AAMC FL Practice Exam

Which process results in the net gain of 2 ATP in cellular respiration?

1. Aerobic respiration

2. Krebs cycle

3. Fermentation

4. Glycolysis

The correct answer is: Fermentation

The correct choice, fermentation, is a metabolic process that enables cells to produce ATP in the absence of oxygen. During fermentation, glucose is partially oxidized to produce energy, and the net gain of ATP is typically 2 per molecule of glucose. This occurs because, while glycolysis generates 4 ATP molecules, 2 ATP are consumed in the early steps of the glycolytic pathway, resulting in a net production of 2 ATP. In contrast, aerobic respiration and the Krebs cycle yield significantly more energy. Aerobic respiration can generate up to 36-38 ATP per glucose molecule due to the complete oxidation of glucose in the presence of oxygen. The Krebs cycle, specifically, is part of aerobic respiration and does not produce ATP directly; rather, it generates reduced coenzymes (NADH and FADH2) that will later contribute to ATP production through oxidative phosphorylation. Glycolysis itself, while also generating a net gain of 2 ATP, is not the direct answer here as fermentation is fundamentally the process that occurs in anaerobic conditions leading to the same net ATP yield. Understanding the distinction between these processes clarifies why fermentation is the correct choice for a process that specifically results in a net gain of 2 ATP when oxygen is