Instant source of energy/most common respiratory substrate is

Options

A. glucose
B. fructose
C. sucrose
D. all

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
- The question asks for the instant source of energy or the most common respiratory substrate in plants.

2. **Defining Respiratory Substrate:**
- A respiratory substrate is an organic molecule that is broken down in the body to release energy, specifically for the production of ATP (adenosine triphosphate).

3. **Characteristics of a Good Respiratory Substrate:**
- The ideal respiratory substrate should be:
- An organic molecule.
- Simple in structure.
- Easily available in the organism.

4. **Identifying the Common Respiratory Substrate:**
- Among various organic molecules, glucose is recognized as a primary respiratory substrate.
- Glucose is a simple sugar that is readily available in the body and is easily broken down during cellular respiration.

5. **Energy Yield from Glucose:**
- When glucose is metabolized, it can yield up to 38 molecules of ATP, making it an efficient source of energy.

6. **Conclusion:**
- Therefore, the instant source of energy or the most common respiratory substrate in plants is **glucose**.

---