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The Q-cycle mechanism of the bc₁ complex explains how the electron transfer from ubihydroquinone (quinol, QH₂) to cytochrome (cyt) c (or c₂ in bacteria) is coupled to the pumping of protons across the membrane. The efficiency of proton pumping depends on the effectiveness of the bifurcated reaction at the Q_o-site of the complex. This directs the two electrons from QH₂ down two different pathways, one to the high potential chain for delivery to an electron acceptor, and the other across the membrane through a chain containing heme b_L and b_H to the Q_i-site, to provide the vectorial charge transfer contributing to the proton gradient. In this review, we discuss problems associated with the turnover of the bc₁ complex that center around rates calculated for the normal forward and reverse reactions, and for bypass (or short-circuit) reactions. Based on rate constants given by distances between redox centers in known …