Energy exists in different forms, some of which are electrical, heating, chemical, luminous, among others.
Chemical energy in biological systems is based on the formation-breaking of bonds: to form a bond, energy must be expended, while when a bond is broken, energy is released.
Often, these processes require the participation of enzymes within the organisms; enzymes are proteins that decrease the activation energy of a reaction. For example, if a lot of energy is needed to break a bond, the enzyme will help to lower the energy required.
In biological systems such as humans, the energy molecule is ATP, which releases energy when a bond is broken and a phosphate group is released, leaving ADP as a product. And although it is an efficient process, the laws of thermodynamics explain that no process is 100% efficient, and therefore, some amount of energy is always released in the form of heat. Unlike chemical energy, heat energy is not stored in bonds and cannot be catalyzed by enzymes or utilized by biological systems.