Final answer:
NADH is oxidized to regenerate NAD+ in order to sustain the glycolytic pathway. The presence of oxygen enables this via the electron transport chain, while fermentation pathways achieve this in anoxic environments.
Step-by-step explanation:
NADH is oxidized to regenerate NAD+ to be re-used in another round of glycolysis. The continued progress of glycolysis is contingent upon the presence of NAD+, which acts as an electron carrier. During glycolysis, NAD+ is reduced to NADH as it accepts electrons released during the conversion of glucose into pyruvate. To maintain the flow of glycolysis, NADH must be re-oxidized back into NAD+.
If oxygen is present, this re-oxidation occurs via the electron transport chain in cellular respiration, ultimately producing ATP. In the absence of oxygen, fermentation pathways, such as lactic acid fermentation, accomplish the re-oxidation of NADH, allowing glycolysis to continue. This process is vital for sustaining the cell's energy supply, especially during anaerobic conditions.