ATP synthase is a complex enzyme found in the mitochondria of eukaryotic cells and the plasma membrane of prokaryotic cells. It acts as a molecular motor that uses the proton gradient across the membrane to generate ATP from ADP and inorganic phosphate.

Here's a breakdown of the process:

* Electron transport chain: Electrons are passed along a chain of proteins in the mitochondrial membrane, releasing energy.

* Proton pumping: This energy is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.

* ATP synthase activity: The proton gradient drives protons back across the membrane through ATP synthase. This flow of protons spins a rotor within the enzyme, which drives the synthesis of ATP from ADP and phosphate.

So, ATP synthase is the key enzyme responsible for the final step in cellular respiration – the conversion of ADP to ATP, the cell's primary energy currency.