Defining a living thing is a difficult proposition, as is defining “life”—that property possessed by living things. However, a living thing possesses certain properties that help define what life is.
Living things have a level of complexity and organization not found in lifeless objects. At its most fundamental level, a living thing is composed of one or morecells. These units, generally too small to be seen with the unaided eye, are organized into tissues. A tissue is a series of cells that accomplish a shared function. Tissues, in turn, form organs, such as the stomach and kidney. A number of organs working together compose an organ system. An organism is a complex series of various organ systems.
Living things exhibit a rapid turnover of chemical materials, which is referred to asmetabolism. Metabolism involves exchanges of chemical matter with the external environment and extensive transformations of organic matter within the cells of a living organism. Metabolism generally involves the release or use of chemical energy. Nonliving things do not display metabolism.
All living things are able to respond to stimuli in the external environment. For example, living things respond to changes in light, heat, sound, and chemical and mechanical contact. To detect stimuli, organisms have means for receiving information, such as eyes, ears, and taste buds.
To respond effectively to changes in the environment, an organism must coordinate its responses. A system of nerves and a number of chemical regulators calledhormones coordinate activities within an organism. The organism responds to the stimuli by means of a number of effectors, such as muscles and glands. Energy is generally used in the process.
Organisms change their behavior in response to changes in the surrounding environment. For example, an organism may move in response to its environment. Responses such as this occur in definite patterns and make up the behavior of an organism. The behavior is active, not passive; an animal responding to a stimulus is different from a stone rolling down a hill. Living things display responsiveness; nonliving things do not.
Growth requires an organism to take in material from the environment and organize the material into its own structures. To accomplish growth, an organism expends some of the energy it acquires during metabolism. An organism has a pattern for accomplishing the building of growth structures.
During growth, a living organism transforms material that is unlike itself into materials that are like it. A person, for example, digests a meal of meat and vegetables and transforms the chemical material into more of himself or herself. A nonliving organism does not display this characteristic.
A living thing has the ability to produce copies of itself by the process known asreproduction. These copies are made while the organism is still living. Among plants and simple animals, reproduction is often an extension of the growth process. For example, bacteria grow and quickly reach maturity, after which they split into two organisms by the process of asexual reproduction. Asexual reproduction involves only one parent, and the resulting cells are generally identical to the parent cell.
More complex organisms engage in a type of reproduction called sexual reproduction, in which two parents contribute to the formation of a new individual. During this process, a new combination of traits can be produced. The process is generally more complex than asexual reproduction, requiring that parents find one another, then (usually) care for their offspring before it can live independently. Nonliving things have no such ability or requirements.
Populations of living organisms have the ability to adapt to their environment through the process of evolution. During evolution, changes occur in populations, and the organisms in the population become better able to metabolize, respond, and reproduce. They develop abilities to cope with their environment that their ancestors did not have.
Evolution also results in a greater variety of organisms than existed in previous eras. This proliferation of populations of organisms is unique to living things.
The environment influences the living things that it surrounds. Ecology is the study of relationships between organisms and their relationships with their environment. Living things can alter their environment, but nonliving things cannot. Living things, for example, may migrate or hibernate if the environment becomes difficult to live in.