Figure 20.12 The Q cycle. In the first half of the cycle, two electrons of a bound QH2 are transferred, one to cytochrome c and the other, after passing through cytochrome b, to a bound Q in a second binding site to form the semiquinone radical anion Q•–. The newly formed Q dissociates and enters the Q pool. In the second half of the cycle, a second QH2 also gives up its electrons, one to a second molecule of cytochrome c and the other to reduce Q•–to QH2. This second electron transfer results in the uptake of two protons from the matrix. The path of electron transfer is shown in red.