So in yeast cells, for example, they produce ATP and to continually produce the ATP the Pyruvate basically undergoes these two steps to produce ethanol as well as regenerate the NAD plus that can then go into our glycolysis cycle to produce even more ATP molecules. So this is how we actually regenerate and we reuse our NAD plus molecules. Now, let's move on to a more common cycle that takes place in human cells known as the lactic acid fermentation. So in certain cells, including human cells, bacterial cells as well as fungi cells when oxygen supply runs low, for example during some type of vigorous exercise too many Pyruvate molecules and too many NADH molecules are produced. And because not all pyruvates and NADH molecules can be placed through the citric acid cycle and the electron transport chain at the same exact time we have a build up of NADH molecules that takes place. And so we have a very little amount of NAD plus present in the cell and we need NAD plus for glycolysis to take place.