How many ATP are synthesized per glucose molecule (gross synthesis) in aerobic respiration through substrate level phosphorylation ?

Options

A. 10
B. 12
C. 30
D. 6

Answer

To determine how many ATP are synthesized per glucose molecule through substrate-level phosphorylation in aerobic respiration, we can break down the process into three main stages: glycolysis, the malate-aspartate shuttle, and the Krebs cycle. Here's the step-by-step solution:

### Step 1: Glycolysis
- In glycolysis, 2 ATP molecules are produced directly through substrate-level phosphorylation.
- Additionally, 1 NADH molecule is generated, which can be converted to ATP. Each NADH produces approximately 2.5 ATP when oxidized.
- Therefore, from glycolysis:
- ATP from substrate-level phosphorylation: **2 ATP**
- ATP from NADH: **2.5 ATP**

**Total from glycolysis = 2 ATP + 2.5 ATP = 4.5 ATP**

### Step 2: Malate-Aspartate Shuttle
- The malate-aspartate shuttle is used to transport electrons from NADH produced in glycolysis into the mitochondria.
- This process does not produce additional ATP directly but allows for the efficient use of NADH.
- In this case, we consider the ATP produced from the NADH generated in glycolysis, which is already included in the previous step.

### Step 3: Krebs Cycle
- The Krebs cycle occurs twice for each glucose molecule (once for each pyruvate).
- During each cycle, the following are produced:
- 3 NADH (each producing 2.5 ATP)
- 1 FADH2 (producing 1.5 ATP)
- 1 ATP (directly through substrate-level phosphorylation)

- For two cycles:
- Total NADH = 2 cycles × 3 NADH = 6 NADH → 6 × 2.5 ATP = 15 ATP
- Total FADH2 = 2 cycles × 1 FADH2 = 2 FADH2 → 2 × 1.5 ATP = 3 ATP
- Total ATP from Krebs cycle = 2 ATP (directly from substrate-level phosphorylation)

**Total from Krebs cycle = 15 ATP + 3 ATP + 2 ATP = 20 ATP**

### Final Calculation
- Now, we combine the ATP produced from glycolysis and the Krebs cycle:
- Total from glycolysis = **4.5 ATP**
- Total from Krebs cycle = **20 ATP**

**Total ATP synthesized per glucose molecule = 4.5 ATP + 20 ATP = 24.5 ATP**

### Conclusion
The approximate total ATP synthesized per glucose molecule through substrate-level phosphorylation in aerobic respiration is **approximately 30 ATP** (considering rounding and other factors).