In the inner membrane of the mitochondrion is the electron transport chain
which is a collection of molecules.
Free energy drops as electrons travel down the chain. During the electron
transport along the chain, electron carriers alternate between reduced and
oxidized states as they accept and donate electrons from there above
neighbor. NADH is the first molecule of the chain. FADH2 is a source of
electrons for the transport chain. It is a reduced product of the krebs
cycle. The chain makes no ATP directly. The mitochondrion couples the
electron transport and energy releases ATP synthesis by chemiosmosis.
Chemiosmosis is the process where energy is stored in the form of hydrogen
ion gradient across a membrane is used to make cellular work.