FADH₂ delivers electrons to the ETC at a different entry point than NADH. Which statement correctly describes the consequence of this difference?
AFADH₂ produces more ATP per molecule than NADH because it enters at a later complex
BFADH₂ produces fewer ATP per molecule than NADH because it bypasses Complex I, which pumps 4 protons
CFADH₂ and NADH produce equal ATP because they both ultimately reduce oxygen
DFADH₂ produces fewer ATP because it carries fewer electrons than NADH
NADH enters at Complex I (which pumps ~4 protons per electron pair), while FADH₂ donates electrons directly to CoQ, bypassing Complex I. Fewer protons pumped means a smaller proton gradient contribution, which yields fewer ATP via ATP synthase. The standard estimates are ~2.5 ATP per NADH versus ~1.5 ATP per FADH₂. Both carry 2 electrons; the difference is in how many proton pumps they engage.
Question 2 True / False
Oxygen participates throughout the entire electron transport chain as electrons are passed between complexes.
TTrue
FFalse
Answer: False
Oxygen's role is restricted to Complex IV, where it serves as the final electron acceptor. It is reduced there to water (O₂ + 4H⁺ + 4e⁻ → 2H₂O). Between Complexes I–III, electrons are transferred among iron-sulfur clusters, ubiquinone (CoQ), and cytochrome c — none of which involve molecular oxygen. This is why oxygen deprivation halts the entire chain: it removes the terminal acceptor that keeps electrons flowing.
Question 3 Short Answer
Explain why a cell in brown adipose tissue can generate heat from glucose even when ATP synthase is inhibited.
Think about your answer, then reveal below.
Model answer: Brown adipose tissue expresses uncoupling protein 1 (UCP1), which creates a channel allowing protons to flow back across the inner mitochondrial membrane without passing through ATP synthase. The proton gradient is dissipated as heat rather than captured as ATP, but electron flow through the chain continues uninterrupted.
ATP synthase is not a gatekeeper of the ETC itself — it is the device that harvests the proton gradient. As long as electrons can flow from NADH/FADH₂ to oxygen via the complexes, and protons can return to the matrix by any route, the chain keeps running. Uncoupling 'short-circuits' the energy harvest without stopping respiration. This mechanism is how newborns and hibernating animals generate body heat without shivering.