OrganismAn 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.