24.+Fungi


 * Fungi** are **heterotrophic** decomposers, meaning they must obtain organic compounds that other organisms have synthesized because they cannot make their own food. Some Fungi are **saprobes** which feed on and decompose organic wastes while others are **parasites** which withdraw nutirents from the tissues of a living host. Fungi have a **symbiotic** relationship with several organisms, meaning that the organisms interact. A relationship is **mutualistic** if the interaction benefits both or does not harm one. As they are growing on top of matter they perform **extracellular digestion and absorption.** Fungi grow, they secrete digestive enzymes and absorb breakdown products; this process is unique to all fungi and benefits both fungi and plants because nutrients are cycled within communites. Fungi make it easier for plants to absorb water and minerals, and turn dead organic material into nutrients so that plants can grow. Plants photosynthesize sugars to feed the fungi. Some bacteria also decompose matter. The mutualistic relationships between mycelia and plant roots are called **mycorrizae**. Fungi evolved 900 million years ago. According to fossil records, there are three major lineages: **zygomycetes**, **sac** **fungi**, and **club** **fungi**.

Fungi often reproduce asexually but can reproduce sexually when the opportunity presents itself through the usage of **spores** and nonmotile spores.Spores are reproductive cells or multicelled structures which germinate after dispersal from the parent. As for the means of asexual reproduction, **hyphae** contribute to the production of asexual spores. The hyphae cells have chitin-reinforced walls and form the **mycelium,** a mesh of branched filaments which exists in multicelled organisms. These filaments/hyphae have a good surface-to-volume ratio for absorbing nutrients.Other means of asexual reproduction is fragmentation and budding. An example of budding is can be found in yeast cells which holds thousands of //Saccharomyces cerevisiae// spores. In warm, moist spots the spores give rise to cells that reproduce by budding. //Candida albicans// reproduces by budding and causes yeast infections of the vagina, mouth, intestines and skin. Examples of asexual spores include Sporangiospores and Conidia. Condia is derived from the word meaning "dust" because conidiospores drift away like dust after the slightest jiggling. Asexual haploid spores form on a sporangium perched on top of a stalk (at the tip of the hypha). Sexual spores are produced through meiosis.

The mycelium is the collective mass of nutrient-absorbing filaments called hyphae which make up the majority of the fungus. When there is little nutrition left in the place the fungus is growing in, the fungus begins to reproduce. Fungi can reproduce sexually. This occurs when two hyphae with different genes (i.e. part of different organisms) come into contact with one another. These two cells are now called gametangia, but do not produce actual gametes. Single cells fuse-**plasmogamy** occurs, meaning that the cytoplasm of each cell fuses to form a single membrane encasing two nuclei and multiple organelles.This new cell is **dikaryotic//,//** having two haploid nuclei. Then, the cell undergones **karyogamy**, the fusing of the two nuclei. As cells in the hyphae of fungi are haploid, once the nuclei fuse, a single diploid nucleus forms. The nucleus divides by mitosis, resulting in two new haploid nuclei. These daughter cells germinate into fruiting bodies, which contain haploid spores. These spores are scattered into the environment, where they colonize new land and grow into new mycelia.

As mentioned earlier, spores are reproductive cells of multi-celled structures. They can be classified into four kinds. **Zygospores** are formed by zygomycetes through meiosis and decay. They are thick-walled sexual spores enclosed in a thin, clear covering. After a zygospore has finished meiosis, it keeps dividing, so it is called a zygote. A tall, sturdy hypha grows from it and a sporangium forms around the ballooned-out tip of the hypha. Inside the spore sac are hundres of tiny haploid spores and each spore can give rise to a mycelium. These spores are very resistant to the environment. **Ascospores** are sac shaped cells in an ascus. Asci are enclosed in reproductive structures of tightly interwoven hyphae.**Basidiospores** are club-fungi spores which form on the gill margins on club-shaped structures. They are distributed by air currents and can germinate and give rise to a haploid mycelium whn they land on a suitable site. No one has discovered what kind of sexual spores are produced, if any, these organisms produce so they are called **Imperfect Spores.** They cannot be assigned to any of the recognizable taxonomy groups. Mushrooms are part of the basidiomycetes which produce basidospores. Mushrooms are the actual reproductive bodies and are short lived; their mycelium are buried beneath the soil.

Fungi are classified by the type of spores they produce. **Zygomycetes** produce zygosopores. Most zygomycetes reproduce asexually. Bread mold is an example of this kind of fungi. **Ascomycota** produce ascospores. These are spores that form inside asci, or sacs formed by hyphae. Some yeasts are ascospores. Yeasts, which are single-celled and often reproduce asexually, reproduce sexually when two cells fuse and become a spore-producing sac. Byproducts of yeasts performing alcohal fermentation have been used by humans for centuries. Yeasts live in environments rich in polysacchrides, such as the nectar of floweres. Truffles, an expensive, edible fungus, are ascospores. //Aspergillus//and //penicillium// are other examples of ascospores. Most edible fungi are club fungi (**basidiomycetes//)//** which contain spores in club-shaped structures on top of stalks. Basidiomycetes reproduce sexually. Club fungi have gills that hold the spores. Wind blowing through the gills disperses the spores. Rusts and smuts are types of club fungi which destroy crops. Glomeromycota do not produce zygospores but are a small group of fungi that occur only in mutualistic associations with roots of plants.

Lichens are an example of a symbiotic relationship between fungi and plants. Lichens are single vegetative bodies with a fungi intertwined with one or more photosynthetic species (normally algae). They colonize in extreme environments such as Antarctica, but die easily in areas with intense air pollution. Since they are hypersensitive to air quality, they are indicators of deteriorating environmental conditions. They survive in such hostile environments by absorbing minerals from rocks and nitrogen from air

Starr, Cecie, and Ralph Taggart. //__Biology : The Unity and Diversity of Life.__// 10th ed. Belmont: Brooks/Cole, 2003.

Pack, Phillip E. //Cliffs AP:// //AP Biology//. 3rd ed. Hoboken: Wiley Publishing, Inc., 2007.

1) Tube Lichen: http://www.naturephoto-cz.com/photos/others/tube-lichen-22219.jpg 2) Mycelium and Hyphae: http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/MajorDivisions/KingdomFungi/Fungi/92386a.jpg