Death is the irreversible cessation of all biological functions that sustain a living organism. The remains of a former organism normally begin to decompose shortly after death. Death eventually and inevitably occurs in all organisms. Some organisms, such as Turritopsis dohrnii, are biologically immortal, however they can still die from means other than aging. Death is generally applied to whole organisms; the equivalent for individual components of an organism, such as cells or tissues, is necrosis. Something that is not considered an organism, such as a virus, can be physically destroyed but is not said to die, as a virus is not considered alive in the first place.
The human skull is used universally as a symbol of death.
A flower, a skull, and an hourglass stand for life, death, and time in this 17th-century painting by Philippe de Champaigne.
French – 16th-/17th-century ivory pendant, Monk and Death, recalling mortality and the certainty of death (Walters Art Museum)
The Premature Burial, Antoine Wiertz's painting of a man buried alive, 1854
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not. It is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. All life over time eventually reaches a state of death and none is immortal. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Viruses in particular make definition difficult as they replicate only in host cells. Life exists all over the Earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles.
Life
Animal corpses, like this African buffalo, are recycled by the ecosystem, providing energy and nutrients for living organisms.
Adenoviruses as seen under an electron microscope
Cyanobacteria dramatically changed the composition of life forms on Earth by leading to the near-extinction of oxygen-intolerant organisms.