In biology, an organism is any contiguous living system. In at least some form, all types of organisms are capable of responding to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole.
An organism may be either unicellular or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism composed of more than one cell.
All organisms living on Earth are divided into the eukaryotes and prokaryotes based on the presence or absence of true nuclei in their cells. The prokaryotes represent two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus, and contain additional membrane-bound compartmentalization called organelles. Fungi, animals and plants are examples of kingdoms of organisms that are eukaryotes.
In 2002 Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria. See Branching order of bacterial phyla.
The term “organism” 1st 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.
There has been a great deal of recent 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.
The word organism may broadly be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. However, many sources propose definitions that exclude viruses and theoretically possible man-made non-organic life forms. Viruses are dependent on the biochemical machinery of a host cell for reproduction.
In multicellular terms, “organism” usually describes the whole hierarchical assemblage of systems themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent.
A superorganism is an organism consisting of many individuals working together as a single functional or social unit.
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 didn’t exist, viral evolution would be impossible. This isn’t true for cells. If viruses didn’t exist, the direction of evolution could be different; however, the ability to evolve would not be affected. As for the reproduction, viruses totally rely on hosts’ machinery to replicate themselves. The discovery of viral megagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong on the tree of life. The presence of these genes suggested that viruses could metabolize in the past. It was found later that the genes coding for energy and protein metabolism have cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts.
All organisms are classified by the science of alpha taxonomy into either taxa or clades.
To give an example, Homo sapiens is the Latin binomial equating to modern humans. All members of the species sapiens are, at least in theory, genetically able to interbreed. Several species may belong to a genus, but the members of different species within a genus are usually unable to interbreed to produce fertile offspring. Homo only has one surviving species (sapiens), Homo erectus, Homo neanderthalensis, etc. having become extinct thousands of years ago; some scientists argue for interbreeding between H. sapiens and H. neanderthalensis with fertile progeny. Several genera belong to the same family and so on up the hierarchy. Eventually, the relevant kingdom (Animalia, in the case of humans) is placed into one of the three domains depending upon certain genetic and structural characteristics.
All living organisms known to science are given classification by this system such that the species within a particular family are more closely related and genetically similar than the species within a particular phylum.
Since viruses aren’t living organisms, their classification is a challenging task. At first, viruses were classified according to their hosts: plant viruses, animal viruses, bacteriophages. Later, they were classified by the disease that they cause. For example, respiratory viruses, enterics. Now, viruses are classified based on the nucleic acid content, capsid symmetry and the presence or absence of the envelope.
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, they aren’t 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, 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 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 sequence of 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.
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.
All organisms consist of monomeric units called cells; some contain a single cell 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 to allow for reproduction, digestion, etc. Many multicell organisms consist of several organ systems, which coordinate to allow for life.
The cell theory, 1st developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within 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 so DNA is unbound within the cell, eukaryotic cells have nuclear membranes.
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.
Related Sites for Organism
- Organism | Define Organism at Dictionary.com read Organism
- Genetics | Define Genetics at Dictionary.com read Organism
- Zebrafish Information Network – ZFIN: The Zebrafish Model Organism … read Organism