A carbon source is a carbon-containing molecule that is used by an organism to synthesise biomass. Such sources may be organic or inorganic. Heterotrophs must use organic molecules as a source of both carbon and energy. In contrast, autotrophs may use inorganic materials as a source for both, such as inorganic chemical energy (chemolithotrophs) or light (photoautotrophs). The carbon cycle, which begins with an inorganic carbon source (such as carbon dioxide) and progresses through the biological carbon fixation process, includes the biological use of carbon as one of its components.