Placing organisms into categories is useful so that instead of describing a slew of characteristics, we can simply use broad categories as reference points to inform us not only about the nature of an individual, but also about its relationship to other similar organisms.
A new organism classified as a vertebrate, for example, will be commonly understood to have a spine composed of vertebrae. For scientists, taxonomic groups are touchstones of understanding: a foundation upon which to build new knowledge. This metaphor communicates the fundamental importance of taxonomy, but it implies a stability that taxonomic classification lacks. Classifying fungi as plants has led to some curious events. This pairing of fungi with plants is a present problem: misclassification matters because how we classify organisms affects how we understand, support financially and culturally and engage with them.
Why were fungi ever considered plants? Today, we know that fungi are not plants, but the botanical history of fungi provides an interesting perspective on our scientific biases, on how we classify organisms and how these impact our collective knowledge. Taxonomic classifications are in constant flux, as we refine our understanding of the incredible diversity that surrounds us. Even in the age of genomics, we have only just scratched the surface of this diversity.
Today, we have the luxury of molecular tools for classification, but taxonomic classifications can be traced back before the discovery of DNA, the concept of evolution and the invention of the microscope.
Early classifications were limited by the tools and views available to them. Fungi, once considered plant-like organisms, are more closely related to animals than plants. Fungi are not capable of photosynthesis: they are heterotrophic because they use complex organic compounds as sources of energy and carbon.
Some fungal organisms multiply only asexually, whereas others undergo both asexual reproduction and sexual reproduction with alternation of generations. Most fungi produce a large number of spores, which are haploid cells that can undergo mitosis to form multicellular, haploid individuals. Like bacteria, fungi play an essential role in ecosystems because they are decomposers and participate in the cycling of nutrients by breaking down organic and inorganic materials to simple molecules.
Fungi often interact with other organisms, forming beneficial or mutualistic associations. For example most terrestrial plants form symbiotic relationships with fungi. The roots of the plant connect with the underground parts of the fungus forming mycorrhizae. Through mycorrhizae, the fungus and plant exchange nutrients and water, greatly aiding the survival of both species Alternatively, lichens are an association between a fungus and its photosynthetic partner usually an alga.
Fungi also cause serious infections in plants and animals. For example, Dutch elm disease, which is caused by the fungus Ophiostoma ulmi , is a particularly devastating type of fungal infestation that destroys many native species of elm Ulmus sp. The elm bark beetle acts as a vector, transmitting the disease from tree to tree.
Accidentally introduced in the s, the fungus decimated elm trees across the continent. Many European and Asiatic elms are less susceptible to Dutch elm disease than American elms. In humans, fungal infections are generally considered challenging to treat. Unlike bacteria, fungi do not respond to traditional antibiotic therapy because they are eukaryotes.
Fungal infections may prove deadly for individuals with compromised immune systems. Fungi have many commercial applications. The food industry uses yeasts in baking, brewing, and cheese and wine making. Many industrial compounds are byproducts of fungal fermentation. Fungi are the source of many commercial enzymes and antibiotics. Fungi are unicellular or multicellular thick-cell-walled heterotroph decomposers that eat decaying matter and make tangles of filaments.
Fungi are eukaryotes and have a complex cellular organization. As eukaryotes, fungal cells contain a membrane-bound nucleus where the DNA is wrapped around histone proteins.
A few types of fungi have structures comparable to bacterial plasmids loops of DNA. Fungal cells also contain mitochondria and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus.
Unlike plant cells, fungal cells do not have chloroplasts or chlorophyll. Many fungi display bright colors arising from other cellular pigments, ranging from red to green to black. The poisonous Amanita muscaria fly agaric is recognizable by its bright red cap with white patches. Pigments in fungi are associated with the cell wall.
They play a protective role against ultraviolet radiation and can be toxic. The poisonous Amanita muscaria : The poisonous Amanita muscaria is native to temperate and boreal regions of North America. The rigid layers of fungal cell walls contain complex polysaccharides called chitin and glucans. Chitin, also found in the exoskeleton of insects, gives structural strength to the cell walls of fungi. The wall protects the cell from desiccation and predators.
Fungi have plasma membranes similar to other eukaryotes, except that the structure is stabilized by ergosterol: a steroid molecule that replaces the cholesterol found in animal cell membranes. Most members of the kingdom Fungi are nonmotile. The vegetative body of a fungus is a unicellular or multicellular thallus. Dimorphic fungi can change from the unicellular to multicellular state depending on environmental conditions.
Unicellular fungi are generally referred to as yeasts. Example of a unicellular fungus : Candida albicans is a yeast cell and the agent of candidiasis and thrush. This organism has a similar morphology to coccus bacteria; however, yeast is a eukaryotic organism note the nucleus.
Most fungi are multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of slender thread-like structures called hyphae singular, hypha , whereas the reproductive stage can be more conspicuous. The mass of hyphae is a mycelium. It can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue. Example of a mycelium of a fungus : The mycelium of the fungus Neotestudina rosati can be pathogenic to humans.
The fungus enters through a cut or scrape and develops a mycetoma, a chronic subcutaneous infection.
Most fungal hyphae are divided into separate cells by endwalls called septa singular, septum a, c. In most phyla of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hypha. They are described as perforated septa. The hyphae in bread molds which belong to the Phylum Zygomycota are not separated by septa.
Instead, they are formed by large cells containing many nuclei, an arrangement described as coenocytic hyphae b. Fungi thrive in environments that are moist and slightly acidic; they can grow with or without light. A bright field light micrograph of c Phialophora richardsiae shows septa that divide the hyphae. Like animals, fungi are heterotrophs: they use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants.
In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. However, unlike most animals, which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order: digestion precedes ingestion. First, exoenzymes are transported out of the hyphae, where they process nutrients in the environment.
Then, the smaller molecules produced by this external digestion are absorbed through the large surface area of the mycelium.
As with animal cells, the polysaccharide of storage is glycogen rather than the starch found in plants. Fungi are mostly saprobes saprophyte is an equivalent term : organisms that derive nutrients from decaying organic matter. They obtain their nutrients from dead or decomposing organic matter, mainly plant material.
Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily-absorbable glucose molecules. The carbon, nitrogen, and other elements are thus released into the environment.
Because of their varied metabolic pathways, fungi fulfill an important ecological role and are being investigated as potential tools in bioremediation. Some fungi are parasitic, infecting either plants or animals. Fungi can reproduce asexually by fragmentation, budding, or producing spores, or sexually with homothallic or heterothallic mycelia.
Perfect fungi reproduce both sexually and asexually, while imperfect fungi reproduce only asexually by mitosis.
0コメント