Final answer:
During intense exercise, the switch from aerobic respiration to lactic acid fermentation in muscle cells is caused by a decrease in oxygen availability, leading to a decrease in ATP production through aerobic respiration and an increase in lactic acid production.
Step-by-step explanation:
In intense exercise, the cells in a muscle switch from aerobic respiration to lactic acid fermentation due to a decrease in oxygen availability to the muscle cells. When muscle cells don't get enough oxygen, they cannot produce a lot of ATP through aerobic respiration. Instead, they produce lactic acid as a by-product of the small amount of ATP they make in the absence of oxygen.
This switch to lactic acid fermentation allows the recycling of the enzyme NADH to NAD+, which is needed for glycolysis, and enables the continuation of glycolysis to generate energy in the form of ATP. While lactic acid fermentation provides a quick burst of high-intensity energy during intense exercise, it is not as efficient as aerobic respiration, resulting in the accumulation of lactic acid and contributing to muscle fatigue.