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. The end product is a molecule of lactic acid. 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.