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why do cellular respiration occur in cells only
Cellular respiration refers to the biochemical pathway by which cells release energy from the chemical bonds of food molecules and provide that energy for the essential processes of life. All living cells must carry out cellular respiration. It can be aerobic respiration in the presence of oxygen or anaerobic respiration. Prokaryotic cells carry out cellular respiration within the cytoplasm or on the inner surfaces of the cells. More emphasis here will be placed on eukaryotic cells where the mitochondria are the site of most of the reactions. The energy currency of these cells is ATP, and one way to view the outcome of cellular respiration is as a production process for ATP.
     Cellular respiration allows organisms to use (release) energy stored in the chemical bonds of glucose (C6H12O6). The energy in glucose is used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a process in which the energy in glucose is transferred to ATP. In respiration, glucose is oxidized and thus releases energy. Oxygen is reduced to form water.    The carbon atoms of the sugar molecule are released as carbon dioxide (CO2).
    The complete breakdown of glucose to carbon dioxide and water requires two major steps: 1) glycolysis and 2) aerobic respiration.  Glycolysis produces two ATP. Thirty-four more ATP are produced by aerobic pathways if oxygen is present.
    In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP.


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