Biological energy: Whenever matter is broken down its chemical energy is released as heat. Non biological system can utilize this heat energy directly in the performance  of work, or   may easily convert it into mechanical  electrical or other forms of energy. Because these system are an isothermal i.e. they do not maintain  constant temperatures. Biological system   on the other hand are isothermal they maintain either a fixed body temperature or an optimum range of temperature equal to that of the environment. A rise  or fall in  body  temperature  by  a few degrees   from the constant or optimum   temperature  has an adverse effect on  the structure  and function  of living  cells. Thus living  systems cannot  use heat  energy  directly  to perform  work. These therefore require an energy exchanges i.e.  an intermediary   compound  capable of receiving  energy of chemical  oxidation directly from the oxidative or  energy release processes, storing  this energy  in its  molecules and supplying  it directly  to the energy consuming  processes .Obviously, the  energy stored by this    compound is the energy used   by  living  system ,and  this  is called  Biological energy.