The Kingdom Fungi includes eukaryotic, generally multicellular organisms with an unusual form of multicellularity. Their cells are not entirely divided by cell walls, so cytoplasm and even nuclei can flow between individual cells. The cell walls are made of chitin, a protein also found in the exoskeletons of arthropods, which reflects the close relationship between the Fungi and Animalia kingdoms. They are more closely related to each other than they are to plants.
Fungi are split between acting as parasites, which prey on insects, plants, and animals, and saprophytes, which primarily aid in the breakdown of dead organic material. Fungi can also live in symbiotic relationships with other organisms. Lichens are the result of a symbiotic relationship between a fungus and a photosynthesizing organism. Other fungi live in symbiosis with plant roots to increase the root surface area.
Fungi can be divided into three groups based on relatedness and shared reproductive habits: Basidiomycota, Ascomycota, and Zygomycota. Basidiomycota are characterized by the presence of basidia and dikaryotic fruiting bodies and include the mushrooms, puffballs, and shelf fungi. Ascomycota are characterized by the presence of asci and include such diverse organisms as the cup fungi and mildews as well as the unicellular fungi, yeast. Zygomycota are characterized by the presence of a thick-coated zygospore and include bread and other molds, plant and animal parasites, as well as many plant-root symbionts.