“FIVE BASIC TASTES are recognized today: salty, sweet, bitter, sour and umami. Salty and sour taste sensations are both detected through ion channels. Sweet and bitter and umami, however, are detected by way of G protein-coupled taste receptors.” Taste modifiers exist, including miraculin and curculin for sweet and sterubin for bitter masking.
http://en.m.wikipedia.org/wiki/Taste_receptor