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BIO153 Ch 31 Notes.pdf

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University of Toronto Mississauga
Christoph Richter

Freeman,Biological Science, 4e, Chapter 31 Chapter 31 - Fungi Learning Objectives: Students should be able to ... • Describe the ecological importance of fungi. • Describe at least four types of symbiotic relationships that fungi can have with other organisms, and give an example of each. • List some fungal adaptations associated with the absorption of nutrients. • Describe how fungal life cycles differ from animal or plant life cycles. • Identify the distinct reproductive structures of the four traditional groups of fungi. Lecture Outline y What are the fungi? o Fungi are one of the three major lineages of multicellular eukaryotes. o Fungi absorb carbon and energy from other organisms. o Fungi may be decomposers or parasitic or beneficial symbiotes. o Fungi play a key role in nutrient cycling in terrestrial ecosystems. I. Why Do Biologists Study Fungi? A. Fungi provide nutrients for land plants. 1. Mycorrhizal fungi are fungi that live in close association with plant roots. 2. The fungi provide the plant with water and key nutrients such as nitrogen and phosphorus, in exchange for sugars. 3. Without these fungi, plant growth is greatly slowed; therefore, these fungi are important to agricultural productivity. (Fig. 31.1) B. Fungi speed the carbon cycle on land. 1. Carbon cycling during the Carboniferous period a. The coal deposits of the Carboniferous may be the result of a scarcity of saprophytic (decomposing) fungi at the time. b. The lack of fungi allowed large deposits of dead plant tissue to build up, eventually producing coal. c. Biologists hypothesize that because peatlands have an acidic pH, the fungi could not grow in them. 2. Carbon cycling at the end-Permian a. A "fungal spike" (a large increase in fungal fossils) coincides with the end-Permian mass extinction. (Fig. 31.2) b. The end-Permian extinction may have involved a massive die-off of land plants, resulting in a population explosion of saprophytic fungi. 3. Carbon cycling at present a. Today, saprophytic fungi speed the terrestrial carbon cycle. (Fig. 31.2) b. Fungi are almost the only organisms capable of digesting both cellulose and lignin, recycling carbon into glucose and carbon dioxide. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 31 c. Without fungi, carbon would be trapped in the wood for a very long time, and terrestrial environments would be far less productive. C. Fungi have important economic impacts. 1. About 31 species of fungi cause human disease. Examples: athlete's foot, diaper rash, vaginitis, pneumonia, thrush. 2. Soil-dwelling fungi have been the source of many important antibiotics. 3. Certain fungi destroy billions of dollars of crops each year. (Fig. 31.3) 4. Fungi have destroyed significant numbers of trees in North America. Examples: chestnut blight, Dutch elm disease. 5. Many fungi are used to make food. a. Mushrooms are a popular edible fungus. b. Yeasts are used to make bread, beer, wine and other foods. II. How Do Biologists Study Fungi? A. Millions of species of fungi probably remain undiscovered. 1. A British study found that there are probably at least six species of fungi per vascular plant species in the British Isles. 2. A Panama study found that the leaves of just two tree species housed 418 fungal species. 3. A direct sequencing study found that 49 species of fungi (many previously unknown) were found on the roots of just one species of grass. B. Analyzing morphological traits 1. Fungi have only two growth forms: a. Single-celled yeasts (Fig. 31.4a) b. Multicellular, filamentous mycelia. (Fig. 31.4b) 2. The nature of the fungal mycelium a. Mycelia can be very long-lived and very large. b. Mycelia constantly grow in the direction of food sources and die back when food is scarce. c. Mycelia are made up of individual filaments called hyphae. (Fig. 31.5a) 3. The nature of hyphae a. A hypha may have just one haploid nucleus or several (heterokaryotic). (1) Most heterokaryotic hyphae are dikaryotic (two nuclei per cell, one from each parent). b. A hypha is a long, narrow filament that branches frequently. c. Each hypha is subdivided by septa (walls) with pores through which cytoplasm, organelles, and other material can flow. (Fig. 31.5b) (1) Due to this flow, a fungal mycelium is intermediate between a multicellular organism and a large unicellular organism. d. Hyphae are so thin that they can penetrate tiny fissures in the soil and absorb nutrients that plant roots cannot reach. 4. Mycelia have a large surface area. a. The shape of mycelia gives fungi the highest surface-area-to- volume ratio of all multicellular organisms. