What does respiration usually Begin?

Respiration usually begins with the process of glycolysis, where glucose is broken down in the cytoplasm of the cell to produce pyruvate, ATP, and NADH. This process does not require oxygen and is the first step in both aerobic and anaerobic respiration. In aerobic respiration, pyruvate then enters the mitochondria, where it undergoes the Krebs cycle and oxidative phosphorylation, producing additional ATP. In contrast, anaerobic respiration leads to fermentation, which occurs in the absence of oxygen.