Answer:
During fermentation, NADH that was produced during glycolysis is converted back to NAD+.
Step-by-step explanation:
Glycolysis includes a breakdown of a glucose molecule into two molecules of pyruvate. During these reactions, NAD+ serves as an electron donor and is reduced into NADH. For one molecule of glucose, two molecules of NAD+ are reduced into NADH. To continue the process of glycolysis, the NAD+ supply should be continued.
Electron transport chain oxidized NADH into NAD+ in presence of oxygen. In the absence of oxygen, either reduction of pyruvate or any other substance uses NADH as an electron donor and the NAD+ is regenerated. This is called fermentation. During lactic acid fermentation, the reduction of pyruvate into lactate regenerates the NAD+.