Sentence:  A few types of molecules get sensed by receptors on the tongue. Protons coming off of acids ping receptors for "<WSD>sour</WSD>." Sugars get received as "sweet." Bitter, salty, and the proteinaceous flavor umami all set off their own neural cascades. 
Meaning of the word marked with <WSD> tags:  Having an acidic, sharp or tangy taste.