Fungi absorb food after digesting it outside their bodies. Fungi absorb food after digesting it outside their bodies. Fungi

Transcription

1 Fungi absorb food after digesting it outside their bodies Fungi are absorptive heterotrophic eukaryotes, secrete powerful enzymes to digest their food externally, and acquire their nutrients by absorption. Fungi absorb food after digesting it outside their bodies Most fungi consist of a mass of threadlike hyphae ( 菌絲 ) making up a mycelium ( 菌絲體 ). Hyphal cells are separated by cross-walls with pores large enough for ribosomes, mitochondria, and to cross, are sometimes multinucleate without cross-walls, and have a huge surface area to secrete digestive enzymes and absorb food. Figure 7.B Reproductive structure Hyphae Spore-producing structures (tips of hyphae) Mycelium Animation: Fungal Reproduction and Nutrition Right click on animation / Click play

2 Fungi absorb food after digesting it outside their bodies Fungal hyphae are surrounded by a cell wall made of chitin instead of cellulose ( 纖維素 ). Some fungi are parasites and obtain their nutrients at the expense of living plants or animals. Fungi absorb food after digesting it outside their bodies Mycorrhizae (plural) represent a symbiotic relationship between fungi and plant root cells and are present in nearly all vascular plants. Mycorrhizal fungi absorb phosphorus and other essential materials from the soil and make them available to the plant. Sugars produced by the plant through photosynthesis nourish the mycorrhizal fungi. Fungi produce spores in both asexual and sexual life cycles Fungi produce huge numbers of asexual spores, each of which can germinate to form a new fungus. Fungi produce spores in both asexual and sexual life cycles In many fungi, sexual fusion of haploid hyphae leads to a heterokaryotic stage, in which cells contain two genetically distinct haploid. Hours or centuries may pass before parental fuse to form a short-lived diploid phase. Zygotes undergo meiosis to produce haploid spores. Fungi produce spores in both asexual and sexual life cycles In asexual reproduction, spore-producing structures arise from haploid mycelia that have undergone neither a heterokaryotic stage or meiosis. Many fungi that reproduce sexually can also produce spores asexually. (unfused ) Diploid (n) Spore-producing structures Spores (n) Asexual Mycelium reproduction Germination stage Fusion of cytoplasm Germination Spores (n) Sexual reproduction Fusion of Meiosis Spore-producing structures Zygote (n)

3 Fungi produce spores in both asexual and sexual life cycles Molds ( 黴 ) are any rapidly growing fungus that reproduces asexually by producing spores. Yeasts are single-celled fungi that reproduce asexually by cell division or budding. There are over 00,000 described fungi species. Suspected but as yet undescribed species may number as many as.5 million. Sexual reproductive structures are often used to classify fungi. Fungi and animals may have diverged from a flagellated unikont ancestor more than billion years ago. Chytrids ( 壺菌 ) are the only fungi with flagellated spores and earliest lineage of fungi. Chytrid fungi are common in lakes, ponds, and soil and linked to the widespread decline of amphibian species. Chytrids ( 壺菌 ) Zygomycetes (zygote fungi) ( 接合菌 ) Glomeromycetes (arbuscular mycorrhizal fungi) ( 球囊菌 ) Ascomycetes (sac fungi) ( 原囊菌 ) Basidiomycetes (club fungi) ( 擔子菌 ) Zygomycetes, or zygote fungi ( 接合菌 ) are characterized by their protective zygosporangium ( 接合孢子囊 ), where zygotes produce haploid spores by meiosis. This diverse group includes fast-growing molds that attack bread peaches, strawberries, sweet potatoes, and some animals.

4 Figure 7.6C Glomeromycetes ( 球囊菌 ) form a distinct type of mycorrhizae, in which hyphae that invade plant roots branch into treelike structures known as arbuscules ( 叢枝體 ). About 90% of all plants have symbiotic partnerships with glomeromycetes. Glomeromycete: an arbuscule in a root cell Ascomycetes ( 原囊菌 ), or sac fungi Ascomycetes form saclike structures called asci ( 菌囊 ), which produce spores in sexual reproduction, live in marine, freshwater, and terrestrial habitats, and range in size from unicellular years to elaborate ( 綿密 ) morels ( 羊肚菌 ) and cup fungi. Some ascomycetes live with green algae or cyanobacteria in symbiotic associations called lichens. Edible morels Cup fungus Basidiomycetes, or club fungi ( 擔子菌 ), include common mushrooms, puffballs ( 馬勃 ), and shelf fungi ( 托架真菌 ) and are named for their club-shaped, spore-producing structure called a basidium ( 擔子 ). These fungi include important forest decomposers and particularly destructive plant parasites called rusts ( 銹斑菌 ) and smuts ( 黑穗菌 ).

