ATP synthase
/ A-T-P SIN-thays /
A water wheel turns when a stream pushes its paddles, and the spinning shaft can grind grain or drive a mill. The cell has a real molecular machine that works on the same principle: a flow of particles makes part of it physically spin, and the spinning does useful chemical work. That machine is ATP synthase, arguably the smallest rotary motor known.
ATP synthase is a large protein embedded in a membrane, with two main parts. One part is a channel that lets protons flow through, down their gradient; as they pass, they make a rotor at the center of the protein spin, much like water turning a turbine. The other part is a head that sticks out into the cell's interior. The mechanical turning of the rotor forces the head to change shape repeatedly, and each twist squeezes a phosphate onto ADP to forge a new ATP. So a flow of protons is literally turned into the chemical bonds of ATP, through real rotation.
This single enzyme makes most of the ATP in your body, both in mitochondria during respiration and in chloroplasts during photosynthesis. The fact that the same spinning machine, running on the same proton-gradient principle, appears across bacteria, plants, and animals tells us it is extraordinarily ancient. Remarkably, it can run backward too: spending ATP to pump protons, which some cells use to build their gradients.
In a mitochondrion, protons flowing through ATP synthase spin its rotor about a hundred times a second, and each turn stamps out several molecules of ATP from ADP and phosphate.
ATP synthase is a real rotary motor: proton flow spins it, and the spin makes ATP.
Despite the name, ATP synthase normally makes ATP, not breaks it — but it is reversible, and under some conditions it runs backward to pump protons at the cost of ATP.