An organism (from Greek: ὀργανισμός, organismos) is any individual entity that embodies the properties of life. A living organism is an entity, unicellular or multicellular, subject to the laws of the physical world and under the control of the systems that express the information it contains. This information is primarily encoded in the genome and in otherwise transmitted genetic material, such as in cellular organelles, and is subject to all typical mechanisms of expression, including those emphasized in epigenetics.
The peculiarities of each organism are the specificity of form and chemical composition, the ability to perceive and respond to stimuli from the environment, and the ability to reproduce, giving rise to other organisms with similar characteristics.
Just as the organism is subordinate to the purpose of the being’s survival, so the parts are subordinate to the organism: this is the philosophical concept of the organism in Aristotle and Descartes, although the latter calls the organism a “machine”; along these lines, Leibniz calls the organism a “divine machine” or “natural automaton. Kant, on the other hand, distinguishes the machine from the organism in that the former has its productive cause outside itself, while in the organism the parts are together cause and effect of each other. According to Kant, the metaphysical dispute between finalism and mechanism does not touch the concept of the organism; however, the cosmic finalism of the organism (supported by vitalism and the various metaphysical schools) falls away, while its internal finalism remains.
Organisms turn out to be related to each other. Every living organism known to date derives from a network, which can be compared in a first approximation to an arborescent phylogenetic lineage, that is common to all other organisms, regardless of the time of separation between the evolutionary lines. Every existing life form is descended from one or a few common ancestors that appeared on Earth billions of years ago, possesses metabolic pathways, reproduces, transmits information to its offspring, and organizes its structures. These properties form the core of the biological concept of life, an emergent peculiarity that distinguishes it from non-living entities.
These characteristics are conventionally represented by certain aspects common to all living organisms:
- Evolution: evolves in relation to all other living organisms.
- Order: is structured.
- Encoding: contains within itself the information and instructions that control and define its structure and function.
- Regulation: results in autonomous maintenance of homeostasis.
- Growth and Development: results in autonomous growth capability.
- Energy: represents an open thermodynamic system capable of absorbing, storing, transforming and releasing energy to the environment.
- Irritability, sensitivity or motility: results in an autonomous ability to respond to external stimuli.
In a broader sense, organisms may also be capable of and possess, as a whole:
- Reproductive ability: capable of reproducing fertile offspring that will give rise to adult-like organisms.
- Evolutionary capacity: capable of varying its genotype and phenotype, giving rise to novel anatomical structures, physiological pathways, and genomic combinations that have not previously occurred within the phylogenetic lineage to which it belongs (evolutionary divergence) or that have occurred within previously separate phylogenetic lines (evolutionary convergence).
These characteristics form the basis of the sufficiently common view of a living thing as a known organism of the terrestrial biota. Other definitions deviate in one or more points from the previous one, generated by the study of the natural sciences.
Synthetic biology, which can lead to the genesis of organisms endowed with so-called synthetic life, such as Mycoplasma laboratorium, and issues of exobiology and hypothetical biochemistry can lead to a more general definition of the concept of a living organism.
Known organisms span a dimensional and temporal range from the 80,000 years and more than 6,600 tons of a single, millennia-old poplar genome to tiny mycoplasmas 200 nanometers in diameter that “live” (duplication time) a few minutes, with masses of less than a billionth of a gram.
On Earth, the life cycles so far known are developed around the elements CHNOPS (which stands for carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, represents the six most important chemical elements whose covalent combinations make up most biological molecules on Earth), but this does not imply, in principle, that there may be other series of elements around which alternative life systems can be created. The discovery in 2010 in Mono Lake, California, of a bacterium capable of using arsenic instead of phosphorus would have shown the existence of organisms with a biochemistry completely different from what has been known so far, especially with regard to nucleic acids, but subsequent studies have denied the results. Exobiologists hypothesize organisms based on the chemistry of silicon instead of carbon.
From the property of Order we derive the characteristics of:
- Cellularity: all living things are made up of elementary structural and functional units, called cells capable of performing all the functions proper to living things. Cells, in fact, are born, feed themselves, grow, reproduce and die. The simplest living organisms are made up of a single microscopic cell; so, for example, bacteria, many species of algae and yeasts. Other organisms, such as plants, animals, and almost all fungi, consist of a large number of cells. The cells that make up a multicellular organism may be tightly welded to one another or may be relatively free and independent. Either way, they are in chemical communication with each other, they may themselves constitute the organism, or they may be aggregated to form a community of cells called an organism.
