Editing Primary nutritional groups (section)

== Primary sources of reducing equivalents ==

[[Organotroph|''Organotrophs'']] use organic compounds as [[electron donor|electron/hydrogen donors]].<br>
[[Lithotroph|''Lithotrophs'']] use inorganic compounds as electron/hydrogen donors.

The [[electron]]s or hydrogen atoms from [[reducing equivalent]]s (electron donors) are needed by both phototrophs and chemotrophs in [[Redox|reduction-oxidation reactions]] that transfer energy in the anabolic processes of [[ATP synthesis]] (in heterotrophs) or [[biosynthesis]] (in autotrophs). The electron or hydrogen donors are taken up from the environment.

Organotrophic organisms are often also heterotrophic, using organic compounds as sources of both electrons and carbon. Similarly, lithotrophic organisms are often also autotrophic, using inorganic sources of electrons and {{CO2}} as their inorganic carbon source.

Some lithotrophic bacteria can utilize diverse sources of electrons, depending on the availability of possible donors.

The organic or inorganic substances (e.g., oxygen) used as electron acceptors needed in the catabolic processes of aerobic or anaerobic [[Cellular respiration|respiration]] and [[fermentation]] are not taken into account here.

For example, plants are lithotrophs because they use water as their electron donor for the electron transport chain across the thylakoid membrane. Animals are organotrophs because they use organic compounds as electron donors to synthesize ATP (plants also do this, but this is not taken into account). Both use oxygen in respiration as electron acceptor, but this character is not used to define them as lithotrophs.