The respiratory quotient (R.Q) or respiratory ratio is

Options

A. `R.Q = ("volume of" O_(2) "evolved")/("volume of " CO_(2) "consumed")`
B. `R.Q = ("volume of " CO_(2) "evolved")/("volume of " O_(2) "consumed")`
C. `R.Q = ("volume of " O_(2) "consumed")/("volume of" CO_(2) "evolved")`
D. `R.Q = ("volume of " CO_(2) " consumed")/("volume of " O_(2) "consumed")`

Answer

### Step-by-Step Solution

1. **Understanding the Respiratory Quotient (R.Q)**:
- The respiratory quotient (R.Q), also known as the respiratory ratio, is a measure used in respiration to assess the ratio of carbon dioxide produced to oxygen consumed during metabolism.

2. **Analyzing the Options**:
- We have four options to evaluate:
- **Option 1**: RQ = Volume of oxygen evolved / Volume of carbon dioxide consumed.
- **Option 2**: RQ = Volume of carbon dioxide evolved / Volume of oxygen consumed.
- **Option 3**: RQ = Volume of oxygen consumed / Volume of carbon dioxide evolved.
- **Option 4**: RQ = Volume of carbon dioxide consumed / Volume of oxygen consumed.

3. **Evaluating Each Option**:
- **Option 1**: This is incorrect because it states the volume of oxygen evolved instead of consumed.
- **Option 2**: This is correct. RQ is defined as the volume of carbon dioxide evolved to the volume of oxygen consumed.
- **Option 3**: This is incorrect as it reverses the relationship, stating oxygen consumed over carbon dioxide evolved.
- **Option 4**: This is also incorrect because it refers to the volume of carbon dioxide consumed instead of evolved.

4. **Conclusion**:
- The correct definition of the respiratory quotient (R.Q) is given by **Option 2**: RQ = Volume of carbon dioxide evolved / Volume of oxygen consumed.

5. **Additional Information**:
- The R.Q values for different substrates are:
- Carbohydrates: RQ = 1
- Proteins: RQ = 0.9
- Lipids: RQ = 0.7