Final answer:
Pyruvate is converted into lactic acid in the absence of adequate oxygen through anaerobic respiration, which allows muscles to continue producing ATP for short periods of intense activity.
Step-by-step explanation:
In the presence of a limited oxygen supply, pyruvate is converted into lactic acid through a process called anaerobic respiration or fermentation. This conversion is crucial as it allows the recycling of NADH back into NAD+, which is necessary for glycolysis to continue producing ATP. This anaerobic pathway is significant when oxygen delivery to the muscle cells can't keep up with demand, such as during intense exercise, allowing for short-term ATP production.
The lactic acid produced is then transported from the muscle to the liver where, under oxygen-rich conditions, it can be converted back into pyruvate and eventually glucose through the Cori cycle. However, accumulation of lactic acid can contribute to muscle fatigue. Anaerobic respiration is a quick, albeit inefficient, method to produce ATP when oxygen is scarce.