Carbon compounds are the energy source of cellular respiration. These molecules are oxidized, that is, they lost their electrons, in order create energy for the cell. For example, we could start with a large lipid chain of 16 carbon atoms, that will experience oxidation in order to create conditions for ATP synthesis, that finally will become one carbon atom. Single carbon atoms are exhaled in respiration as CO2 (Carbon dioxidde). So, carbon atoms are the source of energy in cellular respiration (as carbon molecules), and finally they serve as a mean to exhale the metabolic waste, as CO2.