Which process of ATP synthesis occurs without oxygen and produces lactic acid?

The process of ATP synthesis that occurs without oxygen and produces lactic acid is anaerobic fermentation. During anaerobic fermentation, glucose is partially broken down to generate ATP in the absence of oxygen. This process results in the conversion of pyruvate, a product of glycolysis, into lactic acid.

Lactic acid fermentation primarily occurs in muscle cells when oxygen levels are low, such as during intense exercise. In this situation, the muscle cells rely on anaerobic metabolism to quickly generate ATP to sustain energy needs. While this process is less efficient than aerobic processes in terms of ATP yield, it allows for rapid energy production in situations where oxygen is scarce.

In contrast, aerobic respiration involves the complete oxidation of glucose in the presence of oxygen, producing carbon dioxide and water along with a much higher yield of ATP. Oxidative phosphorylation is a stage of aerobic respiration where ATP is produced using energy derived from electron transport and chemiosmosis. Cellular respiration encompasses both aerobic and anaerobic pathways, but specifically in the context of the question, it would not center only on the anaerobic production of lactic acid. Thus, anaerobic fermentation is the key process that matches the criteria of the question, leading to the production of lactic acid without oxygen.