The gas that makes up about 21% of the Earth's atmosphere is oxygen (O₂). It is a vital component for the survival of most life forms on Earth, as it is required for cellular respiration, a process through which organisms generate energy. Oxygen is produced primarily through photosynthesis by plants and phytoplankton, which convert carbon dioxide and sunlight into glucose and oxygen. The presence of oxygen in the atmosphere is crucial not just for living organisms, but also for maintaining atmospheric stability and supporting various ecological processes.