An alternative method used to create ATP is through oxidative phosphorylation, which takes place during cellular respiration. This process utilizes the oxidation of NADH to NAD, yielding 3 ATP, and of FADH to FAD, yielding 2 ATP. The potential energy stored as an electrochemical gradient of protons (H) across the inner mitochondrial membrane is required to generate ATP from ADP and P (inorganic phosphate molecule), a key difference from substrate-level phosphorylation. This gradient is exploited by ATP synthase acting as a pore, allowing H from the mitochondrial intermembrane space to move down its electrochemical gradient into the matrix and coupling the release of free energy to ATP synthesis. Conversely, electron transfer provides the energy required to actively pump H out of the matrix.