What occurs if oxygen is present after glycolysis?

When oxygen is present after glycolysis, the process typically follows the pathway leading to the citric acid cycle, also known as the Krebs cycle. This cycle takes place in the mitochondria and uses the pyruvate produced from glycolysis. In the presence of oxygen, pyruvate is converted into acetyl-CoA, which then enters the citric acid cycle. During this cycle, acetyl-CoA undergoes a series of reactions that produce energy-rich molecules such as ATP, NADH, and FADH2. These molecules are crucial for the next stage of cellular respiration, where they will be used in the electron transport chain to generate a significant amount of ATP through oxidative phosphorylation.

This process contrasts markedly with fermentation, which occurs when oxygen is absent. Fermentation allows glycolysis to continue by reoxidizing NADH to NAD+ so that glycolysis can perpetuate, but it does not yield the same energy output as the citric acid cycle. Therefore, the presence of oxygen is vital for the continuation of aerobic respiration through the citric acid cycle and ultimately enhances energy production for the cell.