What type of respiration occurs in the absence of oxygen?

Fermentation is a type of respiration that occurs in the absence of oxygen. During fermentation, cells convert sugars into acids, gases, or alcohol in order to generate energy. This process allows for ATP production when the aerobic pathway (which requires oxygen) is not available.

When oxygen is scarce, organisms may rely on fermentation to sustain their energy needs. For example, yeast undergoes alcoholic fermentation and produces ethanol and carbon dioxide, while some bacteria may undergo lactic acid fermentation, resulting in lactic acid as a byproduct. Overall, fermentation is crucial for maintaining energy production under anaerobic conditions.

The other processes listed—such as aerobic respiration, oxidative phosphorylation, and substrate-level phosphorylation—either require oxygen or are part of pathways that generally depend on an aerobic environment for maximum efficiency in energy production.