:Alternate uses: see Life (disambiguation) and Living (disambiguation)Life is a multi-faceted concept with no simple definition;
this article is confined to the primary meanings in biology; articles on life in other senses are included in the article life (disambiguation).
1 Defining the concept of life
Life has a number of senses with a biological meaning —
- "life" may refer to the ongoing process of which living things are a part;
- "life" may refer to the period between birth and death of an organism;
- "life" may refer to the state of something that has been born and has yet to die, i.e., that which makes a living thing alive.
The remainder of this section focuses on the last biological sense — how can one tell when an entity is a living thing?
It would be relatively straightforward to offer a practical set of guidelines if one's only concern were life on Earth as we know it (see biosphere), but as soon as one considers questions about life's origins on Earth, or the possibility of extraterrestrial life, or the concept of artificial life, it becomes clear that the question is fundamentally difficult and comparable in many respects to the problem of defining intelligence.
1.1 A conventional definition
In biology, an entity has traditionally been considered to be alive if it exhibits all the following phenomena at least once during its existence:
- Growth
- MetabolismMetabolism in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste products. It is major process of living organisms, and because this process can happen, consuming, transforming and storing energyThis article is about the scientific concept. Energy use by humans is discussed in other articles''. Energy generally and qualitatively speaking, is the property (or the quantity of the property) of doing things or supplying power. The expressions energy/ massMass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are two different quantities called mass Inertial mass; growing by absorbing and reorganizing mass; excreting waste
- Motion, either moving itself, or having internal motion
- ReproductionReproduction is the creation of one thing as a copy of, product of, or replacement for a similar thing, e. photocopying and the making of replicas. It is perhaps most commonly used in the context of biological reproduction and sex: Sexual reproduction is, the ability to create entities that are similar to itself
- Response to stimuliA stimulus is the following: In physiology, a stimulus (physiology) is something external that elicits or influences a physiological or psychological activity or response. In psychology, anything effectively inpinging upon any of the sensory apparatuses o - the ability to measure properties of its surrounding environmentAn environment is a complex of external factors that acts on a system and determines its course and form of existence. An environment may be thought of as a superset, of which the given system is a subset. An environment may have one or more parameters, p, and act upon certain conditions.
These criteria are not without their uses, but their disparate nature makes them unsatisfactory from a number of perspectives; in fact, it is not difficult to find counterexamples and examples that require further elaboration. For example, according to the above definition, one could say:
- fireFor other uses see fire (disambiguation). bonfire The word fire is used to refer to the combination of the brilliant glow and large amount of heat released during a rapid, self-sustaining exothermic oxidation process of combustible gases ejected from a fu is alive. (This could be remedied by adding the requirement of locality, where there is an obvious feature that delineates the spatial extension of the living being, such as a cell membrane.)
- male mules are not alive as they are sterile and cannot reproduce.
- viruses are not alive as they do not grow and cannot reproduce outside of a host cell.
Biologists who are content to focus on terrestrial organisms often note some additional signs of a "living organism", including these:
- Living organisms contain molecular components such as: carbohydrates, lipids, nucleic acids, and proteins.
- Living organisms require both energy and matter in order to continue living.
- Living organisms are composed of at least one cell.
- Living organisms maintain homeostasis.
- Species of living organisms will evolve.
All life on Earth is based on the chemistry of carbon compounds. Some assert that this must be the case for all possible forms of life throughout the universe; others describe this position as ' carbon chauvinism'.
