Oxygen is necessary for cellular respiration (energy is obtained from glucose). Energy is needed for life-sustaining processes such as growth, repair of tissues, movement, reproduction and excretion. Even though glucose is high in energy, it must be converted into a form which can be used by living cells. This process involves oxygen combining with the glucose, through a series of enzyme-controlled reactions, through which chemical energy is released as ATP. This is known as the oxidation of glucose.
Carbon dioxide is produced in all living cells as a waste product of chemical respiration. It must be removed from the cells, to prevent a change in the pH of cells, the bloodstream and the body. When carbon dioxide reacts with water, carbonic acid is produced. If carbonic acid is built up it is toxic and it can change the pH of cells and bloodstream – thus affecting the homeostatic balance within an organism. A low ph reduces enzyme efficiency, which affects cell functioning – hence the removal of carbon dioxide is essential as it affects the functioning of enzymes.