Five different types of taste receptors have been identified in humans. These ar
ID: 3474207 • Letter: F
Question
Five different types of taste receptors have been identified in humans. These are salty, sour, sweet, bitter and umami. Taste receptors consist of specific types of proteins incorporated into the plasma membrane of taste receptor cells, which are clustered together in structures called taste buds. All five receptor types are present in each taste bud, which are distributed over most of the tongue surface area. Olfactory receptors are located in the nasal cavity and consist of several thousand different types, each type specific for a different type of odorant molecule (ligand).
a) Think about what it is about proteins that allow ligands to bind to them when answering the following. If you plug your nose, red wine and black coffee taste exactly the same. Explain why this is so.
b) Many different types of molecules are perceived as sweet, including different types of sugars and artificial sweeteners. Provide a molecular explanation for why this is possible, given that there is only one type of “sweet” taste receptor (that we know of!).
Explanation / Answer
a.Rather than binding specific ligands,olfactory receptors display affinity for range of odor molecules and conversly a single odorant molecule may bind to a number of olfactory receptors with varying affinities,which depend on molecular volumes.Once the odorant has bind to odor receptor,it undergoes structural changes and it binds and activates the olfactory type G-protein on the inside of olfactory receptor neuron.It inturn activates the lyase adenylate cyclase which converts ATP to cyclic AMP.The cyclic AMP opens cyclic nucleotide gated ion channels which allow calcium and sodium ions to enter into cell, depolarising the olfactory receptor neuron amd beginnig an action potential which carries the information to brain.
When the nose is plugged,the action potential doesnot take place and taste or sensation is not carried to brain and hence red wine and black coffee tastes exactly same.
b.The heterodimer of Tas1R2 and Tas1R3 are broadly acting swewt taste receptors which mediates mammalian sweet taste towards natural and artificial sweeteners and sweet tasting proteins.Although specific regions determine the activation of receptors by these sweetners have been identified, the molecular mechanism of species dependent sweet taste remain elusive.
Due to primary sensory tansmission process of sweet taste perception, natural sweet ligands and sweet proteins are produced which are considered as only one type of sweet.
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