If two molecules of glucose were to proceed through the citric acid cycle, how many ATP would be produced?
The correct answer and explanation is:
Correct Answer: 2 molecules of glucose would produce 24 ATP through the citric acid cycle.
Explanation (300 words):
To determine how many ATP are produced from two molecules of glucose via the citric acid cycle (Krebs cycle), we first need to understand the relationship between glucose metabolism and the citric acid cycle.
Each glucose molecule (C₆H₁₂O₆) is broken down during glycolysis into two molecules of pyruvate. Each pyruvate is then converted into one molecule of acetyl-CoA, which enters the citric acid cycle. So, one glucose yields two acetyl-CoA, and therefore, enters the cycle twice.
Now let’s consider the ATP equivalents produced per turn of the citric acid cycle (i.e., per acetyl-CoA):
- 3 NADH → 3 NADH × 2.5 ATP = 7.5 ATP
- 1 FADH₂ → 1 FADH₂ × 1.5 ATP = 1.5 ATP
- 1 GTP (converted to ATP) = 1 ATP
Total per cycle (per acetyl-CoA):
7.5 + 1.5 + 1 = 10 ATP
Since one glucose produces 2 acetyl-CoA, the citric acid cycle runs twice for each glucose, yielding:
- 10 ATP × 2 = 20 ATP per glucose
Therefore, for two glucose molecules, the citric acid cycle runs four times, yielding:
- 10 ATP × 4 = 40 ATP from 2 glucose is incorrect if only citric acid cycle is considered.
We must remember: 10 ATP per acetyl-CoA refers to ATP equivalents, but the question asks specifically how many ATP are produced by the citric acid cycle itself, not including the ETC.
So, per acetyl-CoA, the cycle directly produces:
- 1 ATP (GTP)
Thus, 2 glucose → 4 acetyl-CoA → 4 ATP directly from the citric acid cycle.
But if the question refers to total ATP equivalents generated from the citric acid cycle including NADH and FADH₂ contributions, then:
- 4 turns × 10 ATP = 40 ATP
✅ Final Clarified Answer: 20 ATP per glucose, so 40 ATP from two glucose molecules in the citric acid cycle (counting NADH, FADH₂, and GTP).