Kingdom is one of the largest units of biological classification. Every organism belongs to a particular kingdom. Scientists group organisms in a kingdom because they share certain basic characteristics. The number of kingdoms has changed over time, as scientists have learned more about how species are related to one another.
Today, many biologists recognize six kingdoms: (1) Bacteria, (2) Archaea, (3) Protista (protists), (4) Plantae (plants), (5) Fungi, and (6) Animalia (animals). They often group these kingdoms into three larger groups called domains. The kingdoms Bacteria and Archaea each belong to a domain of the same name, while the other four kingdoms belong to the domain Eukaryota, also called Eucarya or Eukarya. Bacteria and Archaea have prokaryotic cells, which lack a nucleus. Scientists refer to both groups as prokaryotes. All members of Eukaryota have eukaryotic cells that include a nucleus. Such organisms are known as eukaryotes. See Domain ; Eukaryote .
The kingdom Bacteria includes all true bacteria and the algaelike cyanobacteria. Bacteria live in nearly every environment. These single-celled prokaryotes often form colonies, or loosely organized groups of similar cells. Bacteria cause many diseases of plants and animals. See Bacteria .
The kingdom Archaea consists of prokaryotes that differ in fundamental ways from bacteria. These single-celled organisms often live in such harsh environments as deep-sea hydrothermal vents and highly saline (salty) pools. Some can obtain energy from sulfur or other inorganic chemicals. Many ecosystems need large numbers of Archaea to function properly. See Archaea .
The kingdom Protista, or Protoctista, contains such microscopic organisms as amoebas, dinoflagellates, and protozoans. Most protists consist of a single cell or simple colonies of cells. See Protist .
The kingdom Plantae consists of such multicellular organisms as mosses, ferns, shrubs, and trees. Nearly all plants make their own food using sunlight in a complex process called photosynthesis (see Photosynthesis ). Some flowering plants become parasites on other plants and absorb their food. Plants play an essential role in the cycle of nature, and they provide food and often shelter for animals, fungi, and protists. See Plant .
The kingdom Fungi contains mostly multicellular organisms that obtain food by absorbing organic matter. This kingdom includes lichens, mushrooms, and yeasts. Fungi secrete proteins called enzymes into the plants, animals, and decaying organisms on which they live. The enzymes help to break organic molecules into nutrients (nourishing substances) that the fungi can absorb. See Fungi .
The kingdom Animalia, also known as Metazoa, consists of multicellular organisms, some of which have the most complex body features of all living things. The cells of all but the simplest animals are organized into tissues that perform special tasks. These tissues form such organs as the brain, heart, and stomach, which in turn form organ systems. Animals depend on plants and other organisms for their food. See Animal .
Not all scientists use this six kingdom classification scheme. Many biologists classify protists in two or more separate kingdoms. Others divide the domain Archaea or the domain Bacteria into multiple kingdoms.
