When a cell lacks adequate oxygen for cellular respiration, it resorts to fermentation as an alternative method of generating ATP. Fermentation allows the cell to continue producing energy (ATP) in the absence of oxygen, although it is a less efficient process compared to cellular respiration.

In fermentation, glucose is partially broken down without the use of oxygen. The end products of fermentation vary depending on the specific type of fermentation and the organism involved. Some common examples include:

* Lactic acid fermentation: This type of fermentation produces lactic acid as the major product. It occurs in some bacterial species and muscle cells when oxygen is limited. Lactic acid fermentation is responsible for the production of yogurt, cheese, and other fermented dairy products.

* Alcoholic fermentation: This type of fermentation produces ethanol (alcohol) and carbon dioxide as the major products. It occurs in yeast and certain bacterial species. Alcoholic fermentation is used in the production of alcoholic beverages, such as beer, wine, and spirits.

Fermentation serves as a backup mechanism for cells when oxygen is scarce, ensuring that they can still generate ATP and maintain cellular functions. However, fermentation is less efficient than cellular respiration, yielding only a small amount of ATP compared to the complete breakdown of glucose through cellular respiration.