Which of the following is the first stage of cellular respiration?

The first stage of cellular respiration is glycolysis. This process occurs in the cytoplasm of the cell and does not require oxygen, making it an anaerobic process. During glycolysis, one glucose molecule is broken down into two molecules of pyruvate, generating a net gain of two molecules of ATP (the energy currency of the cell) and two molecules of NADH, which are important for the next stages of cellular respiration.

Glycolysis sets the stage for the subsequent processes—if oxygen is present, pyruvate will enter the Krebs cycle, followed by the electron transport chain, where the majority of ATP is produced. In contrast, fermentation occurs in anaerobic conditions and serves as an alternative pathway for producing energy when oxygen is not available, but it is not considered the first stage in the context of aerobic respiration. Understanding glycolysis as the initial step highlights its critical role in cellular respiration and energy production.