Fungi Are Eukaryotic Heterotrophs with Cell Walls Made of Chitin

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Summary

21-1 The Kingdom Fungi

Fungi are eukaryotic heterotrophs with cell walls made of chitin.

Chitin is a complex carbohydrate. Fungi do not ingest their food. Instead, fungi digest food outside their bodies and then absorb it. All fungi except for yeasts are multicellular. Multicellular fungi are made up of thin filaments called hyphae (singular: hypha). Each hypha is only one cell thick. The bodies of multicellular fungi are made of hyphae tangled together into a thick mass called a mycelium. The mycelium allows a large surface area to come into contact with the food source through which the fungi grow. The fruiting body of a fungus is a reproductive structure growing from the mycelium in the soil beneath it. In a mushroom, the fruiting body is the aboveground part of the mushroom.

Most fungi reproduce both asexually and sexually.

• Asexual reproduction can occur when cells or hyphae break off and begin to grow on their own. Some fungi also make spores. In some fungi, spores are formed in structures called sporangia. Sporangia are found at the tips of hyphae called sporangiophores.

• Sexual reproduction in fungi usually involves two different mating types. One type is called “ + ” (plus), and the other is called “ —” (minus). When the hyphae of a “ + ” fungus meets the hyphae of a “ — ” fungus, they fuse together in the same cell. After a period of growth and development, the nuclei form a diploid zygote. The diploid zygote enters meiosis, and produces haploid spores.

Spores of fungi exist in almost every environment. Many fungi produce dry, almost weightless spores that are easily scattered by wind. For these spores to grow, they must land in a favorable environment. Temperature, moisture, and food conditions must be in the right combination. Most spores, therefore, do not grow into mature organisms.

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21-2 Classification of Fungi

There are four main groups of fungi: common molds, sac fungi, club fungi, and imperfect fungi. Fungi are classified according to their methods of reproduction and their structure.

Common molds (phylum Zygomycota) grow on meat, cheese, and bread. These fungi have a life cycle that includes a zygospore. A zygospore is a resting spore that contains zygotes formed during the sexual phase of the mold’s life cycle.

Common molds include the black bread mold. During the sexual phase in black bread mold, hyphae from different mating types fuse to form gamete-making structures called gametangia. Black bread mold has two kinds of hyphae.

• The rootlike hyphae that go through the bread’s surface are rhizoids.

• The stemlike hyphae that run along the surface of bread are stolons.

Sac fungi (phylum Ascomycota) include the large cup fungi and the unicellular yeasts. These fungi are named for the ascus, a reproductive structure that contains spores. The life cycle of an ascomycete includes both sexual and asexual reproduction.

In sexual reproduction, haploid hyphae from two different mating types (+ and —) grow close together to make a fruiting body. An ascus forms within the fruiting body. Two nuclei of different mating types fuse within the ascus to form a diploid zygote. Asexual reproduction in cup fungi and yeast are different.

• In cup fungi, tiny spores called conidia form at the tips of specialized hyphae called conidiophores.

• Asexual reproduction in yeast occurs by cell division. This process is called budding.

Club fungi (phylum Basidiomycota) include mushrooms, shelf fungi, and puffballs. These fungi have a reproductive structure that resembles a club. The cap of the fruiting body of a basidiomycete is made up of tightly packed hyphae. The lower side of the cap has gills, thin blades of tissue lined with basidia. A basidium is a spore-bearing structure. Two nuclei in each basidium fuse to form a diploid zygote cell. The zygote cell undergoes meiosis, forming clusters of spores called basidiospores. A single mushroom can produce billions of basidiospores.

Imperfect fungi (phylum Deuteromycota) include fungi that are not placed in other phyla because researchers have never been able to observe a sexual phase in their life cycles.

An example of an imperfect fungus is Penicillium notatum, a mold that grows on fruit. It is the source of the antibiotic penicillin.

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21-3 Ecology of Fungi

All fungi are heterotrophs. Some fungi are saprobes. Saprobes are organisms that obtain food from decaying organic matter.

Fungi play an essential role in maintaining balance in ecosystems. Fungi recycle nutrients as they break down the remains and wastes of other organisms. Many fungi feed by releasing digestive enzymes that break down organic material into simple molecules. In breaking down this matter, fungi help recycle nutrients and essential chemicals. Without such decomposers, the energy-rich compounds that organisms accumulate would be lost forever.

Parasitic fungi cause plant and animal diseases. A few cause diseases in humans.

• Fungal diseases in plants include corn smut and wheat rust.

• Fungal diseases in humans include athlete’s foot, ringworm, thrush, and yeast infections of the female reproductive tract.

Some fungi are symbiotes that form mutualistic relationships in which both partners benefit.

• Lichens are an association between a fungus and green alga, a cyanobacterium, or both. The alga or cyanobacterium provides the fungus with a source of energy by carrying out photosynthesis. The fungus provides the photosynthetic organism with water and minerals and shades it from intense sunlight.

• Mutualistic associations of plant roots and fungi are called mycorrhizae. The hyphae of fungi aid the plant in absorbing water and minerals. The fungi also release enzymes that free nutrients from the soil. The plant provides the fungi with the products of photosynthesis. The presence of mycorrhizae is needed for the growth of many plants.

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