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 31 b. The large surface area is important because fungi make their living via absorption; however, it also makes fungi susceptible to dying out. c. Students should be able to explain how mycelia are an adaptation for an absorptive lifestyle. 5. Fungi can perform sexual reproduction in one of four ways. a. Chytrids have swimming gametes and spores. (Fig. 31.6a) b. Zygomycetes produce a distinctive spore-producing structure called a zygosporangium, which develops when haploid hyphae from two individuals meet. (Fig. 31.6b) c. Basidiomycetes (mushrooms, puffballs, etc.) form spores in small pedestal-like structures called basidia at the ends of hyphae. (Fig. 31.6c) d. Ascomycetes produce spores with specialized cells called asci. (Fig. 31.6d) e. Students should be able to identify the four types of reproductive structures observed in fungi. C. Evaluating molecular phylogenies 1. Fungi are closely related to animals. (Fig. 31.7) a. Animals and fungi both produce chitin, have similar flagella, and use glycogen to store energy. 2. What is the relationship among the major fungal groups? (Fig. 31.8) a. Microsporidians are actually fungi. b. Chytrids and zygomycetes form a paraphyletic group that is poorly resolved. c. A group of zygomycetes called the Glomeromycota are monophyletic. d. Basidomycota and Ascomycota are each monophyletic groups, and together they form a monophyletic group. e. The sister group to fungi consists of choanoflagellates and animals. D. Experimental studies of mutualism 1. Plant–fungi symbiosis may be mutualistic (both benefit), parasitic (one benefits and one is harmed), or commensalistic (one benefits and the other is unaffected). 2. Biologists hypothesize that the evolution of plant–fungi symbiosis is mutualistic and has played a significant role in the diversification of fungi. a. Experiment: Add certain isotopes of C, N, and P to the air and soil to track the transfer of nutrients between plants and fungi. (Fig. 31.9) (1) Plants provide sugars to the fungi. (2) Fungi provide nitrogen and phosphorus to the plants. III. What Themes Occur in the Diversification of Fungi? A. The diversification of fungi was driven by the evolution of novel methods for absorbing nutrients from a diverse array of food sources. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 31 B. Fungi participate in several types of mutualism. 1. Most plants are covered from tips to roots with mutualistic fungi. 2. Ectomycorrhizal fungi (EMF) (Fig. 31.10a) a. EMF are often basidiomycetes (or occasionally ascomycetes). b. They form a dense network of hyphae (0.1 mm thick) around plant roots, extending between root cells and into the soil. c. They are abundant in regions with warm summers and cold winters, and they are especially common on trees of temperate and boreal forests. d. EMF are the dominant nutrient-gathering organs in temperate forests. (1) They secrete peptidases that break down proteins in decaying material, take up the amino acids, and transfer them to the host plant. (2) They also take up and transfer phosphorus to the host plant. (3) Experiment: Birch seedlings grown in forest soil in pots without EMF cannot acquire sufficient N and P for healthy growth. 3. Arbuscular mycorrhizal fungi (AMF; endomycorrhizal fungi) (Fig. 31.10b) a. AMF are members of Glomeromycota. b. Highly branched hyphae penetrate the cell walls of root cells and contact the plasma membrane directly. (1) This increases the contact area for the exchange of molecules. c. AMF occur in 80% of land plant species, especially in grasslands and tropical forests (areas with long, warm growing seasons). d. They deliver phosphorus to plants but not nitrogen. (1) Nitrogen is readily available in these environments because plant tissues degrade rapidly in warm climates. (2) But phosphorus is often scarce because it leaches out of soil. (3) Radioactive phosphorus was used to demonstrate that AMF deliver phosphorus to their host plants. e. AMF contain large quantities of a glycoprotein called glomalin, which enriches the organic content of soil. 4. Are endophytes mutualists? a. Endophytes are fungi that live in the aboveground tissues of a plant. b. Some may be beneficial. For example, endophytes in some grasses and in morning glories produce compounds that deter herbivores. c. Others may be commensals, since some studies have not documented any benefit to the host plant. 5. Mutualisms with other species a. Lichens are a mutualistic partnership between an ascomycete and either a cyanobacterium or an alga. b. Some ant species farm fungi inside their colonies, harvesting them for food. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 31 6. Students should be able to describe evidence that mutualism occurs in EMF, AMF, and the endophytes of grasses. C. What adaptations make fungi such effective decomposers? 1. Extracellular digestion a. Fungi secrete digestive enzymes outside their hyphae into their food, so digestion actually o
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