AP BIOLOGY

Detailed explanation-1: -When oxygen is not present or if an organism is not able to undergo aerobic respiration, pyruvate will undergo a process called fermentation. Fermentation does not require oxygen and is therefore anaerobic. Fermentation will replenish NAD+ from the NADH + H+ produced in glycolysis.

Detailed explanation-2: -Fermentation can occur in the absence and presence of oxygen both. In the presence of oxygen, it is called aerobic fermentation. It occurs at the beginning of the fermentation process and is an intense and quick phase.

Detailed explanation-3: -In the absence of oxygen, NAD+ may not be regenerated via the electron transport chain (which depends on oxygen as the terminal electron acceptor). Fermentation allows NAD+ to be regenerated in the absence of oxygen via lactic acid fermentation or alcohol fermentation.