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In biology and ecology, an organism (in Greek organon = instrument) is a living being.

The origin of life and the relationships between its major lineages are controversial. Two main grades may be distinguished, the prokaryotes and eukaryotes. The prokaryotes are generally considered to represent two separate domains, called the Bacteria and Archaea, which are not closer to one another than to the eukaryotes. The gap between prokaryotes and eukaryotes is widely considered a major missing link in evolutionary history. Two eukaryotic organelles, namely mitochondria and chloroplasts, are generally considered to be derived from endosymbiotic bacteria.

The phrase complex organism describes any organism with more than one cell.

Characteristics common to many organisms include:

These are not universal, however. Many organisms are incapable of independent movement, and do not respond directly to their environment. Microorganisms such as bacteria may not conduct respiration, using alternate chemical pathways instead. And many organisms are incapable of reproduction.

Biological Organization

Environmental Organization



The following articles are entry points for information about the classification of organisms:


Viruses are not typically considered to be organisms because they are not capable of independent reproduction or metabolism. However, according to the United States Code, they are considered to be microorganisms in the sense of biological weaponry and malicious use. This controversy is problematic, though, since some parasites and endosymbionts are incapable of independent life either. Although viruses do have enzymes and molecules characteristic of living organisms, they are incapable of surviving outside a host cell and most of their metabolic processes require a host and its 'genetic machinery'. The origin of such parasites is uncertain, but it appears most likely that they are derived from their hosts.

Life span

One of the basic parameters of organism is its life span. Some animals live as short as one day, while some plants can live thousands of years. Aging is important when determining life span of most organisms, bacterium, a virus or even a prion.

External links

See also

Last updated: 10-23-2005 14:07:44
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