FUNGI

FUNGI

Fungi are almost entirely multicellular (with yeast, Saccharomyces cerviseae, being a prominent unicellular fungus), heterotrophic (deriving their energy from another organism, whether alive or dead), and usually having some cells with two nuclei (multinucleate, as opposed to the more common one, or uninucleate) per cell. Ecologically this kingdom is important (along with certain bacteria) as decomposers and recyclers of nutrients. Economically, the Fungi provide us with food (mushrooms; Bleu cheese/Roquefort cheese; baking and brewing), antibiotics (the first of the wonder drugs, penicillin, was isolated from the fungus Penicillium), and crop parasites (doing several million dollars per year of damage). First classified as plants, fungi are now considered different enough from plants to be placed in a separate kingdom.

Body Plans and Nutritional Modes: Fungi contain unicellular, multinucleate, and multicellular forms. They are classified on the basis of their reproductive spores and the nature of their multinucleate or multicellular filaments known as hyphae. Fungal cells have cell walls containing the carbohydrate chitin. Fungi are absorptive heterotrophs: they breakdown food by secreting digestive enzymes onto a substrates and then absorbing the resulting small food molecules. Fungal hyphae have a small volume but large surface area, enhancing the fungal absorptive capacity. Fungal hyphae are collectively termed a mycelium. Some fungi are saprophytes, they obtain their food from the decaying bodies of plants and animals. Other fungi are parasites.Fungi usually grow better in an environment with a pH of 5, which is too acidic for the growth of most common bacteria. Most fungi are more resistant to osmotic pressure than bacteria and can therefore grow in high sugar or salt concentrations.They can grow on substances with a very low moisture content, generally too low to support the growth of bacteria. They require somewhat less nitrogen than bacteria for an equivalent amount of growth and are capable of metabolizing complex carbohydrates, such as lignin, that most bacteria cannot use for nutrients.

Molds: The thallus of a mold consists of long filaments of cells joined together; these filaments are called hyphae. Hyphae can grow to immense proportions. In most molds, hyphae contain cross-walls called septa, which divide them into distinct, uninucleate cell-like units. These hyphae are called septate hyphae. In few classes of fungi, the hyphae contain no septa and appear as long contionous cells with many nuclei. These are called coenocytic hyphae. Even fungi with septate hyphae, there are usually openings in the septa that make the cytoplasm of adjacent cells continous, these fungi are actually coenocytic organisms too. Hyphae grow by elongating tips. Each part of a hypha is capable of growth, and when a fragment breaks off, it can elongate to form a new hypha. The propotion of a hypha that obtains nutrients is called vegetative hypha; tha propoition concerned with reproduction is the reproductive or aerial hypha. Aerial hyphae often bear reproductive spores. When environmental conditions are suitable, the hyphae grow to form filamentous mass called mycelium, which is visible to the unaided eye.

Yeasts: are nonfilamentous, unicellular fungi that are typically spherical or oval. Yeasts undergo fission, known as fission yeasts, such as Schizosaccharomyces, divide evenly to produce new cells. Yeasts that undergo budding, or budding yeasts, such as Saccharomyces divide unevenly. Most yeasts are facultative anaerobes capable of anaerobic growth. If given access to oxygen, yeasts perform aerobic respiration to metabolize carbohydrates to carbon dioxide and water; denied oxygen they ferment carbohydrates and produce carbon dioxide and etanol. This fermentation is used in the brewing, wine-making and baking industries. Species of Saccharomyces produce ethanol in brewed beverages and carbon dioxide for leavening of dough.

Some fungi, most notably the pathogenic species exhibit dimorphism-two forms of growth. Such fungi can grow either as mold or as yeast. The mold like forms produce vegetative and aerial hyphae; the yeast like forms reproduce by budding.

