Question at position 3 When is lactic acid produced in the cell? When creatine phosphate runs outWhen oxygen builds up in the cellWhen the cell relies on aerobic respirationWhen the cell relies on anaerobic respiration

Question

Question at position 3 When is lactic acid produced in the cell? When creatine phosphate runs outWhen oxygen builds up in the cellWhen the cell relies on aerobic respirationWhen the cell relies on anaerobic respiration

BlackTom AI · Accepted Answer

Lactic acid is produced during anaerobic glycolysis when pyruvate is reduced to lactate to regenerate NAD+.

1) Relevant knowledge points and concepts
- Glycolysis breaks one glucose into two pyruvate, yielding a net 2 ATP and 2 NADH. 
- Under aerobic conditions, pyruvate is transported into mitochondria and oxidized to acetyl-CoA for the Krebs cycle and oxidative phosphorylation, where NADH is reoxidized by the electron transport chain (ETC). 
- Under anaerobic conditions (insufficient O2 or impaired mitochondrial respiration), the ETC cannot reoxidize NADH fast enough. Cells regenerate NAD+ by reducing pyruvate to lactate via lactate dehydrogenase: pyruvate + NADH + H+ → lactate + NAD+.

2) Step-by-step reasoning
- Glycolysis produces 2 NADH per glucose. To continue glycolysis when oxygen is low, NAD+ must be available. 
- With limited oxygen, the ETC slows, so NADH accumulates. Converting pyruvate to lactate regenerates NAD+, allowing glycolysis to continue and produce ATP anaerobically. 
- Therefore lactate formation is a metabolic adaptation to sustain ATP production without oxygen.

3) Calculations / intermediate steps
- Glucose → 2 pyruvate + net 2 ATP + 2 NADH.
- If O2 available: pyruvate → acetyl-CoA → Krebs → ETC; NADH → NAD+ via ETC.
- If O2 limited: pyruvate + NADH → lactate + NAD+ (restores NAD+ to permit further glycolysis and 2 ATP per glucose).

4) Why the correct answer is correct
- "When the cell relies on anaerobic respiration" directly describes the condition that forces pyruvate to be reduced to lactate to regenerate NAD+, so lactic acid accumulates.

5) Why other options are incorrect
- "When creatine phosphate runs out": creatine phosphate provides immediate ATP buffering for seconds; its depletion may precede reliance on glycolysis but does not itself produce lactate. 
- "When oxygen builds up in the cell": abundant O2 prevents lactate formation because oxidative phosphorylation handles NADH reoxidation. 
- "When the cell relies on aerobic respiration": aerobic metabolism directs pyruvate into mitochondria, not to lactate.

6) Conclusion
Final answer: When the cell relies on anaerobic respiration. This matches the provided correct answer.

Question at position 3 When is lactic acid produced in the cell? When creatine phosphate runs outWhen oxygen builds up in the cellWhen the cell relies on aerobic respirationWhen the cell relies on anaerobic respiration单项选择题

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