Therefore:

• When ammonium is added to cells grown on nitrate and then starved (with or without CM), it can be taken up, but not by unstarved cells, since ammonium permease was never made.
  
  
• When nitrate is added to cells grown on ammonium and then starved (with or without CM), it can be utilized, but not by unstarved cells, since CM prevents synthesis of the reductase.