Adenosine triphosphate (ATP) synthase contains a rotary motor involved in biological energy conversion requiring H+ to beformed and transferred across a membrane.  The resulting movement of the H+ ions creates a charge gradient potential (battery effect) across the membrance that facilitates the rotational movement of the ATPase.  The ultimate structure (including a stator protein extention) causes the large proteins at the head of the ATPase that holds ADP (Adensonine Diphosphate) to bring inorganic moieties of phosphate in molecular juxposition to each other, therefore greatly reducing the energy to form ATP (Adenosine Triphosphate).  The rotational ability of this nanomotor is sufficient in such numbers, that the formation of ATP is sustainable.  The Larger ATP generating protion of the complex protein structure is located on the cytoplasmic side (The two disk structures represent the phospholipid boundries, interior is the hydrophobic region of the long chains comprizing the lipid).  The smaller portion of the ATPase protein is shown on the extracellular side.  The model is from the PDB databank, 2XOK, and the final STL was generated by the server Orientations of Proteins in Membranes. The file is an 187,373 kb STL file that is watertight and printable.  A color image of the STL file can be found here.  Reference: Molecular Architecture of the Rotary Motor in ATP Synthase.Stock, D., Leslie, A.G., Walker, J.E. (1999) Science 286: 1700-1705.