So that implies that we must have some way to regenerate the NAD plus molecules by oxidizing NADH back to NAD plus. And since under anaerobic conditions, the NADH molecules do not actually go onto the electron transport chain, another method of regenerating the NAD plus molecules is via the process of fermentation. And there are two types of fermentation processes. So we have alcohol fermentation and lactic acid fermentation. So let's begin with alcohol fermentation. So certain types of bacterial cells as well as these cells basically undergo this process of alcohol fermentation.