An organism is an individual living system capable of carrying out the fundamental processes associated with life. Organisms maintain an organised physical structure, acquire and use energy and materials, regulate internal conditions, respond to their surroundings, grow or develop according to biological programmes, reproduce directly or participate in reproduction, and contain hereditary information that can be transmitted through generations.
The word organism is related to the idea of an organised living system, reflecting the coordinated arrangement of structures and processes that allows life to persist. Organisms range enormously in complexity, from single celled bacteria and archaea to multicellular animals, plants, fungi and other complex life forms. A living organism should not be defined merely by movement, because many organisms do not move visibly, nor by reproduction alone, because an individual organism can be sterile. The concept instead concerns an integrated biological system whose components interact to maintain the system and allow it to participate in evolutionary processes.
The simplest organisms are unicellular, meaning that one cell performs all functions necessary for life. A bacterium may therefore contain a cell membrane, genetic material, ribosomes, metabolic enzymes and other structures within one cell, while that same cell obtains nutrients, produces energy, removes waste, responds to environmental conditions and divides to produce descendants. Multicellular organisms distribute biological functions among specialised cells.
A human, for example, consists of trillions of cells organised into tissues, organs and organ systems, with nervous, circulatory, respiratory, digestive, immune, endocrine and other systems interacting continuously. Plants similarly contain specialised roots, stems, leaves, vascular tissues and reproductive structures. Multicellularity therefore does not replace cellular organisation but builds higher levels of organisation upon it. In every case, the organism is more than an arbitrary collection of molecules because its components are coordinated through biochemical, genetic, electrical and physical interactions.
Organisms are distinguished from nonliving structures by the integrated biological processes occurring within them. A crystal can grow by adding molecules to its structure, but its growth does not involve genetic information, metabolism or regulated cellular reproduction. A fire can consume material, release energy and spread, but it does not possess a genome or cellular organisation and does not maintain itself through biological homeostasis.
Viruses present a more complicated boundary case because they contain genetic material and undergo evolution but generally lack independent metabolism and cellular organisation and require host cells for replication. Consequently, whether viruses should be called organisms depends partly on the definition being used. Biology commonly distinguishes cellular life from acellular infectious agents, while recognising that the boundary between living and nonliving systems is scientifically interesting rather than perfectly simple.
Organisms have hereditary information, usually encoded in DNA, although some viruses use RNA. In cellular organisms, DNA contains genes and regulatory sequences that influence the production of proteins and functional RNA molecules and thereby affect cellular structure and behaviour. Genetic information is copied during reproduction or cell division, and mutations and other forms of genetic variation provide material on which natural selection and other evolutionary processes can act. Heredity therefore connects the individual organism with populations and species over time.
An organism is not simply the expression of its genes, however, because phenotype results from interactions among genetic factors, developmental processes, environmental conditions and chance. Identical genetic material can sometimes produce different outcomes in different environments. The organism consequently exists within a continuous relationship between inherited biological information and its physical, chemical and ecological surroundings.
Metabolism is another defining feature of organisms. Metabolism comprises the interconnected chemical reactions through which organisms obtain, transform, store and use matter and energy. Plants, algae and some microorganisms can capture energy from sunlight through photosynthesis, whereas animals obtain chemical energy by consuming organic material. Chemotrophic microorganisms can obtain energy from inorganic compounds or other chemical reactions. Cellular respiration and related pathways release usable energy from molecules and support processes such as movement, synthesis, transport and maintenance.
Organisms must also regulate internal conditions, a process commonly described as homeostasis. Temperature, water balance, ion concentrations, acidity, glucose concentration and many other variables may be controlled within functional ranges. Homeostasis does not mean that internal conditions remain perfectly constant; biological regulation usually involves dynamic adjustment around suitable operating ranges.
Organisms interact continuously with their environments. They may detect light, temperature, pressure, chemicals, predators, competitors, nutrients, pathogens and other environmental factors and alter their behaviour or physiology in response. Plants can alter growth direction in response to light and gravity, bacteria can move towards favourable chemical conditions, and animals can change behaviour in response to sensory information. Ecologically, an organism is therefore not an isolated object but a participant in networks involving other organisms and physical processes.
Individuals consume resources, release waste, modify habitats, compete, cooperate, reproduce and sometimes form symbiotic relationships. These interactions influence survival and reproductive success and therefore influence evolutionary change in populations. The organism concept is consequently connected to ecology, physiology, genetics, cell biology, developmental biology and evolutionary biology.
Organisms can be classified according to cellular organisation, evolutionary relationships, nutrition, reproduction, morphology, genetics and other characteristics. Cellular organisms are broadly divided into prokaryotic and eukaryotic forms based on cellular structure, although modern classification places prokaryotes into the domains Bacteria and Archaea and eukaryotes into Eukarya. Eukaryotic organisms include animals, plants, fungi and numerous groups of unicellular organisms. Organisms may reproduce asexually, sexually or through combinations of reproductive strategies.
They may be autotrophic, heterotrophic or mixotrophic depending on how they acquire carbon and energy. These categories are useful but do not completely define the organism concept. Modern biology increasingly emphasises evolutionary relationships and genetic evidence rather than relying solely on visible characteristics, because organisms with superficially similar forms can be distantly related while apparently different organisms can share close evolutionary ancestry.
Understanding what constitutes an organism is fundamental because nearly every area of biology ultimately concerns living systems at one or more levels of organisation. Cell biology examines the smallest autonomous units of cellular life, physiology examines how organisms function, genetics examines hereditary information, ecology examines interactions among organisms and environments, and evolution explains how populations change across generations. The concept also has practical importance in medicine, agriculture, conservation, biotechnology and astrobiology.
Identifying whether something is an organism affects how scientists interpret reproduction, disease, environmental impact and evolutionary history. Modern research also investigates unusual biological entities, including giant viruses, obligately symbiotic organisms and organisms with highly reduced genomes, showing that life has many forms that do not fit simple textbook descriptions. The most useful general definition therefore treats an organism as an integrated living system possessing biological organisation and participating in processes such as metabolism, regulation, heredity, interaction and evolution.
