##### Anaerobic Respiration

What if oxygen is not present? If oxygen is not present, no electron acceptor exists to accept the electrons at the end of the electron transport chain. If this occurs, then NADH accumulates. After all the $$NAD^+$$ has been converted to NADH, the Krebs cycle and glycolysis both stop (both need $$NAD^+$$ to accept electrons). When this happens, no new ATP is produced, and the cell soon dies.

Anaerobic respiration is a method cells use to escape this fate. Two common metabolic pathways, alcohol and lactic acid fermentation, are slightly different, but the objective of both processes is to replenish $$NAD^+$$ so that glycolysis can proceed once again. Anaerobic respiration occurs in the cytosol alongside glycolysis.