Fungi range in form and size from unicellular yeasts to large mushrooms and puffballs. Yeasts are unicellular, do not have flagella and reproduce asexually by budding or transverse fission, or sexually by spore formation. Multicellular forms such as moulds have long, branched, threadlike filaments called hyphae, which aggregate together to form a tangled mycelium (Figure 8.1a). In some fungi the hyphae have crosswalls or septa (sing: septum) separating cells, which may nevertheless be joined by one or more pores, which permit cytoplasmic streaming, a form of internal transport. Such hyphae are said to be septate; others have no crosswalls and are therefore coenocytic (i.e. many nuclei within a single plasma membrane, Figure 8.1b).
Many fungi are dimorphic, that is, they exist in two distinct forms. Some fungi that cause human infections can change from the yeast form in the human to a mycelial form in the environment in response to changes in nutrients, and environmental factors such as CO2 concentration and temperature. This change in body form is known as the YM shift; in fungi associated with plants, the shift often occurs the other way round, i.e. the mycelial form exists in the plant and the yeast form in the environment.
One of the features that caused taxonomists finally to remove fungi from the plant kingdom was the distinctive chemical nature of the fungal cell wall. Whereas plant and algal cells have walls composed of cellulose, the cell wall of fungi is made up principally of chitin (Figure 3.17b), a strong but flexible polysaccharide that is also found in the exoskeleton of insects. It is a polymer whose repeating subunit is N-acetylglucosamine, a compound we encountered when discussing peptidoglycan structure (Chapter 3).
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