---> Fermentation/Glycolysis
In the absence of oxygen, cells can undergo two sets of anaerobic metabolic pathways to produce energy: glycolysis and fermentation.
Glycolysis: Glycolysis is the first step in cellular respiration. It is an anaerobic process that takes place in the cytoplasm of the cell. During glycolysis, glucose is broken down into two molecules of pyruvate, which yields a net of two ATP molecules and two NADH molecules.
Fermentation: Fermentation is an anaerobic process that takes place when oxygen is not available for cellular respiration. In fermentation, pyruvate is converted into either lactic acid or ethanol, depending on the type of organism. This process regenerates the NAD+ required for glycolysis to continue, allowing for the continued production of ATP through glycolysis.
Overall, these two pathways allow cells to continue producing ATP even in the absence of oxygen. However, they are less efficient than aerobic respiration, which produces much more ATP per glucose molecule through the use of oxygen in the electron transport chain.