The primary energy carrier between the Krebs cycle and the electron transport system is :

Options

A. NADH
B. ADP
C. `FADH_2`
D. `CO_2`

Explanation

To answer the question "The primary energy carrier between the Krebs cycle and the electron transport system is:", we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Krebs Cycle**:
- The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It takes place in the mitochondria and produces energy carriers.

2. **Identify Energy Carriers Produced**:
- During the Krebs cycle, several energy carriers are produced, including NADH and FADH2. These carriers store energy in the form of electrons.

3. **Role of NADH and FADH2**:
- NADH (Nicotinamide adenine dinucleotide) and FADH2 (Flavin adenine dinucleotide) are crucial for the next stage of cellular respiration, which is the electron transport system (ETS).

4. **Electron Transport System (ETS)**:
- The ETS is the final stage of aerobic respiration, where the electrons carried by NADH and FADH2 are transferred through a series of proteins in the inner mitochondrial membrane, leading to the production of ATP.

5. **Determine the Primary Carrier**:
- Among the energy carriers produced in the Krebs cycle, NADH is the primary carrier that transfers electrons to the ETS. While FADH2 also carries electrons, NADH is produced in greater quantities and is considered the main energy carrier.

6. **Conclusion**:
- Therefore, the primary energy carrier between the Krebs cycle and the electron transport system is **NADH**.

### Final Answer:
The primary energy carrier between the Krebs cycle and the electron transport system is **NADH**.

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