In biology and ecology, 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.
The phrase complex organism describes any organism with more than one cell.
Characteristics common to many organisms include:
- Sensitivity to stimuli
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.
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. This 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.
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.
- NCBI Taxonomy entry: root (rich)
- Species 2000 Indexing the world's known species. Species 2000 has the objective of enumerating all known species of plants, animals, fungi and microbes on Earth as the baseline dataset for studies of global biodiversity. It will also provide a simple access point enabling users to link from here to other data systems for all groups of organisms, using direct species-links.
- The Tree of Life.
- BBCNews: 27 September, 2000, When slime is not so thick Citat: "...It means that some of the lowliest creatures in the plant and animal kingdoms, such as slime and amoeba, may not be as primitive as once thought...."
- BBCNews, 18 December, 2002, 'Space bugs' grown in lab Citat: "...Bacillus simplex and Staphylococcus pasteuri...Engyodontium album...The strains cultured by Dr Wainwright seemed to be resistant to the effects of UV - one quality required for survival in space...."
- BBCNews, 19 June, 2003, Ancient organism challenges cell evolution Citat: "..."It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both,"..."
- Saint Anselm College: Survey of representatives of the major Kingdoms Citat: "...Number of kingdomss has not been resolved...Bacteria present a problem with their diversity...Protista present a problem with their diversity...", Interactive Syllabus for General Biology - BI 04, Saint Anselm College, Summer 2003
- Jacob Feldman: Stramenopila