Figure 2. The molecular structure of the F0 subunit of the ATP Synthase complex found in the bacterial outer membrane. The F0 complex has three parts, here labeled a, b and c. The first of these, "a", consists of a single protein that spans the membrane eight times. The second unit, "b", consists of two subunits that pass through the membrane once. The third unit, "c", is present in 9 to 12 copies that each span the membrane twice. Together the c units form a "rotor" with a hollow cylindrical center.