For each acetyl Co-A oxidized by the citric acid cycle, what is the energy gain?

Options

A. Two molecules of NADH, one `FADH_2` and one nucleoside triphosphate
B. Three molecules of NADH, one `FADH_2` and one nucleoside triphosphate
C. Two molecules of NADH, one `FADH_2` and two nucleoside triphosphate
D. Three molecules of NADH, one `FADH_2` and two nucleoside triphosphate

Answer

To determine the energy gain from the oxidation of each acetyl CoA in the citric acid cycle (Krebs cycle), we need to analyze the products formed during this process.

### Step-by-Step Solution:

1. **Identify the Reactants**: The main reactant in the citric acid cycle is acetyl CoA. The other reactants include 3 NAD+, 1 FAD, 1 ADP, and water.

2. **Understand the Products**: When acetyl CoA is oxidized in the citric acid cycle, the following products are generated:
- 3 molecules of NADH + H+
- 1 molecule of FADH2
- 1 molecule of ATP (adenosine triphosphate)
- 2 molecules of CO2 (carbon dioxide)

3. **Energy Carriers**: The NADH and FADH2 produced are crucial as they are energy carriers that will later contribute to ATP production during oxidative phosphorylation.
- Each NADH can generate approximately 2.5 ATP during oxidative phosphorylation.
- Each FADH2 can generate approximately 1.5 ATP.

4. **Calculate Total ATP Yield**:
- From 3 NADH: 3 NADH × 2.5 ATP/NADH = 7.5 ATP
- From 1 FADH2: 1 FADH2 × 1.5 ATP/FADH2 = 1.5 ATP
- From 1 ATP directly produced in the cycle: 1 ATP

5. **Total Energy Gain**: Now, we can sum up the ATP produced:
- Total ATP = 7.5 ATP (from NADH) + 1.5 ATP (from FADH2) + 1 ATP = 10 ATP

Thus, the total energy gain from the oxidation of one acetyl CoA in the citric acid cycle is approximately **10 ATP**.