Kaplan Nursing Entrance Practice Exam

What is converted to acetyl-CoA before entering the Krebs cycle?

1. Glucose

2. Fructose

3. Pyruvate

4. Citric Acid

The correct answer is: Pyruvate

Acetyl-CoA is a crucial molecule that acts as a key entry point into the Krebs cycle, also known as the citric acid cycle. This conversion process occurs primarily in the mitochondrial matrix. When glucose undergoes glycolysis, it is broken down into two molecules of pyruvate. Each pyruvate is then converted to acetyl-CoA through a process called pyruvate decarboxylation, where a carbon atom is removed from pyruvate, releasing CO2, and a coenzyme A molecule is added, forming acetyl-CoA. Once formed, acetyl-CoA enters the Krebs cycle, where it is further oxidized to generate energy in the form of ATP, NADH, and FADH2. This conversion from pyruvate to acetyl-CoA is pivotal because it links glycolysis, which happens in the cytosol, to the aerobic pathway of respiration in the mitochondria. Other substances mentioned, like glucose and fructose, are carbohydrates that can be converted into pyruvate but do not directly enter the Krebs cycle. Citric acid, also known as citrate, is a product of the Krebs cycle itself and serves as an intermediate rather than a precursor. Therefore, pyruvate is