Which process is primarily associated with ATP generation in the absence of oxygen?

The process primarily associated with ATP generation in the absence of oxygen is glycolysis. Glycolysis is a metabolic pathway that occurs in the cytoplasm of the cell, where one molecule of glucose is broken down into two molecules of pyruvate. This pathway does not require oxygen and can occur whether oxygen is present or not, making it anaerobic in nature.

Throughout glycolysis, a net gain of two ATP molecules is produced from one molecule of glucose, through substrate-level phosphorylation. Additionally, glycolysis generates NADH, which can be used in anaerobic fermentation processes, such as lactic acid fermentation or alcoholic fermentation, to regenerate NAD+, allowing glycolysis to continue in the absence of oxygen.

In contrast, processes such as the Krebs cycle, the electron transport chain, and oxidative phosphorylation rely on oxygen as the final electron acceptor to produce ATP efficiently. These pathways are part of aerobic respiration and do not work under anaerobic conditions, making them unsuitable for ATP generation when oxygen is absent. Thus, glycolysis stands out as the main process for ATP production when oxygen is unavailable.