Scientific Definition of Living-Being

Defining “living” has always been challenging. Life is an old notion, complex and very difficult to delimit. With passing time, the definition varies. Indeed, History shows the progression of this notion. But how does this definition change according to various scientific domains? Does life mean the same for a physicist or for a biologist?

From the physicist’s point of view, the question of life is a current matter. Among the diversity of definitions, we can quote the point of view of Schrödinger for whom life feeds from negative entropy. This definition, which is not the last one but maybe the first one to get the agreement of the majority of the physicists’ community, was permitted by Carnot's work and especially the second principle of thermodynamics, a conclusion of which is that living-beings can maintain a low-constant entropy only because they can export entropy to its environment. However, the question of life is still a concern to physicists because its solving depends not only on a better knowledge of fundamental living mechanisms but also because new technologies such as informatics and synthetic biology are challenging its answer.

According to biologists, the definition of living-being is based on a number of required properties shared by all living organisms. This definition is not fixed, however, and it is still challenging to get everyone to agree upon a clear and definitive definition. Thus, biology specifies that an entity is alive if it possesses the following properties:

• Grow or develop: the entity has the faculty to grow or to become mature, to produce more or larger cell, to change its structure. At a certain point the entity must reach a mature structure that allows it to reproduce
• Be confined: the entity must have a defined border with the environment
• Have a special metabolism: the entity must be able to feed itself, to transform and stock energy, to use external nutriments from the environment (chemical components from organic or inert matter) or to use stored molecules in its mass, to produce energy, to perform a task and to produce wastes
• Respond to environmental changes: the entity must be able to detect environmental variations (close or far away), and to act in an appropriate way in response. In other words, the entity must respond to stimuli.
• Reproduce: the entity must be able to give birth to similar entities in an autonomous way (sexually or asexually).

These are some of the properties that are used by biologists to help defining life, but this list is not exhaustive and other properties can be considered (for example, the maintenance of a dynamic equilibrium between a stable inner milieu and the environment- homeostasis). It shows that the definition of life is not set, as the parameters used to define it can change. Furthermore, certain organisms are considered as belonging to living being although do not satisfy all the properties presented above. For instance, some biologists consider virus as living-being. However, viruses do not use their own metabolism and cannot reproduce themselves without a host cell. Certain symbiotic organisms also do not possess all the above properties, showing once again the limits of the present definitions.

The development of synthetic biology in the last decade adds another layer of complexity to problem of defining life. Indeed, the design of bacterial chassis with minimal genome may involve the loss of mechanisms used by bacteria to evolve (such as conjugation, transposition for example) as these mechanisms are considered as undesirable in a factory bacterium. Such bacteria, enable to evolve, could be considered as not being alive, according the previous definition. Yet, our survey and interviews show that for a large majority of people, these bacteria are still alive, even though some consider that they could be “new living beings”.