Humans breathe in oxygen to survive. This vital gas is a crucial component of the air, making up about 21% of the Earth's atmosphere. Oxygen is essential for cellular respiration, a process in which cells convert glucose and oxygen into energy, carbon dioxide, and water. This energy is necessary for all bodily functions, including movement, growth, and the maintenance of bodily systems. The oxygen we breathe is primarily generated by plants through photosynthesis, which converts carbon dioxide and sunlight into organic materials and releases oxygen as a byproduct. Maintaining a balance of oxygen in the atmosphere is vital for the survival of most life forms.