Here's how it happens:

* Electrons are passed down the chain: Electrons, carried by NADH and FADH₂, are passed from one electron carrier molecule to another within the electron transport chain. This process releases energy.

* Protons are pumped across the membrane: The energy released from electron movement is used to pump protons (H⁺ ions) from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space.

* Proton gradient is established: This pumping creates a proton gradient, with a higher concentration of protons in the intermembrane space.

* Protons flow back through ATP synthase: Protons flow back across the membrane down their concentration gradient through a protein complex called ATP synthase.

* ATP is produced: This flow of protons drives the production of ATP (adenosine triphosphate), the primary energy currency of the cell.

* Water is formed: At the end of the chain, the low-energy electrons are passed to molecular oxygen (O₂), which combines with protons to form water.

The overall reaction is:

4H⁺ + 4e⁻ + O₂ → 2H₂O