An incorrect statement regarding respiratory substrates is :-

Options

A. When proteins are used as respiratory substrate, the volume of `CO_(2)` evolved is less than the volume of `O_(2)` consumed
B. Only freshly formed biomolecules are used as respiratory substrate
C. Pure proteins and fats are never used as respiratory substrates
D. Sucrose directly does not enter into glycolysis

Answer

**Step-by-Step Solution:**

1. **Understanding Respiratory Substrates**:
- Respiratory substrates are organic substances that are oxidized during respiration to release energy. Common examples include carbohydrates, fats, and proteins.

2. **Analyzing the Statements**:
- We need to evaluate the given statements regarding respiratory substrates to identify the incorrect one.

3. **Statement 1**: "When proteins are used as respiratory substrate, the volume of CO2 evolved is less than the volume of O2 consumed."
- This statement is **correct**. When proteins are used, the respiratory quotient (RQ) is less than 1 (approximately 0.8), indicating that less CO2 is produced compared to the O2 consumed.

4. **Statement 2**: "Only freshly formed biomolecules are used as respiratory substrates."
- This statement is also **correct**. Freshly formed biomolecules like carbohydrates, fats, and proteins are typically utilized in respiration.

5. **Statement 3**: "Pure proteins or fats are never used as respiratory substrates."
- This statement is **incorrect**. While proteins and fats are not the primary substrates, they can be used in respiration, especially during specific conditions like germination of protein-rich seeds.

6. **Statement 4**: "Sucrose directly does not enter into glycolysis."
- This statement is **correct**. Sucrose must first be hydrolyzed into glucose and fructose by the enzyme invertase before it can enter glycolysis.

7. **Conclusion**:
- The incorrect statement regarding respiratory substrates is: "Pure proteins or fats are never used as respiratory substrates."

**Final Answer**: The incorrect statement is: "Pure proteins or fats are never used as respiratory substrates."

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