In biology, an organism (from Greek: οργανισμός, organismos)
is any individual entity that exhibits the properties of life. It is a
synonym for "life form".
Organisms are classified by taxonomy into specified groups such as the
multicellular animals, plants, and fungi; or unicellular
microorganisms such as a protists, bacteria, and archaea. All types
of organisms are capable of reproduction, growth and development,
maintenance, and some degree of response to stimuli. Humans are
multicellular animals composed of many trillions of cells which
differentiate during development into specialized tissues and organs.
An organism may be either a prokaryote or a eukaryote. Prokaryotes are
represented by two separate domains—bacteria 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). 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, of which only about 1.2 million have been
documented. More than 99% of all species, amounting to over five
billion species, that ever lived are estimated to be extinct.
In 2016, a set of 355 genes from the last universal common ancestor
(LUCA) of all living organisms living was identified.
2.1 Non-cellular life
5.1 Last universal common ancestor
6 Location of the root
6.2 Horizontal gene transfer
6.3 Future of life (cloning and synthetic organisms)
7 See also
9 External links
The term "organism" (from Greek ὀργανισμός, organismos,
from ὄργανον, organon, i.e. "instrument, implement, tool,
organ of sense or apprehension") 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.
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". 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. A
superorganism is an organism consisting of many individuals working
together as a single functional or social unit.
There has been controversy about the best way to define the
organism and indeed about whether
or not such a definition is necessary. Several
contributions are responses to the suggestion that the category of
"organism" may well not be adequate in biology.[page needed]
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
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. 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 a cellular origin. Most likely, these genes
were acquired through horizontal gene transfer from viral hosts.
Organisms are complex chemical systems, organized in ways that promote
reproduction and some measure of sustainability or survival. The same
laws that govern non-living chemistry govern the chemical processes of
life. It is generally the phenomena of entire organisms that determine
their fitness to an environment and therefore the survivability of
their DNA-based genes.
Organisms clearly owe their origin, metabolism, and many other
internal functions to chemical phenomena, especially the chemistry of
large organic molecules. Organisms are complex systems of chemical
compounds that, through interaction and environment, play a wide
variety of roles.
Organisms are semi-closed chemical systems. Although they are
individual units of life (as the definition requires), they are not
closed to the environment around them. To operate they constantly take
in and release energy. Autotrophs produce usable energy (in the form
of organic compounds) using light from the sun or inorganic compounds
while heterotrophs take in organic compounds from the environment.
The primary chemical element in these compounds is carbon. The
chemical properties of this element such as its great affinity for
bonding with other small atoms, including other carbon atoms, and its
small size making it capable of forming multiple bonds, make it ideal
as the basis of organic life. It is able to form small three-atom
compounds (such as carbon dioxide), as well as large chains of many
thousands of atoms that can store data (nucleic acids), hold cells
together, and transmit information (protein).
Compounds that make up organisms may be divided into macromolecules
and other, smaller molecules. The four groups of macromolecule are
nucleic acids, proteins, carbohydrates and lipids. Nucleic acids
(specifically deoxyribonucleic acid, or DNA) store genetic data as a
sequence of nucleotides. The particular sequence of the four different
types of nucleotides (adenine, cytosine, guanine, and thymine) dictate
many characteristics that constitute the organism. The sequence is
divided up into codons, each of which is a particular sequence of
three nucleotides and corresponds to a particular amino acid. Thus a
DNA codes for a particular protein that, due to the
chemical properties of the amino acids it is made from, folds in a
particular manner and so performs a particular function.
These protein functions have been recognized:
Enzymes, which catalyze all of the reactions of metabolism
Structural proteins, such as tubulin, or collagen
Regulatory proteins, such as transcription factors or cyclins that
regulate the cell cycle
Signaling molecules or their receptors such as some hormones and their
Defensive proteins, which can include everything from antibodies of
the immune system, to toxins (e.g., dendrotoxins of snakes), to
proteins that include unusual amino acids like canavanine
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 membrane 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
All organisms consist of structural units called cells; some contain a
single cell (unicellular) and others contain many units
(multicellular). Multicellular organisms are able to specialize cells
to perform specific functions. A group of such cells is a tissue, and
in animals these occur as four basic types, namely epithelium, nervous
tissue, muscle tissue, and connective tissue. Several types of tissue
work together in the form of an organ to produce a particular function
(such as the pumping of the blood by the heart, or as a barrier to the
environment as the skin). This pattern continues to a higher level
with several organs functioning as an organ system such as the
reproductive system, and digestive system. Many multicellular
organisms consist of several organ systems, which coordinate to allow
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.
There are two types of cells, eukaryotic and prokaryotic. Prokaryotic
cells are usually singletons, while eukaryotic cells are usually found
in multicellular organisms. Prokaryotic cells lack a nuclear membrane
DNA is unbound within the cell; eukaryotic cells have nuclear
All cells, whether prokaryotic or eukaryotic, have a membrane, which
envelops the cell, separates its interior from its environment,
regulates what moves in and out, and maintains the electric potential
of the cell. Inside the membrane, a salty cytoplasm takes up most of
the cell volume. All cells possess DNA, the hereditary material of
genes, and RNA, containing the information necessary to build various
proteins such as enzymes, the cell's primary machinery. There are also
other kinds of biomolecules in cells.
All cells share several similar characteristics of:
Reproduction by cell division (binary fission, mitosis or meiosis).
Use of enzymes and other proteins coded by
DNA genes and made via
RNA intermediates and ribosomes.
Metabolism, including taking in raw materials, building cell
components, converting energy, molecules and releasing by-products.
The functioning of a cell depends upon its ability to extract and use
chemical energy stored in organic molecules. This energy is derived
from metabolic pathways.
