Fungi

Fungi are eukaryotes: their cells contain nuclei and other membrane-bound structures. In standard biological classification, they form a kingdom distinct from plants and animals; evolutionary evidence places them closer to animals than to plants. Fungi do not photosynthesize. Most obtain food by releasing enzymes into their surroundings, breaking down complex organic matter and absorbing the resulting nutrients. Their cell walls commonly contain chitin, rather than the cellulose typical of plants.

The group includes microscopic yeasts and molds as well as larger forms such as mushrooms. In many multicellular fungi, fine filaments called hyphae grow and branch into a network called a mycelium. A mushroom is a reproductive structure produced by some fungi, not the whole organism; much of the fungus may be out of sight in soil, wood or another substrate.

Fungal growth depends on the resources available and on the species. Hyphae can extend through a food source, while secreted enzymes make nutrients from complex material accessible for absorption. Some fungi are single-celled; others build extensive networks. Many produce spores that disperse and grow into new fungal individuals. Reproductive cycles vary: species may reproduce sexually, asexually or by more than one route.

Because fungi feed by absorption and have diverse life cycles, their appearances can be misleading. A familiar mushroom is only one stage or structure in the life of some fungi, while many species never produce a mushroom-like form visible to people.

Many fungi are decomposers. By breaking down dead plants, animals and other organic material, they help return nutrients to ecosystems. Others live in close association with other organisms. In mycorrhizal partnerships, fungal networks associated with plant roots help plants obtain water and nutrients, while plants supply the fungi with sugars made through photosynthesis. Lichens are associations between fungi and photosynthetic partners, often algae.

Not all relationships are beneficial to the host. Fungi can also be parasites or pathogens of plants and animals. Their interactions with hosts, competitors and other microbes help shape ecosystem processes, and ecological research increasingly treats fungi as important connections between organisms and their environments.

People use fungi in food production and fermentation. Yeasts help leaven bread and are used to make alcoholic drinks; fungi also contribute to some cheeses and produce enzymes and medicines. These uses depend on particular species and controlled processes, not on fungi as a whole.

Other fungi cause disease. Human fungal infections range from common skin or nail conditions to serious illnesses, including infections acquired from environmental spores. Risk is higher for some people with weakened immune systems. Since fungi are biologically different from bacteria, antibiotics used against bacterial infections do not treat fungal disease; antifungal medicines are used instead, and some fungi can develop resistance to them.