Owing to certain characteristics which make fungi quite different from both plants and animals, fungi are classified in their own kingdom. Like plants, fungi are sedentary and have cell-wall-bound cells. Unlike plants, fungi's cell walls are made from chitin, a polysaccharide containing nitrogen, not from cellulose. Also unlike plants, fungi are heterotrophs: fungi's cells do not contain chlorophyll, and, consequently, fungi must obtain nutrients in an already synthesized form. Because fungi cannot ingest particulate food-stuffs, they must live in or on their food. Fungi are thus parasitic or saprophytic. Whereas some parasitic fungi can absorb nutrients directly from their host, other parasitic fungi and all saprophytic fungi must secrete enzymes to digest surrounding food-stuffs which are then absorbed into the organism via root like rhizoids (found in larger fungi) or haustoria (found in smaller fungi).
Fungi are eukaryotic and range from being unicellular to multicellular, but multicellular fungi do not have cell walls or membranes separating individual cells. Thus, the cytoplasm is continuous among the cells. Cells are organized into branched filaments, or hyphae; all a fungus's hyphae are collectively called a mycelium. Fungi can reproduce asexually and sexually; in both reproduction cycles, the haploid stage is dominant. The different ways fungi reproduce helps distinguish between the
