Fungi are a kingdom of eukaryotic organisms that obtain nutrients by absorbing organic substances from their surroundings. They include unicellular yeasts, filamentous molds, and organisms that produce mushrooms and other conspicuous reproductive structures. Unlike plants, fungi do not perform photosynthesis. Molecular evidence places them closer to animals than to plants. Their activities range from recycling dead organic matter to forming partnerships with other organisms and causing disease. The scientific study of fungi is called mycology. (openstax.org)
Structure and nutrition
A typical fungal cell contains a membrane-bound nucleus and internal organelles. Its wall generally contains chitin and glucans, rather than the cellulose characteristic of plant cell walls. The cell membrane commonly contains ergosterol, a sterol that contributes to membrane function. These features distinguish fungi from both plants and bacteria and are important in identifying fungi and understanding their physiology. (openstax.org)
Most filamentous fungi grow as narrow, branching tubes called hyphae. Together, these form a mycelium, which constitutes much of the organism’s feeding body. Hyphae often have cross-walls, or septa, with pores that permit movement of materials between compartments. Other hyphae lack regular septa and contain multiple nuclei within a continuous cytoplasm. Yeasts generally grow as individual cells; some fungi can alternate between yeast-like and filamentous forms. A mushroom is usually a reproductive structure, not the entire fungus. (openstax.org)
Fungi are absorptive heterotrophs. They release enzymes that break complex substances down outside their cells, then absorb the resulting smaller compounds. This method allows different species to exploit dead material, living hosts, or resources supplied by symbiotic partners. Growth through a substrate brings hyphae into contact with nutrients, while their narrow shape provides a large surface for absorption. (openstax.org)
Reproduction and life cycles
Fungi reproduce through diverse sexual and asexual processes. Asexual reproduction includes budding in yeasts, fragmentation of hyphae, and production of spores. Spores can disperse and germinate when conditions permit, establishing new growth. They are reproductive cells rather than seeds, and their production does not necessarily involve sexual reproduction. (openstax.org)
Sexual reproduction commonly involves three distinguishable events: plasmogamy, the fusion of cytoplasm; karyogamy, the fusion of nuclei; and meiosis, which reduces chromosome number. Cytoplasmic and nuclear fusion need not occur together. In many fungi, genetically distinct haploid nuclei coexist before eventually fusing. A dikaryotic condition specifically contains two separate haploid nuclei within a compartment and is prominent in the life cycles of many mushroom-forming fungi. (openstax.org)
Life cycles differ substantially among groups. Some fungi have extended haploid stages, others maintain a dikaryotic mycelium, and yeasts may alternate between haploid and diploid growth. Consequently, neither mushroom production nor a single reproductive pattern defines the kingdom. (openstax.org)
Diversity and classification
Fungal taxonomy combines structural and reproductive characteristics with molecular evidence. Phylogenetic analyses of DNA sequences have revised relationships that were previously inferred mainly from appearance. Familiar terms such as “yeast” and “mold” describe growth forms, not single evolutionary lineages; similar forms occur in different branches of the kingdom. (pmc.ncbi.nlm.nih.gov)
Two major groups are Ascomycota, whose sexual spores develop within sac-like asci, and Basidiomycota, whose sexual spores develop on basidia. Ascomycota includes morels, truffles, and many yeasts and molds. Basidiomycota includes most familiar cap-and-stalk mushrooms, bracket fungi, rusts, and smuts. Other lineages include chytrids, many of which produce swimming spores, and fungi associated with arbuscular mycorrhizae. (openstax.org)
Older classifications grouped numerous fungi within Zygomycota or assigned species lacking a known sexual stage to Deuteromycota. Molecular studies showed that these categories did not adequately represent evolutionary relationships. Their former members have been reassigned, and the delimitation of several fungal lineages remains an active research subject. (pmc.ncbi.nlm.nih.gov)
Ecological roles
Fungi occupy terrestrial and aquatic habitats, including soils, decaying wood, living tissues, and seawater. Many drive decomposition, returning nutrients from dead organisms to their surroundings. Their breakdown of organic material is an important part of the carbon cycle and influences nutrient availability in ecosystems. Other fungi obtain resources as parasites or predators rather than decomposers. (openstax.org)
Mycorrhizae are associations between fungi and plant roots. Fungal hyphae can extend beyond the roots’ immediate surroundings and supply mineral nutrients and water; the plant supplies organic compounds. Different kinds of mycorrhizae differ in the arrangement of fungal tissue around or within roots. Lichens involve a fungus associated with photosynthetic partners, usually algae or cyanobacteria, and can inhabit exposed environments unsuitable for many other organisms. (openstax.org)
Human uses and disease
Fungi provide food directly and transform ingredients during food production. Edible mushrooms include shiitake, morels, and chanterelles. Yeasts conduct fermentation in bread making and alcoholic beverage production, converting sugars into products including ethanol and carbon dioxide. Selected molds contribute to cheese ripening. Industrial applications also use fungal enzymes and other metabolic products. (openstax.org)
Fungi produce compounds used in medicines, including penicillin. They are also important experimental organisms: Saccharomyces cerevisiae and Neurospora crassa have contributed to research in genetics, cell function, and metabolism. Their relatively convenient cultivation allows researchers to investigate processes shared with other eukaryotes. (openstax.org)
Some fungi cause crop diseases, food spoilage, or infections in animals and humans. Only a few hundred fungal species are known to make people sick. Human infections range from superficial conditions, such as ringworm, to invasive disease; people with weakened immune systems are at higher risk for many fungal infections. These pathogenic species represent a small part of fungal diversity. (cdc.gov)
Discovery and conservation
The total number of fungal species is uncertain because many remain undescribed or difficult to distinguish by appearance alone. Kew’s 2023 assessment estimated approximately 2.5 million species worldwide, with a proposed range of two to three million. This is an estimate, not a count of formally named species. DNA-based surveys and examination of preserved collections help reveal previously unrecognized diversity. (kew.org)
Fungal conservation is constrained by limited knowledge of distributions and population trends. Kew reported in 2023 that only a small fraction of known fungal species had received formal conservation assessments, leaving substantial gaps in understanding their status and threats. (kew.org)