A cell wall is a structural layer surrounding the outer membrane of some types of cells. The cell wall serves many functions, such as protecting the cell from external threats, assisting in the transport of materials into the cell, and giving the cell shape and structure. Most prokaryotic organisms, plants, and fungi have cell walls. In contrast, few other eukaryotes or multicellular animals have cell walls.
The exact composition and structure of the cell wall differ depending on the species and stages of an organism’s development, though the different kinds of cell wall play a similar functional role. One of the major roles of the cell wall is to provide shape and structure to organisms that lack internal supporting structures like bones in vertebrates or hydrostatic cavities in invertebrates. They also keep out foreign pathogens and prevent the cell from bursting from over-expansion. Most cell walls also mediate the organism’s interactions with the environment and so contain various structure for signaling and transferring resources in and out of the cell.
Types Of Cell Walls
Plant cells have rigid thick cell walls composed primarily of cellulose and pectin. The plant cell wall can be divided into the primary and secondary wall. Cellulose molecules form small chains that attract to each other via hydrogen bonding, creating a dense crisscrossing matrix of cellulose microfibrils. Chains of the polysaccharide pectin intertwine through the matrix, providing extra strength and cohesion of the microfibrils Secondary constituents include various proteins and glycoproteins that support the cellulose and allow for transport across the cell wall.
One of the main functions of the cell wall in plants is to give cells structure and rigidity. Central vacuoles fill with fluid and exert turgor pressure on the cell wall, causing the plant to stand upright. The tough and flexible cell wall also protects the cells integrity from mechanical stress or environmental effects. There is still considerable debate about the exact mechanism for biosynthesis of the cell wall in plants, It is known that cellulose fibrils are formed in the plasma of the cell, but it is still unknown the mediating process that directly integrates the microfibrils into the cell wall.
Fungi are distinguished from plant cells and prokaryotes due to the presence of chitin and chitosan in their cell walls. Chitin is a polysaccharide similar in structure to cellulose and is also a major component of the exoskeletons of arthropods like insects or crustaceans, and fish scales. Chitin polymers fixate themselves on the cell membrane pointing outwards and interlink to make a tensile and robust basket-like scaffold for other proteins and glycoproteins to attach to. The chitin scaffold provides most of the structural strength of the fungal cell wall and is responsible for keeping the organism’s shape.
The various components of the fungal cell wall are covalently cross-linked with each other. The outside part of the cell wall in most fungi secretes a gel-like substance composed of various polymers that differ by species. Many fungi also contain melanin in their cell walls, which contribute to their distinctive colors. Lastly, fungal cell walls tend to contain hydrophobins, hydrophobic amino acids that link together to form a water-repelling coating. Still not much is known about the process of cell wall assembly in fungi, but the process is thought to involve over 1,200 different genes on average.
Prokaryotic Cell Walls
Most archaea and bacteria have cell walls, but their structure can differ widely depending on species. Whereas plant cell walls are composed of cellulose and fungal cell walls are composed of chitin, bacterial cell walls are composed primarily of peptidoglycan, a polymer made out of sugars and amino acids. There are two main categorizations of bacteria cell walls, Gram-positive and Gram-negative. Their names derive from the characteristic reactions the types of cell have to Gram staining, a technique used to stain cells for microscope viewing. Gram-positive bacteria have thick cell walls made out of many layers of peptidoglycan and Gram-negative bacteria have relatively thin walls. The thick walls of Gram-positive bacteria absorb the staining solution and appear as a crystal violet color under the microscope. Gram-negative bacteria have cell walls to thin to absorb the staining solution and so appear different colors, typically a pale pinkish-red.
The classification of a bacteria as Gram-positive and Gram-negative usually indicates how effective traditional antibiotics will be treating the bacteria. Despite having thicker cell walls, Gram-positive bacteria are more receptive to antibiotics due to the lack of outer membrane. Some bacteria, such as those in the family Mycoplasma lack cell walls entirely so antibiotics are not as effective. Archaea are unique from bacteria in that their cell walls do not contain peptidoglycan. Archaea have been observed with 4 different types of cell walls: pseudopeptidoglycan, polysaccharides, glycoproteins, and a simple layer of surface proteins. Archaea can also be Gram-positive or Gram-negative as well.
Other Eukaryotic Cell Walls
Although not common, there are a handful of non-plant non-fungi eukaryotes that have cell walls. Various algae, for example, contain cell walls made of cellulose and photosynthesize, but lack other defining tissues and cell structure found in land plants. Slime molds also have cell walls containing cellulose, although they are similar to fungi in many other ways. Various other single-cell protists have cell walls, though the composition differs between species.
In summation, the cell wall is a versatile structure found in a wide range of cell types. Most cell walls serve to protect the cell interior and give it shape and size. Cell walls are generally the cell’s first line of defense against the exterior world and so end up mediating the organism’s interactions with the environment.