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Autotroph
An autotroph ("self-feeding", from the Greek ''autos'' "self" and ''trophe'' "nourishing") or producer, is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water, in contrast to heterotrophs as consumers of autotrophs. They do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and also create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide. Phototrophs (green plants and algae), a type of autotroph, convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon.
Autotrophs can be photoautotrophs or chemoautotrophs. Phototrophs use light as an energy source, while chemotrophs utilize electron donors as a source of energy, whether from organic or inorganic sources; however in the case of autotrophs, these electron donors come from inorganic chemical sources. Such chemotrophs are lithotrophs. Lithotrophs use inorganic compounds, such as hydrogen sulfide, elemental sulfur, ammonium and ferrous iron, as reducing agents for biosynthesis and chemical energy storage. Photoautotrophs and lithoautotrophs use a portion of the ATP produced during photosynthesis or the oxidation of inorganic compounds to reduce NADP+ to NADPH to form organic compounds.
==Variants==
Some organisms rely on organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy. Such organisms are not defined as autotrophic, but rather as heterotrophic. An organism that obtains carbon from organic compounds but obtains energy from light is called a ''photoheterotroph'', while an organism that obtains carbon from organic compounds but obtains energy from the oxidation of inorganic compounds is termed a ''chemoheterotroph'', ''chemolithoheterotroph'', or ''lithoheterotroph''.
Evidence suggests that some fungi may also obtain energy from radiation. Such radiotrophic fungi were found growing inside a reactor of the Chernobyl nuclear power plant.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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