Answer:
Fermentation replaces the Krebs cycle when there is inadequate oxygen and lasts for a much shorter duration, which depends on the individual.
Explanation:
The best way to describe this is when someone is running. While they are exerting their maximum effort, they breathe very heavily to make up for a lack of oxygen. The muscles they are using take up more oxygen than the lungs can obtain, so the cells switch over from the more efficient Krebs Cycle as a last-ditch effort to provide the energy that individual needs.
This is a very inefficient way of producing ATP because it only yields 2 ATP per glucose molecule, as opposed to roughly 26 ATP through the Krebs Cycle.
One common misconception here is that lactic acid buildup occurs during this process, and while that is partly true, the lactic acid would suffocate the cells and lead to death if lactic acid were to build up in the way people believe.
As for how long Fermentation can occur, it depends on the fitness of the individual, as the Fermentation process would likely only last for a few seconds for an out-of-shape individual and over an hour in a long-distance runner.
It also depends on whether or not the person has trained their muscle for endurance or short bursts of strength. A sprinter cannot use Fermentation for long but a marathoner may use Fermentation for the last hour of their run.
