Carbon source (biology)

An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs produce complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,^[1] generally using energy from light or inorganic chemical reactions.^[2] Autotrophs do not need a living source of carbon or energy and are the producers in a food chain, such as plants on land or algae in water. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and as stored chemical fuel. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide.

A heterotroph (/ˈhɛtərəˌtroʊf, -ˌtrɒf/;^[3][4] from Ancient Greek ἕτερος (héteros) 'other' and τροφή (trophḗ) 'nutrition') is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain,5819/415836/rav65819_ch07.pdf |title=How Cells Harvest Energy |publisher=McGraw-Hill Higher Education |access-date=2010-10-10 |archive-url=https://web.archive.org/web/20120731144141/http://highered.mcgraw-hill.com/sites/dl/free/0072965819/415836/rav65819_ch07.pdf |archive-date=2012-07-31 |url-status=dead}}</ref> and many parasitic plants. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition.^[5] The term is now used in many fields, such as ecology, in describing the food chain. Heterotrophs occupy the second and third trophic levels of the food chain while autotrophs occupy the first trophic level.^[6]