In which of the following steps of aerobic respiration of glucose, carbon dioxide is evolved ?
I. Glycolysis
II. Link reaction
III. Krebs cycle
IV. Election transport system

Options

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

Answer

To determine in which steps of aerobic respiration carbon dioxide (CO2) is evolved, let's analyze each step mentioned in the question:

### Step 1: Glycolysis
- **Process**: Glycolysis is the first step of aerobic respiration where glucose is broken down into two molecules of pyruvate.
- **CO2 Production**: During glycolysis, no carbon dioxide is released. The process involves the conversion of glucose into pyruvate without any decarboxylation occurring.

### Step 2: Link Reaction (Pyruvate Oxidation)
- **Process**: After glycolysis, the pyruvate molecules enter the mitochondrial matrix where they are converted into acetyl-CoA. This process is known as the link reaction.
- **CO2 Production**: During this conversion, one carbon atom is removed from each pyruvate molecule in the form of carbon dioxide. Therefore, carbon dioxide is evolved in this step.

### Step 3: Krebs Cycle (Citric Acid Cycle)
- **Process**: The acetyl-CoA produced in the link reaction enters the Krebs cycle, where it undergoes a series of reactions to produce energy carriers (NADH, FADH2) and ATP.
- **CO2 Production**: In the Krebs cycle, two carbon atoms are released as carbon dioxide during the decarboxylation reactions that occur at various steps in the cycle. Thus, carbon dioxide is also evolved in this step.

### Step 4: Electron Transport System
- **Process**: The electron transport system is the final step of aerobic respiration, where electrons are transferred through a series of proteins in the inner mitochondrial membrane, leading to ATP synthesis.
- **CO2 Production**: No carbon dioxide is produced in this step. The primary function is to generate ATP using the electrons carried by NADH and FADH2.

### Conclusion
Based on the analysis:
- **Glycolysis**: No CO2 evolved.
- **Link Reaction**: CO2 evolved.
- **Krebs Cycle**: CO2 evolved.
- **Electron Transport System**: No CO2 evolved.

Thus, the steps in which carbon dioxide is evolved are the **Link Reaction** and the **Krebs Cycle**.

### Final Answer:
**II. Link Reaction and III. Krebs Cycle**

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