Oxygen is the essential element for cellular respiration in humans. During this process, oxygen molecules are taken in and used to convert glucose into adenosine triphosphate (ATP), which is the energy currency of cells. Cellular respiration occurs mainly in the mitochondria and involves various biochemical pathways, including glycolysis, the Krebs cycle, and the electron transport chain. The presence of oxygen allows for aerobic respiration, which is more efficient than anaerobic processes, yielding significantly more ATP per glucose molecule. This is crucial for sustaining the high energy demands of human tissues and organ functions.