Element possessing 6 protons per atom and 4 valence electrons.

Carbon is the most common element, by , that is found in organisms. It also forms the basis of the of such as carbohydrates, proteins, nucleic acids, and lipids.

Carbon is also found in small but significant quantities in the , as carbon dioxide, that is, as part of the carbon cycle. See also hydrocarbon.

The more carbon found in a molecule, generally the more energy potentially associated with that molecule on a per molecule basis. Important also in terms of energy content, however, are carbon-to-hydrogen bonds (C-H). A lipid containing the same number of carbon atoms as a carbohydrate, for example, will possess more energy than the carbohydrate because on a per-carbon basis lipids possess more C-H bonds than do carbohydrates.