The majority of cellular energy in the form of adenosine triphosphate (ATP) is synthesized by the ubiquitous F1F0 ATP synthase. Power for ATP synthesis derives from an electrochemical proton (or Na+) gradient, which drives rotation of membranous F-0 motor components. Efficient rotation not only requires a significant driving force (Delta mu H+), consisting of membrane potential (Delta psi) and proton concentration gradient (Delta pH), but also a high proton concentration at the source P side. In vivo this is maintained by dynamic proton movements across and along the surface of the membrane. The torque-generating unit consists of the interface of the rotating c ring and the stator a subunit. Ion translocation through this unit involves a sophisticated interplay between the c-ring binding sites, the stator arginine, and the coupling ions on both sides of the membrane. c-ring rotation is transmitted to the eccentric shaft gamma-subunit to elicit conformational changes in the catalytic sites of F-1, leading to ATP synthesis.