The correct answer is (C) (i) 2.70 and (ii) 0.67.

E. coli Respiratory Chains

E. coli uses branched electron transport chains for NADH oxidation during oxidative phosphorylation. NDH-1 (proton-pumping NADH:ubiquinone oxidoreductase) feeds electrons to ubiquinone (UQ), then to cytochrome bo3 (proton-pumping quinol oxidase). NDH-2 (non-proton-pumping) also reduces UQ, but pairs with cytochrome bd (non-proton-pumping quinol oxidase). Protons for ATP synthesis come solely from these complexes, with H+/ATP = 3 given.

Proton Stoichiometries

Per NADH (2e- transferred to O2):

• NDH-1 pumps 4 H+/2e-.

• Cytochrome bo3 pumps 4 H+/2e-.

• Total for (i): 8 H+/2e-.

• NDH-2 pumps 0 H+/2e-.

• Cytochrome bd pumps 2 H+/2e-.

• Total for (ii): 2 H+/2e-.

ATP Calculations

ATP per NADH = (total H+/2e-) / (H+/ATP).

• (i): 8 / 3 ≈ 2.67 (rounded to 2.70 in options).

• (ii): 2 / 3 ≈ 0.67.

Option Analysis

• (A) Reverses values; ignores NDH-1 pumping.

• (B) Assumes uniform 3 ATP/NADH; ignores branch differences.

• (C) Matches calculations: 2.70 for NDH-1/bo3, 0.67 for NDH-2/bd.

• (D) Underestimates bo3 (treats as non-pumping).

NADH oxidation in E. coli oxidative phosphorylation generates varying ATP yields depending on the respiratory pathway used—NDH-1 with cytochrome bo3 complex or NDH-2 with cytochrome bd complex. This key concept, assuming H+/ATP = 3, is critical for CSIR NET life sciences preparation, highlighting bacterial bioenergetics differences from mitochondria. Understanding proton pumping stoichiometries unlocks the exact ATP per NADH values.

NDH-1 and Cytochrome bo3 Pathway

NDH-1 pumps 4 H+/2e- from NADH to UQ, while cytochrome bo3 pumps 4 H+/2e- during quinol oxidation to O2, totaling 8 H+/NADH. Divided by 3 H+/ATP yields approximately 2.67 ATP, often listed as 2.70 in exam contexts. This high-efficiency path dominates under aerobic conditions.

NDH-2 and Cytochrome bd Pathway

NDH-2 transfers electrons without proton pumping (0 H+/2e-), and cytochrome bd pumps only 2 H+/2e-, totaling 2 H+/NADH. This gives 2/3 ≈ 0.67 ATP per NADH, suiting low-oxygen microaerobic environments where efficiency trades for speed.

Why Values Differ from Eukaryotes

Unlike mitochondrial Complex I (≈10 H+/2e-) yielding ~2.5 ATP/NADH (H+/ATP=4), E. coli’s simpler chains and given H+/ATP=3 produce lower, pathway-specific yields. NDH-2/bd minimizes pmf buildup for rapid NADH turnover.

CSIR NET Exam Insights

Question Q78 tests proton stoichiometries: NDH-1 (4), bo3 (4) vs NDH-2 (0), bd (2). Option (C) 2.70/0.67 aligns precisely. Use for revision: sketch chains, calculate H+, divide by 3.