Q.35 Oxidation of one molecule of FADH2 in the respiratory chain forms how many molecules of ATP?
- 1 through complex I
- 2.5 through complexes I and IV
- 1.5 through complexes II and IV
- 2 through complexes I and III
Oxidation of one FADH₂ molecule in the respiratory chain yields 1.5 ATP by entering at Complex II, bypassing Complex I’s proton pumping. This makes option 3 correct, as confirmed by standard bioenergetics in glycolysis and TCA cycle contexts.
Correct Answer
1.5 through complexes II and IV
Option Analysis
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1 through complex I
Incorrect. FADH₂ skips Complex I (NADH entry point, ~4 H⁺ pumped); it enters at Complex II, yielding fewer protons (~6 total) and thus ~1.5 ATP, not 1. -
2.5 through complexes I and IV
Incorrect. 2.5 ATP is for NADH (Complexes I, III, IV; ~10 H⁺). FADH₂ bypasses I, using only II, III, IV for lower yield. -
1.5 through complexes II and IV
Correct. FADH₂ donates to Complex II (succinate dehydrogenase), electrons flow to III (via Q) and IV (cytochrome c), pumping ~6 H⁺ (~1.5 ATP via ATP synthase; ~4 H⁺/ATP). -
2 through complexes I and III
Incorrect. FADH₂ avoids Complex I; older estimates used integers (2 ATP), but modern P/O ratios give 1.5. No IV involvement here.
ETC Entry Points
FADH₂ from TCA cycle (e.g., succinate dehydrogenase) enters electron transport chain (ETC) at Complex II, unlike NADH at Complex I. This bypasses ~4 H⁺ pumping, reducing total protons to ~6 H⁺ across Complexes II-III-IV, yielding 1.5 ATP via chemiosmosis.
Proton Pumping Breakdown
Electrons from FADH₂ reduce ubiquinone (Q) at Complex II (no H⁺ pump), then Complex III pumps 4 H⁺, and Complex IV pumps 2 H⁺. ATP synthase uses ~4 H⁺/ATP (plus 1 for transport), netting ~1.5 ATP per FADH₂.
Carrier Entry Complex Protons Pumped ATP Yield NADH I ~10 2.5 FADH₂ II ~6 1.5 Exam Relevance
Key for calculating total ATP from glucose (~30-32 ATP): 2 FADH₂ per glucose give 3 ATP. Common error: Using outdated 2 ATP values. Focus on bypass mechanism for competitive exams.
Plant/Microbe Context
In plant mitochondria (your botany focus), FADH₂ yield remains ~1.5 ATP; alternative oxidases may lower it further, affecting respiration efficiency.
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