<P> Some lithotrophic bacteria can utilize diverse sources of electrons, depending on availability of possible donors . </P> <P> It should be noted that the organic or inorganic substances (e.g., oxygen) used as electron acceptors needed in the catabolic processes of aerobic or anaerobic respiration and fermentation are not taken into account here . </P> <P> For example, plants use water as electron donor to biosynthesis, being lithotrophs . Animals use organic compounds as electron donors to synthesize ATP, being organotrophs (plants also do it, 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 . </P> <P> Heterotrophs: Organic compounds are metabolized to get carbon for growth and development . Autotrophs: Carbon dioxide (CO) is used as source of carbon . </P>

The class of compounds that are the most important source of cellular energy are the