A living organism is an organised biological system that possesses the characteristics associated with life and is capable of carrying out processes such as metabolism, maintaining internal conditions, responding to its environment and reproducing either independently or as part of a life cycle. Organisms include bacteria, archaea, protists, fungi, plants and animals, while viruses occupy a disputed position because they lack cellular organisation and independent metabolism and can reproduce only by using host cellular machinery.
A living organism is not merely a collection of biological molecules. It possesses organisation, boundaries, regulated chemical processes and mechanisms that allow its components to interact as a functioning system. Most organisms are composed of one or more cells, and cellular organisation is generally regarded as a fundamental property of life on Earth. Organisms may be unicellular, consisting of a single cell, or multicellular, consisting of many specialised cells that cooperate within tissues, organs and organ systems.
A defining characteristic of organisms is organisation. At the molecular level, organisms contain nucleic acids, proteins, lipids, carbohydrates, ions and numerous other substances arranged into functional structures. Cells provide the basic organisational units of cellular life, containing membranes and molecular machinery that maintain a distinction between the organism and its environment. In multicellular organisms, cells can specialise and become organised into tissues, organs and interconnected systems. Organisation also occurs at larger scales, because organisms interact with populations, communities and ecosystems.
Biological organisation is dynamic rather than static. Molecules are continuously synthesised and degraded, membranes are renewed, damaged structures are repaired and energy is transferred through metabolic pathways. This continual activity allows an organism to maintain an organised state despite constant exchange of matter and energy with its surroundings.
Metabolism is another central characteristic of living organisms. Metabolism comprises the network of chemical reactions through which organisms obtain, transform, store and use energy and materials. Catabolic reactions break molecules down and can release usable energy, while anabolic reactions construct complex molecules and generally require energy. Cellular respiration, photosynthesis, protein synthesis, DNA replication and nutrient processing are examples of processes linked to metabolism. Organisms obtain energy in different ways.
Plants, algae and some microorganisms can capture light energy through photosynthesis, while animals obtain chemical energy from food and many microorganisms use inorganic or organic chemical substances as energy sources. Metabolic reactions are controlled by enzymes, regulatory systems and cellular conditions. Without controlled energy and material transformations, the organised processes required for biological maintenance could not continue.
Homeostasis refers to the regulation of internal conditions within tolerable ranges despite changes in the external environment. Living organisms regulate variables such as temperature, water balance, ion concentrations, acidity and nutrient availability. In animals, nervous and endocrine systems coordinate many homeostatic responses, while plants regulate water loss, gas exchange, ion transport and other processes through cellular and physiological mechanisms. Microorganisms also regulate internal conditions through membrane transport, metabolic regulation and molecular responses.
Homeostasis does not mean that internal conditions remain absolutely constant. Biological variables usually fluctuate within ranges, and those ranges can change with developmental stage, activity, environmental conditions and circadian rhythms. The ability to regulate internal conditions is therefore better understood as dynamic stability maintained through feedback mechanisms.
Reproduction is the process by which organisms produce new individuals or reproductive units. Sexual reproduction generally involves genetic contributions from two reproductive cells or genetic lineages, while asexual reproduction can produce offspring without the fusion of gametes. Bacteria commonly reproduce through binary fission, while many plants and fungi can reproduce through both sexual and asexual mechanisms.
Reproduction itself is not required for every individual organism to remain alive, because sterile individuals can still be living organisms, but reproduction is essential for the continuation of biological lineages. Heredity allows biological information to pass between generations, primarily through DNA in cellular organisms. Mutation and genetic recombination introduce or rearrange variation, while natural selection and other evolutionary mechanisms influence the distribution of inherited characteristics in populations.
Living organisms respond to stimuli and interact continuously with their surroundings. A stimulus can be physical or chemical and may include changes in temperature, light, pressure, nutrient availability, chemical concentration, sound or mechanical forces. Plants respond to light through processes such as phototropism and adjust growth according to environmental signals. Animals can detect sensory information through specialised receptors and coordinate rapid or complex responses through nervous and hormonal systems.
Microorganisms can move toward favourable chemical conditions or away from harmful ones through behaviours such as chemotaxis. At the molecular level, cells respond through receptors, signalling pathways, gene regulation and changes in metabolism. Responsiveness therefore operates across many scales, from molecular interactions to coordinated behaviour.
Growth and development are also associated with life. Growth involves an increase in size, mass, cell number or biological material, while development involves organised changes in structure and function over an organism's life cycle. In multicellular organisms, development involves processes such as cell division, differentiation, migration and programmed cell death. A human develops from a fertilised egg through progressively organised stages, while a flowering plant develops roots, stems, leaves, flowers and reproductive structures through regulated growth.
Some organisms undergo dramatic developmental transformations, such as insects that pass through larval and adult stages. Growth does not necessarily mean simple enlargement because biological development can involve restructuring and specialisation. These processes are controlled by genetic programmes interacting with environmental conditions and cellular signalling systems.
The definition of life has important boundaries and unresolved questions. Organisms are generally cellular and exhibit metabolism, regulation, heredity, reproduction at the lineage level, responsiveness and evolutionary capacity, but no single characteristic alone is sufficient in every case. Viruses possess genetic material and evolve but lack independent cellular metabolism and cannot reproduce without host machinery. Prions consist of abnormal proteins and can propagate structural information but are not cellular organisms and contain no nucleic acid.
Some biological entities can remain dormant for long periods while showing little detectable metabolic activity, yet become active when conditions improve. Artificial systems and hypothetical extraterrestrial life also raise questions about whether Earth's familiar characteristics are universally necessary for life. Consequently, the concept of a living organism is grounded in a collection of interconnected biological properties rather than one simple test.
