The chemiosmotic coupling hypothesis of oxidative phosphorylation proposes that adenosine triphosphate (ATP) is formed because

A proton gradient forms across the inner mitochondrial membrane

There is a change in the permeability of the inner mitochondrial membrane towards adenosine diphosphate (ADP)

High energy bonds are formed in mitochondrial proteins

ADP is pumped out of the matrix into the intermembrane space