The Q cycle (named for quinol) describes a series of reactions that describe how the sequential oxidation and reduction of the lipophilic electron carrier Coenzyme Q (CoQ) between the ubiquinol and ubiquinone forms, can result in the net movement of protons across a lipid bilayer (in the case of the mitochondria, the inner mitochondrial membrane).
The Q cycle was first proposed by Peter D. Mitchell, though a modified version of Mitchell's original scheme is now accepted as the mechanism by which Complex III moves protons (i.e. how complex III contributes to the biochemical generation of the proton or pH, gradient, which is used for the biochemical generation of ATP).
The first reaction of Q cycle is the 2-electron oxidation of ubiquinol by two oxidants,  c (Fe) and ubiquinone:
: CoQH + cytochrome c (Fe) + CoQ  → CoQ + CoQ + cytochrome c (Fe) + 2 H (intermembrane)
The second reaction of the cycle involves the 2-electron oxidation of a second ubiquinol by two oxidants, a fresh c (Fe) and the CoQ' produced in the first step:
: CoQH + cytochrome c (Fe) + CoQ  + 2 H (matrix)→ CoQ + CoQH + cytochrome c (Fe) + 2 H (intermembrane)
These net reactions are mediated by electron-transfer mediators including a Rieske 2Fe-2S cluster (shunt to  c) and  c  (shunt to CoQ  and later to CoQ) 
In chloroplasts, a similar reaction is done with plastoquinone by cytochrome b6f complex.