Organisms that use aerobic cell respiration need oxygen to live and produce carbon dioxide as a waste product; this is usually done by breathing mostly.
Breathing is the process that supplies oxygen to the body and remove carbon dioxide that is produced.
There are 2 forms of respiration; Aerobic and Anaerobic.
Aerobic respiration requires oxygen and is used by most organisms as it produces more oxygen than anaerobic respiration, giving the organisms the advantage.
In aerobic respiration glucose reacts with oxygen in the mitochondria of the cells to release energy. Carbon dioxide and water are by-products of the reaction.
Glucose + Oxygen → Carbon Dioxide + Water + Energy
C6H12O6 + 6O2 → 6CO2 + 6H2O + 2900kJ
Anaerobic respiration does not require oxygen, and is used by some organisms that can only respire anaerobically, such as bacteria. Some organisms can turn to anaerobic respiration when there is no oxygen available, like yeast and some plant tissues like water-logged roots in soil.
In anaerobic respiration the glucose is only partially broken down, and lactic acid is produced - together with a much smaller amount of energy.
Glucose → Lactic Acid + Energy
C6H12O6 → 2C3H6O3 + 120kJ
When anaerobic respiration occurs in yeast it is called fermentation. In this case ethanol (alcohol) is produced instead of lactic acid, and this reaction is used in the brewing of alcoholic drinks.
Glucose → ethanol + carbon dioxide + energy
Oxygen debt
During vigorous exercise the body needs a lot more energy. It gets this by breathing deeper and faster and rushing the oxygen to the muscles in dilated blood vessels. This extra oxygen is then used to release more energy, needed to meet the higher level of demand. Soon a point is reached when the body cannot breathe any faster or harder, and aerobic respiration alone cannot meet the enhanced energy demands. So muscle cells get the extra energy they need by respiring anaerobically.
Anaerobic respiration produces lactic acid, which accumulates in the muscles and causes muscle fatigue and cramps. To avoid damage to the cells, lactic acid is broken down to carbon dioxide and water immediately after the exercise has finished. This is an oxidisation reaction, and requires oxygen.
This extra oxygen needed to neutralise the harmful effects of anaerobic respiration is called an oxygen debt. In order to get the extra oxygen to 'pay back' the debt, the body continues to breathe deeply for some time after vigorous activity has ceased. When all the lactic acid in the muscles is broken down the oxygen debt has been repaid and normal aerobic respiration resumes.