"Biological form" redirects here. For the informal taxonomic term in botany, see Race (biology) § Physiological race.
"Form of life" redirects here. For the philosophical concept, see Form of life (philosophy).
Polypores are often parasitic on hosts such as trees.

In biology, an organism is any contiguous living system, such as an animal, plant, fungus, archaeon, or bacterium. All known types of organisms are capable of some degree of response to stimuli, reproduction with variation,[1] growth and development and homeostasis.[2] An organism consists of one or more cells; when it has one cell it is known as a unicellular organism; and when it has more than one it is known as a multicellular organism. Most unicellular organisms are of microscopic scale and are thus loosely described 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 domainsbacteria 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).[3] Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes.

Estimates on the number of Earth's current species range from 10 million to 14 million,[4] of which only about 1.2 million have been documented.[5] More than 99% of all species, amounting to over five billion species,[6] that ever lived are estimated to be extinct.[7][8] In 2016, a set of 355 genes from the last universal common ancestor (LUCA) of all living organisms living was identified.[9][10]


The term "organism" (from Greek ὀργανισμός, organismos, from ὄργανον, organon, i.e. "instrument, implement, tool, organ of sense or apprehension"[11][12]) first appeared in the English language in 1703 and took on its current definition by 1834 (Oxford English Dictionary). It is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kant's 1790 Critique of Judgment.[13]


An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction".[14] Many definitions exclude viruses and possible man-made non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction.[15] A superorganism is an organism consisting of many individuals working together as a single functional or social unit.[16]

There has been controversy about the best way to define the organism[17][18][19][20][21][22][23][24][25] and indeed about whether or not such a definition is necessary.[26][27] Several contributions[28] are responses to the suggestion that the category of "organism" may well not be adequate in biology.[29]

Non-cellular life

Main article: Non-cellular life

Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism. This controversy is problematic because some cellular organisms are also incapable of independent survival (but are capable of independent metabolism and procreation) and live as obligatory intracellular parasites. Although viruses have a few enzymes and molecules characteristic of living organisms, they have no metabolism of their own; they cannot synthesize and organize the organic compounds from which they are formed. Naturally, this rules out autonomous reproduction: they can only be passively replicated by the machinery of the host cell. In this sense they are similar to inanimate matter. While viruses sustain no independent metabolism, and thus are usually not classified as organisms, they do have their own genes, and they do evolve by mechanisms similar to the evolutionary mechanisms of organisms.

The most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly. Some scientists argue that viruses neither evolve, nor self- reproduce. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. This is not true for cells. If viruses did not exist, the direction of cellular evolution could be different, but cells would nevertheless be able to evolve. As for the reproduction, viruses totally rely on hosts' machinery to replicate.[30] The discovery of viral megagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong in the tree of life. The presence of these genes suggested that viruses were once able to metabolize. However, it was found later that the genes coding for energy and protein metabolism have cellular origin. Most likely, these genes were acquired through horizontal gene transfer from viral hosts.[30]








These protein functions have been recognized:

  1. Enzymes, which catalyze all of the reactions of metabolism
  2. Structural proteins, such as tubulin, or collagen

A bilayer of phospholipids makes up the membrane of cells that constitutes a barrier, containing everything within the cell and preventing compounds from freely passing into, and out of, the cell. Due to the selective permeability of the phospholipid membraine only specific compounds can pass through it. In some multicellular organisms they serve as a storage of energy and mediate communication between cells. Carbohydrates are more easily broken down than lipids and yield more energy to compare to lipids and proteins. In fact, carbohydrates are the number one source of energy for all living organisms.




The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.



All cells share several similar characteristics of:[31]


Last universal common ancestor

Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, a paper in the scientific journal Nature suggested that these 3.5 Gya (billion years old) geological formations contain fossilized cyanobacteria microbes. This suggests they are evidence of one of the earliest known life forms on Earth.

The last universal common ancestor (LUCA) is the most recent organism from which all organisms now living on Earth descend.[32] Thus it is the most recent common ancestor of all current life on Earth. The LUCA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era).[33][34] The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[35] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[36][37] Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct,[7][8] there are currently 10–14 million species of life on Earth.[4]



There is strong evidence from genetics that all organisms have a common ancestor. For example, every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary. Horizontal gene transfer makes it more difficult to study the last universal ancestor.[38] However, the universal use of the same genetic code, same nucleotides, and same amino acids makes the existence of such an ancestor overwhelmingly likely.[39]


Main article: Reproduction

Sexual reproduction is widespread among current eukaryotes, and was likely present in the last common ancestor.[40] This is suggested by the finding of a core set of genes for meiosis in the descendants of lineages that diverged early form the eukaryotic evolutionary tree.[41] and Malik et al.[42] It is further supported by evidence that eukaryotes previously regarded as "ancient asexuals", such as Amoeba, were likely sexual in the past, and that most present day asexual amoeboid lineages likely arose recently and independently.[43]

In prokaryotes, natural bacterial transformation involves the transfer of DNA from one bacterium to another and integration of the donor DNA into the recipient chromosome by recombination. Natural bacterial transformation is considered to be a primitive sexual process and occurs in both bacteria and archaea, although it has been studied mainly in bacteria. Transformation is clearly a bacterial adaptation and not an accidental occurrence, because it depends on numerous gene products that specifically interact with each other to enter a state of natural competence to perform this complex process.[44] Transformation is a common mode of DNA transfer among prokaryotes.[45]

Horizontal gene transfer




Future of life (cloning and synthetic organisms)

Modern biotechnology is challenging traditional concepts of organism and species. Cloning is the process of creating a new multicellular organism, genetically identical to another, with the potential of creating entirely new species of organisms. Cloning is the subject of much ethical debate.

In 2008 the J. Craig Venter Institute assembled a synthetic bacterial genome, Mycoplasma genitalium, by using recombination in yeast of 25 overlapping DNA fragments in a single step. The use of yeast recombination greatly simplifies the assembly of large DNA molecules from both synthetic and natural fragments.[48] Other companies, such as Synthetic Genomics, have already been formed to take advantage of the many commercial uses of custom designed genomes.


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External links
