ATP synthase rotor ring. Molecular model showing the rotor ring component of ATP synthase, an important enzyme that provides energy for cells through the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. The rotation of the rotor ring structure are part of the process of proton transport across the cell membrane. The rotors of ATP synthases turn about 100 times every second. This example is a sodium-driven ATP synthase rotor ring from the bacterium Fusobacterium nucleatum.