The chemical element crucial for respiration in living organisms is oxygen (O). Oxygen is essential for aerobic respiration, a process that allows cells to produce energy by converting glucose and oxygen into carbon dioxide, water, and adenosine triphosphate (ATP), which cells use for various functions. Most multicellular organisms, including humans, rely on oxygen taken in through respiration to support metabolic processes. The presence of oxygen in the atmosphere is also vital for sustaining life, influencing ecosystems and the behavior of various species.