Final answer:
In the electron transport chain, NADH and FADH2 donate their electrons to the respiratory chain by passing them along carriers, which are reduced when they receive the electron and oxidized when passing it on. This process releases energy that is used to pump H+ ions across the inner membrane, creating a proton gradient.
Step-by-step explanation:
The electrons released from NADH and FADH2 are passed along the chain by each of the carriers, which are reduced when they receive the electron and oxidized when passing it on to the next carrier. Each of these reactions releases a small amount of energy, which is used to pump H+ ions across the inner membrane. The accumulation of these protons in the space between the membranes creates a proton gradient with respect to the mitochondrial matrix.