In biology, heterotrophy is the construction of body components (metabolism) from already existing organic compounds. Animals and fungi as well as most bacteria and archaea are heterotrophic. Organic substances help them as energy sources and to compose endogenous substances. So-called heterotrophs or consumers cannot form organic substances exclusively from inorganic substances. The counterpart of heterotrophy is autotrophy, where the carbon needed to build up organic matter exclusively comes from inorganic compounds. This is especially the case with primary producers that perform photosynthesis (especially plants), i.e. carbon dioxide is a source of carbon and light is an energy source for them.