Which would be the last substrate to be used in respiration ?

Options

A. Fat
B. Protein
C. Organic acid
D. starch

Answer

### Step-by-Step Solution:

1. **Understanding Respiration in Plants**:
- Respiration is a metabolic process where complex molecules are broken down into simpler molecules, releasing energy that can be utilized by the organism.

2. **Identifying Primary Substrates**:
- The primary substrate for respiration in plants is carbohydrates, specifically starch. Carbohydrates have a high respiratory quotient and energy yield, making them the first choice for energy production.

3. **Secondary Substrates**:
- If carbohydrates are not available, the next substrate used is fats. Fats also provide a high energy yield, although their respiratory quotient is lower than that of carbohydrates.

4. **Tertiary Substrates**:
- In the absence of both carbohydrates and fats, organic acids can be utilized as substrates for respiration. However, they are not commonly used and have a high respiratory quotient.

5. **Last Resort Substrate**:
- The last substrate to be used in respiration is proteins. This occurs only when all other substrates (carbohydrates, fats, and organic acids) have been exhausted. The use of proteins for respiration is rare and is referred to as protoplasmic respiration.

6. **Final Answer**:
- Therefore, the last substrate to be used in respiration is **proteins**.