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Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have died), or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Biology is the science concerned with the study of life and living organisms.
Living things generally undertake nutrition of their own, excrete, reproduce, move, have respiration, have senses, and grow over time. They also often need to feed on either organic or, very rarely, non-organic material. Living organisms can either be made of a single living compartment called a cell (unicellular) or multiple cells (multicellular). Abiogenesis is the process in which organic compounds are made from non-organic compounds. All the life on Earth is carbon-based or made of organic compounds such as DNA.
There is a long-standing debate whether carbon, the main element within organic compounds, is necessary for any form of life to exist; others argue that there could be life - extraterrestrial if any - that could potentially be based on other elements (see Carbon chauvinism).