What processes are involved in anaerobic cellular respiration?

Anaerobic cellular respiration occurs in the absence of oxygen and involves processes that allow cells to generate energy without relying on aerobic pathways. Glycolysis is the first step in both aerobic and anaerobic respiration; it breaks down glucose into pyruvate, producing a small amount of ATP in the process.

In the absence of oxygen, pyruvate does not enter the Krebs Cycle and the Electron Transport Chain, both of which require oxygen to function efficiently. Instead, the process shifts to fermentation, which allows for the regeneration of NAD+, an essential coenzyme that must be available for glycolysis to continue. During fermentation, pyruvate from glycolysis is converted to either lactic acid (in animals) or ethanol and carbon dioxide (in yeast), depending on the organism. This way, cells can maintain their energy production under anaerobic conditions by continuously supplying ATP through glycolysis and recycling NAD+ via fermentation.

In contrast, the Krebs Cycle, which is a part of aerobic respiration, and the Electron Transport Chain require oxygen and do not play a role in anaerobic processes. Therefore, the combination of glycolysis and fermentation is what defines anaerobic cellular respiration, making this answer the most appropriate choice.