More complete is an open system, namely, in its simplest form,

  
    
      
        R
        
          
            →
            
          
        
        A
        
          
            →
            
               
               
              p
              (
              S
              )
               
               
            
          
        
        B
        
          
            →
            
               
               
              q
               
               
            
          
        
        ,
      
    
    {\displaystyle R{\xrightarrow {}}A{\xrightarrow {\ \ p(S)\ \ }}B{\xrightarrow {\ \ q\ \ }},}
  
where R stands for the rate of production of A, p(S) is the activation rate coefficient explicitly expressing its dependence on the stimulus intensity S and q represents the rate coefficient of removal from the state B.
The following schematic represents the activity:

  
    
      
        A
        
          
            →
            
               
               
              p
               
               
            
          
        
        B
      
    
    {\displaystyle A{\xrightarrow {\ \ p\ \ }}B}
  
where p is the activation rate coefficient. It is customary that p is called a rate constant, but, since the p stands for measure of the intensity of the stimulus causing the activation, p may be variable (non-constant).