The carbon dioxide breathed out is a by-product of the process of cell respiration, as is water. In this process, energy is produced in the mitochondria of cells. This energy is our physical "life force". Both oxygen and glucose are required for this.
 
        
             
        
        
        
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Combustion is the burning of a substance. It could be complete or could be incomplete. It changes the form of carbon and so it plays it's role in cycle
        
             
        
        
        
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The ozone layer protects the Earth against most UVB coming from the sun. It is always important to protect oneself against UVB, even in the absence of ozone depletion, by wearing hats, sunglasses, and sunscreen. However, these precautions will become more important as ozone depletion worsens.
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