Final answer:
In the absence of oxygen, pyruvic acid is converted into acetaldehyde, which then is further reduced to ethanol while releasing carbon dioxide and regenerating NAD+.
Step-by-step explanation:
Pyruvic acid undergoes a biochemical process known as fermentation in the absence of oxygen. During fermentation, pyruvic acid is first transformed into acetaldehyde by the removal of a carboxyl group, releasing carbon dioxide.
This reaction is catalyzed by pyruvate decarboxylase, an enzyme that requires coenzyme derived from vitamin B1 (thiamine). Next, acetaldehyde is reduced to ethanol by the enzyme alcohol dehydrogenase, which uses NADH and converts it back to NAD+, facilitating the continuation of glycolysis.