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 more cells. These units, generally too small
to be seen with the naked 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
Living things exhibit a
rapid turnover of chemical materials, which is referred to as metabolism. 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 called hormones 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 as reproduction. These copies are made while the organism is still
living. Among plants and simple animals, reproduction is often an extension of
the growth process. 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.
Asexual reproduction involves only one parent, and the resulting cells
are generally identical to the parent cell. For example, bacteria grow and
quickly reach maturity, after which they split into two organisms by a process
of asexual reproduction called binary
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. Both biotic factors (living things) and abiotic factors (nonliving
things) can alter the environment. Rain and sunlight are non-living components,
for example, that greatly influence the environment. Living things may migrate
or hibernate if the environment becomes difficult to live in.