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How does ATP synthase use protons to make ATP?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

In the mitochondrion, the synthesis of ATP is driven by the chemiosmotic process, which converts electrochemical energy into chemical bond energy.12

During cellular respiration, the electron-transport chain pumps protons (H⁺ ions) across the highly folded inner mitochondrial membrane (the crista) from the matrix to the intermembrane space. Because the inner membrane is impermeable to these ions, this action establishes a significant electrochemical gradient—a difference in both pH and electrical potential—across the membrane.12

The enzyme complex ATP synthase, also known as F₁F₀-ATPase, acts as a specialized proton pore embedded within this membrane. When ADP and inorganic phosphate (Pᵢ) are bound to the enzyme, the accumulated protons flow back into the mitochondrial matrix through the F₀ portion of the complex. This movement of protons down their electrochemical gradient releases energy, which the F₁ portion of the enzyme uses to catalyze the conversion of ADP and Pᵢ into ATP.12