Living things have a variety of common characteristics.
- Organization. Living things
exhibit a high level of organization, with multicellular organisms
being subdivided into cells, and cells into organelles, and organelles
into molecules, etc.
- Homeostasis. Homeostasis is the
maintenance of a constant (yet also dynamic) internal environment in
terms of temperature, pH, water concentrations, etc. Much of our own
metabolic energy goes toward keeping within our own homeostatic
limits. If you run a high fever for long enough, the increased
temperature will damage certain organs and impair your proper
functioning. Swallowing of common household chemicals, many of which
are outside the pH (acid/base) levels we can tolerate, will likewise
negatively impact the human body's homeostatic regime. Muscular
activity generates heat as a waste product. This heat is removed from
our bodies by sweating. Some of this heat is used by warm-blooded
animals, mammals and birds, to maintain their internal temperatures.
- Adaptation. Living things are
suited to their mode of existence. Charles Darwin began the
recognition of the marvellous adaptations all life has that allow
those organisms to exist in their environment.
- Reproduction and heredity. Since
all cells come from existing cells, they must have some way of
reproducing, whether that involves asexual (no recombination of
genetic material) or sexual (recombination of genetic material). Most
living things use the chemical DNA (deoxyribonucleic acid) as the
physical carrier of inheritance and the genetic information. Some
organisms, such as retroviruses (of which HIV is a member), use RNA
(ribonucleic acid) as the carrier. The variation that Darwin and
Wallace recognized as the wellspring of evolution and adaptation, is
greatly increased by sexual reproduction.
- Growth and development. Even
single-celled organisms grow. When first formed by cell division, they
are small, and must grow and develop into mature cells. Multicellular
organisms pass through a more complicated process of differentiation
and organogenesis (because they have so many more cells to develop).
- Energy acquisition and release.
One view of life is that it is a struggle to acquire energy (from
sunlight, inorganic chemicals, or another organism), and release it in
the process of forming ATP (adenosine triphosphate).
- Detection and response to stimuli
(both internal and external).
- Interactions. Living things
interact with their environment as well as each other. Organisms
obtain raw materials and energy from the environment or another
organism. The various types of symbioses (organismal interactions with
each other) are examples of this.