What is the primary substance produced in muscle cells during anaerobic respiration?

During anaerobic respiration, which occurs in muscle cells when oxygen levels are low, the primary substance produced is lactic acid. This process allows cells to generate energy without oxygen by breaking down glucose. When oxygen is not available, the conversion of glucose does not proceed through the usual aerobic path that results in carbon dioxide and water; instead, it leads to the formation of lactic acid.

Lactic acid accumulates in the muscles and is responsible for the feeling of fatigue and soreness often associated with intense exercise. The production of lactic acid reflects the body’s need for rapid energy in situations when the oxygen supply cannot meet the demand, such as during vigorous physical activity.

Other substances listed, such as carbon dioxide, glucose, and oxygen, do not align with the primary outputs of anaerobic respiration. Carbon dioxide is typically produced during aerobic respiration, glucose is the starting substrate, and oxygen is not a byproduct of this anaerobic process but rather a reactant in aerobic respiration.