How are NADH and FADH2 different in their roles in cellular respiration?

NADH and FADH2 are both crucial electron carriers in cellular respiration, but they enter the electron transport chain at different points, which affects their energy contributions. The correct answer highlights that NADH enters the electron transport chain at a higher energy level than FADH2.

When NADH donates its electrons to the electron transport chain, it does so at Complex I, which is positioned at a higher energy level compared to where FADH2 donates its electrons, which is at Complex II. The higher energy entry point of NADH allows for its electrons to be passed through more protein complexes in the chain, ultimately resulting in the pumping of more protons across the mitochondrial membrane. This creates a larger proton gradient, which is used by ATP synthase to generate more ATP.

As a result of this difference in energy levels and the pathways they take, NADH typically leads to the production of approximately 2.5 ATP molecules, while FADH2 leads to the production of about 1.5 ATP molecules. This distinction illustrates the efficiency of NADH as an energy carrier compared to FADH2 in the context of ATP synthesis during aerobic respiration.