Oxidative phosphorylation and the ETC

FADH2 is a better electron donor than NADH because it allows more H+ ions to be pumped across the membrane.

All of the electrons that enter the transport chain (ETC) come from NADH and FADH2 molecules which are produced during earlier phases of cellular respiration.

NADH and FADH2 are single-use molecules so once they donate their electrons, they cannot be used again.

NADH is the oxidized form of NAD+ and FADH2 is the reduced form of FADH.