- Taste (Gustatory)- There are 5 different types of tastants perceptible to mammals. These include sweet, sour, salty, bitter, and umami (taste of monosodium glutamate). It appears that different cells expressing different tastant receptors (TR) mediate taste. These receptors could be ligand-gated ion channels, or work through ligand-induced conformational changes in the receptor which are sensed by intracellular proteins (usually G proteins, that are discussed in the next chapter section).
- Sweet - Cells that recognize and respond to sweet tastants express receptors from the TIR family. The receptor appears to be active as a heterodimer. Dimers of T1R2 and T1R3 recognize sucrose and saccharin. The bound receptor is coupled to intracellular G proteins.
- Sour - This sense would protect us from excessively acidic foods. Huang et recently found an ion channel, PKD1L1 (Polycystic-Kidney-Disease-Like Ion Channel), that is necessary in mice for the sour sensation. Mice breed to lack this gene could not detect sour stimuli.
- Salty - Taste cells with receptors necessary for salt sensation have not yet been identified.
- Bitter - Bitter taste perception, required to prevent the ingestion of toxic substances, is mediated by the T2Rs family of receptors, mediated by the G-protein, gustducin.
- Umami - The G-protein coupled receptor for this taste consists of a dimer of TIR1 and T1R3.
- Smell (Olfactory) - tba
- Sight (Visual)- TBA
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