.
The composition of the cell wall varies depending on the organism. For example, in plants, the cell wall is generally arranged in three layers: primary cell wall, middle lamella, and secondary cell wall.
The primary cell wall is situated closest to the inside of the plasma membrane and is composed mainly of carbohydrates like pectin, cellulose, and hemicellulose along with smaller amounts of minerals that form a network along with structural proteins to form the cell wall.
The middle lamella is a thin layer between two adjacent plant cells that contain pectin. The secondary cell wall is present in some plant cells and provides additional strength to withstand mechanical stress.
In fungi, chitin is a glucose derivative that forms layers in their tough outer covering called exoskeletons. Chitin is also found in fungal walls along with glucans which are other glucose polymers. Bacteria usually contain peptidoglycan in their walls while archaea have pseudo-peptidoglycan or polysaccharides like glycoproteins or glycolipids.
Cell Wall Structure
Plant Cell Walls
Plant cell walls are complex structures that provide support and protection to the cell. They are composed of several components, including cellulose microfibrils, hemicellulose, pectin, lignin, and soluble protein. The primary function of the cell wall is to maintain the shape and integrity of the cell.
Cellulose microfibrils are long chains of glucose molecules that form a network within the cell wall. Hemicellulose is another type of polysaccharide that helps to cross-link cellulose fibers and prevent them from adhering together in an uncontrolled manner.
Pectin is a gel-like substance that fills the spaces between cellulose fibers and provides flexibility to the cell wall. Lignin is a complex polymer that provides rigidity to the cell wall and makes it waterproof.
The composition of the plant cell wall varies depending on the type of plant cell. All plant cells have their origin in dividing cells, as the cell plate forms during cytokinesis to create a new partition wall between daughter cells. The new cells are usually produced in special regions called meristems, where they synthesize, assemble, and remodel their cell walls.
Algae Cell Walls
Algal cell walls are composed of a diverse array of fibrillar, matrix, and crystalline polymers interacting with various ions and water. The composition of the cell wall varies among different algal groups due to ancient evolutionary origins and ecological pressures of modern earth habitats.
In many classes of algae, cellulose is already the main structural element of the wall, although remarkable variations of the fibrillary structure exist. The thickness of the outer layer varies since it includes components that the cell takes up from its surrounding.
This indicates the main function of the cell wall of simple, single-celled algae: it mediates between the cell and its surrounding. It protects not only the cell but serves, too, communication with cells of the same or other types. It has to be permeable for metabolites and regulators and/or to carry.
The Chlorophyceae are the largest group of green algae and exhibit great morphological diversity ranging from motile unicells to large filaments to blade-like thalli.
The extracellular coverings of Chlorophycean algae are also very diverse and consist of a distinct assortment of “cell walls.” In Chlorella, for example, the wall contains cellulose and, in some species, it is coated by a highly resistant outer stratum consisting of “algaenan”.
Cell walls from several Ulva species comprise two major polysaccharide components: soluble ulvans and cellulose; two minor polysaccharides: xyloglucan (XG) and sulfated galactans (SGs).
Algal cells are eukaryotic cells that contain three types of double-membrane-bound organelles: nucleus, chloroplasts, mitochondria; as well as several single-membrane-bound organelles including endoplasmic reticulum (ER), Golgi apparatus (GA), lysosomes, peroxisomes, contractile or noncontractile vacuoles.
Fungi Cell Walls
The fungal cell wall is a characteristic structure of fungi and is composed mainly of glucans, chitin, and glycoproteins. The inner cell wall consists of a core of covalently attached branched β-(1,3) glucan with 3 to 4% interchain and chitin in most fungal species.
Chitin is a structurally important component of the fungal cell wall located closest to the plasma membrane. The composition of the outer layer varies depending on the fungal species, morphotype, and growth stage.
The components of the cell wall are essential in protecting fungi and shifting host immune responses in favor of fungal growth allowing dissemination in the host.
Chitin contributes to the strength of the cell wall even though it is present in minor quantities in some fungi such as Cryptococcus neoformans. In Cryptococcus, eight chitin synthases and three potential regulatory proteins coordinate and regulate chitin deposition in the cell wall.
The fungal cell wall is a dynamic structure that protects cells from environmental stress and prevents various effects caused by changes in osmotic pressure. Biosynthesis is a unique feature of the fungal cell wall.
They possess a complicated cellular structure comprising membrane-bound nuclei wrapped around histone proteins. Pigments are associated with the cell wall and vary from black to other colors depending on the species.
Bacteria and Archaea Cell Walls
Both bacteria and archaea have cell walls that protect them, but the chemical composition of the cell wall varies between the two domains. Bacterial cell walls contain peptidoglycan, which is composed of polysaccharide chains cross-linked by unusual peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine.
In contrast, archaeal cell walls are composed of S-layers and lack peptidoglycan molecules with the exception of methanobacteria who have pseudopeptidoglycan in their cell wall.
Both bacteria and archaea have a plasma membrane that is composed of phospholipids organized into two layers forming a structure called a phospholipid bilayer.
The plasma membranes of archaea have some unique properties different from those of both bacteria and eukaryotes. For instance, in some species, the opposing phospholipid tails are joined into a single tail, forming a monolayer instead of a bilayer.