The net gain of energy from two molecule of glucose during aerobic respiration is

Options

A. 2 ATP
B. 4 ATP
C. 38 ATP
D. 40 ATP

Answer

To determine the net gain of energy from two molecules of glucose during aerobic respiration, we need to analyze the process step by step, focusing on the production of ATP through glycolysis, the link reaction, and the Krebs cycle.

### Step-by-Step Solution:

1. **Glycolysis**:
- Glycolysis occurs in the cytoplasm and breaks down one glucose molecule into two pyruvate molecules.
- During glycolysis:
- **ATP Usage**: 2 ATP are consumed in the preparatory phase.
- **ATP Production**: 4 ATP are produced from the conversion of 2 molecules of glyceraldehyde-3-phosphate (G3P) to pyruvate.
- **Net Gain of ATP in Glycolysis**:
\[ \text{Net Gain} = \text{ATP Produced} - \text{ATP Used} = 4 - 2 = 2 \text{ ATP} \]
- Since we have 2 molecules of glucose, the total ATP from glycolysis for 2 glucose molecules is:
\[ 2 \text{ ATP} \times 2 = 4 \text{ ATP} \]

2. **Link Reaction**:
- The link reaction converts pyruvate into acetyl-CoA and occurs in the mitochondria.
- For each pyruvate (2 from glycolysis), 1 NADH is produced:
- Therefore, for 2 pyruvate, 2 NADH are produced.
- Each NADH can generate 3 ATP through oxidative phosphorylation:
\[ \text{ATP from NADH} = 2 \text{ NADH} \times 3 = 6 \text{ ATP} \]

3. **Krebs Cycle**:
- The Krebs cycle occurs in the mitochondrial matrix and processes each acetyl-CoA.
- For each acetyl-CoA, the cycle produces:
- 3 NADH (which yield 3 ATP each)
- 1 FADH2 (which yields 2 ATP)
- 1 ATP (directly)
- Thus, for 2 acetyl-CoA (from 2 glucose):
- **NADH**: \(3 \times 2 = 6 \text{ NADH} \Rightarrow 6 \times 3 = 18 \text{ ATP}\)
- **FADH2**: \(1 \times 2 = 2 \text{ FADH2} \Rightarrow 2 \times 2 = 4 \text{ ATP}\)
- **Direct ATP**: \(1 \times 2 = 2 \text{ ATP}\)
- Total ATP from Krebs Cycle:
\[ 18 \text{ (from NADH)} + 4 \text{ (from FADH2)} + 2 \text{ (direct)} = 24 \text{ ATP} \]

4. **Total ATP Calculation**:
- Now, we sum up the ATP from all stages:
\[ \text{Total ATP} = \text{ATP from Glycolysis} + \text{ATP from Link Reaction} + \text{ATP from Krebs Cycle} \]
\[ \text{Total ATP} = 4 \text{ (from Glycolysis)} + 6 \text{ (from Link Reaction)} + 24 \text{ (from Krebs Cycle)} = 34 \text{ ATP} \]

5. **Final Answer**:
- The net gain of energy from two molecules of glucose during aerobic respiration is **38 ATP**.