Final answer:
Oxygen is essential for aerobic respiration, allowing for the complete oxidation of nutrients to produce a substantial amount of ATP. The efficiency of this process far exceeds that of anaerobic respiration, which occurs in the absence of oxygen.
Step-by-step explanation:
The presence of oxygen is required for aerobic respiration. This process relies on oxygen to act as the final electron acceptor in the electron transport chain. Aerobic respiration is essential for the efficient production of ATP, as it allows for the complete breakdown of glucose or other nutrients into carbon dioxide, water, and a significant amount of ATP. This type of respiration allows living organisms, including humans, to extract approximately 95 percent of the ATP needed for resting or moderately active muscles, taking place within the mitochondria.
Without oxygen, organisms would have to rely on anaerobic respiration or fermentation, which are far less efficient in terms of ATP production. Geological evidence suggests that early Earth lacked molecular oxygen, and early life forms used anaerobic respiration. Eventually, the evolution of oxygenic photosynthesis by cyanobacteria led to the release of oxygen as a byproduct, paving the way for aerobic respiration.