A student argues that FADH₂ and NADH should produce the same ATP yield because both donate exactly two electrons to the electron transport chain. What is wrong with this reasoning?
AFADH₂ actually donates three electrons, not two, so the comparison is incorrect
BNADH donates its electrons to Complex I while FADH₂ enters at Complex II, bypassing one proton-pumping step and therefore contributing less to the proton gradient
CFADH₂ is less stable than NADH and loses energy as heat before reaching the ETC
DThe number of electrons donated determines ATP yield only for NADH, not for flavin carriers
The number of electrons transferred is necessary but not sufficient information to determine ATP yield. What matters is where those electrons enter the ETC. NADH feeds electrons to Complex I, which pumps protons across the inner mitochondrial membrane. FADH₂, because it is covalently bound to succinate dehydrogenase (Complex II), delivers electrons directly to ubiquinone at Complex II — bypassing Complex I entirely. That skipped proton-pumping step is why FADH₂ yields ~1.5 ATP versus NADH's ~2.5 ATP.
Question 2 Multiple Choice
What is the key structural difference between FAD and NAD⁺ that explains why FADH₂ cannot travel freely through the cell to deliver electrons to different acceptors?
AFAD is larger than NAD⁺ and cannot diffuse through the mitochondrial matrix
BFAD carries electrons at a different redox potential that prevents interaction with soluble acceptors
CFAD is a prosthetic group permanently bound to its enzyme, while NAD⁺ is a cosubstrate that binds, accepts electrons, and then diffuses away as NADH
DFADH₂ is immediately re-oxidized before it can diffuse, while NADH is stable enough to travel
This is the defining distinction. NAD⁺ functions as a cosubstrate: it binds temporarily, picks up electrons (becoming NADH), and dissociates — free to carry electrons to Complex I or other acceptors. FAD is a prosthetic group: it is covalently or very tightly non-covalently bound to its enzyme and never floats free. FADH₂ therefore cannot shop around for different electron acceptors; it must donate electrons to whatever redox partner its enzyme is positioned to contact. This is why succinate dehydrogenase is simultaneously a citric acid cycle enzyme and a respiratory chain component.
Question 3 True / False
Because FAD is permanently bound to its enzyme, FADH₂ cannot transfer electrons to acceptors other than the one its enzyme directly contacts.
TTrue
FFalse
Answer: True
Unlike NADH, which diffuses freely and can donate electrons to Complex I anywhere in the mitochondrial matrix, FADH₂ never leaves its enzyme. The electrons it carries must be transferred to whatever redox partner the enzyme's active site is positioned to interact with. For succinate dehydrogenase, that partner is ubiquinone (coenzyme Q), located in the inner mitochondrial membrane immediately adjacent to Complex II. This positional constraint is not a limitation — it's a feature that ensures electrons from specific substrates feed into specific points in the chain.
Question 4 True / False
FADH₂ and NADH produce identical amounts of ATP per molecule because both carry two electrons to the same final electron acceptor (molecular oxygen).
TTrue
FFalse
Answer: False
Both ultimately deliver electrons to O₂ via the ETC, but they enter at different points: NADH at Complex I, FADH₂ at Complex II. Complex I pumps protons across the inner membrane; Complex II does not. Because the proton gradient drives ATP synthase, bypassing Complex I means fewer protons are pumped per electron pair, producing a smaller gradient and less ATP. FADH₂ yields ~1.5 ATP; NADH yields ~2.5 ATP — a 40% difference that matters significantly in fatty acid oxidation, where FADH₂ is generated in each β-oxidation cycle.
Question 5 Short Answer
Why does FADH₂ generate less ATP than NADH per molecule, even though both donate two electrons to the electron transport chain and both ultimately reduce molecular oxygen?
Think about your answer, then reveal below.
Model answer: NADH donates electrons to Complex I, which pumps protons across the inner mitochondrial membrane, contributing to the proton gradient that drives ATP synthase. FADH₂, being permanently bound to succinate dehydrogenase (Complex II), delivers electrons directly to ubiquinone at Complex II — a step that does not pump protons. By bypassing Complex I, FADH₂ contributes to only two proton-pumping complexes (III and IV) rather than three (I, III, and IV). Fewer protons pumped means a smaller gradient and less ATP synthesized: ~1.5 ATP for FADH₂ versus ~2.5 ATP for NADH.
The key principle here is that ATP yield depends not on how many electrons are transferred but on how many protons are pumped per electron pair — which is determined by where electrons enter the chain, not just where they exit.