Final answer:
In the absence of oxygen and fermentation, glycolysis halts at the step where NAD+ is regenerated from NADH because without this regeneration, the pathway cannot continue and this would halt glycolysis, specifically post the oxidation of glyceraldehyde-3-phosphate.
Step-by-step explanation:
In cells that cannot carry out fermentation and are in the absence of oxygen, glycolysis would halt at the step where the cell must regenerate NAD+ from NADH. This regeneration is crucial because if NAD* is not available, the pathway cannot continue and therefore would result in a halt, specifically after glyceraldehyde-3-phosphate has been oxidized to 1,3-bisphosphoglycerate. During this step, NAD+ is typically reduced to NADH, and without a means of recycling NADH back to NAD+ through fermentation, the available NAD+ would be rapidly depleted, halting glycolysis.
Without the ability to carry out fermentation or aerobic respiration, a cell will not be able to produce additional ATP from the electron transport chain, thereby relying solely on the very limited amount of ATP that can be generated through substrate-level phosphorylation during glycolysis. If glycolysis is interrupted, cells that lack mitochondria, such as mature mammalian red blood cells, would eventually die, as they cannot tap into any other energy sources.