Consider the remarkable structure of the F1Fo ATP synthase shown in the following schematic drawing. Answer the following question(s) based on the structure.

-In Escherichia coli, the c ring of ATP synthase is composed of 12 c subunits. For one fully productive revolution of this ring, how many protons are transported across the membrane for each molecule of ATP produced? Write down your answer as a number, rounded to the nearest integer, e.g. 17.