I believe it's the theory that cells are the building blocks of life and without cells, there would be no life at all. Its also stated cells come from pre-existing cells and each organism has either one or more.
 
        
             
        
        
        
One of the most general forms of discomfort or pain one feels at the time of strenous work out is a burning sensation in the muscles or lungs, which goes away after some time, that is, after stopping the activity. This is a result of an accumulation of lactic acid. 
Lactic acid is a by-product of the procedure the body goes through when it requires to generate energy more briskly that it does usually, like when one exercises.
The muscles functioning generally produce energy aerobically, that is, by using oxygen, however, when one push himself or herself at the time of workout and enough oxygen is not accessible, then these muscles start producing energy anaerobically, resulting in production of lactic acid as a by-product and ultimately causing burning sensation. 
 
        
                    
             
        
        
        
Explanation:
the Krebs cycle occurs on the mitochondrial matrix and generates a pool of chemical energy from the oxidation of pyruvate, the end product of glycolysis. pyruvate is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA, a 2-carbon molecule