Respiration releases the energy stored in glucose so that cells can use it. It takes place in living cells, and it is not the same thing as breathing.
Respiration is the breakdown of organic molecules, usually glucose, to release energy for life processes: movement, growth, repair, keeping warm and building new molecules.
Living cells carry out respiration to release energy for cellular processes. In eukaryotic cells, most of the reactions of aerobic respiration take place in the mitochondria. Breathing is the movement of air in and out of the lungs. Respiration is a chemical reaction inside cells.
Aerobic respiration uses oxygen:
glucose + oxygen → carbon dioxide + water, releasing energy.
It releases much more energy per glucose molecule than the anaerobic route, and it is the usual way cells release energy.
When there is not enough oxygen, cells can respire anaerobically.
In human muscle during hard exercise: glucose → lactic acid. Lactic acid builds up, which is associated with muscle fatigue, and it must later be removed. After exercise, extra oxygen is needed while the body does this. Some of the lactic acid is carried to the liver and converted back into glucose.
In yeast and some other microorganisms the process is fermentation: glucose → ethanol + carbon dioxide. This is used in brewing, for the ethanol, and in baking, where the carbon dioxide makes bread rise.
The reactants differ: aerobic respiration needs oxygen, anaerobic does not.
The products differ: aerobic gives carbon dioxide and water; anaerobic gives lactic acid in animals, or ethanol and carbon dioxide in yeast.
The energy released differs: aerobic respiration releases far more per molecule of glucose, because the glucose is broken down completely. Anaerobic respiration breaks it down only part of the way. In human muscles it cannot support intense activity for long, because it releases much less energy per glucose molecule and lactic acid accumulates.
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