Section: Biological and Biochemical Foundations of Living Systems

14) Which of the following molecules will stop being produced first when oxygen is no longer supplied to the cell?

1. Oxaloacetate
2. Pyruvate
3. Water
4. Adenosine triphosphate

This question is testing your understanding of the utilization of oxygen in the production of metabolic energy. Under aerobic conditions, oxygen serves as the final electron accepter in the electron transport chain, which yields water and ATP as final products. In the absence of oxygen, the cell switches from aerobic respiration to anaerobic respiration. As a result, the ETC chain will stop functioning and water will no longer be produced. Oxaloacetate (choice A), an intermediate in the citric acid cycle, is controlled by negative feedback from the ETC. Therefore, the production of oxaloacetate will eventually stop, but the production of water will stop first. Thus, C is the best answer. Without ATP production occurring via aerobic respiration, anaerobic respiration will become the source of ATP for the cell. As a result, glycolysis will increase in order to meet the energy requirements of the cell resulting in an increase in pyruvate (choice B), a product of glycolysis.