Answer is: carbon dioxide and water are released during cellular respiration.
Chemical reaction of cellular respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
There are 18 oxygen atoms (six in glucose and twelve in six molecules of oxygen) in left side of chemical reaction and also 18 oxygen atoms (twelve in six molecules of carbon dioxide and six in six molecules of water) at the right.
