How much of the energy released during aerobic respiration is approximately conserved in the form of ATP?

Options

A. `20%`
B. `40%`
C. `60%`
D. `100%`

Answer

**Step-by-Step Solution:**

1. **Understanding Aerobic Respiration**: Aerobic respiration is a process in which glucose is broken down in the presence of oxygen to produce energy. This process consists of several stages: glycolysis, oxidative decarboxylation, and the Krebs cycle.

2. **Calculating ATP from Glycolysis**:
- During glycolysis, a total of 4 ATP molecules are produced. However, 2 ATP molecules are consumed in the process, leaving a net gain of 2 ATP.
- Additionally, 2 NADH molecules are produced. Each NADH can generate approximately 3 ATP through oxidative phosphorylation.
- Therefore, from glycolysis, the total ATP produced is:
\[ \text{Net ATP from glycolysis} = 2 \text{ (from ATP)} + 2 \times 3 \text{ (from NADH)} = 2 + 6 = 8 \text{ ATP} \]

3. **Calculating ATP from Oxidative Decarboxylation**:
- In this step, 2 NADH molecules are formed, which can also generate ATP.
- Thus, the total ATP from oxidative decarboxylation is:
\[ 2 \times 3 = 6 \text{ ATP} \]

4. **Calculating ATP from the Krebs Cycle**:
- The Krebs cycle produces 6 NADH, 2 FADH2, and 2 ATP.
- The ATP generated from NADH and FADH2 is calculated as follows:
- From 6 NADH: \( 6 \times 3 = 18 \text{ ATP} \)
- From 2 FADH2: \( 2 \times 2 = 4 \text{ ATP} \)
- Therefore, the total ATP from the Krebs cycle is:
\[ 18 + 4 + 2 = 24 \text{ ATP} \]

5. **Total ATP Calculation**:
- Now, we sum up all the ATP produced from each stage:
\[ \text{Total ATP} = 8 \text{ (from glycolysis)} + 6 \text{ (from oxidative decarboxylation)} + 24 \text{ (from Krebs cycle)} = 38 \text{ ATP} \]

6. **Energy Conservation Percentage**:
- The total energy released during aerobic respiration can be estimated to be around 686 kcal per mole of glucose.
- Since 1 mole of glucose yields approximately 38 ATP, and each ATP is equivalent to about 7.3 kcal, the total energy conserved in ATP is:
\[ 38 \text{ ATP} \times 7.3 \text{ kcal/ATP} = 277.4 \text{ kcal} \]
- To find the percentage of energy conserved as ATP:
\[ \text{Percentage} = \left( \frac{277.4 \text{ kcal}}{686 \text{ kcal}} \right) \times 100 \approx 40.4\% \]

7. **Final Answer**:
- Therefore, approximately **40%** of the energy released during aerobic respiration is conserved in the form of ATP.

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