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 one molecule of glucose (C6H12O6) is broken down into two molecules of pyruvate (C3H4O3).
- **CO2 Production**: No carbon dioxide is produced during glycolysis. The process involves the conversion of glucose into pyruvate without the release of CO2.

### Step 2: Link Reaction (Pyruvate Oxidation)
- **Process**: After glycolysis, the pyruvate molecules are transported into the mitochondria, where they undergo the link reaction. Here, each pyruvate is converted into acetyl CoA.
- **CO2 Production**: During this conversion, one carbon atom is removed from each pyruvate molecule, resulting in the release of one molecule of CO2 for each pyruvate. Since two pyruvate molecules are produced from one glucose molecule, two CO2 molecules are released in total during this step.

### Step 3: Krebs Cycle (Citric Acid Cycle)
- **Process**: Acetyl CoA enters the Krebs cycle, where it undergoes a series of reactions to produce energy carriers (NADH and FADH2) and ATP.
- **CO2 Production**: During the Krebs cycle, two carbon atoms are released as CO2 for each acetyl CoA that enters the cycle. Since two acetyl CoA are generated from one glucose molecule, a total of four CO2 molecules are produced during this cycle.

### Step 4: Electron Transport System
- **Process**: The electron transport system is the final stage of aerobic respiration where NADH and FADH2 are oxidized to produce ATP.
- **CO2 Production**: No carbon dioxide is produced in this step. The process focuses on the transfer of electrons and the creation of a proton gradient to synthesize ATP.

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

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

### Final Answer
The correct answer is **Option 4 (II and III)**.

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