Organisms

An organism is any individual living thing that exhibits the characteristics of life: it maintains internal order (homeostasis), carries out metabolism, grows, develops, responds to stimuli, and reproduces. Organisms range from single cells to complex multicellular bodies; they are composed of cells that may be prokaryotic or eukaryotic. They extract energy and matter from their environment and adapt to changing conditions over time.

Organisms are classified into domains and kingdoms based on cellular organization and genetic relations. Current consensus recognizes three domains: Bacteria, Archaea, and Eukarya. Within Eukarya, life is grouped into kingdoms such as plants, animals, fungi, and various protists. Organisms may be unicellular or multicellular, with levels of organization including tissues, organs, and organ systems.

Metabolism encompasses all chemical processes that sustain life, including energy capture through photosynthesis or chemosynthesis, and energy use through respiration or fermentation. Many organisms interact with other species and their environment, forming ecosystems. Some rely on symbiosis, while others decompose or recycle nutrients.

Reproduction can be sexual or asexual, producing offspring and providing genetic continuity. Development follows genetic programming and environmental cues. Evolution acts on populations, yielding diversity through mutation, recombination, and natural selection, which shapes organismal traits over generations.

Studying organisms involves multiple disciplines, from genetics and physiology to ecology and taxonomy. The term ‘organism’ is sometimes debated for entities at the edge of life, such as certain viruses, which possess genetic material but lack independent metabolism; many scientists exclude them as organisms in a strict sense.