A fungus is a eukaryotic organism belonging to the kingdom Fungi that absorbs nutrients from organic material and includes yeasts, molds and multicellular fruiting bodies such as mushrooms.
Fungi differ from plants and animals in both structure and metabolism. Their cells contain nuclei and other organelles typical of eukaryotes, but they lack chlorophyll and do not perform photosynthesis. Instead, they acquire energy by secreting enzymes that break down complex substrates outside the cell and absorbing the resulting small molecules. Their cell walls are composed primarily of chitin and glucans rather than cellulose. Many fungi grow as thread-like hyphae that branch and interconnect to form a mycelium, which infiltrates substrates such as soil, wood or decaying matter. Yeasts are unicellular fungi that reproduce by budding or fission. Reproductive cycles include both asexual spore formation and sexual stages involving plasmogamy, karyogamy and meiosis. Fungi are classified into phyla such as Ascomycota, Basidiomycota and Zygomycota based on reproductive structures and molecular data. Fungi play essential roles in ecosystems as decomposers, symbionts and pathogens. Saprophytic fungi recycle nutrients by decomposing cellulose, lignin and other polymers in dead plants and animals. Mycorrhizal fungi form mutualistic associations with plant roots, enhancing uptake of water and minerals while receiving carbohydrates. Lichens are partnerships between fungi and photosynthetic algae or cyanobacteria that colonize rocks and tree bark. On the other hand, pathogenic fungi such as Candida albicans, Cryptococcus neoformans and Histoplasma capsulatum cause infections in humans, while rusts and smuts damage crops. In biotechnology, fungi are used to produce bread, beer, wine and cheese, and as sources of antibiotics like penicillin and immunosuppressants like cyclosporine. Fungal metabolites also include toxins such as aflatoxins from Aspergillus species and ergot alkaloids from Claviceps. Fungi are a diverse kingdom of organisms that occupy nearly every habitat on Earth. Their ability to degrade complex substrates, form symbiotic relationships and produce bioactive compounds makes them both valuable and potentially harmful to humans and other organisms. Related Terms: Mycelium, Hyphae, Yeast, Mold, MycologyEcological roles and significance