At the bottom of the bacterial flagellum, where it meets the cell membrane, a motor protein acts as an engine. This engine gets its power from proton motive force, the flow of protons (hydrogen ions) across the cell membrane. This is caused by a concentration gradient (a difference in the number of protons on each side of the membrane) created by the cell's metabolism. (Bacteria called "Vibrio" use two kinds of flagella, one of which is powered by sodium ions instead.) Flagella are very efficient, meaning not much energy is wasted. They can push bacteria as fast as 60 times their length every second. The motor at the bottom of the flagellum has a similar structure to ATP synthase. Bacteria called spirillum have twisted bodies with flagella on both ends. Spririllum bacteria spin around as they move.