Respiration is an exothermic reaction (releases energy) that supplies all the energy needed by living organisms. This reaction happens continuously in all living cells. The reaction can be aerobic (with oxygen) or anaerobic (without oxygen).
In aerobic respiration, glucose reacts with oxygen in the mitochondria of cells to give carbon dioxide, water and energy.
In plant and yeast cells, glucose is converted into ethanol and carbon dioxide.
In yeast cells, this is called fermentation. This is an important step in the manufacture (making) of both bread and alcohol.
Glucose is not broken down completely, making it a less efficient way of transferring energy than aerobic respiration.
In animals, glucose is converted to lactic acid:
Anaerobic respiration happens when insufficient (not enough) oxygen reaches the muscles during periods of intense activity.
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