How many substrate level phosphorylations take place in one turn of TCA cycle?

Options

A. 4
B. 3
C. 2
D. 1

Answer

To determine how many substrate level phosphorylations occur in one turn of the TCA (Tricarboxylic Acid) cycle, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Substrate Level Phosphorylation**:
- Substrate level phosphorylation is a process where a phosphate group is directly transferred from a substrate to ADP or GDP to form ATP or GTP. This is distinct from oxidative phosphorylation, which occurs in the electron transport chain.

2. **Overview of the TCA Cycle**:
- The TCA cycle, also known as the Krebs cycle or Citric Acid cycle, begins with the condensation of Acetyl-CoA with oxaloacetate to form citrate. The cycle involves a series of reactions that ultimately regenerate oxaloacetate.

3. **Identifying Key Steps in the TCA Cycle**:
- The TCA cycle includes several key steps where various substrates are converted into different compounds. The important steps to note for substrate level phosphorylation are:
- Conversion of succinyl-CoA to succinate.

4. **Recognizing the Substrate Level Phosphorylation Step**:
- In the TCA cycle, the only step that involves substrate level phosphorylation occurs when succinyl-CoA is converted to succinate. During this conversion, ATP (or GTP, depending on the cell type) is produced.

5. **Conclusion**:
- Since there is only one step in the TCA cycle that results in the direct production of ATP or GTP through substrate level phosphorylation, we conclude that **there is 1 substrate level phosphorylation in one turn of the TCA cycle**.

### Final Answer:
The number of substrate level phosphorylations that take place in one turn of the TCA cycle is **1**.

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