Organism

An organism 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 prokaryote and
eukaryotes is widely considered as a major missing link in evolutionary
history. Two eukaryotic organelles, namely mitochondria and chloroplasts,
are generally considered to be derived from endosymbiotic bacteria.

Complex organism is used to describe any organism with more than one cell.

Characteristic common to many organisms include:

   * Movement
   * Feeding
   * Respiration
   * Growth
   * Reproduction
   * Sensitivity to stimuli

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

Viruses are not typically considered to be organisms because they are not
capable of independent reproduction or metabolism, requiring a host. This is
problematic, though, since some extreme parasites or endosymbionts are not
capable of independent life. Although viruses do have enzymes and molecules
characteristic of living organisms, they are not capable 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.

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.