Answer:
ATP
Explanation:
In the first very important biochemical katabolic (degradation) pathway called glycolysis 1 part of glucose is step by step degraded to 2 parts pyruvate under aerobic as well as anaerobic conditions. Apart from 2 pyruvates there are also 2 ATP's and 2 NADH's created. This is called substrate-level phosphorylation. Glycolysis is a metabolic pathway that is almost identical in all living organisms, which implies that glycolysis is a very old biochemical system in the history of life. Researchers have found that the reactions that constitute glycolysis occur metal-catalyzed under the oxygen-free conditions of the Archean oceans, also in the absence of enzymes. Glycolysis could thus have originated from chemical constraints of the prebiotic world.
Under aerobic conditions the pyruvate would then enter the citric acid cycle (CAC, aka tricaboxylic acid cycle (TCA) or Krebs cycle), where GTP, NADH and FADH2 is produced, followed by oxidative phosphorylation (aka respiration), where the NADH's and FADH2's are converted to ATP's. In oxidative phosphorylation, the energy for ATP formation is derived from an electrochemical proton gradient generated across the inner mitochondrial membrane (or, in the case of bacteria, the plasma membrane). In the whole aerobic katabolic pathway (glycolysis, CAC and oxidative phosphorylation) of 1 molecule glucose between 30 and 38 molecules of ATP are produced, depending on the efficiency of of the whole process, where by far most of the ATP's are produced in oxidative phosphorylation. For that reason, fermentation is rarely utilized when oxygen is available. The exception being obligate anaerobes, which cannot tolerate oxygen.
However, under anaerobic conditions, pyruvate cannot enter the CAC but may be converted into different products that sill contain chemical energy in a process called fermentation, where alcohol and lactic acid fermentation are the most well known. In fermentation the 2 molecules of NADH from glycolysis are used for the reduction of pyruvate to either alcohol or lactic acid. Fermentation occurs in the presence of microorganisms like yeasts ( e.g. Saccharomyces cerevisiae) or bacteria (e.g. Lactobacillus acidophilus). As the fermentation reactions only use the 2 NADH's from glycolysis and no ATP, the 2 ATP's from glycolysis are left over. This makes sense, because at least a small amount of ATP is generated by fermentation. If no ATP would be generated the whole process of fermenting glucose wouldn't be of any advantage to the fermenting organism.
So the answer is: 2 molecules of ATP are produced during the fermentation of 1 molecule of glucose.
