Electrons are brought to the electron transport system by the oxidation of NADH and FADH2.
The electron transport chain is a group of proteins located within the inner membrane of the mitochondria which transport electrons. Electrons are passed from one member of the transport chain to another in a series of redox reactions, forming a proton gradient which is then used for ATP production.
NADH and FADH2 are reduced electron carriers which are oxidized into NAD+ and FAD near the beginning of the electron transport chain.