Fermentation is the process that cells use when they do not have enough oxygen for respiration. Fermentation is an anaerobic process, meaning that it does not require oxygen. During fermentation, glucose is broken down into smaller molecules, such as ethanol or lactate, and energy is released in the form of ATP.

There are two main types of fermentation:

* Alcoholic fermentation is the process by which yeast converts glucose into ethanol and carbon dioxide. This process is used to make wine, beer, and other alcoholic beverages.

* Lactic acid fermentation is the process by which bacteria convert glucose into lactic acid. This process is used to make yogurt, cheese, and other fermented dairy products.

Fermentation is an important process for cells because it allows them to continue to produce energy even when oxygen is not available. This process is essential for the survival of many organisms, including yeast, bacteria, and some plant cells.