The question of what defines life is central to biology, and scientists use a set of universal characteristics to distinguish a living organism from an inanimate object. While an intuitive answer might point to movement as a clear sign of life, the standard definition of a living entity rests on a combination of specific, interconnected biological processes. Evaluating whether something is alive requires looking beyond simple motion to assess a suite of mandatory requirements that must all be present.
The Essential Requirements for Defining Life
Life is defined by the simultaneous presence of several characteristics shared by all organisms. The first is organization, meaning all living things are composed of one or more cells, the fundamental unit of life. These cells contain coordinated structures like organelles that carry out specific functions. Another element is metabolism, which represents the sum of all chemical reactions that convert energy and nutrients into the substances necessary for life, such as generating adenosine triphosphate (ATP) for cellular work.
Organisms must also exhibit several other characteristics:
- Homeostasis, the ability to maintain a stable internal environment despite external fluctuations, such as regulating body temperature or internal pH levels.
- Growth and development, involving an increase in size and the orderly maturation of the organism over time.
- Reproduction, ensuring that genetic information is passed on to the next generation.
- Response to stimuli, which is an organism’s ability to sense and react to changes in its surroundings, like a plant growing toward light.
Movement is often a component of this response, but it is not a standalone criterion for existence.
Movement: Locomotion Versus Internal Activity
When considering movement in a biological context, it is helpful to distinguish between two main types: locomotion and internal activity. Locomotion refers to the macroscopic movement of an entire organism from one place to another, such as a cheetah running or a bacterium using a flagellum to swim. This type of movement is what most people associate with being alive. However, many organisms, including plants and numerous marine animals like sponges and corals, are sessile, meaning they are fixed in place and do not exhibit locomotion as adults.
Despite lacking the ability to change their physical location, sessile organisms are still highly active internally. This internal activity includes microscopic movements necessary for survival and growth. For instance, plants exhibit cytoplasmic streaming, the movement of cytoplasm within their cells to circulate nutrients, organelles, and genetic material. Other internal movements involve cell division for growth, nutrient transport through vascular tissues, or the slow, directed bending of stems and roots in response to environmental cues like gravity and light.
Why Movement Is Often Misleading
While movement is a common characteristic of living things, it is neither a sufficient nor a necessary condition for defining life. Movement is not sufficient because many non-living objects move without meeting any of the other criteria for life. Examples include water flowing in a river, wind currents, the slow drift of tectonic plates, or the growth of crystals, which expands its structure by adding material from the outside. These phenomena are driven by external physical forces, such as gravity or pressure, and lack the internal, self-sustaining organization of a living cell.
Movement is also not necessary because, as seen with sessile organisms, an entity can be alive without ever locomoting. A mature tree, for example, is alive because it metabolizes energy, grows, reproduces, and maintains homeostasis, even though it cannot walk away. The complexity of life lies in the source and purpose of the movement, which for living things is self-initiated, regulated by an internal genetic program, and aimed at sustaining the organism. Ultimately, an object must possess all the defining characteristics—from cellular organization and metabolism to reproduction and homeostasis—to be classified as a living entity.