第24章菌類

Fungi are eukaryotes, and as such, have a complex cellular organization. As eukaryotes, fungal cells contain a membrane-bound nucleus. The DNA in the nucleus is represented by multiple linear molecules wrapped around histone proteins, as is observed in other eukaryotic cells. A few types of fungi have accessory genomic structures comparable to bacterial plasmids (loops of DNA); however, the horizontal transfer of genetic information that occurs between one bacterium and another rarely occurs in fungi. Fungal cells also contain mitochondria and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus.

Unlike plant cells, fungal cells do not have chloroplasts or chlorophyll. Many fungi display bright colors arising from other cellular pigments, ranging from red to green to black. The poisonous Amanita muscaria (fly agaric) is recognizable by its bright red cap with white patches (Figure 24.2). Pigments in fungi are associated with the cell wall and play a protective role against ultraviolet radiation. Some fungal pigments are toxic to humans.

Like plant cells, fungal cells have a thick cell wall. The rigid layers of fungal cell walls contain complex polysaccharides called chitin and glucans. Chitin (N-acetyl-D-glucosamine), also found in the exoskeleton of arthropods such as insects, gives structural strength to the cell walls of fungi. The wall provides structural support and protects the cell from desiccation and some predators. Fungi have plasma membranes similar to those of other eukaryotes, except that the structure is stabilized by ergosterol: a steroid molecule that replaces the cholesterol found in animal cell membranes. Most members of the kingdom Fungi are nonmotile. However, flagella are produced by the spores and gametes in the primitive Phylum Chytridiomycota.

The vegetative body of a fungus is a unicellular or multicellular thallus. Unicellular fungi are called yeasts. Multicellular fungi produce threadlike hyphae (singular hypha). Dimorphic fungi can change from the unicellular to multicellular state depending on environmental conditions. Saccharomyces cerevisiae (baker’s yeast) and Candida species (the agents of thrush, a common fungal infection) are examples of unicellular fungi (Figure 24.3).

Most fungi are multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of hyphae, whereas the reproductive stage can be more conspicuous. The mass of hyphae is a mycelium (Figure 24.4). It can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue. Although individual hyphae must be observed under a microscope, the mycelium of a fungus can be very large, with some species truly being “the fungus humongous.” The giant Armillaria solidipes (honey mushroom) is considered the largest organism on Earth, spreading across more than 2,000 acres of underground soil in eastern Oregon; it is estimated to be at least 2,400 years old.

Most fungal hyphae are divided into separate cells by endwalls called septa (singular, septum) (Figure 24.5a, c). In most phyla of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hypha. They are described as perforated septa. The hyphae in bread molds (which belong to the Phylum Zygomycota) are not separated by septa. Instead, they are formed by large cells containing many nuclei (multinucleate), an arrangement described as coenocytic hyphae (Figure 24.5b).

Fungi thrive in environments that are moist and slightly acidic, and can grow in dark places or places exposed to light. They vary in their oxygen requirement. Most fungi are obligate aerobes, requiring oxygen to survive. Other species, such as members of the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes, in that they only use anaerobic respiration because oxygen will disrupt their metabolism or kill them. Yeasts are intermediate, being facultative anaerobes. This means that they grow best in the presence of oxygen using aerobic respiration, but can survive using anaerobic respiration when oxygen is not available. The alcohol produced from yeast fermentation is used in wine and beer production.