Ubiquinone (pink) delivers two electrons at a time to cytochrome b-c1 (red).

As each electron makes its way through the complex, a hydrogen ion, or proton, is pumped from the matrix space of the mitochondria into the intermembrane space, helping to maintain the proton gradient.

After affecting the pumping of a proton across the membrane, the electron leaves cytochrome b-c1 and enters the mobile carrier protein, cytochrome c (purple).