At the end of the Kreb's cycle, but before the electron transport chain, the oxidation of glucose has produced a net gain of -

Options

A. `3CO_(2),"5 NAD"H_(2),"1 FAD"H_(2),"2 ATP"`
B. `6CO_(2),"10 NAD"H_(2),"2 FAD"H_(2),"4 ATP"`
C. `6CO_(2),"10 NAD"H_(2),"2 FAD"H_(2),"38 ATP"`
D. None of the above is correct

Answer

To determine the net gain of ATP from the oxidation of glucose at the end of the Krebs cycle, but before the electron transport chain (ETC), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Process**: The oxidation of glucose occurs through several stages: glycolysis, the link reaction (conversion of pyruvate to acetyl-CoA), and the Krebs cycle (Citric Acid Cycle).

2. **Glycolysis**: In glycolysis, one molecule of glucose is converted into two molecules of pyruvate. This process produces a net gain of 2 ATP and 2 NADH.

3. **Link Reaction**: Each pyruvate is converted into acetyl-CoA, releasing one molecule of CO2. Since there are two pyruvate molecules from one glucose, this results in the release of 2 CO2. This reaction also produces 2 NADH (one for each pyruvate).

4. **Krebs Cycle**: Each acetyl-CoA enters the Krebs cycle. For each turn of the Krebs cycle (which occurs twice for one glucose molecule), the following is produced:
- 2 CO2
- 3 NADH
- 1 FADH2
- 1 ATP (or GTP, which is energetically equivalent)

Since one glucose molecule generates two acetyl-CoA, the total yield from the Krebs cycle for one glucose molecule is:
- 4 CO2 (2 from each cycle)
- 6 NADH (3 from each cycle)
- 2 FADH2 (1 from each cycle)
- 2 ATP (or GTP, 1 from each cycle)

5. **Total Yield Before ETC**: Now, we can summarize the total yield before entering the electron transport chain:
- From glycolysis: 2 ATP + 2 NADH
- From link reaction: 2 NADH
- From Krebs cycle: 2 ATP + 6 NADH + 2 FADH2

Adding these together:
- Total ATP = 2 (from glycolysis) + 2 (from Krebs cycle) = 4 ATP
- Total NADH = 2 (from glycolysis) + 2 (from link reaction) + 6 (from Krebs cycle) = 10 NADH
- Total FADH2 = 2 (from Krebs cycle) = 2 FADH2
- Total CO2 = 6 CO2 (2 from link reaction + 4 from Krebs cycle)

6. **Final Answer**: Therefore, at the end of the Krebs cycle, but before the electron transport chain, the oxidation of glucose has produced a net gain of **4 ATP**.