membrane. (Unlike the receptor proteins for some of the other tastes
described below, the salt taste receptor proteins have not yet been
conclusively identified.) Thus, when high (relatively) concentrations
of Na appear in the mouth after ingestion of salt, the Na is believed
to flow through sodium ion channels in the salt taste receptor cells,
triggering a neural signal to the brain. Ingestion of some amount of
sodium and other related elemental cations (potassium, calcium,
magnesium) is essential for survival, and the taste of salt—at least in
moderation—is generally experienced as pleasant.
Sour. What we call “sour” is the taste of acids: citric acid from grape-
fruit and lemon, acetic acid from vinegar, lactic acid from sauerkraut
and yogurt. The defining feature of acids is the release of hydrogen
ions (H) in solution. And sour taste is associated with hydrogen ions.
The German word for acid is saur. The proteins on the taste receptor
cells related to the experience of sour appear to be channels sensi-
tive to hydrogen ions. As for the salt receptors, they have not been
conclusively identified. When (relatively) high concentrations of H
appear in the mouth after ingestion of acidic substances, some kind of
positive charge is believed to flow through channels in the sour taste
receptor cells, triggering a neural signal to the brain.
Bitter. The proteins that initiate the signals associated with the
perceptual experience of bitterness are not ion channels; they are
G-protein-coupled receptors (GPCRs). In humans, more than thirty
different GPCR proteins are distributed over the receptor cells associ-
ated with bitter taste. When a ligand floating around in the interior of
the mouth comes into contact with microvilli of a bitter taste receptor
cell and binds to a GPCR, this initiates an intracellular signaling cas-
cade leading to release of neurotransmitter and generation of a signal