Which type of metabolism occurs in the absence of oxygen, producing mainly lactic acid?

The correct answer is anaerobic metabolism, which refers to the process of generating energy in the absence of oxygen. During anaerobic metabolism, glucose is broken down to produce energy, primarily resulting in the formation of lactic acid as a byproduct. This type of metabolism is crucial in situations where oxygen supply is limited, such as during intense exercise, allowing cells to continue producing energy even when aerobic pathways are not available.

Lactic acid accumulation is often responsible for the muscle fatigue experienced during vigorous activity, demonstrating the body's reliance on anaerobic processes when there's insufficient oxygen for aerobic respiration.

Aerobic metabolism, in contrast, occurs in the presence of oxygen and primarily produces carbon dioxide and water as byproducts, along with a higher yield of energy compared to anaerobic processes. Fermentation refers to various metabolic processes that also occur in the absence of oxygen but can produce other byproducts like ethanol, depending on the organism, and is generally considered a broader category that includes anaerobic pathways. Cellular respiration encompasses both aerobic and anaerobic processes but typically refers to the complete breakdown of glucose with oxygen, highlighting that anaerobic metabolism is just a subset of the overall cellular respiration process.