Question

The most plausible explanation for a sudden increase of the respiratory quotient (RQ) of a microbial culture is that

• A. cells are dying
• B. the maintenance rate is decreasing
• C. the fermentation rate is increasing relative to respiration rate
• D. yield of biomass is increasing

Hint:

Bioprocess controllers monitor the Respiratory Quotient (RQ) in real-time.
A sudden rise in RQ above 0 is an indicator of oxygen limitation or over-feeding, which triggers a shift to fermentative metabolism.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Respiratory Quotient (RQ) is a dimensionless metabolic parameter defined as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed over a given period.
Key Formula or Approach:
The formula for the respiratory quotient is:
\[ \text{RQ} = \frac{\text{Moles of }\text{CO}_2\text{ produced}}{\text{Moles of }\text{O}_2\text{ consumed}} \]

Step 2: Detailed Explanation:
The RQ value depends on the metabolic pathways active in the microbial culture.
During aerobic respiration on carbohydrates (such as glucose), the complete oxidation of the substrate consumes one mole of oxygen for every mole of carbon dioxide produced, resulting in an RQ of approximately $0$:
\[ \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} \implies \text{RQ} = \frac{6}{6} = 0 \]
If microbes metabolize lipids or proteins, which are more reduced than carbohydrates, they require more oxygen for complete oxidation, resulting in RQ values between $0.7$ and $0.8$.
A sudden, significant increase in the RQ value (often above $0$) indicates a shift in metabolism from respiration to fermentation.
Fermentation pathways (such as ethanol or mixed-acid fermentation) are anaerobic processes that break down organic substrates without using oxygen as a terminal electron acceptor.
However, these pathways still produce carbon dioxide during decarboxylation steps (such as the conversion of pyruvate to acetaldehyde by pyruvate decarboxylase).
Because carbon dioxide is produced while oxygen consumption drops to near zero, the denominator in the RQ equation decreases relative to the numerator, causing the RQ value to rise.
Therefore, a sudden increase in RQ indicates that the fermentation rate is rising relative to the aerobic respiration rate.

Step 3: Final Answer:
The sudden rise in RQ is explained by an increase in the fermentation rate relative to the respiration rate.