The human tongue has taste buds that respond to different substances, with evolution programming us to find nutritious things tasty. Sugar tastes good because it is highly digestible and offers a condensed source of calories. The tongue has clusters of chemical sensors that determine flavor, with the geometric shape of molecules playing a role. Both sugar and saccharin have similar prongs that lead to a sweet taste sensation. Bitter substances have different spacing of hydrogen atoms, leading to a bitter taste.
The human tongue responds to a range of different substances, registering them as various tastes. Evolution has programmed our sense of taste to find the nutritious things tasty and the non-nutritious things tasteless, for the most part. Humans are flexible about what they eat compared to many animals, hence our omnivore status, but there are many types of organic material that we are unable to digest and therefore perceive as unpalatable. Sugar is highly digestible and offers a very condensed source of calories, so to us it tastes good and has a distinct flavor that we call sweet. All mammals except cats can taste and like this substance and are more inclined to eat bad-tasting foods if they contain it.
Scientists now know that the tongue is covered in tiny clusters of chemical sensors called taste buds. It is the geometric shape of the incoming molecules that determines their flavor. Some foods have multiple molecules that all contribute to their overall taste sensation. There are several types of sugar, but by far the most frequently consumed by humans is a molecule called sucrose. There are also other molecules, such as saccharin, that taste sweet even though they are not sugar, although the taste sensation is slightly different.
The reason that two different classes of molecules produce the same sweet taste was confirmed in the 1960s. Both have a double set of hydrogen atoms sticking out of their surface like prongs, ready to bind with receptors in the tongue. In sugar, these two atoms are between 2.5 and 4 angstroms apart, where one angstrom is a length approximately equal to the width of two hydrogen atoms. Saccharin has a completely different molecular structure, but the same prongs with similar spacing, leading to a similar taste sensation.
Bitter-tasting substances also have two hydrogen atoms sticking out for bonding, another pole, but the average separation of these two atoms is 1.5 angstroms. The different spacing of hydrogen atoms of a few atomic diameters is enough to produce the high-level effect of sweet or bitter. This is why sugar tastes sweet and quinine tastes bitter.
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