Final answer:
During exercise, when oxygen is being used faster than it can be replaced, muscle cells in animals undergo fermentation to provide energy. Fermentation is an important way of producing ATP without oxygen and occurs in the absence of the other stages of aerobic respiration.
Step-by-step explanation:
When oxygen is being used faster than it can be replaced in muscle cells of animals, such as during exercise, fermentation takes place to provide cells with energy. Fermentation is an important way of making ATP without oxygen and involves glycolysis, but not the other two stages of aerobic respiration. Human muscle cells use fermentation when they cannot get enough oxygen to meet their energy needs through aerobic respiration.