= Solution
At low <ATP> demand, the <proton-motive force> is high and much of the remaining oxygen consumption supports <proton leak>. Increasing leak conductance then creates a substantial extra demand for proton pumping and strongly changes respiratory flux. At high phosphorylation demand, proton return through <ATP synthase> is large, the <proton-motive force> is lower, and leak contributes a much smaller part of total proton return. \b[Control of oxygen consumption shifts toward substrate supply and the respiratory machinery, so the leak <flux control coefficient> becomes small.] The plot's value near zero does not mean that the physical leak current vanishes; the separate maximal-rate table likewise gives a small nonzero coefficient, $0.04$.
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