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Fungi are heterotrophic organisms that have wide-ranging functions as ecological decomposers, sources of nutrition, and the basis of antibiotic medications.

Fungi are an ecologically important group of organisms that are also utilized by humans in a variety of ways. Many fungi are decomposers that break down the wastes and dead materials of other organisms. As such, they play critical roles in food webs and nutrient cycling within an ecosystem. Some fungi are regularly used in food production, such as yeasts used in bread and beer making. Others, such as certain mushrooms, are directly consumed by humans and other organisms for nutrition. In addition, the discovery of a fungus releasing a substance with antibiotic properties, penicillin, revolutionized medicine. Fungi are heterotrophic, meaning that they acquire energy by consuming other organisms. However, some form symbiotic relationships with organisms that make their own food. A symbiotic relationship between plant roots and fungus found within the soil is called a mycorrhiza. The fungus provides minerals in exchange for organic compounds. Some fungi are parasitic, primarily of plant hosts. Human illnesses caused by fungal infection include athlete's foot and ringworm (which does not involve a worm). Each fungus is composed of a web of tiny hyphae. A hypha (plural, hyphae) is a small filament that makes up a fungal body. Hyphal cells are surrounded by a protective tubular cell wall that contains chitin, a polysaccharide used for structure in fungi and some animals, such as insects. In some fungi, the hyphal cells share a single continuous cellular interior fluid. The web of hyphae can be enormous, stretching multiple kilometers underground. The largest known individual organism on earth is a fungus living in Oregon that covers 3.4 square miles (8.8 square kilometers).

Reproductive bodies, such as mushrooms, develop out of the hyphae to produce haploid gametes called spores. Spores can travel long distances or remain dormant for a long time. Most fungi reproduce both sexually and asexually. However, some reproduce using only one of these strategies. Many fungi have an unusual heterokaryotic stage in their life cycle, which means that they have nuclei from multiple individuals in the same cell. This occurs during reproduction, when the haploid nuclei of two different individuals inhabit the same cell but have not yet fused. When the nuclei eventually fuse, they form a diploid cell. There are currently five recognized phyla of fungi: Ascomycota, Basidiomycota, Zygomycota, Chytridiomycota, and Glomeromycota. The first three groups are distinguished by the shape of their reproductive structures. The Chytrids are the only group in which organisms have cells with motile structures, flagella. One chytrid is responsible for a disease, chytridiomycosis, that is causing death in amphibian populations across the world. Members of Glomeromycota are known for their ability to form mycorrhizae.
Fungal bodies are composed of hyphae filaments forming a web called a mycelium. Different fungi have different reproductive structures, but most use spores.