Final answer:
To enable pyruvate entry into the Krebs cycle, the presence of oxygen is required, which allows the transformation of pyruvate into acetyl CoA in mitochondria.
Step-by-step explanation:
The end product of glycolysis is pyruvate, and for this pyruvate to enter the Krebs cycle, a specific condition must be met. The correct answer to the question is D - Oxygen. In the presence of oxygen, pyruvate undergoes a transformation where it's converted into acetyl coenzyme A (acetyl CoA) in a process that involves the decarboxylation of pyruvate (removal of carbon dioxide) and the reduction of NAD+ to NADH.
This conversion process takes place within the mitochondria of the cell. Once transformed into acetyl CoA, the molecule can continue to the Krebs cycle where it undergoes further oxidation, contributing to the production of ATP, NADH, and FADH2 molecules. In summary, oxygen is crucial for pyruvate to enter the Krebs cycle because it enables the aerobic environment necessary for the conversion of pyruvate to acetyl CoA, which is then fed into the Krebs cycle for energy extraction.