An organism (from Ancient Greek ὄργανον (órganon) 'instrument, implement, tool', and Ancient Greek -ισμός (-ismós)) is any biological living system that functions as an individual life form. All organisms are composed of cells. The idea of organism is based on the concept of minimal functional unit of life. Three traits have been proposed to play the main role in qualification as an organism:
- noncompartmentability – structure that cannot be divided without its functionality loss,
- individuality – the entity has simultaneous holding of genetic uniqueness, genetic homogeneity and autonomy,
- distinctness – genetic information has to maintain open-system (a cell).
Organisms include multicellular animals, plants, and fungi; or unicellular microorganisms such as protists, bacteria, and archaea. All types of organisms are capable of reproduction, growth and development, maintenance, and some degree of response to stimuli. Most multicellular organisms differentiate into specialized tissues and organs during their development.
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 from a philosophical point of view, whether such a definition is necessary. Problematic cases include colonial organisms: for instance, a colony of eusocial insects fulfils criteria such as adaptive organisation and germ-soma specialisation. If so, the same argument would include some mutualistic and sexual partnerships as organisms. If group selection occurs, then a group could be viewed as a superorganism, optimised by group adaptation. Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately rather than demanding that an organism should have all of them; if so, there are multiple dimensions to biological individuality, resulting in several types of organism.
Other views include the idea that an individual is distinguished by its immune response, separating self from foreign; that "anti-entropy", the ability to maintain order, is what distinguishes an organism; or that Shannon's information theory can be used to identify organisms as capable of self-maintaining their information content. Finally, it may be that the concept of the organism is inadequate in biology.
Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism. Although viruses have a few enzymes and molecules like those in living organisms, they have no metabolism of their own; they cannot synthesize the organic compounds from which they are formed. In this sense, they are similar to inanimate matter. Viruses have their own genes, and they evolve. Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, 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. As for reproduction, viruses rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms, but the genes have a cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts.
This section needs additional citations for verification. (June 2023)
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 for life.
The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor 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 have a membrane consisting of a lipid bilayer and embedded proteins 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 in addition share:
- Reproduction by cell division (binary fission, mitosis or meiosis).
- Use of enzymes and other proteins coded by DNA genes and made via messenger 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.
There is strong evidence from genetics that all organisms have a common ancestor. In particular, 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. However, the universal use of the same genetic code, same nucleotides, and same amino acids makes the existence of such an ancestor overwhelmingly likely. The first organisms were possibly anaerobic and thermophilic chemolithoautotrophs that evolved within inorganic compartments at geothermal environments.
The last universal common ancestor 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 last universal common ancestor lived some 3.5 to 3.8 billion years ago, in the Paleoarchean era.
Human intervention Edit
Modern biotechnology is challenging traditional concepts of organisms 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. 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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