Carbon source refers to any carbon-containing molecule (carbohydrate, amino acid, fatty acid, CO2) used by an organism for the synthesis of its organic molecules. Carbon is a basic element for sustaining life as we know it and the main element in all classes of macromolecules. All cells require carbon as a major nutrient. On a dry weight basis, a typical cell is 50 % carbon. Depending on their carbon source, organisms are either heterotrophs, requiring one or more organic compounds as their carbon source, or autotrophs, where CO2 is the carbon source.