Advanced Placement (AP) Biology Practice Exam

Lactic acid fermentation primarily occurs in certain bacteria and muscle cells when oxygen is scarce or during intense exercise. The main product of this anaerobic process is lactic acid, which is generated from the reduction of pyruvate. During fermentation, glucose is first broken down through glycolysis to produce pyruvate, and then, in the absence of oxygen, the pyruvate is converted into lactic acid. This process allows for the regeneration of NAD+, which is essential for glycolysis to continue, allowing the cell to produce ATP even in low-oxygen conditions. While other options represent products of different metabolic processes—such as oxygen, which is produced in photosynthesis; ethanol, which is the main product of alcoholic fermentation; and glucose, which is a reactant for both types of fermentation—lactic acid specifically represents the key product resulting from the fermentation process under anaerobic conditions in certain organisms.