Q.19 The most plausible explanation for a sudden increase of the respiratory quotient (RQ) of a
microbial culture is that
(A) cells are dying
(B) yield of biomass is increasing
(C) the fermentation rate is increasing relative to respiration rate
(D) the maintenance rate is decreasing
Understanding Sudden Increase in Respiratory Quotient (RQ) in Microbial Cultures
Respiratory quotient (RQ) measures the ratio of CO₂ produced to O₂ consumed in microbial metabolism, typically around 1.0 for complete aerobic respiration of carbohydrates. A sudden RQ increase signals a metabolic shift, often toward anaerobic processes. The correct answer is option (C): the fermentation rate is increasing relative to the respiration rate.
What is Respiratory Quotient?
RQ indicates substrate utilization and metabolic state in microbial cultures. For glucose aerobic respiration, RQ equals 1 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Values above 1 suggest fermentation dominance, producing extra CO₂ without equivalent O₂ use, as in ethanol fermentation (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂).
Correct Answer Explanation
Fermentation generates CO₂ anaerobically while respiration requires O₂, raising RQ when fermentation accelerates relative to respiration. This occurs under oxygen limitation or high substrate loads, common in dense microbial cultures shifting to mixed acid or alcoholic fermentation.
Option Analysis
| Option | Explanation | Why Incorrect |
|---|---|---|
| (A) Cells are dying | Cell death may release substrates but typically lowers overall gas exchange, not specifically raising RQ suddenly. Autolysis affects metabolism minimally compared to active shifts. | Does not directly increase CO₂/O₂ ratio. |
| (B) Yield of biomass is increasing | Higher biomass yield ties to efficient aerobic growth (RQ ~1), not sudden RQ spikes. Biomass synthesis consumes CO₂ equivalents, stabilizing RQ. | Contradicts RQ elevation mechanism. |
| (C) The fermentation rate is increasing relative to respiration rate | Fermentation boosts CO₂ without O₂ consumption, directly elevating RQ >1. This matches sudden shifts in oxygen-limited fermentors. | Correct: Matches physiological evidence. |
| (D) The maintenance rate is decreasing | Maintenance energy uses respiration (RQ ~1); its decrease reduces total O₂ uptake proportionally, not causing RQ surge. | No impact on CO₂/O₂ imbalance. |
