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• The fungi are a distinct kingdom of eukaryotic organisms characterized by a filamentous growth form, lack of chlorophyll and motile cells, chitin-rich cell walls, and external digestion of food by the secretion of enzymes.
• Fungal filaments, called hyphae, collectively make up the fungus body, which is called the mycelium.
• In many fungi, the two kinds of nuclei that will eventually undergo syngamy occur together in hyphae for a long period before they fuse. Meiosis occurs immediately after the formation of the zygote in all fungi; the zygote, therefore, is the only diploid nucleus of the entire life cycle in these organisms.
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1. What is
a hypha? What is the advantage to having incomplete septa?
2. What is the composition of the fungal cell wall? Why is this
composition an advantage to the fungi?
3. Which fungal nuclei are diploid? Which are haploid? To what
do the following terms refer: heterokaryotic, homokaryotic, dikaryotic,
and monokaryotic?
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• There are three phyla of fungi: Zygomycota, the zygomycetes; Ascomycota, the ascomycetes; and Basidiomycota, the basidiomycetes.
• Zygomycetes form septa only when gametangia or sporangia are cut off at the ends of their hyphae; otherwise, their hyphae are multinucleate. Most hyphae of ascomycetes and basidiomycetes have perforated septa through which the cytoplasm, but not necessarily the nuclei, flows freely.
• Cells within the heterokaryotic hyphae of ascomycetes are multinucleate; those within the heterokaryotic hyphae of the basidiomycetes are dikaryotic. Zygotes in ascomycetes form within sac-like structures known as asci, and those in basidiomycetes form within structures known as basidia.
• Asexual reproduction in zygomycetes takes place by means of spores from multinucleate sporangia; in ascomycetes, it takes place by means of conidia. Asexual reproduction in basidiomycetes is rare.
• The imperfect fungi are not a phylum but rather, a
group of fungi in which sexual reproduction has not been observed.
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4. What are
the three reproductive structures that occur in fungi? How do they differ?
5. Fungi are nonmotile. How are they dispersed to new areas?
6. What are the ascomycete asexual spores called? Do the nonreproductive
hyphae of this division have septa?
7. To what phyla do the yeasts belong? How do they differ from
other fungi? Is it more likely that this characteristic is primitive or
degenerate?
8. What are the imperfect fungi? Which phylum seems to be best
represented in this group? By what
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• Lichens are mutualistic symbiotic systems involving fungi (almost always ascomycetes), which derive their nutrients from green algae, cyanobacteria, or both.
• Mycorrhizae are mutualistic symbiotic associations
between fungi and plants. Endomycorrhizae, more common, involve zygomycetes,
while ectomycorrhizal fungi are mainly basidiomycetes.
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9. What are
lichens? Which fungal phylum is best represented in the lichens?
10. What are mycorrhizae? How do endomycorrhizae and ectomycorrhizae
differ?
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