= Solution
<ATP synthase> combines a <Cell membrane>[membrane] $F_o$ proton-conducting rotor with an $F_1$ catalytic head. The c-ring and central gamma shaft rotate relative to a stator and the alpha/beta head. <Proton> flow down its <electrochemical potential> can drive conformational changes in three beta catalytic sites to bind substrates, form and release <ATP>. Reverse operation uses <ATP hydrolysis> to turn the shaft and pump <Proton>[protons]. The immobilized $F_1$ assay isolates this latter rotation; it does not require the <Cell membrane>[membrane] <proton> motor to be present.
\Image[/past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2016-atp-motor-and-waiting-times.png]
{title=ATP-synthase organization and single-step versus two-step waiting-time distributions}
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