Life cycle: Filamentous fungi can reproduce asexually by fragmentation of their hyphae. In addition both sexual and asexual reproduction of fungi occurs by the formation of spores. Fungal spores, however are quite different from bacterial endospores. Bacterial endospores are formed so the bacterial cell can survive adverse environmental conditions. A single vegetative bacterial cell forms only one endospore, which eventually germinate to produce a single vegetative bacterial cell. This process is not reproduction because it does not increrase the total number of bacterial cells. But after a mold forms a spore, the detaches from the parent and germinates into a new mold. Unlike bacterial endospote this is a true reproductive spore. Although fungal spores can survive for extended periods in dry or hot environments, most do not exhibit the extreme tolerence and longevity of bacterial endospores. Spores are formed from areal hyphae in a number of different ways, depending on the species. Fungal spores can be either asexual or sexual. Asexual spores are formed by hyphae of one organism. When these spores germinate, they become organisms that are genetically identical to the parent. Sexual spores result from the fusion of nuclei from two opposite mating strains of the same species of fungus. Organisms that grow from sexual spores have genetic characteristics of both parental strains. The different types of asexual spores seen in fungi includes:

(a) Conidiospores: unicellular or multicellular spores that is not included in a sac. There are two types of conidiospores i.e an arthrospore formed by fragmentation of septate hypha into single thickend cells and blastospores, formed during budding (as seen in Candida sp).

(b) Chalamydospores: thich walled spore formed by rounding and enlargement within hyphal segment.

(c) Sporangiospore: an asexual spore formed within sporangium, or a sac at the end of aerial hypha (as seen in Rhizopus).

Classification of fungi:

Over 60,000 species of fungi are known. Fungi are classified by their method of reproduction (both sexual and asexual). Historically they have been divided into four taxonomic divisions: Deuteromycota, Zygomycota, Ascomycota, and Basidiomycota.

1. Deuteromycota: Fungi are placed in the category Deuteromycota if they have not yet been found to produce sexual spores. Members of this division produce asexual chalamydospores arthrospores and conidiospores; budding also occurs. Deuteromycota have septate hyphae. Some of them are expecially pathogenic species exhibit dimorphic growth. ex: Candida albicans.

2. Zygomycota: (or conjugation fungi) The zygomycete hyphae do not have one nucleus per cell, but rather have long multinucleate, haploid hyphae (coenocytic hyphae) that comprise their mycelia. Asexual reproduction is by spores produced in stalked sporangia (sporangiospores). Sexual reproduction involved the fusion of haploid mating hyphae to produce a diploid zygospore. There are less than 1000 species of zygomycetes. Common bread molds (Rhizopus) are in this group, as are a few species that parasitize plants and animals. Most zygomycetes feed on dead or decaying plant and animal material.(ex: Rhizopus, Mucor etc)

3. Ascomycota (or sac fungi): This division contains more than 30,000 species of unicellular (yeasts) to multicellular fungi. Yeasts reproduce asexually by budding and sexually by forming a sac (or ascus). An ascoppore results from the fussuin of nuclei of two cells that can be either morphologically similar or dissimilar. The asexual spores are usually conidiospores produced in long chains to form conidiophore. (ex: Saccharomyces, Aspergillus sp etc).

4. Basidiomycota (or club fungi): Mushrooms, toadstools, and puffballs are commonly encountered basidiomycetes. These conspicuous features of the fungi are the reproductive structures known as fruiting bodies. Sexual reproduction involves the formation of basidiospores on club-shaped cells known as basidia. (ex: Agaricus campestris , Amanita phalloides etc..

Lichens and Mycorrhizae: Lichens are a symbiosis between a photosynthetic organism (alga or cyanobacterium) and a fungus (sac or club). Mycorrhizae are fungi (usually a zygomycete or basidiomycete) symbiotic with the roots of plants. Both relationships are mutualistic: both parties benefit. Fungi provide nutrients from the substrate, the phototroph provides food. Plants with mycorrhizae grow better: the plant gets nutrients from the fungus in exchange for carbohydrates.