The primary gas in Earth's atmosphere responsible for breathing is oxygen (O2). Making up approximately 21% of the atmosphere, oxygen is essential for life as it is required by most living organisms for cellular respiration. During this process, oxygen is used to convert glucose and other nutrients into energy, producing carbon dioxide and water as byproducts. Plants contribute to the oxygen supply through photosynthesis, where they utilize sunlight to convert carbon dioxide and water into glucose and oxygen. The balance between oxygen production and consumption is vital for maintaining the health of ecosystems and supporting animal life, including humans.