Taste reception occurs at the apical tip of taste cells that form taste buds. Each onion shaped taste bud has is composed of 50–100 taste cells that possess microvilli. Each single taste bud contains 50–100 taste cells representing all 5 taste sensations. Embedded in the cell membranes of these taste cells are receptor proteins. Each of the taste receptors are transmembrane proteins which function either by physically binding to a flavor ingredient (sweet, bitter and umami) or by acting as a channel to allow ions to flow directly into a taste cell (salty and sour). This interaction triggers a signaling cascade that culminates with signals to the brain through a network of taste nerve fibers. 

In the case of sweet, umami and bitter tasting it is the size and shape and the chemical groups on a tastant molecule which determine any taste it will have as they will determine its ability to dock onto one or more of the different receptor proteins (See schematic or full reference). A single taste cell seems to be restricted to expressing only a single type of receptor strongly supporting the (labelled-line model) as evidenced by data using gene-knockout mice (see data). And further data here, where targeted expression of a novel bitter receptor to bitter (T2R-expressing) cells resulted in dose-dependent aversion to the specific bitter tastant. In marked contrast, directing expression of the same receptor to sweet cells produces animals that are strongly attracted to this bitter tastant.