How do organisms generate ATP in the absence of oxygen?

Organisms generate ATP in the absence of oxygen through anaerobic pathways. This process allows them to produce energy without relying on oxygen as the terminal electron acceptor in cellular respiration. Anaerobic pathways, such as fermentation, enable organisms to convert glucose into ATP along with byproducts like lactic acid or ethanol, depending on the organism type.

In contrast, aerobic respiration, which involves oxygen and is more efficient at producing ATP, and oxidative phosphorylation, which depends on the electron transport chain functioning in the presence of oxygen, do not apply in anaerobic conditions. Thus, anaerobic pathways provide an essential alternative for energy production when oxygen is limited or unavailable.