How many ATP are formed in ETS from reduced NAD generated in one turn cycle of Krebs' cycle ?

Options

A. 3
B. 6
C. 12
D. 9

Explanation

To determine how many ATP are formed in the Electron Transport System (ETS) from the reduced NAD (NADH) generated in one turn of the Krebs cycle, we can 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 enzymatic reactions that produce energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

2. **Identify NADH Production**: In one turn of the Krebs cycle, three molecules of NAD+ are reduced to three molecules of NADH. This occurs at three specific steps:
- Conversion of isocitrate to alpha-ketoglutarate (NAD+ is reduced to NADH).
- Conversion of alpha-ketoglutarate to succinyl-CoA (NAD+ is reduced to NADH).
- Conversion of malate to oxaloacetate (NAD+ is reduced to NADH).

3. **ATP Yield from NADH**: Each NADH molecule produced in the Krebs cycle can generate approximately 2.5 to 3 ATP molecules when it donates electrons to the electron transport chain. For the sake of this calculation, we will use the common value of 3 ATP per NADH.

4. **Calculate Total ATP from NADH**: Since three NADH molecules are produced in one turn of the Krebs cycle:
\[ \text{Total ATP} = \text{Number of NADH} \times \text{ATP per NADH} = 3 \times 3 = 9 \text{ ATP} \]

5. **Conclusion**: Therefore, the total number of ATP formed in the ETS from the reduced NAD generated in one turn of the Krebs cycle is 9 ATP.

### Final Answer:
The answer is **9 ATP**.

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