When NADH is the electron donor to the respiratory chain, 2.5 ATP are formed, but when FADH2 is the electron donor, only 1.5 ATP are formed. This difference occurs because .

When NADH is the electron donor, more protons are pumped which create the electrochemical potential of the proton-motive force to drive ATP synthesis while when FADH2 is the electron, lesser protons are pumped resulting in the production of lesser amount of ATP than NADH. This is because NADH transfers electrons to complex I while FADH2 transfers electrons to complex II.

Explanation: Electron transport chain is made up of four unique electron-carrier complexes namely: complex I, II, III and IV. Complex I and II catalyze the transfer of electrons to ubiquinone from NADH and Succinate respectively, complex III carries electrons from ubiquinone to cytochrome c while complex IV transfers electron from cytochrome c to oxygen. For every pair of oxygen transferred to oxygen, four protons are pumped out by complex I, four protons are pumped out by complex III while two protons are pumped out by complex IV.

When NADH is the electron donor, electron transfer starts from NADH to complex I and a total of 10 protons are pumped out in the sequence for each pair of electrons transferred to oxygen. But when FADH2 is the electron donor, electron transfer starts from FADH2 to complex II and a total of six protons are pumped out for each pair of electron transferred to oxygen. The protons pumped provide proton-motive force which drives the synthesis of ATP, therefore the higher the number of protons pumped, the higher the number of ATP generated while the lesser the number of protons pumped the fewer the number of ATP generated.