The tongue's tip is mostly sensitive to what is sweet and salty. Its sides favour what is sour. Taste is a chemical sense, depending on what substances do when broke down, and affected by saliva. Lose Na and the result will be a salt craving. 7.Bitter and sweet substances do not ionize.
Sweetness likely is related to sugar content.
Bitterness may be related to many poisons, and deters us. It often has strong motivational effects. Rotten-meat smell is an emetic. Some scents attract. Menstrual cycles can be influenced by smell.