First, we will address the fact that yeast can undergo both aerobic and anaerobic respiration.
It can undergo aerobic respiration because they have mitochondria! Therefore, a molecule of glucose will be broken down into pyruvate through glycolysis. The aerobic part of respiration occurs when this pyruvate then is moved into the citric acid cycle, where oxygen will be used to help create energy (the citric acid cycle).
Yeast cells can also undergo anaerobic respiration. The same beginning step of glycolysis is required just as it is in aerobic respiration. In conditions of low oxygen, the yeast cell will no longer undergo aerobic respiration since the lack of oxygen will make oxidative phosphorylation non-functional.
Instead, the pyruvate generated after glycolysis is done will undergo fermentation, where ethanol and carbon dioxide are the end product. The sole purpose of this is to regenerate NAD+ to continue glycolysis.
A key note here is that in anaerobic respiration, glycolysis is the only pathway producing ATP. In aerobic respiration, BOTH glycolysis and the citric acid cycle produce ATP but the citric acid cycle produces a lot more.
