What happens to pyruvate in the absence of oxygen?

In the absence of oxygen, pyruvate undergoes a process called fermentation to be converted into lactic acid. This is particularly relevant in anaerobic conditions where organisms, such as certain bacteria and animal cells (like muscle cells), require a way to regenerate NAD+ for glycolysis to continue producing ATP. When oxygen is not present, the electron transport chain cannot function, so cells switch to fermentation pathways.

During this conversion, each pyruvate molecule is transformed into lactic acid, which allows the continued production of ATP through glycolysis despite the lack of oxygen. This process is why lactic acid can build up in muscles during intense exercise, resulting in the familiar feeling of muscle fatigue. This pathway provides a quick source of energy but is less efficient compared to aerobic respiration which uses oxygen and involves the Krebs cycle and oxidative phosphorylation for higher ATP yield.

The other options represent pathways that depend on the presence of oxygen, as pyruvate typically enters the Krebs cycle and is transformed into acetyl CoA when oxygen is available, and pyruvate does not get stored as glycogen or converted to lactic acid in all scenarios. The biochemical pathways are designed to adapt to the availability of oxygen, with lactic acid fermentation being a direct and efficient