Looking at your tongue in the mirror, you can see collections of little bumps clustered on the sides and tip. If you stick your tongue out very far, you see larger flattened pegs on the posterior area. These macroscopic structures are papillae, and all over their surfaces are the taste buds, which are in turn made up of several types of cells, including the taste sensory cells. Although an individual taste bud cannot be seen without a microscope, it looks something like a balloon with a small opening at the tongue surface: this is the taste pore. Into the pore come food and drink molecules, fitting into membrane receptors located on small finger-like protrusions called microvilli at the tops of taste sensory cells. The microvilli increase the surface area of the cell.
How do these cells begin the process that leads to recognizing tastes? As mentioned in Section 1, the membrane receptors on sensory cells contain molecular pockets that accommodate only compounds with certain chemical structures. According to current research, humans can detect five basic taste qualities: salt, sour, sweet, bitter, and umami (the taste of monosodium glutamate and similar molecules). Investigations of the molecular workings of the first four show that salt and sour receptors are types of ion channels, which allow certain ions to enter the cell, a process that results directly in the generation of an electrical signal.
Sweet and bitter receptors are not themselves ion channels, but instead, like olfactory receptors, accommodate parts of complex molecules in their molecular pockets. When a food or drink molecule binds to a sweet or a bitter receptor, an intracellular "second messenger" system (usually using cyclic AMP) is engaged. After several steps, concluding with the opening of an ion channel, the membrane of the taste receptor cell produces an electrical signal. (The second messenger system is a signaling mechanism used in many sensory nerve cells as well as in other cells in the body.)
Although humans can distinguish only five taste qualities, more than one receptor probably exists for some of these. This is supported by the finding that some people cannot detect certain bitter substances but do respond to others, indicating that only one kind or class of bitter receptor is missing, probably as the result of a small genetic change.
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