It is generally accepted that Q molecule binds at site 1 and gets reduced by electron transfer from the N2 FeS cluster. Recent structural data shows that both short-and long-tailed Q molecules can bind in the Q tunnel with the head group residing within electron transfer distance (<14 Å) to the N2 FeS cluster . In contrast to earlier suggestions , the Q tunnel does not appear to be entirely insulated from the bulk N phase. Instead, a number of cavities have been identified that link the N aqueous phase to the Q tunnel (Fig. ) (see also ). This raises two important questions: what the roles of these cavities are, and what kind of gating mechanisms are required to prevent the premature protonation of anionic charges that may accumulate after Q reduction in the Q tunnel (section 2).