Living on land means reptiles can't rely on absorbing oxygen through their skin like amphibians. All reptiles have lungs they use for breathing -- even those who live most of their lives near or in water, such as crocodiles, must surface to breathe. Lungs allow reptiles to venture far away from aquatic environments.
        
                    
             
        
        
        
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Key molecules that contain carbon include proteins, nucleic acids, carbohydrates and lipids. Carbon is an integral part of many biological processes, including reproduction, photosynthesis and respiration. We often assume that life in other parts of the universe, if we ever find it, will be carbon-based.
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