Escherichia coli can generate energy under anaerobic growth conditions by fermentation, converting glucose to a mixture of ethanol and lactate. Under these conditions, the F1Fo ATP synthase is not required for ATP synthesis. Instead, the enzyme runs in reverse, hydrolyzing ATP and pumping protons out of the cell across the cytoplasmic membrane. This is a strategy that appears to waste ATP, so what is the physiological rationale for the ATP synthase running backwards when ATP is being made by fermentation? [Tip: you may need to do some research to answer this question, but also think about the mechanism of flagellar rotation].