Figure 9.4 Energy-Yielding Metabolic Pathways Energy-yielding reactions can be grouped into five metabolic pathways: glycolysis, pyruvate oxidation, the citric acid cycle, the respiratory chain/ATP synthesis, and fermentation. (A) The three lower pathways occur only in the presence of O₂ and are collectively referred to as cellular respiration. (B) When O₂ is unavailable, glycolysis is followed by fermentation.