Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

As the name implies, each ATP consists of a tiny molecule (an adenosine) attached to three molecules of phosphate (a phosphorus atom surrounded by oxygen atoms). These three phosphates are bound to each other in a chain, one on top of the other, storing energy in the chemical bonds between each phosphate. When the last of these phosphates is broken off using water, the tiny quantity of energy that binds it to the second phosphate is liberated along with one hydrogen ion (H+), leaving behind an ADP (adenosine diphosphate). This liberated energy powers almost everything done by every cell in the body like firing nerves, making proteins, and contracting muscles. And, critically, ATPs are rechargeable. By breaking down chemical bonds in sugar and fat molecules, cells acquire the energy to restore ADPs to ATPs by adding back the lost phosphate.