What process do animals undergo when oxygen availability is low during exercise?

When oxygen availability is low during exercise, animals primarily undergo lactic acid fermentation. This process allows for the production of energy (ATP) without the need for oxygen, which is vital during intense physical activity when oxygen supply to muscles cannot meet demand.

Lactic acid fermentation takes place in the cytoplasm of cells and begins with glycolysis, where glucose is converted into pyruvate. Under low oxygen conditions, the pyruvate is then converted into lactic acid, and this shift helps in regenerating NAD+, which is essential for glycolysis to continue producing ATP.

This energy production mechanism is crucial for short bursts of high-intensity activity, such as sprinting, where quick energy is required even when oxygen is not sufficiently available. The buildup of lactic acid can lead to muscle fatigue, but this process allows the animal to continue exercising in the absence of sufficient oxygen.

In contrast, aerobic respiration, which is typically the process used during lower intensity and longer duration activities when oxygen is available, primarily relies on the Krebs cycle, where further breakdown of pyruvate occurs. Alcohol fermentation is a process that occurs primarily in yeast and some plant cells, which is not applicable to animal physiology during exercise.