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These Escherichia coli cells provide an example of a prokaryotic microorganism.
A polypore mushroom has a parasitic relationship with its host.

In biology, an organism is any contiguous living system, such as an animal, plant or bacterium. More than 99 percent of all species, amounting to over five billion species,<ref name="Book-Biology">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> that ever lived on Earth are estimated to be extinct.<ref name="StearnsStearns2000">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name="NYT-20141108-MJN">{{#invoke:citation/CS1|citation |CitationClass=news }}</ref> Estimates on the number of Earth's current species range from 10 million to 14 million,<ref name="MillerSpoolman2012">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> of which about 1.2 million have been documented and over 86 percent have not yet been described.<ref name="PLoS-20110823">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

All known types of organisms are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. An organism consists of one or more cells; when it has one cell is a unicellular organism; and when it has more than one it is known as 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).<ref name=cavaliersmith1987>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes.

In 2002, Thomas Cavalier-Smith, author of Branching order of bacterial phyla, proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria.<ref name=cavaliersmith2002>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Organism sections
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