Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt. Human muscle cells behave as facultative anaerobes. They make ATP by glycolysis and lactic acid fermentation when O 2 is scarce such as during the early stages of strenuous exercise. Lactic acid accumulates as a waste productand may cause muscle fatigue. The lactic acid is gradually carried away by the blood to the liver.
Lactic acid is converted back to pyruvic acid by liver cells.
That process requires oxygen, thus fermentation in muscle cells results in an oxygen debt that is paid back when you continue to breathe heavily after you have stopped exercising. The primary organic compounds of cells make up a metabolic pool. These molecules can also be used for biosynthesis. Check out your understanding of respiration by trying the following quizz. Simply memorizing the answers to these questions will not help you.