First respiratory substrate :

Options

A. Lipid
B. Protein
C. Carbohydrates
D. both (a) and (c)

Answer

**Step-by-Step Solution:**

1. **Understanding Respiratory Substrates**:
- Respiratory substrates are the food molecules that are broken down during respiration to release energy.

2. **Identifying the Primary Function of Carbohydrates**:
- The primary function of carbohydrates is to provide energy to the organism. This makes carbohydrates the most important respiratory substrate.

3. **Role of Other Macromolecules**:
- While carbohydrates are the primary respiratory substrates, fats and proteins can also serve as substrates under certain conditions.
- Fats can be broken down into fatty acids and glycerol, which can then enter the respiration pathway.
- Proteins are broken down into amino acids, which undergo deamination to form organic acids that can enter the respiration pathway.

4. **Order of Preference for Respiratory Substrates**:
- The order of preference for respiratory substrates is:
1. Carbohydrates (e.g., glucose)
2. Fats
3. Proteins

5. **Conclusion**:
- Therefore, the first respiratory substrate is carbohydrates, specifically glucose.

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