The protonmotive Q cycle is the mechanism by which complex III transfers electrons from ubiquinol to cytochrome c, linking this process to translocation of protons across the membrane. This cycle is complicated by the fact that both ubiquinol is oxidised and ubiquinone is reduced during this process. Through a complex series of electron transfers, Complex III consumes two molecules of ubiquinol (QH2) and two molecules of oxidized cytochrome c, generates one molecule of ubiquinone (Q) and two molecules of reduced cytochrome c, regenerates one molecule of ubiquinol (QH2), and mediates the translocation of two protons from the mitochondrial matrix to the mitochondrial intermembrane space. The overall reaction can be summed up as

2QH2 + 2cyt c (ox.) + Q + 2H+ (matrix) = 2Q + 2cyt c (red.) + QH2 + 4H+ (intermemb. space)