The light reactions drive the formation of a concentration gradient of H+ ions which ultimately generate ATP by ATP synthase when the H+ ions flow across the concentration gradient across the thylakoid membrane. The electron transport chain utilizes the same mechanism, but instead of using energy from the sun, it requires electron carriers such as NADH and FADH2 to drop electrons off at the chain to drive the formation of the H+ concentration gradient across the inner mitochondrial membrane. This gradient also ultimately generates ATP by ATP synthase when the H+ ions flow across the inner membrane of the mitochondria.