Which two steps of the citric acid cycle produce CO₂?

• A. 1 and 3
• B. 3 and 4
• C. 2 and 4
• D. 2 and 3

Answer: (B)

Verified step by step guidance

1. Understand that the citric acid cycle, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide.
2. Recall that during the citric acid cycle, carbon dioxide (CO2) is released as a byproduct in two specific steps.
3. Identify the steps in the citric acid cycle where decarboxylation occurs, which is the process of removing a carboxyl group and releasing CO2.
4. The first decarboxylation occurs when isocitrate is converted into alpha-ketoglutarate, releasing CO2. This is catalyzed by the enzyme isocitrate dehydrogenase.
5. The second decarboxylation occurs when alpha-ketoglutarate is converted into succinyl-CoA, releasing another CO2 molecule. This step is catalyzed by the enzyme alpha-ketoglutarate dehydrogenase.