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Mulitcellular Organisms


Bread Mold Life Cycle Fungi The oldest fossils of fungi date back 460 million years ago. New knowledge on evolutionary lineage shows that they are more closely related to animals than plants. The first vascular plant fossils (dated back to about 400 million years ago) have petried mycorrhizae - indicating that plants and fungi moved onto land together. Mycorrhizae are the roots of fungus, which lives in plant roots where it provides the plant with minerals and the plant gives organic nutrients in return.

Figure 10-29 Bread Mold Life Cycle [view large image]

Figure 10-30 Fungi [view large image]

Fungi lack chloroplasts, as do animals. Thus the process of photosynthesis, which is very common and important in plant life because it serves as their source for food, is absent. Fungi obtain food by heterotrophic process. They secrete enzymes into the nutritive materials. These exoenzymes degrade those substances to which they are adapted, and the resulting compounds must be absorbed through the wall of the cell while leaving the residues (wastes) outside.

The bodies of all fungi, except unicellular yeast, are made up of filaments called hyphae. A hypha is an elongated cylinder containing a mass of cytoplasm and many haploid nuclei, which may or may not be separated by cross wall. A collection of hyphae is called a mycelium.

Fungi reproduce in accordance with the haplontic life cycle as shown in Figure 10-29. This figure shows the life cycle of the black bread mold. Asexually, a mycelium gives rise to spore-producing sporangia. Sexually, the tip ends of hyphae from opposite mating strains can fuse, giving a zygote, the only diploid portion of the cycle. After a period of dormancy, meiosis is followed by germination of the zygospore and production of a sporangium. Windblown spores, an adaptation to land, produce mycelia.

There are many different types of fungi as shown in Figure 10-30. There are sac fungi with saclike cells to store the spores; some have shape like a cup. Yeasts are sac fungi that do not form fruiting bodies; they carry out the fermentation: "glucose => carbon dioxide + alcohol" for either baking or production of wines and beers. The blue-green molds (notably penicillium) are also sac fungi; they are used to provide flavor in cheeses and to produce antibiotic penicillin. Club fungi include the mushroom, which store the spores in the gills under the cap. Some mushrooms are poisonous while other types of fungi cause diseases such as althlete's foot and vaginal infection.

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