Functional Structure
Main article: Gustatory system- Sweetness
Sweetness is produced by the presence of sugars, some proteins, and a few other substances. It is often connected to aldehydes and ketones, which contain a carbonyl group.Sweetness is detected by a variety of G protein-coupled receptors coupled to a G protein that acts as an intermediary in the communication between taste bud and brain, gustducin. These receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which account for sweet sensing in humans and other animals.
- Sourness
Sourness is acidity, and, like salt, it is a taste sensed using ion channels. Hydrogen ion channels detect the concentration of hydronium ions that are formed from acids and water. In addition, the taste receptor PKD2L1 has been found to be involved in tasting sour.
- Saltiness
Saltiness is a taste produced best by the presence of cations (such as Na+, K+ or Li+) and, like sour, it is tasted using ion channels.
Other ions of the alkali metals group also taste salty, but the less sodium-like the ion is, the less salty the sensation. As the size of lithium and potassium ions is close to that of sodium, they taste similar to salt. In contrast, the larger rubidium and cesium ions do not taste as salty.
Other monovalent cations, e.g., ammonium, NH+
4, and divalent cations of the alkali earth metal group of the periodic table, e.g., calcium, Ca2+, ions, in general, elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue.
- Bitterness
Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 are responsible for the human ability to taste bitter substances. They are identified not only by their ability to taste certain bitter ligands, but also by the morphology of the receptor itself (surface bound, monomeric).
- Umami
The amino acid glutamic acid is responsible for umami, but some nucleotides (inosinic acid and guanylic acid) can act as complements, enhancing the taste.
Glutamic acid binds to a variant of the G protein-coupled receptor, producing an umami taste.
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