Fungi Fungi are eukaryotic heterotrophs that digest food externally and absorb the the digested materials through their body walls. Most fungi, including molds and mushrooms, are multicellular and macroscopic. Yeasts, though, are unicellular and microscopic. Like plants, fungal cells have walls, but these walls are made of chitin, not cellulose. Fungi are classified by the way they produce sexual spores. Hyphae are the basic structure of threadlike filaments that grow from a fungal spore. Hyphae grow into a mycelium – a cottony mass covering and distributed within whatever the fungus is feeding on. Inside the hyphae are cross walls called septa. Within the septa there can be one to many nuclei depending on the species. Cytoplasm flows through pores in the septa This is a cross-section of hyphae. It shows septa, pore and cytoplasm. Fungi are heterotrophs. Most are saprobes, decomposing dead organisms. Some fungi have symbiotic relationships with other organisms. Parasitic fungi absorb nutrients from their host through specialized hyphae called haustoria. Lichens are among the most fascinating organisms on this planet. Their very structure is unique: a symbioses of two organisms -- a fungus and algae -- so complete that they behave and look like an entirely new being. A lichen can literally eat stones, survive severe cold, and remain dormant for long periods without harm. What type of relationship is this? Mycorrhiza is another symbiotic association in which a fungus lives in close contact with roots of a plant partner. What type of symbiosis is this? Fungi use extra cellular digestion – breaking down their food outside the cell then absorbing the nutrients. Fungi may reproduce sexually or asexually. Most produce spores which grow when they encounter favorable conditions. Budding is a form of asexual reproduction in which mitosis takes place and a new organism grows out and eventually separates from the parent cell. A sporangium is a sac or case in which spores are produced. This is a Rhizopus sporangium. The four phyla of fungi are identified by the shape and presence of their sporangium. Phylum Zygomycota’s common names are “bread molds” and “pin molds.” Why do they carry these names? Two views of a Zygomycota’s sporangium. Phylum Zygomycota has stolons which grow horizontally across the surface of the food source and produce Rhiziods which grow down to anchor the mycelium to the food source. This is the site of most extracellular digestion and absorbtion. Phylum Zygomycota is characterized by zygospores – thick walled spores capable of withstanding unfavorable conditions. Phylum Ascomycota are commonly called “sac fungi.” Phylum Ascomycota is characterized by ascospores – sexual spores produced inside an Ascus – a saclike structure in which ascospores are produced. This phylum also has conidiaspores asexual spores produced in conidia. Phylum Basidiomycota is also the “Club Fungi”. It is characterized by basidia – club shaped hyphae that produce basidiaspores during sexual reproduction.
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