Each molecule of pyruvic acid entering the Kreb's cycle produces

Options

A. 3
B. 2
C. 4
D. 6

Answer

To solve the question regarding how many molecules of CO2 are produced from each molecule of pyruvic acid entering the Krebs cycle, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Pyruvic Acid**:
- Pyruvic acid is a three-carbon compound (C3H4O3).

2. **Conversion to Acetyl CoA**:
- Before entering the Krebs cycle, pyruvic acid is converted into acetyl coenzyme A (acetyl CoA), which is a two-carbon compound (C2H3O).
- During this conversion, one molecule of CO2 is released.
- Therefore, from one molecule of pyruvic acid, we have already released 1 CO2.

3. **Entering the Krebs Cycle**:
- Acetyl CoA combines with oxaloacetic acid (a four-carbon compound) to form citric acid (a six-carbon compound).
- The citric acid then undergoes a series of transformations in the Krebs cycle.

4. **CO2 Production in the Krebs Cycle**:
- As citric acid is converted through various intermediates (isocitric acid, alpha-ketoglutaric acid, succinic acid, fumaric acid, malic acid) back to oxaloacetic acid, additional CO2 molecules are released:
- From citric acid to isocitric acid: No CO2 is released.
- From isocitric acid to alpha-ketoglutaric acid: 1 CO2 is released.
- From alpha-ketoglutaric acid to succinyl CoA: 1 CO2 is released.
- From succinyl CoA to succinic acid: No CO2 is released.
- From succinic acid to fumaric acid: No CO2 is released.
- From fumaric acid to malic acid: No CO2 is released.
- From malic acid to oxaloacetic acid: No CO2 is released.

5. **Total CO2 Produced**:
- Adding it all up:
- 1 CO2 from the conversion of pyruvic acid to acetyl CoA.
- 1 CO2 from isocitric acid to alpha-ketoglutaric acid.
- 1 CO2 from alpha-ketoglutaric acid to succinyl CoA.
- Thus, the total CO2 produced from one molecule of pyruvic acid entering the Krebs cycle is 3 CO2.

6. **Final Answer**:
- Therefore, each molecule of pyruvic acid entering the Krebs cycle produces **3 molecules of CO2**.