If NAD+ is unavailable, the second half of glycolysis:

1. Step six needs NAD+ to accept the electrons stripped from the sugar. 2. NADH must be continuously oxidized back into NAD+ in order to keep this step going. 3. If NAD+ is not available, the second half of glycolysis slows down or stops. 4. That dependence is why fermentation exists: it regenerates NAD+ when oxygen is absent. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 7 'Cellular Respiration', sections 7.1-7.7_