In oxidative phosphorylation, why does $F_A{D}_2$ oxidation yield less ATP than $NADH$ oxidation?

• A. $F_A{D}_2$ transfers electrons directly to Complex IV, skipping Complexes I through III, which eliminates proton pumping.
• B. $F_A{D}_2$ inhibits ATP synthase, reducing the coupling of the proton gradient to ATP formation.
• C. $F_A{D}_2$ is oxidized in the cytosol and requires a shuttle that consumes ATP before electrons enter the electron transport chain.
• D. $F_A{D}_2$ donates electrons to Complex II, bypassing Complex I, so fewer protons are pumped across the inner mitochondrial membrane.

Answer: (D)

Verified step by step guidance

1. Understand that oxidative phosphorylation generates ATP by using the proton gradient created by electron transport through complexes I, III, and IV in the mitochondrial inner membrane.
2. Recognize that NADH donates electrons to Complex I, which pumps protons across the membrane, contributing significantly to the proton gradient used for ATP synthesis.
3. Note that FADH2 donates electrons to Complex II, which does not pump protons, meaning electrons from FADH2 bypass Complex I and thus contribute less to the proton gradient.
4. Since fewer protons are pumped when electrons enter at Complex II, the resulting proton motive force is smaller, leading to less ATP produced per FADH2 oxidized compared to NADH.
5. Conclude that the key reason FADH2 oxidation yields less ATP than NADH oxidation is because FADH2 electrons enter the electron transport chain downstream of Complex I, resulting in fewer protons being pumped and a reduced proton gradient.