The mobile carrier protein cytochrome c (purple) transfers electrons, one at a time, to cytochrome oxidase (orange). Four electrons must be transferred to the oxidase comples in order for the next major reaction to occur.

The next major event is the reaction of the four electrons, a molecule of O2 (oxygen), and eight protons.

The reaction results in the pumping of four hydrogen ions across the inner membrane into the intermembrane space, and the release of two H20 (water) molecules into the matrix space.