Answer:
Fermentation takes over the krebs cycle in anerobiotic conditions. The lack of oxygen prevents oxidative phosphorylation and the formation of new oxidized compounds which can only be reformed through the reduction of pyruvate to lactate: through this process glycolysis can continue to function and we can have the production of ATP. The problem is that only 2 ATPs are formed: this is not enough to sustain the life of the cell so, over a period of time that varies from cell to cell (for example the myocyte survives for 40 min, nerve cells fastly die in ischemia), it will go to death if no new oxygen arrives.