Kaplan Nursing Entrance Practice Exam

What is the first molecule added in the initial reaction of the Krebs cycle?

1. Glucose

2. A 2-carbon molecule

3. A 2-carbon fatty acid

4. Acetyl-CoA

The correct answer is: A 2-carbon molecule

The first molecule added in the initial reaction of the Krebs cycle is Acetyl-CoA. The Krebs cycle, also known as the citric acid cycle or TCA cycle, begins with the combination of Acetyl-CoA, a two-carbon product derived from the breakdown of carbohydrates, fats, and proteins, with oxaloacetate, which is a four-carbon molecule. This combination forms citrate, a six-carbon compound, which then undergoes a series of transformations throughout the cycle. Understanding the role of Acetyl-CoA as the initial molecule is crucial because it serves as the entry point for energy metabolism derived from various macronutrients. Therefore, recognizing Acetyl-CoA's significance is essential in the study of cellular respiration and energy production within the body. It also highlights the interconnectedness of metabolic pathways, as Acetyl-CoA is generated from the breakdown of glucose, fatty acids, and amino acids before entering the Krebs cycle.