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AP Biology: The Wonderful World of Fungus

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Sophia Barnes

on 25 March 2011

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Transcript of AP Biology: The Wonderful World of Fungus

Fungi are a diverse, helpful, and widespread category of eukaryotic organisms. Fungi are heterotrophs that feed by absorbtion. Fungi do not ingest their food - they release exoenzymes that break down complex molecules into simple organic compounds to be absorbed. Exoenzymes are powerful hydrolytic enzymes. They are released into the environment for digestion. Saprobic fungi (decomposers, or saprobs) breakdown and absorb nutrients from nonliving organic material. Parasitic fungi absorb nutrients from the cells of living hosts Mutualistic fungi also absorb nutrients from living species, but they also function to benefit the host. Most fungi are multicellular, which helps them absorb nutrients. Fungal bodies come from a network of hyphae.
Hyphae are composed of tubular cell walls that surround the plasma membrane and cytoplasm. Hyphae form a mass called mycelium that embed itself in the fungi's food source (Ex: Dirt). This maximizes the surface area to volume ratio. The cell walls of fungus contain chitin. Chitin is a strong and flexible polysaccaride that contains nitrogen. Mycelium can be so densly packed that 1cubic centimeter can contain 1 km of hyphae that have 300 square centimeters of exposed for absorbtion. Mycelium grow rapidly. Cytoplasmic streaming channels protiens and nutrients to the tips of hyphae, extending them into the soil. This increases surface area. Most hyphae are septate, meaning their cells are independant but connected through very large pores. Even nuclei and ribosomes can travel through these pores. Some hyphae are coenocytic. These cells are continuous cytoplasic streaming masses with perhaps thousands of nuclei. This is accomplished by repeated divison of nuclei without cytoplasmic division. Some hyphae can live off of other living things. Mycorrhizae can deliver phosphate ions and other materials to plants, while the plants supply fungi with organic nutrients. Haustoria can penetrate their host's tissues. Ectomycorrhizal fungi form sheaths over the surface of the root. Endomycorrhizal fungi extend their hyphae through the root cell wall. Fungi produce spores through sexual or asexual lifestyles. Fungi produce vast numbers of spores that can be carried long distances. When they land, they germinate and produce new mycelium. Most fungi are diploid only during sexual cycles. Fungi release pheromones, which are molecules that signal when other, different hyphae are near. These signals relate to sexual reproduction. The union of cytoplasm of two parent mycelia is known as plasmogamy. In fungi where haploid parent nuclei do not fuse right away, they are called heterokaryon. With dikaryotic fungi, two nuclei are in a cell but divide separately. Karyogamy is when haploid nuclei of parent fungi fuse. This does not always happen, and is the only diploid stage. Some mold grows asexually. Mold grows quickly and mycelia produce spores. Yeasts also are asexual fungi that inhabit moist habitats and reproduce by simple cell division. Deuteromycetes, or imperfect fungi, have no known sexual stage of life. Fungi descended from an aquatic, single celled, flagellated protist. Fungi, animals, and their protistan relatives fit into a category labeled opisthokonts. Opisthokonts are living things that descended from animals with posterior flagellum. Evidence suggests that the anscestors of fungi were single celled, and fungi and animals evolved into multicellular beings seperately. Fungi have been on land for a long time. Some of the earliest plants have evidence of mycorrhizae on their roots. Fungi have radiated into a diverse set of lineages. Chytrids are the earliest divergance of fungus. They are commonly found in lakes and soil. Chytrids are the only fungi with flagellated spores, called zoospores. Zygomycetes are a very diverse group. It represents fast growing molds, parasites, symbionts, and other things. Rhizopus stolonifer, or simply black bread mold, has a typical reproductive cycle out of zygomycetes. Cells are coenocytic, but contain septa in reproductive structures. Each sporangium can release hundreds of haploid spores. Some can even aim their spores. Zygomycete reproduction tutorial: http://bcs.whfreeman.com/thelifewire/content/chp31/31020.html Zygosporangium are produced by plasmogamy. These are the structures in which karyogamy and meiosis occur. See page 614 of 'Biology.' They are multinucleate structures. First, they are heterokaryotic with many haploid nuclei. Then, karyogamy (nucleus fusion) produces diploid zygosporangium. These fungi produce asexually in ideal conditions and sexually in unfavorable ones. Microsporidia are unicellular parasities of animals and plants. They mostly pose a risk to those with compromised immune systems. They are different than most eukaryotes: they have no real mitochondria. While it may appear that they are primitive, they are actually highly evolved.
Glomeromycetes all have arbuscular mycorrhizae where the tips of the hyphae push into the plant's root cells and form tiny structures called arbuscules. 90% of plants have mutualistic relationships with Glomeromycetes. Ascomycetes are a group of fungus distinguished by their production of sexual spores in asci. Fruiting bodies, known as ascocarps, are where sexual stages take place. More than 40% of ascomycetes live with cyanobacteria in lichens. Ascomecytes use conidia to produce large numers of spores asexually. They are produced at the tips of specialized hyphae called conidiophores. Page 617 of "Biology" outlines the reproductive cycle. Basidomycetes are molds, mutualists that form mycorrhizae, rusts, and smuts. The phylum is named for the basidium cells present during the transient diploid stage. The common name for basidomycetes, Club Fungus, comes from the club shape of these cells. These fungi are very important wood and plant decomposers because they can process lingin well. Basidocarps are the sexually reproducing fruiting bodies. Mushrooms are basidocarps. The reproductive cycle of these fungi is described on page 619 of 'Biology.' Fungi have a powerful impact on ecsystems and human welfare. Fungi are decomposers, which means they break down organic material and release needed nutrients from decaying matter. Fungi also form important symbiotic relationships, such as mycorrhizae, lichens, and animal relations. Lichens are symbiotic associations of millions of photosynthetic microorganisms held in a mass of fungal hyphae. Soredia are small clusters of hyphae embedded with algae. Fungi were originally used to produce penecillin. There are about 30,000 parasitic fungi. Mycosis is a general term for a fungal infection in humans or animals. Works Cited:
Campbell, N. & Reece, J. (2005). <i>Biology.</i> San Fransisco, CA: Pearson.
Photographs. Pathogenic Fungi. By S. Kilvington. Department of Microbiology and Immunology, 1996. Web. 20 Mar. 2011. <http://cmbi.bjmu.edu.cn/www-learn/micro-ac-uk/mbchb/6a.html>.
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