The gas we breathe in is oxygen. Oxygen is essential for human survival, as it plays a critical role in cellular respiration, the process by which cells convert nutrients into energy. Earth’s atmosphere contains about 21% oxygen, a level that is maintained through photosynthesis performed by plants, algae, and certain bacteria. These organisms absorb carbon dioxide and release oxygen as a by-product, contributing significantly to the balance of gases in the atmosphere. Ensuring adequate oxygen levels is vital for the health of most living organisms, emphasizing the importance of both plant life and the Earth's ecosystems.