How does NADH convert to NAD+ during fermentation?

NADH converts to NAD+ during fermentation primarily through the process of donating electrons. In fermentation, NADH serves as a reducing agent, where it donates electrons to an electron acceptor, typically an organic molecule like pyruvate. This donation of electrons is essential for regenerating NAD+, which is required for glycolysis to continue.

In glycolysis, NAD+ is necessary to accept electrons during the oxidation of glucose. If NAD+ is not regenerated, glycolysis cannot proceed, leading to a halt in ATP production. Through fermentation, the electrons from NADH are transferred to molecules such as pyruvate or acetaldehyde, converting them into lactate or ethanol, respectively, while simultaneously regenerating NAD+. This cycle allows the cell to maintain its energy production under anaerobic conditions when oxygen is scarce or absent.

The other options relate to processes that are either indirect or not part of the mechanism by which NADH is converted to NAD+. Hence, the focus on the electron donation aspect accurately captures the essential biochemical reaction taking place during fermentation.