Free oxygen is oxygen that isn't combined with other elements such as carbon or nitrogen, and aerobic organisms need it to live. (sciencedaily.com)
This is a series of reactions in which aerobic organisms release energy. (aminoacidstudies.org)
Atmospheric carbon dioxide derives from multiple natural sources including volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms; man - made sources of carbon dioxide come mainly from the burning of various fossil fuels for power generation and transport use. (sciencedaily.com)