Final answer:
To keep glycolysis running, NADH must be continuously oxidized back into NAD+. In the presence of oxygen, NADH is readily oxidized, while in the absence of oxygen, fermentation can provide the oxidation of NADH.
Step-by-step explanation:
Glycolysis is the first pathway used in the breakdown of glucose to extract energy
The continuation of the glycolysis reaction depends on the availability of the oxidized form of the electron carrier, NAD+. NADH, which is the reduced form, must be continuously oxidized back into NAD+ in order to keep glycolysis running.
In the presence of oxygen, NADH can be readily oxidized, indirectly releasing high-energy electrons that are used to produce ATP. However, in the absence of oxygen, an alternate pathway called fermentation can provide the oxidation of NADH to NAD+.