How many of the following are the substrates involved in the Kreb's cycle?
Citric acid, succinic acid, hydrochloric acid, a-ketoglutaric acid, fumaric acid, malic acid, pyruvic acid, phosphoglyceric acid, acetic acid

Options

A. six
B. four
C. five
D. three

Answer

To determine how many of the listed compounds are substrates involved in the Krebs cycle, we will analyze each compound one by one.

### Step-by-Step Solution:

1. **Identify the Krebs Cycle**: The Krebs cycle, also known as the TCA (tricarboxylic acid) cycle or citric acid cycle, is a series of chemical reactions used by aerobic organisms to release stored energy through the oxidation of acetyl-CoA.

2. **List the Compounds**: The compounds given in the question are:
- Citric acid
- Succinic acid
- Hydrochloric acid
- α-ketoglutaric acid
- Fumaric acid
- Malic acid
- Pyruvic acid
- Phosphoglyceric acid
- Acetic acid

3. **Evaluate Each Compound**:
- **Citric acid**: Involved in the Krebs cycle (substrate).
- **Succinic acid**: Involved in the Krebs cycle (substrate).
- **Hydrochloric acid**: Not involved in the Krebs cycle (not a substrate).
- **α-ketoglutaric acid**: Involved in the Krebs cycle (substrate).
- **Fumaric acid**: Involved in the Krebs cycle (substrate).
- **Malic acid**: Involved in the Krebs cycle (substrate).
- **Pyruvic acid**: Not directly a substrate in the Krebs cycle; it is converted to acetyl-CoA before entering the cycle.
- **Phosphoglyceric acid**: Not involved in the Krebs cycle (not a substrate).
- **Acetic acid**: Not involved in the Krebs cycle (not a substrate).

4. **Count the Substrates**: From the evaluation:
- Substrates involved in the Krebs cycle: Citric acid, Succinic acid, α-ketoglutaric acid, Fumaric acid, Malic acid.
- Total number of substrates = 5.

5. **Conclusion**: The total number of substrates from the given list that are involved in the Krebs cycle is **5**.

### Final Answer:
5 substrates are involved in the Krebs cycle.

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