What is the main energy yield from glycolysis?

• A. 2 ATP
• B. 4 ATP
• C. 36 ATP
• D. 38 ATP

Answer: (A)

The main energy yield from glycolysis is 2 ATP molecules. During this metabolic pathway, glucose is broken down into two molecules of pyruvate through a series of enzymatic reactions. In the process, there are two key phases: the energy investment phase, where 2 ATP are consumed, and the energy payoff phase, where 4 ATP are generated. The net gain, therefore, is 4 ATP produced minus the 2 ATP used, resulting in a total of 2 ATP.

It's important to note that glycolysis does not fully oxidize glucose; it occurs in the cytoplasm and serves as the first step in both aerobic and anaerobic respiration. Following glycolysis, the pyruvate can enter the mitochondria for further oxidation in the presence of oxygen, which would yield significantly more ATP, but those reactions occur after glycolysis itself. Therefore, while glycolysis initiates the breakdown of glucose and generates ATP, the immediate and direct yield from glycolysis alone is 2 ATP.