Now what about lactic acid fermentation? So lactic acid fermentation is used by many prokaryotic cells so bacterial cells that we're going to discuss in just a moment as well as many eukaryotic cells. In fact, the cells of our body also use under some circumstances, lactic acid fermentation. For instance, when we're exercising strenuously the skeletal muscles at some point will not be able to get enough oxygen and at that moment in time they have to switch to the process of lactic acid fermentation because that allows them to continually generate those ATP molecules. And so that's exactly what allows the buildup of lactate. So lactic acid in our body and that eventually causes fatigue.