The citric acid cycle, also known as the Krebs cycle, is indeed a crucial part of aerobic respiration, yet it does not directly require oxygen to function. This can seem paradoxical, but it can be understood with a closer look at the processes involved in respiration.
The citric acid cycle’s main role is to process acetyl-CoA, which is derived from carbohydrates, fats, and proteins, into electron carriers like NADH and FADH2. These carriers then enter the electron transport chain, where oxygen plays a vital role as the final electron acceptor. The reliance on oxygen comes into play after the cycle has generated these high-energy molecules.
In other words, while the cycle itself does not use oxygen, it is a crucial step that prepares the cells for the aerobic part of respiration. The products of the citric acid cycle are essential for the electron transport chain, which generates ATP using the energy released from electrons as they are passed along a series of proteins.
Without oxygen, the electron transport chain cannot function properly, which limits ATP production. Therefore, while oxygen is not required for the cycle itself, it is essential for the energy extraction process that occurs afterward. This interdependence highlights how the citric acid cycle serves as a bridge between glycolysis and the aerobic respiration stages that utilize oxygen.