What is the net gain in a Krebs cycle ?

Options

A. `1 FADH_(2), 2 NADH_(2) "and " 1 ATP`
B. `2 FADH_(2), 2 NADH_(2) "and " 2 ATP`
C. `1 FADH_(2), 3 NADH_(2) " and " 1 ATP`
D. `1 FADH_(2), 6 NADH_(2) " and " 2 ATP`

Answer

To determine the net gain in the Krebs cycle, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Krebs Cycle**:
- The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a crucial part of cellular respiration that occurs in the mitochondria. It follows glycolysis, where glucose is broken down into pyruvic acid.

2. **Starting Molecules**:
- Each glucose molecule, after glycolysis, is converted into two molecules of pyruvic acid. Each pyruvic acid is then converted into acetyl CoA, which enters the Krebs cycle.

3. **Key Outputs of the Krebs Cycle**:
- For each turn of the Krebs cycle (which processes one acetyl CoA), the following products are generated:
- **3 NADH**: These are electron carriers that will be used in the electron transport chain.
- **1 FADH2**: Another electron carrier.
- **1 ATP (or GTP)**: This is the direct energy currency produced.

4. **Total Outputs from Two Turns**:
- Since one glucose molecule produces two acetyl CoA molecules, the Krebs cycle turns twice for each glucose molecule. Therefore, we double the outputs:
- **NADH**: 3 NADH x 2 = 6 NADH
- **FADH2**: 1 FADH2 x 2 = 2 FADH2
- **ATP**: 1 ATP x 2 = 2 ATP

5. **Calculating ATP Yield**:
- Each NADH can produce approximately 3 ATP in the electron transport chain.
- Each FADH2 can produce approximately 2 ATP.
- Therefore, the total ATP yield from the products of the Krebs cycle is calculated as follows:
- From NADH: 6 NADH x 3 ATP/NADH = 18 ATP
- From FADH2: 2 FADH2 x 2 ATP/FADH2 = 4 ATP
- From direct ATP: 2 ATP

6. **Total Net Gain**:
- Adding these together gives:
- Total ATP = 18 ATP (from NADH) + 4 ATP (from FADH2) + 2 ATP = 24 ATP

### Final Answer:
The net gain from the Krebs cycle per glucose molecule is **24 ATP**.