This is a smaller part of a larger, more general reaction called cellular respiration, where a simple sugar, such as glucose, is oxidized and produces energy (ATP), carbon dioxide, and water. The overall reaction looks something like this:
C6H12O6 + O2 ---> energy (ATP) + CO2 + H2O
The part of the equation you are asking about is the energy production part, where adenosine triphosphate (ATP) is produced by an enzyme causing the addition of a free phosphorous ion to adenosine diphosphate (ADP). The enzyme causes the extra phosphorous to be added, usually as the result of a H+ ion being passed through the membrane of the mitochondria. That equation looks like this:
ADP + P ---> ATP
ATP is the molecule that energy is stored into, to be used by the animal cell as it conducts it's life processes. The glucose that makes the ATP is secured by the digestive system, while the oxygen is brought into the reaction via the respiratory system.