The reduction of oxygen to form water occurs during the electron transport system of cellular respiration. Cellular respiration that requires oxygen is called aerobic respiration. It is the mechanism by which glucose is transformed into adenosine triphosphate (ATP), which is then used by cells for energy.
The chemical formula for cellular respiration is: C6H12O6 + 6O2 = 6CO2 + 6H2O + 36 ATP.
Glucose is broken down into two pyruvate molecules in a process known as glycolysis, which occurs in a cell's cytoplasm. From there, the pyruvate molecules move into the mitochondrion and enter aerobic respiration. During the process, electrons are combined with hydrogen ions to reduce oxygen and form H2O.