Response to external and internal stimuli such as changes in
temperature, pH or nutrient levels.
Cell contents are contained within a cell surface membrane that
contains proteins and a lipid bilayer.
See also: Origin of life, Earliest known life forms, and Common
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.
Main article: Last universal common ancestor
Further information: Timeline of the evolutionary history of life
The last universal common ancestor (LUCA) is the most recent organism
from which all organisms now living on
Earth descend. 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
Paleoarchean era). The earliest evidence for life on
Earth is graphite found to be biogenic in 3.7 billion-year-old
metasedimentary rocks discovered in Western Greenland and
microbial mat fossils found in 3.48 billion-year-old sandstone
discovered in Western Australia. Although more than 99 percent
of all species that ever lived on the planet are estimated to be
extinct, there are currently 10–14 million species of life on
Information about the early development of life includes input from
many different fields, including geology and planetary science. These
sciences provide information about the history of the
Earth and the
changes produced by life. However, a great deal of information about
Earth has been destroyed by geological processes over the
course of time.
All organisms are descended from a common ancestor or ancestral gene
pool. Evidence for common descent may be found in traits shared
between all living organisms. In Darwin's day, the evidence of shared
traits was based solely on visible observation of morphologic
similarities, such as the fact that all birds have wings, even those
that do not fly.
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
Horizontal gene transfer makes it more
difficult to study the last universal ancestor. However, the
universal use of the same genetic code, same nucleotides, and same
amino acids makes the existence of such an ancestor overwhelmingly
Location of the root
The LUCA used the Wood–Ljungdahl or reductive acetyl–CoA pathway
to fix carbon.
For branching of
Bacteria phyla, see Bacterial phyla.
The most commonly accepted location of the root of the tree of life is
between a monophyletic domain
Bacteria and a clade formed by Archaea
Eukaryota of what is referred to as the "traditional tree of life"
based on several molecular studies. A very
small minority of studies have concluded differently, namely that the
root is in the domain Bacteria, either in the phylum Firmicutes or
that the phylum Chloroflexi is basal to a clade with
Eukaryotes and the rest of
Bacteria as proposed by Thomas
Research published in 2016, by William F. Martin, by genetically
analyzing 6.1 million protein coding genes from sequenced prokaryotic
genomes of various phylogenetic trees, identified 355 protein clusters
from amongst 286,514 protein clusters that were probably common to the
LUCA. The results "depict LUCA as anaerobic, CO2-fixing, H2-dependent
Wood–Ljungdahl pathway (the reductive acetyl-coenzyme A
pathway), N2-fixing and thermophilic. LUCA's biochemistry was replete
with FeS clusters and radical reaction mechanisms. Its cofactors
reveal dependence upon transition metals, flavins, S-adenosyl
methionine, coenzyme A, ferredoxin, molybdopterin, corrins and
selenium. Its genetic code required nucleoside modifications and
S-adenosylmethionine-dependent methylations." The results depict
methanogenic clostria as a basal clade in the 355 lineages examined,
and suggest that the LUCA inhabited an anaerobic hydrothermal vent
setting in a geochemically active environment rich in H2, CO2, and
iron. However, the identification of these genes as being present
in LUCA was criticized, suggesting that many of the proteins assumed
to be present in LUCA represent later horizontal gene transfers
between archaea and bacteria.
Main article: Reproduction
Sexual reproduction is widespread among current eukaryotes, and was
likely present in the last common ancestor. This is suggested by
the finding of a core set of genes for meiosis in the descendants of
lineages that diverged early from the eukaryotic evolutionary
tree. and Malik et al. 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.
In prokaryotes, natural bacterial transformation involves the transfer
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.
Transformation is a common mode of
DNA transfer among prokaryotes.
Horizontal gene transfer
Main article: Horizontal gene transfer
The ancestry of living organisms has traditionally been reconstructed
from morphology, but is increasingly supplemented with
phylogenetics—the reconstruction of phylogenies by the comparison of
genetic (DNA) sequence.
Sequence comparisons suggest recent horizontal transfer of many genes
among diverse species including across the boundaries of phylogenetic
"domains". Thus determining the phylogenetic history of a species can
not be done conclusively by determining evolutionary trees for single
Biologist Peter Gogarten suggests "the original metaphor of a tree no
longer fits the data from recent genome research", therefore
"biologists (should) use the metaphor of a mosaic to describe the
different histories combined in individual genomes and use (the)
metaphor of a net to visualize the rich exchange and cooperative
effects of HGT among microbes."
Future of life (cloning and synthetic organisms)
Modern biotechnology is challenging traditional concepts of organism
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
J. Craig Venter Institute assembled a synthetic bacterial
genome, Mycoplasma genitalium, by using recombination in yeast of 25
DNA fragments in a single step. The use of yeast
recombination greatly simplifies the assembly of large
from both synthetic and natural fragments. 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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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"
Interactive Syllabus for General
Biology - BI 04, Saint Anselm
College, Summer 2003
Jacob Feldman: Stramenopila
NCBI Taxonomy entry: root (rich)
Saint Anselm College: Survey of representatives of the major Kingdoms
Citat: "Number of kingdoms has not been resolved...
Bacteria present a
problem with their diversity...
Protista present a problem with their
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 largest organism in the world may be a fungus carpeting nearly 10
square kilometers of an Oregon forest, and may be as old as 10500
The Tree of Life.
Frequent questions from kids about life and their answers
Elements of nature
Hierarchy of life
Biosphere > Ecosystem > Biocoenosis >
Population > Organism > Organ system >
Organ > Tissue > Cell > Organelle >
Biomolecular complex > Macromolecule > Biomolecule