Complex sentence: In the presence of O 2, NADH and pyruvate are used in respiration; this is oxidative phosphorylation, it generates a lot more ATP in addition to that created by glycolysis, and for that reason cells generally benefit from avoiding fermentation when oxygen is available.
Split into simpler sentences: In the presence of O 2, NADH and pyruvate are used to generate ATP in respiration. For that reason, cells generally benefit from avoiding fermentation when oxygen is available.