What process occurs after glycolysis in the absence of oxygen?

After glycolysis, when oxygen is not available, the process that occurs is fermentation. Glycolysis, which takes place in the cytoplasm, converts glucose into pyruvate and produces a small amount of ATP. Under aerobic conditions (with oxygen), pyruvate is further oxidized in the mitochondria through aerobic respiration, leading to the Krebs cycle and oxidative phosphorylation.

However, in anaerobic conditions (absence of oxygen), cells cannot perform aerobic respiration. Instead, they undergo fermentation to regenerate the electron carriers (NAD+) required to continue glycolysis. Fermentation allows the cell to ensure that ATP production can continue, albeit at a much lower yield per glucose molecule compared to aerobic respiration. There are two main types of fermentation: lactic acid fermentation (common in animal cells) and alcoholic fermentation (common in yeast and some bacteria).

Options that involve processes requiring oxygen, such as aerobic respiration, oxidative phosphorylation, or the electron transport chain, are not applicable in this context since they depend on the presence of oxygen to function effectively. Hence, fermentation is the appropriate answer to indicate the anaerobic process that follows glycolysis when oxygen is absent.