What is the primary purpose of fermentation?

The primary purpose of fermentation is to regenerate NAD+ for glycolysis. In the absence of oxygen, cells rely on fermentation to ensure that glycolysis can continue to produce ATP. Glycolysis requires NAD+ to accept electrons during the conversion of glucose into pyruvate. However, under anaerobic conditions, the supply of NAD+ is depleted because it cannot be recycled through the electron transport chain, as oxygen—the final electron acceptor in aerobic respiration—is unavailable.

Fermentation allows cells to convert pyruvate into either lactic acid or ethanol, depending on the organism, while simultaneously regenerating NAD+. This recycling of NAD+ is crucial because it allows glycolysis to keep occurring, enabling cells to generate small amounts of ATP even when oxygen is not present.

While other options mention processes linked to cellular activity, they don't address the primary role of fermentation. Specifically, the production of ATP under aerobic conditions is characteristic of aerobic respiration, not fermentation. The synthesis of glucose and converting lactic acid back into glucose are processes that do not directly relate to the core function of fermentation during anaerobic respiration.