Jens Skou, at Aarhus University, discovers in 1957 the sodium-potassium pump enzyme, a membrane protein that actively transports sodium ions out of the cell and potassium ions in, against their natural concentration gradient, consuming energy in the form of ATP to do so; this mechanism is essential for maintaining the membrane electrical potential required for nerve-impulse transmission and muscle contraction. Paul Boyer, at UCLA, proposes in the 1970s an extraordinarily unusual rotary mechanism to explain how the ATP synthase enzyme — present in the mitochondria of virtually every living cell — produces the ATP molecule, the cell's main energy currency: Boyer postulates that parts of the enzyme literally rotate mechanically, like a microscopic molecular motor, driven by the flow of protons across the mitochondrial membrane. John Walker, at the MRC Laboratory of Molecular Biology in Cambridge, determines in 1994 the detailed three-dimensional structure of ATP synthase using X-ray crystallography, visually confirming the rotary mechanism proposed by Boyer and revealing ATP synthase as one of the very few true rotary molecular motors known in biology, conceptually comparable to a microscopic mechanical motor able to convert a proton electrochemical gradient directly into chemical energy usable by the cell.