When the respiratory substances are more than one then which respiratory substrates are not used

Options

A. Pure protein
B. Lipid
C. Carbohydrate
D. Both A and B

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which respiratory substrates are not used when multiple respiratory substances are available. The options provided are pure protein, lipid, carbohydrate, and both A and B.

2. **Identifying Respiratory Substrates**: The primary respiratory substrates are carbohydrates, proteins, and lipids (fats). Each of these substrates has a different capacity for oxidation.

3. **Concept of Respiratory Quotient (RQ)**: The respiratory quotient is a measure that helps determine which substrate is being used for respiration. It is calculated as the ratio of carbon dioxide produced to oxygen consumed during respiration. The formula is:
\[ RQ = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}} \]

4. **RQ Values of Different Substrates**:
- **Carbohydrates**: The RQ value is 1. This indicates that the amount of carbon dioxide produced is equal to the amount of oxygen consumed, meaning carbohydrates are completely oxidized.
- **Proteins**: The RQ value is approximately 0.9. This indicates that proteins require more oxygen for complete oxidation compared to the amount of carbon dioxide produced.
- **Lipids (Fats)**: The RQ value is approximately 0.7. This indicates that fats require even more oxygen for complete oxidation, and thus they are not completely oxidized.

5. **Conclusion on Usage of Substrates**: When multiple substrates are present, preference is given to the substrate that can be completely oxidized, which in this case is carbohydrates (RQ = 1). Since proteins and lipids have RQ values less than 1, they are not completely oxidized and thus are not preferred as respiratory substrates.

6. **Final Answer**: The substrates that are not used when more than one respiratory substance is available are **both pure protein and lipid**. Therefore, the correct option is **both A and B**.