In biology, an organism is any contiguous living system, such as a vertebrate, insect, plant or bacterium. Although more than 99 percent of all species of organisms that ever lived on the planet are estimated to be extinct, there are currently 10–14 million species of life on Earth. All known types of organism are capable of some degree of response to stimuli, reproduction, growth and development and self-regulation (homeostasis). An organism consists of one or more cells. An organism with one cell is a unicellular organism; an organism with more than one is a multicellular organism. Most unicellular organisms are of microscopic size and are thus classified as microorganisms. Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs. An organism may be either a prokaryote or a eukaryote. Prokaryotes are represented by two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles (such as mitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes. In 2002, Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria. See the article: Branching order of bacterial phyla (Cavalier-Smith, 2002).
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