Characteristics of Living Organisms
In everyday language, "living things" and "living organisms" mean the same thing, and our Characteristics of Living Things page covers the classic seven-process list (often remembered as MRS GREN) taught at school level. This page takes a slightly more formal biological angle, the broader set of organisational and functional criteria that biologists use when classifying something as a living organism, which overlaps with but is organised a little differently from that list.
Last updated: 8 September 2026
Key characteristics
Cellular organization
Every living organism is composed of one or more cells, the basic structural and functional unit of life, whether that's a single bacterial cell or the trillions of specialised cells in a large animal.
Metabolism
Organisms carry out organised chemical reactions that convert nutrients into usable energy and building materials, sustaining the processes needed to stay alive and grow.
Homeostasis
Living organisms actively regulate their internal environment, such as temperature, pH, or water balance, keeping conditions stable enough for their cells to function even as the external environment changes.
Growth and development
Organisms increase in size and complexity over their lifespan following a genetically programmed pattern, from a single fertilised cell through to a mature adult form in most species.
Reproduction and heredity
Organisms are capable of producing offspring and passing genetic information to the next generation, which is what allows a species to continue beyond any individual's lifespan.
Response to the environment
Living organisms detect and react to changes in their surroundings, from a single-celled organism moving away from a toxin to complex animal behaviour, allowing them to survive in a changing environment.
Adaptation through evolution
Over many generations, populations of organisms change in response to environmental pressures through natural selection, a defining characteristic of life at the population and species level rather than the individual level.
Frequently asked questions
What is the difference between this page and Characteristics of Living Things?
They describe overlapping ideas from two different angles. Our Characteristics of Living Things page uses the classic school-level list of seven processes (movement, respiration, sensitivity, growth, reproduction, excretion, nutrition). This page covers the broader, more formal set of biological criteria, cellular organisation, metabolism, homeostasis, and evolutionary adaptation, that biologists use in a more academic context. Both describe the same underlying reality.
Are viruses considered living organisms?
Most biologists say no, or place them on the boundary. Viruses lack independent metabolism and cannot reproduce without hijacking a host cell's machinery, which means they fail several of the core criteria on this page even though they do carry genetic material and can evolve.
What is the smallest known living organism?
Among cellular life, some of the smallest known free-living organisms are certain bacteria, such as species in the genus Mycoplasma, which can be under half a micrometre across, right at the theoretical lower size limit for a cell to contain the machinery needed for independent life.