Umm, 1st law of thermodynamics….did you expect to see lesser oxygen atoms?

Yeast undergoes glycolysis just like every other organism on the planet. As you might now, glycolysis produces 2 molecules of ATP, 2 molecules of pyruvate and 2 molecules of NADH. Ok, if oxygen is present, then cellular respiration can go on with the Krebs cycle and oxidative phosphorilation. But the problem is when there is no oxygen present in the medium. the yeast can survive with only 2 molecules of ATP/glucose. But in order for glucose to keep running, there must be free NAD+ atoms to keep accepting electrons in phase 6 of glycolysis. This means that the NADH atoms somehow need to be recycled. The cell does this by the process of fermentation. How? Well, firstly the molecules of pyruvate glycolysis produces are turned into CH3-CH=O(acetaldehyde) and CO2. Then the acetaldehyde molecules accept electrons from NADH and turn to ethanol. This process keeps glycolysis running by producing more NAD+ molecules, but unlike oxidative phosphorilation, produces no ATP during this recycling.