The element essential for breathing and represented by the symbol O is oxygen. Oxygen is a colorless and odorless gas that constitutes about 21% of the Earth's atmosphere. It is vital for the survival of most living organisms as it plays a crucial role in cellular respiration, a process by which cells convert energy from nutrients into adenosine triphosphate (ATP) in the presence of oxygen. This energy is then used for various biological functions. Oxygen also supports combustion and is necessary for the process of burning fuels.