Final answer:
In the absence of oxygen, the NAD+ needed for glycolysis is regenerated through fermentation, which converts NADH back into NAD+ enabling the continuous breakdown of glucose into ATP.
Step-by-step explanation:
The supply of NAD+ required for the production of NADH during glycolysis, in the absence of oxygen, comes from the process of fermentation. Glycolysis, which breaks down glucose into pyruvate, produces ATP and NADH. When oxygen is not present, the NADH cannot be oxidized through aerobic respiration. Instead, fermentation pathways kick in to regenerate NAD+ from NADH.
This allows glycolysis to continue producing ATP. Two main types of fermentation are lactic acid fermentation and alcoholic fermentation, both of which begin with glycolysis. Organisms will use fermentation to ensure the availability of NAD+, which is essential for the continuity of these pathways, especially in the absence of oxygen, thus allowing the continued harvesting of energy from glucose.
During the sixth step of glycolysis, high-energy electrons are extracted from glyceraldehyde-3-phosphate, which then are used to convert NAD+ into NADH. In the absence of oxygen, organisms rely on fermentation as an alternate pathway to recycle the NADH back to NAD+, maintaining the glycolytic flux. This regeneration of NAD+ in fermentation does not result in ATP production and is crucial for the sustenance of energy metabolism under anaerobic conditions.