During carbohydrate metabolism, NADH is produced in which of the following location(s)?
I. Cytosol
II. Mitochondrial matrix
Ill. Nucleus

Options

A. I only
B. II only
C. I and II only
D. I, II, and III

Answer

To determine where NADH is produced during carbohydrate metabolism, we need to analyze the processes involved in glycolysis and the Krebs cycle.

### Step-by-Step Solution:

1. **Identify the Locations**: The question asks about three locations:
- I. Cytosol
- II. Mitochondrial matrix
- III. Nucleus

2. **Glycolysis in the Cytosol**:
- Glycolysis is the first step in carbohydrate metabolism, where one glucose molecule is broken down into two pyruvate molecules.
- This process occurs in the cytosol and produces 2 NADH molecules.

3. **Pyruvate Conversion**:
- After glycolysis, the pyruvate produced in the cytosol is transported into the mitochondria.
- Here, each pyruvate is converted into acetyl coenzyme A (acetyl CoA) by the enzyme pyruvate dehydrogenase.
- This conversion produces an additional 1 NADH molecule per pyruvate. Since there are two pyruvate molecules, this step produces 2 NADH molecules in total.

4. **Krebs Cycle in the Mitochondrial Matrix**:
- The acetyl CoA then enters the Krebs cycle (also known as the citric acid cycle), which occurs in the mitochondrial matrix.
- Each turn of the Krebs cycle produces 3 NADH molecules. Since one glucose molecule generates two acetyl CoA (from the two pyruvate), the Krebs cycle will turn twice for each glucose molecule, resulting in a total of 6 NADH molecules from the Krebs cycle.

5. **Conclusion**:
- Summarizing the production of NADH:
- In the **cytosol**: 2 NADH from glycolysis.
- In the **mitochondrial matrix**: 2 NADH from pyruvate conversion + 6 NADH from the Krebs cycle = 8 NADH.
- In the **nucleus**: NADH is not produced during carbohydrate metabolism.
- Therefore, NADH is produced in the cytosol and mitochondrial matrix, but not in the nucleus.

### Final Answer:
NADH is produced in the following locations:
- I. Cytosol
- II. Mitochondrial matrix

Thus, the correct options are 1 and 2 only.

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