Heterotrophs

A heterotroph (/ˈhɛtərəˌtroʊf, -ˌtrɒf/;trophe = "nutrition") is an organism that cannot fix carbon from inorganic sources (such as carbon dioxide) but uses organic carbon for growth. Heterotrophs can be further divided based on how they obtain energy; if the heterotroph uses light for energy, then it is considered a photoheterotroph, while if the heterotroph uses chemical energy, it is considered a chemoheterotroph.

Heterotrophs contrast with autotrophs, such as plants and algae, which can use energy from sunlight (photoautotrophs) or inorganic compounds (lithoautotrophs) to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon dioxide. These reduced carbon compounds can be used as an energy source by the autotroph and provide the energy in food consumed by heterotrophs. Ninety-five percent or more of all types of living organisms are heterotrophic, including all animals and fungi and most bacteria and protists.

Organotrophs exploit reduced carbon compounds as energy sources, like carbohydrates, fats, and proteins from plants and animals. Photoorganoheterotrophs such as Rhodospirillaceae and purple non-sulfur bacteria synthesize organic compounds by utilization of sunlight coupled with oxidation of inorganic substances, including hydrogen sulfide, elemental sulfur, thiosulfate, and molecular hydrogen. They use organic compounds to build structures. They do not fix carbon dioxide and apparently do not have the Calvin cycle. Chemolithoheterotrophs can be distinguished from mixotrophs (or facultative chemolithotroph), which can utilize either carbon dioxide or organic carbon as the carbon source.

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