Every breath you take...

First the proteins are synthesized and at the same time transported through the pores into mitochondrion. Then they become part of electron transport chain which uses the energy from electron transport to move hydrogen into mitochondrial itermembrane space. This creates electric gradient and the stored energy is used by ATPase to create ATP molecules. ATP are molecules that then power all cell reactions.

This is why you breathe. The oxygen is used as final receptor for both electrons and hydrogens creating water. This is where the food is burnt slowly till it becomes CO₂. Literaly cells power plants.