Energy Transfer in Chemical Reactions

All chemical bonds contain stored chemical energy, which is released as bonds are broken during a chemical reaction.

If overall reactions result in a net release of energy, they are called exergonic reactions, and those that absorb more energy than they release are called endergonic.

In metabolism, exergonic and endergonic reactions are usually coupled so that the energy released by one reaction can be used to power the other.

The total energy present before and after a reaction is conserved, but it can be converted from one form of energy to another.

For instance, the chemical energy captured in the covalent bonds of ATP can be converted to mechanical energy to power muscle movements or to electrical energy to transmit nerve impulses through the body.

But, energy conversions are usually inefficient, and some of the energy is released as heat, like during the  conversion of chemical energy from a  battery into light.

In mammals, the heat produced by metabolic activity helps to maintain the normal body temperature.