5 Figure 7.6E Basidiomycetes Mushrooms A puffball Shelf fungi Fungal groups differ in their life cycles and reproductive structures The life cycle of a black bread mold is typical of zygomycetes. Hyphae reproduce asexually by producing spores in sporangia at the tips of upright hyphae. Fungal groups differ in their life cycles and reproductive structures When food is depleted ( 耗盡 ), the fungus reproduces sexually. Mycelia of join and produce a zygosporangium, a cell containing multiple from two parents. The zygosporangium develops into a thick-walled structure that can tolerate dry, harsh conditions. When conditions are favorable, the parental fuse to form diploid zygotes, which undergo meiosis producing haploid spores. Hyphae of different mating types Cells fuse Zygosporangium (n + n) Fungal groups differ in their life cycles and reproductive structures The life cycle of a mushroom is typical of basidiomycetes. Young zygosporangium (heterokaryotic) Diploid (n) Fusion of Sporangium (on stalk arising from the zygosporangium) Meiosis Spores (n) The heterokaryotic stage begins when mycelia of two fuse, forming a heterokaryotic mycelium, which grows and produces the mushroom. 5

6 Fungal groups differ in their life cycles and reproductive structures In the club-shaped cells called basidia, which line the gills of the mushroom, the haploid fuse, forming diploid. Each diploid nucleus produces haploid spores by meiosis. A mushroom can release as many as a billion spores. If spores land on moist matter that can serve as food, they germinate and grown into haploid mycelia. Diploid (n) Fusion of Diploid Mushroom Mushroom Basidia mycelium Diploid (n) mycelium Diploid (n) Fusion of Diploid Fusion of Diploid Mushroom Meiosis Mushroom Meiosis Basidia Haploid Spore (n) Basidia Haploid Spore (n) 5 Haploid mycelia mycelium Diploid (n) mycelium Diploid (n) 6

7 Parasitic fungi harm plants and animals Figure 7.8A Order Of the 00,000 known species of fungi, about 0% are either parasites or pathogens in or on plants. About 80% of plant diseases are caused by fungi. Between 0 and 50% of the world s fruit harvest is lost each year to fungal attack. A variety of fungi, including smuts and rusts, infect grain crops. Stately ( 挺拔 ) English elms ( 榆樹 ) in Australia, unaffected by Dutch elm disease Ergots Corn smut Ergots ( 麥角菌 ) on rye ( 黑麥 ) Parasitic fungi harm plants and animals Fungi have enormous ecological benefits Only about 50 species of fungi are parasitic on animals. The general term for a fungal infection is mycosis ( 真菌病 ). Skin mycoses include ringworm ( 錢癬 ), named because it appears as circular red areas on the skin, athlete ( 運動員 ) s foot, also caused by the ringworm fungus, vaginal yeast infections, and deadly lung diseases. Fungi supply essential nutrients to plants through symbiotic mycorrhyizae and are essential decomposers in ecosystems, breaking down decomposing leaves, logs, and feces ( 糞便 ) and dead animals. Fungi may also be used to digest petroleum ( 石油 ) products to clean up oil spills, such as the 00 BP (British Petroleum) spill in the Gulf of Mexico. 7

8 Fungi have many practical uses A fungal mycelium Fungi have many practical uses for humans. We eat mushrooms and cheeses modified by fungi. Yeasts produce alcohol and cause bread to rise. Some fungi provide antibiotics that are used to treat bacterial disease. Fungi figure prominently ( 突出 ) in molecular biology and in biotechnology. Yeasts, for example, are often used to study molecular genetics of eukaryotes. Fungi may play a major role in the future production of biofuels from plants. Blue cheese Staphylococcus aureus (bacteria) Penicillium (mold) Zone of inhibited growth Blue cheese Lichens are symbiotic associations of fungi and photosynthetic organisms Lichens consist of algae or cyanobacteria within a mass of fungal hyphae. Many lichen associations are mutualistic. The fungus receives food from its photosynthetic partner. The fungal mycelium helps the alga absorb and retain water and minerals. White rot fungus ( 白腐菌 ) 8

9 Lichens are symbiotic associations of fungi and photosynthetic organisms Lichens are important pioneers on new land, where they help to form soil. Lichens are sensitive to air pollution, because they obtain minerals from the air. Algal cell Fungal hyphae Reindeer moss, a lichen (a) Pine tree, a gymnosperm (b) Puffball, a club fungus 9