Heterotroph



A heterotroph ( heteros = "another", "different" and trophe = "nutrition") is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts 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. More than 95% of all organisms are heterotrophic.

Types
Heterotrophs can be divided into two broad classes: photoheterotrophs and chemoheterotrophs. Photoheterotrophs, including most purple bacteria and green bacteria, produce ATP from light and use organic compounds to build structures. They consume little or none of the energy produced during photosynthesis to reduce NADP+ to NADPH for use in the Calvin cycle, as they do not need to use the Calvin cycle if carbohydrates are available in their diets. Chemoheterotrophs produce ATP by oxidizing chemical substances. There are two types of chemoheterotrophs: chemoorganoheterotrophs and chemolithoheterotrophs.

Chemoorganoheterotrophs (or simply organotrophs) exploit reduced carbon compounds as energy sources, such as carbohydrates, fats, and proteins from plants and animals. Chemolithoheterotrophs (or lithotrophic heterotrophs) such as colorless sulfur bacteria (e.g., Beggiatoa and Thiobacillus) and sulfate-reducing bacteria utilize inorganic substances to produce ATP, 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.

Heterotrophs, by consuming reduced carbon compounds, are able to use all the energy that they obtain from food for growth and reproduction, unlike autotrophs, which must use some of their energy for carbon fixation. Heterotrophs are unable to make their own food, however, and whether using organic or inorganic energy sources, they can die from a lack of food. This applies not only to animals and fungi but also to bacteria.



Ecology
Most heterotrophs are chemoorganoheterotrophs (or simply organotrophs) and utilize organic compounds both as a carbon source and an energy source. The term "heterotroph" very often refers to chemoorganoheterotrophs. Heterotrophs function as consumers in food chains: they obtain organic carbon by eating other heterotrophs or autotrophs. They break down complex organic compounds (e.g., carbohydrates, fats, and proteins) produced by autotrophs into simpler compounds (e.g., carbohydrates into glucose, fats into fatty acids and glycerol, and proteins into amino acids). They release energy by oxidizing carbon and hydrogen atoms present in carbohydrates, lipids, and proteins to carbon dioxide and water, respectively.

All animals and fungi are heterotrophic, as well as most protists and prokaryotes. Some animals, such as corals, form symbiotic relationships with autotrophs and obtain organic carbon in this way. Furthermore, some parasitic plants have also turned fully or partially heterotrophic, while carnivorous plants consume animals to augment their nitrogen supply while remaining autotrophic.