Final answer:
Most electrons/protons in the electron transport chain come from NADH and FADH2, which are oxidized forms of NAD+ and FAD. These molecules donate electrons to power the production of ATP in the mitochondria.
Step-by-step explanation:
The majority of electrons/protons that enter the electron transport chain are in the form of NADH and FADH2. These two molecules are the main energy carriers that donate electrons to the electron transport chain. NADH deposits its electrons at Complex I, initiating the chain of electron transfers through various protein complexes and mobile electron carriers. On the other hand, FADH2 delivers its electrons to Complex II, which then feeds into the chain at a point beyond Complex I.
As the electrons move from carriers with higher reduction potential to those with lower potential, energy is released and captured to synthesize ATP. This process of oxidative phosphorylation occurs in the mitochondria, where NADH and FADH2 are oxidized back into their original forms, NAD+ and FAD, respectively. The movement of electrons and the resulting proton gradient ultimately lead to the reduction of oxygen to water, completing the process.