- Complexity: living things are complex and highly integrated beings. A bacterium, which is one of the smallest forms of life, is made up of about 7000 different chemicals. Each has its own precise biological function and must always be present in the “right” amount for the proper functioning of the bacterium. If we then consider man, we discover that he is made up of at least 10,000 billion cells; these, in turn, are composed of tens of thousands of different chemical substances distributed in numerous microscopic structures (cellular organelles). In the human body, cells are differentiated into about 200 different types. The various types of cells are organized into tissues which, in turn, form organs. The organs constitute the systems and apparatuses and these integrate to form the organism.
From the property of Coding derive the characteristics of:
- Information: the maintenance and transmission from generation to generation of the complexity of living things requires an amount of information that, for even the simplest of them, is greater than that contained in a voluminous encyclopedia. Every structure and every activity, from the single molecule to the whole organism, from birth to death, are encoded in the genome. The first elements of the genome discovered were the genes that are formed by the DNA molecule enclosed in the chromosomes of the cell nucleus. Each gene “contains” information that, from time to time, can be modulated and coordinated with that of other genes. The result is a harmonious and complex system that directs the activities carried out by various cells not only in the adult organism, but also during its growth and development.
From the property of Energy come the characteristics of:
- Metabolism: “Metabolism” means transformation. In fact, every organism undergoes continuous transformations made necessary by the maintenance of its complex structure, growth and continuous adaptations to the environment. More properly, by metabolism we mean that complex of well organized chemical reactions able to exploit external energy to renew, increase or repair the structures of the organism. All this involves, in fact, a continuous transformation of numerous molecules. Nutrition, respiration and excretion are the most evident and manifest expression of the metabolic processes that take place in an organism.
From the property of Reproductive Capacity come the characteristics of:
- Reproduction: each living thing must be able to reproduce at least in one phase of its life cycle, with modes and times often different and proper to each species, that is, it must be able to generate other organisms similar to itself. A unicellular organism duplicates its DNA, grows and divides into two daughter cells that will inherit one of the two copies of DNA. In some multicellular organisms, however, reproduction occurs through the fusion of two cells (called gametes), produced by two individuals of opposite sex. The result of this fusion is called a zygote and is a cell that contains half of the DNA from the father, and half from the mother. The individual that develops from the zygote resembles the parents, but will be different from both of them. In this way, new variants of the same species will appear with each generation.
From the property of Growth come the characteristics of:
- Development: growth is a characteristic property of living organisms. Bacteria grow, albeit slightly, after a reproductive division. Normally, in sexually reproducing organisms, the zygote divides several times to form billions of cells. The growth may be accompanied by the appearance of new cell types, new tissues and new organs, may represent a simple enlargement or even a drastic anatomical and metabolic change as in the case of metamorphosis.
From the property of Evolution come the characteristics of:
- Adaptation: Living organisms can change their anatomy and physiology over time by adapting to their environment. Through reproduction, parents transmit to their child a copy or part of their genes, that is a copy or part of their hereditary material. For this reason the child is not completely equal to the parent, but possesses different hereditary characters. In more could possess also some new characteristic that did not exist in its ancestors. A new character, or mutation, originates as a result of the fact that the hereditary material is transmitted slightly altered from the original. The accumulation of such variations over time and space can lead to the formation of organisms with very different structural characteristics. In this way, in the course of geological eras, new species of living organisms have originated. Careful analysis and in-depth studies testify that the current great variety of living organisms originated through a process called evolution. It is the result of the genetic variations that have taken place in the time between the first forms of life and the present ones.
From the property of Irritability or Motility derive the characteristics of:
- Interaction: all living organisms interact with the environment and with each other. We know that a plant to grow needs water, mineral salts, carbon dioxide, light and oxygen: all these “raw materials” it absorbs from the physical environment. Their greater or lesser availability will influence its growth and its multiplication, moreover they are capable of responding to external stimuli by orienting their leaves and roots in response respectively to sun rays and to the force of gravity. Temperature, rain, wind, latitude and altitude also affect plant life. Plants are the basis of nutrition for animals and humans, who in turn derive “raw materials” and energy from them. Even the simplest organisms such as bacteria or single-celled algae have surface receptors that allow them to distinguish between members of their own species, foreign species, food, etc., and respond appropriately to such stimuli.
There are many cases that are not easy to define. Viruses are a borderline case, since they are not able to reproduce autonomously, but they need a host cell, often a bacterium, they are molecular crystals able to replicate but they do not have their own metabolism. The debate also involves transposable elements of the genome, units consisting of DNA sequences, also known as obligate endonuclear parasites. Some authors believe that these are viruses imprisoned in the genome. Such elements, although capable of reproducing by generating copies of themselves, escape attempts at classification because they are parasitic DNA molecules that can thrive and reproduce exclusively within the cell nucleus.