It is specifically needed in the mammalian kidney as an osmotically active compound which facilitates passive re-uptake of water into the blood. Because of this, salt elicits a pleasant taste in most humans. Sour and salt tastes can be pleasant in small quantities, but in larger quantities become more and more unpleasant to taste. For sour taste this is presumably because the sour taste can signal under-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria which grow in such media. Additionally, sour taste signals acids, which can cause serious tissue damage. Sweet taste signals the presence of carbohydrates in solution. Since carbohydrates have a very high calorie count (saccharides have many bonds, therefore much energy), they are desirable to the human body, which evolved to seek out the highest calorie intake foods. They are used as direct energy (sugars) and storage of energy (glycogen). However, there are many non-carbohydrate molecules that trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin, sucralose, and aspartame. It is still unclear how these substances activate the sweet receptors and what adaptational significance this has had. The savory taste (known in Japanese as "umami") was identified by Japanese chemist Kikunae Ikeda, which signals the presence of the amino acid L-glutamate, triggers a pleasurable response and thus encourages the intake of peptides and proteins.
Gustducin is made of three subunits. When it is activated by the GPCR, its subunits break apart and activate phosphodiesterase, a nearby enzyme, which in turn converts a precursor within the cell into a secondary messenger, which closes potassium ion channels. Also, this secondary messenger can stimulate the endoplasmic reticulum to release Ca2+ which contributes to depolarization. This leads to a build-up of potassium ions in the cell, depolarization, and neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G protein, because of a structural similarity to the relevant GPCR. Umami
Umami, or savory, is an appetitive taste. It can be tasted in soy sauce, meat, dashi and consomme. A loanword from Japanese meaning "good flavor" or "good taste", umami (旨味) is considered fundamental to many East Asian cuisines, such as Japanese cuisine. It dates back to the use of fermented fish sauce: garum in ancient Rome and ge-thcup or koe-cheup in ancient China.Umami was first studied in 1907 by Ikeda isolating dashi taste, which he identified as the chemical monosodium glutamate (MSG). MSG is a sodium salt that produces a strong savory taste, especially combined with foods rich in nucleotides such as meats, fish, nuts, and mushrooms.Some savory taste buds respond specifically to glutamate in the same way that "sweet" ones respond to sugar. Glutamate binds to a variant of G protein coupled glutamate receptors.