Which of the following is NOT a stage of cellular respiration?

Fermentation is not classified as a stage of cellular respiration; rather, it is a metabolic process that occurs in the absence of oxygen and serves as an alternative energy-releasing pathway for organisms. Cellular respiration encompasses three main stages: glycolysis, the citric acid cycle, and the electron transport chain.

Glycolysis is the initial stage, where glucose is broken down into pyruvate, producing a small amount of ATP and NADH. The citric acid cycle further processes pyruvate to generate additional electron carriers, which are then used in the electron transport chain to produce a significant amount of ATP through oxidative phosphorylation. Fermentation, in contrast, is a process that follows glycolysis when oxygen is not available, allowing for ATP production through anaerobic means. Thus, while fermentation can provide energy under specific conditions, it does not form part of the core stages of cellular respiration.