Final answer:
If oxygen is not present, or the organism is incapable of aerobic respiration, NADHs produced at the end of glycolysis can be reoxidized through fermentation. This allows glycolysis to continue without producing ATP through an electron transport chain.
Step-by-step explanation:
If oxygen is not present or an organism is incapable of aerobic respiration, the NADHs produced at the end of glycolysis can be reoxidized through fermentation. Fermentation is a process that uses an organic molecule as the final electron acceptor to regenerate NAD+. This allows glycolysis to continue without producing ATP through an electron transport chain. In contrast, if oxygen is present, the NADH will be directly oxidized and the high-energy electrons will be used to produce ATP.