In which of the following steps of Krebs cycle, `CO_(2)` is evolved ?

Options

A. Isocitric acid `rarr` Oxalosuccinic acid
B. Oxalosuccinic acid `rarr alpha` -ketoglyutaric acid
C. Succinic acid `rarr` Fumaric acid
D. Malic acid `rarr` Oxaloacetic acid

Answer

To determine in which steps of the Krebs cycle carbon dioxide (CO₂) is evolved, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It involves several steps where different substrates are converted into products.

2. **Identify Key Steps**: Within the Krebs cycle, we need to identify the specific steps where CO₂ is released. There are two main steps where this occurs:
- The conversion of **isocitrate** to **alpha-ketoglutarate**.
- The conversion of **alpha-ketoglutarate** to **succinyl-CoA**.

3. **First Step - Isocitrate to Alpha-Ketoglutarate**:
- In this step, isocitrate is converted to alpha-ketoglutarate. During this conversion, one molecule of CO₂ is released.
- An intermediate called **oxalosuccinic acid** is formed transiently, which undergoes decarboxylation to release CO₂.

4. **Second Step - Alpha-Ketoglutarate to Succinyl-CoA**:
- In this step, alpha-ketoglutarate is converted to succinyl-CoA. This conversion also results in the release of another molecule of CO₂.

5. **Conclusion**: Therefore, the two steps in the Krebs cycle where CO₂ is evolved are:
- Isocitrate to alpha-ketoglutarate
- Alpha-ketoglutarate to succinyl-CoA

### Final Answer:
CO₂ is evolved in the following steps of the Krebs cycle:
1. Conversion of isocitrate to alpha-ketoglutarate
2. Conversion of alpha-ketoglutarate to succinyl-CoA