Ch31WordLectureOutline - CHAPTER 31 FUNGI Introduction...

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CHAPTER 31 FUNGI Introduction Ecosystems would be in trouble without fungi to decompose dead organisms, fallen leaves, feces, and other organic materials. This decomposition recycles vital chemical elements back to the environment in forms other organisms can assimilate. Most plants depend on mutualistic fungi that help their roots absorb minerals and water from the soil. Human have cultivated fungi for centuries for food, to produce antibiotics and other drugs, to make bread rise, and to ferment beer and wine. A. Introduction to the Fungi Fungi are eukaryotes and most are multicellular. While once grouped with plants, fungi generally differ from other eukaryotes in nutritional mode, structural organization, growth, and reproduction. Molecular studies indicate that animals, not plants, are the closest relatives of fungi. 1. Absorptive nutrition enables fungi to live as decomposers and symbionts Fungi are heterotrophs that acquire their nutrients by absorption . They absorb small organic molecules from the surrounding medium. Exoenzymes , powerful hydrolytic enzymes secreted by the fungus, break down food outside its body into simpler compounds that the fungus can absorb and use.
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The absorptive mode of nutrition is associated with the ecological roles of fungi as decomposers (saprobes), parasites, or mutualistic symbionts. Saprobic fungi absorb nutrients from nonliving organisms. Parasitic fungi absorb nutrients from the cells of living hosts. Some parasitic fungi, including some that infect humans and plants, are pathogenic. Mutualistic fungi also absorb nutrients from a host organism, but they reciprocate with functions that benefit their partner in some way. 2. Extensive surface area and rapid growth adapt fungi for absorptive nutrition The vegetative bodies of most fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium . Fungal mycelia can be huge, but they usually escape notice because they are subterranean. One giant individual of Armillaria ostoyae in Oregon is 3.4 miles in diameter and covers 2,200 acres of forest, It is at least 2,400 years old, and weighs hundreds of tons. Fungal hyphae have cell walls. These are built mainly of chitin , a strong but flexible nitrogen- containing polysaccharide, identical to that found in arthropods. Most fungi are multicellular with hyphae divided into cells by cross walls, or septa . These generally have pores large enough for ribosomes, mitochondria, and even nuclei to flow from cell to cell. Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei. This results from repeated nuclear division without cytoplasmic division.
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Ch31WordLectureOutline - CHAPTER 31 FUNGI Introduction...

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