Which metabolic pathway occurs in the absence of oxygen?

The correct answer is fermentation, which is a metabolic pathway that takes place in the absence of oxygen. During fermentation, cells can convert sugars into acids, gases, or alcohol, allowing them to generate energy without relying on oxygen for their metabolic processes. This process is particularly important for organisms that live in oxygen-poor environments and for muscle cells in certain conditions.

Fermentation begins with glycolysis, where glucose is broken down into pyruvate, yielding a small amount of ATP. In the absence of oxygen, the pyruvate is then converted into different end products, such as lactic acid in animals or ethanol and carbon dioxide in yeast, allowing for continuous ATP production even without aerobic respiration.

In contrast, aerobic respiration, the Krebs cycle, and oxidative phosphorylation all require oxygen to function. Aerobic respiration fully oxidizes glucose in the presence of oxygen, producing a higher yield of ATP compared to fermentation. The Krebs cycle is a key part of aerobic respiration, where acetyl-CoA is oxidized to produce electron carriers, and oxidative phosphorylation refers to the final stage of aerobic respiration where ATP is generated through the electron transport chain, also requiring oxygen.

Thus, fermentation is a unique metabolic pathway that enables cells to survive and produce energy under anaerobic conditions.