Chemical energy of matter : As is obvious from the above account living cells obtain energy by breaking C-C and C-H bonds of organic matter under the process of catabolism ,It is a long complicated process involving a large number of chemical reactions .It is essentially an oxidative breakdown of energy yielding molecules. Hence complex oxidation requires the use of free oxygen obtained by the organisms from the environment the separated carbon and hydrogen atoms in this breaking combine with the oxygen forming CO2 and H2O respectively .Thus is called cellular respiration.
Cellular fuel cells mostly oxidize glucose for obtaining chemical energy. That is why glucose is called cellular fuel the equation of its oxidation is
C6H12O2 → 6CO2+6H2O + energy
Fats and proteins are also utilized by the cells to some extent energy production.