When oxygen is not available to serve as the final electron acceptor, the electron transport system is unable to function. Electrons are not passed down the cytochrome system, and both FADH2 and NADH are unable to release electrons. If the carrier molecules cannot be returned to their oxidized state, they are unable to accept new electrons. The entire aerobic pathway will shut down.
The cell does have an alternative system which will allow glycolysis to continue despite the lack of oxygen. Anaerobic fermentation will remove hydrogens and electrons from NADH and will remove the end product pyruvate. Together, these actions will allow glycolysis to continue.
Two different anaerobic fermentation pathways are known. Alcoholic fermentation is common in bacteria and yeast cells. In alcoholic fermentation, pyruvate is first decarboxylated to yield a 2-carbon substance acetaldehyde. Acetaldehyde is then reduced as hydrogens are transferred from NADH to acetaldehyde to produce ethyl alcohol.
Once the NAD has been oxidized, glycolysis can continue.
The same result is reached by animal cells through the process of lactic acid fermentation. Here pyruvate is used as the direct acceptor of the hydrogens removed from NADH. The end product is a molecule of lactic acid. Lactic acid [or lactate] is a common by-product of anaerobic respiration in muscle cells.
The steps of anaerobic fermentation do not themselves produce any additional ATP. Their sole value is that, by permitting the continuedglycolytic activity, they allow at least some energy to be recovered in the absence of oxygen.
Some bacteria are obligate anaerobes. They are entirely dependent on anaerobic pathways. Most other organisms are aerobic, but some organisms, and some tissues are able to function in a facultative anaerobic state. That is, they are able to manufacture enough ATP through anaerobic processes to sustain themselves when oxygen is not sufficiently available for aerobic events.