ATP synthase is a membrane protein complex that couples downhill proton movement to ATP formation from ADP and inorganic phosphate.

The overall chemical reaction is

$$\mathrm{ADP+P_i\rightarrow ATP+H_2O},$$

which is thermodynamically unfavorable under many cellular conditions when considered by itself.

ATP synthase couples that reaction to the favorable return of protons down a proton-motive force.

Two coupled sectors

A useful structural division is

• $F_0$, the membrane-embedded sector that provides a proton-conducting rotary mechanism;
• $F_1$, the catalytic sector that binds ADP, phosphate and ATP.

Proton flow through $F_0$ drives rotation of part of the complex. Rotation changes the conformations of catalytic sites in $F_1$, altering their affinities for substrates and product.

The energy conversion chain is

proton-motive force
      ↓ H+ flow
molecular rotation
      ↓ conformational changes
ADP + Pi → ATP

ATP synthase is not a proton channel that merely leaks

A simple proton channel would dissipate the gradient without conserving much of its free-energy decrease as chemical work. ATP synthase instead constrains proton movement so that it is mechanically and chemically coupled to changes in the catalytic sites.

The useful work arises because proton passage and ATP synthesis are parts of one coupled molecular process.

Stoichiometry is not exactly one proton per ATP

Several protons are typically associated with synthesis and cellular delivery of one ATP, and the exact ratio depends on ATP-synthase architecture and associated transport processes.

Therefore introductory ATP-yield calculations should not treat one proton or one reduced electron carrier as corresponding to an exact universal integer number of ATP molecules.

The machine is reversible in principle

If the proton-motive force is weak enough while ATP hydrolysis is strongly favorable, many ATP synthases can operate in reverse: ATP hydrolysis can drive proton pumping.

Direction depends on the combined thermodynamics of ATP formation or hydrolysis and proton transfer across the membrane.

This reversibility emphasizes the core principle: ATP synthase is an energy converter between a transmembrane electrochemical gradient and nucleotide phosphorylation, not a one-way ATP-making reaction in isolation.