Definition
Fungi are eukaryotic organisms (each containing a membrane-bound nucleus) that develop from reproductive bodies called spores. Fungi may be the cause of any number of diseases in humans, animals, and plants; fungal infections are called mycose.
Description
Mycology is the branch of science that studies organisms of the kingdom Fungi. Scientists estimate that over 200,000 species of fungus exist in nature. These species include yeasts, molds, mildews, mushrooms, lichens, and smuts.
There are a number of characteristics that fungi share: they are eukaryotic (containing a nucleus that is bound by a nuclear membrane); they develop from reproductive bodies called spores; their cell walls are composed mostly of chitin, a nitrogen-containing carbohydrate; and they are heterotrophic (they cannot synthesize their own food and therefore absorb food from an external source through their cell walls).
Most fungi obtain their nutrients from dead organic matter and are called saphrophytes. Saphrophytes play an important ecological role in the decomposition of dead plants, animals, and other organic matter: they release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients for use by plants and other organisms. Other fungi are parasites (obtaining their nutrients from a living host organism in a relationship that usually harms the host) or mutualists (involved in a mutually beneficial relationship with another organism).
Another important characteristic of fungi is that they do not contain chlorophyll. Chlorophyll is a green pigment that enables plants and other photosynthetic organisms such as algae and cyanobacteria to absorb energy from sunlight and use it to synthesize carbohydrates (photosynthesis). Because fungi are not reliant on sunlight as an energy source, they can grow in dark or low-light environments and in directions not normally observed in plants.
Morphology
Most fungi may be classified according to two major growth forms: yeasts or molds. Yeasts are round, unicellular (single-celled) organisms that form a vegetative body called a thallus. The thallus may consist of cells in groups or in branched chains called pseudo-hyphae. Examples of yeasts include Saccharomyces cerevisiae, used in making bread and alcoholic beverages; and Candida albicans, the causative agent of yeast infections.
Molds, on the other hand, are composed of long filaments called hyphae (singular, hypha). Hyphae may be further classified as septate (containing cross walls) or aseptate. A mass of hyphae is called a mycelium. Whereas yeast cells each contain a single nucleus, cells in septate hyphae may be uninucleate (containing one nucleus), binucleate (containing two nuclei), or multinucleate (containing many nuclei). An example of a mold is Penicillium roqueforti, used to make blue cheese.
Some fungi are dimorphic: they may exist in either yeast or mold form. What form a fungus assumes depends on environmental factors such as the temperature or nutrients present. Some examples of dimorphic fungi include Histoplasma capsulatum and Coccidioides immitis.
Reproduction
All fungi can reproduce asexually by the production of single-celled structures called spores. The number of chromosomes (structures in the nucleus containing genetic material) remains unchanged when cells duplicate their genetic material and then divide. This is not the ideal state for a fungus and is thus called the imperfect state. (It is often observed in the laboratory when fungi that are normally pathogenic to humans are allowed to reproduce.)
Sexual reproduction can also occur in most fungi and is called the perfect state. In this process, one cell divides to become two haploid cells (each containing a single set of unpaired chromosomes). Two cells can then fuse together to become a diploid cell (containing a full set of chromosomes); that cell can then divide.
Role in human health
Some fungi have been found to be directly or indirectly beneficial to humans, while others are pathogenic (disease-causing). Still others are pathogenic to plants and animals important in the food chain.
Food manufacturing
Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. For instance, S. cerevisiae is commonly used as baker's yeast and in the production of ales. Candida milleri is a yeast used in conjunction with an acid-producing bacteria to yield sourdough bread.
Various species of mushrooms are cultivated specifically for human consumption. These include Agaricus bisporus (accounting for 38% of the world's cultivated mushroom supply), Lentinus edodes (shiitake mushrooms), Volvariella volvacae (the paddy straw mushroom), and the Pleurotus family (oyster mushrooms). Other edible fungi include truffles (fungi of the family Tuber that grow in a special subterranean (mycorrhizal) association with certain trees), morels (of the Morchella family), and the blue-green mold of the Penicillium family that is essential in the production of certain cheeses.
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