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.