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Chapter 31

Chapter 31 Textbook Notes - Fungi

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Christoph Richter

_______________________________________________________________ Chapter 31: Fungi Key Concepts o Fungi are important in part because many species live in close association with land plants. They supply plants with key nutrients and decompose dead wood. They are the master recyclers of nutrients in terrestrial environments. o All fungi make their living by absorbing nutrients from living or dead organisms. Fungi secrete enzymes so that digestion takes place outside their cells. Their morphology provides a large amount of surface area for efficient absorption. o Many fungi have unusual life cycles. It is common for species to have a long-lived heterokaryotic stage, in which cells contain haploid nuclei from two different individuals. Although most species reproduce sexually, very few species produce gametes. Why Do Biologists Study Fungi?  Fungi nourish the plants that nourish us. They affect global warming, because they are critical to the carbon cycle on land. A handful of species can cause debilitating diseases in humans and crop plants. Fungi Provide Nutrients for Land Plants  Mycorrhizal associations between fungi and plant roots allow faster plant growth (Figure 31.1). Fungi Provide Nutrients for Land Plants  Saprophytes are fungi that make their living by digesting dead plant material.  They play a key role in carbon cycling.  The carbon cycle has two basic components: (1) the fixation of carbon by land plants, and (2) the release of CO2 from plants, animals, and fungi as the result of cellular respiration (Figure 31.3).  For most carbon atoms, fungi connect the two components. Fungi Provide Nutrients for Land Plants  Although parasitic fungi cause athlete’s foot, vaginitis, diaper rash, ringworm, pneumonia, and thrush in humans, the incidence of fungal infections in humans is very low.  Their major destructive impact is on crops (Figure 31.4). Fungi Are Key Model Organisms in Eukaryotic Genetics  The yeast Saccharomyces cerevisiae is very important in basic research on cell biology and molecular genetics.  It is easy to culture and manipulate in the lab, grows rapidly, and mutants can easily be created and transferred among individuals. How Do Biologists Study Fungi?  About 80,000 species of fungi have been described and named; about 1000 more are discovered each year.  The fungi are so poorly studied that the known species are widely regarded as a tiny fraction of the real total, estimated at 1.65 million species or even higher. Analyzing Morphological Traits  Fungi have very simple bodies. Two growth forms exist: (1) single–celled forms (yeasts) and (2) multicellular filamentous forms (mycelia) (Figure 31.5).  Some fungi adopt both life–forms. The Nature of the Fungal Mycelium  Mycelia grow out in the direction of food sources and die back in areas where food is running out.  The filaments that make up a mycelium are called hyphae, and most are haploid or heterokaryotic, with two haploid nuclei (Figure 31.6).  Each filament is separated by cell–like compartments called septa.  Because mycelia are composed of branching hyphae, the body of a fungus has a high surface–area– to–volume ratio.  This makes absorption extremely efficient but also makes fungi prone to drying out.  Mycelia are an adaptation to the absorptive lifestyle of fungi. Thus, reproductive organs—not feeding structures—are the only thick, fleshy structures that fungi produce. Reproductive Structures  There are four major groups of fungi based on reproductive structures (Figure 31.7): the Chytridiomycota, the Zygomycota, the Basidiomycota, and the Ascomycota.  Chytridiomycota (chytrids) live primarily in water and have spores and gametes with flagella.  Zygomycota have haploid hyphae of different mating types. Hyphae of different mating types may become yoked together and the cells of the hyphae fuse to form a spore–producing structure called a zygosporangium.  Basidiomycota, or club fungi, have basidia that form at the ends of hyphae and produce spores. Mushrooms, bracket fungi, and puffballs are among the complex reproductive structures this group produces.  Ascomycota, or sac fungi, produce complex reproductive structures. The tips of hyphae inside these structures produce distinctive saclike cells, called asci, that generate spores. Evaluating Molecular Phylogenies  Fungi are more closely related to animals than land plants (Figure 31.8).  The Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota have traditionally been recognized as separate phyla (Figure 31.9).  Chytrids are the most basal group of fungi.  Chytridiomycota and Zygomycota are paraphyletic. A single common ancestor did not give rise to all species within each phylum.  Microsporidians are phylogenetically within the fungi.  The Basidiomycota and Ascomycota are monophyletic. Experimental Studies of Mutualism  Fungi and land plants often have a symbiotic relationship—one of close association.  Mutualism is a symbiotic relationship that provides benefits to both the host and the fungus.  In a parasitic symbiotic relationship, one species benefits at the expense of the other.  In a commensal relationship, one species benefits while the other is unaffected.  Experimental evidence indicates that mycorrhizal fungi and plants are mutualistic (Figure 31.10). What Themes Occur in the Diversification of Fungi?  The evolution of novel methods for absorbing nutrients from a wide array of food sources drove the diversification of fungi. Fungi Participate in Several Types of Mutualisms  Fungi can be involved in both mycorrhizal associations and endophytic (aboveground) associations with plants. Extomycorrhizal Fungi  Ectomycorrhizal fungi (EMF) form
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