noun, plural: living things
Living things are those that display the following characteristics:
- an organized structure, being made up of a cell or cells
- requires energy to survive or sustain existence
- ability to reproduce
- ability to grow
- ability to metabolize
- ability to respond to stimuli
- ability to adapt to the environment
- ability to move
- ability to respire
Thus, a living thing refers to any organism or form that has life. Life is characterized by exhibiting biological processes, e.g. mitosis, reproduction, assimilation, homeostasis, signaling, apoptosis, etc.
Examples of living things include the bacteria, protozoa, plants, fungi, animals, humans, etc. Viruses are not absolutely living or non-living. When outside their host, viruses are inactive and seemingly inanimate. When inside their host, they became active and alive, capable of utilizing the host cell's structures and replicate.
Living things are classified by eight major taxonomic levels: domain, kingdom, phyla, class, order, family, genus, and species. The three-domain system of biological classification as proposed by Carl Woese and others (in 1977) classifies living things into domains: archaea (archaeabacteria), bacteria (eubacteria), and eucarya (eukaryotes).