What is an organism?

Abstract A biological organism can be defined as an entity that uses an energy source and simple chemical compounds from the environment in order to grow and to create copies of itself, based on chemically encoded information. Energy sources can be nonbiological, such as in photosynthetic organisms, or biological, such as in organisms that feed on other organisms or on their remains. The vast majority of animals obtain energy from other organisms, known as heterotrophy. Looking beyond animals at the simplest organisms, we find a range of single-celled organisms which use an array of methods for obtaining energy and surviving. Eukaryotes are single-celled organisms with membrane-bound organelles. The mitochondria of all eukaryotes and the chloroplasts of photosynthetic eukaryotes derive from the endosymbiosis of formerly free-living organisms. Eukaryotes can be divided based on their mode of locomotion: amoeboid, ciliate, or flagellate; the way they obtain energy: photosynthetic or heterotrophic; and their skeleton—no skeleton, or silica or calcium carbonate skeleton.