Q.20 During lactic acid fermentation, net yield of ATP and NADH per mole of glucose is
- (A) 2 ATP and 2 NADH
- (B) 2 ATP and 0 NADH
- (C) 4 ATP and 2 NADH
- (D) 4 ATP and 0 NADH
Lactic acid fermentation occurs in anaerobic conditions (muscle cells, Lactobacillus), converting glucose to lactate while generating minimal ATP. Unlike aerobic respiration (36 ATP), fermentation prioritizes NAD+ regeneration over energy yield. This MCQ tests glycolysis accounting during lactate production.
Correct Answer: (B) 2 ATP and 0 NADH
Lactic acid fermentation yields net 2 ATP and 0 NADH per glucose. Glycolysis produces 4 ATP + 2 NADH but consumes 2 ATP upfront. NADH gets oxidized back to NAD+ during pyruvate → lactate conversion, leaving no net NADH.
Lactic Acid Fermentation Balance Sheet
Glycolysis phase (glucose → 2 pyruvate):
- Consumed: 2 ATP (hexokinase, PFK steps)
- Produced: 4 ATP (2 per GAPDH, 2 per PGK per glyceraldehyde-3-phosphate)
- NADH: 2 NADH (GAPDH step)
- Net so far: +2 ATP, +2 NADH
Fermentation phase (2 pyruvate → 2 lactate):
Pyruvate + NADH → lactate + NAD+ (lactate dehydrogenase)
Net: 0 ATP, -2 NADH (consumed for NAD+ regeneration)
Final yield: 2 ATP, 0 NADH
Explanation of All Options
Each option’s biochemical accuracy:
| Option | ATP Count | NADH Count | Error Type |
|---|---|---|---|
| (A) 2 ATP, 2 NADH | Correct ATP | Wrong NADH | Forgets NADH oxidation |
| (B) 2 ATP, 0 NADH | Correct | Correct | Perfect |
| (C) 4 ATP, 2 NADH | Wrong ATP | Wrong NADH | Gross vs net confusion |
| (D) 4 ATP, 0 NADH | Wrong ATP | Correct NADH | Only ATP investment error |
Reaction Summary
Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Lactate + 2 ATP + 2 H2O + 2 NAD+
Key insight: NAD+ appears on both sides—catalytic cofactor regenerated, not consumed/produced.
Biotechnology Applications
- Yogurt/probiotics: Lactobacillus ferments lactose → lactic acid
- Muscle energy: Sprinting relies on lactate fermentation
- Bioreactor design: Anaerobic conditions maximize lactate yield
Exam Tip
Always account for glycolytic investment (2 ATP) and NADH recycling (0 net NADH)
Memorize: Lactic acid fermentation = 2 ATP, 0 NADH per glucose. Same as alcoholic fermentation.
