Photosynthesis and cellular respiration work together as a team in a never-ending cycle, together they recycle oxygen and other molecules allowing us to breathe. Both metabolic processes recycle oxygen in a very similar way. In Cellular Respiration we have the equation C6H12O6 + 6O2 Ã 6 CO2 + 6 H2O + Energy (Heat + ATP), showing that the exhaust is carbon dioxide and water, and that oxygen is the reactant. In Cellular respiration oxygen is used up to make carbon dioxide, oxygen undergoes several steps to be converted into carbon dioxide, which will later be used by photosynthesis. In the first step where oxygen is found is in the Electron Transport Chain (ETC). Here electron carriers that come from the Krebs cycle and glycolysis, usually NADH and FADH2, carry electrons down a cascade of electron acceptors that gradually take these electrons away. The carrier is pulled down the chain thanks to oxygen that is very electronegative and thus it demands more electrons then the other acceptors, making oxygen the final electron acceptor. Oxygen's high electronegativity allows it to be an acceptor. The Electron Transport Chain doesn't make ATP directly, its main function is to ease electrons from food to oxygen, breaking a large free energy drop into smaller steps that release energy, the process also make water as a byproduct. Cellular respiration ultimately makes carbon dioxide, which is accepted by plants and undergoes photosynthesis.
In photosynthesis the reaction is like that of cellular respiration but backwards, 6 CO2 + 12 H20 + Light energy à C6H12O6 + 6 O2 + 6 H2O. In the reaction oxygen is created but as a byproduct, glucose is the main product. One general misconception about this reaction is that oxygen comes from water and not from carbon dioxide. In photosynthesis oxygen is only created in the light reaction, which is the first part of the metabolic process in the second part, dark reaction, glucose is given off.