Capable of detecting faint traces of smoke, the nose can be a life saver. Sensitive to esters of fruits and flowers, it can be a source of pleasure. Through its associations with the hippocampus, it can serve as a unique gateway to episodic memories. When the brain gathers information through the nose, it does not rely on the electromagnetic spectrum. Like touch and taste, smell is a sense that’s in direct contact with matter.
Decades ago, some chemists imagined that through its shape and functional groups, a particular odiferous molecule was interacting with a single type of receptor. But the simplistic speculation had its shortcomings. The mechanism’s truth turns out to be far more intricate.
Richard Axel and Linda B. Buck won the 2004 Nobel Prize in Physiology for their discoveries of odorant receptors and the organization of the olfactory system. Independent of each other’s work, they experimentally found evidence for the following bouquet of ideas:
(1) For every olfactory receptor cell, there is an odor-receptor, coded by a single gene. The related genes(about 400) that code for olfactory receptors probably make up of the largest gene families.
(2) One receptor can respond to a variety of similar odorous substances, For example, receptor 2 in the diagram can interact with both hexanol(B) and heptanol(D). But the coupling-reaction from one molecule will not necessarily be identical in intensity to that of the other.
(3) There could be a main component of a particular smell, but more often than not, the smell will often be triggered by two or more compounds. For example a rose contains three major constituents: two different ketones and cis-rose oxide. But there are hundreds of more compounds, all of which contribute to its its fragrance. (In 5 decades, the number of odorous compounds discovered in the rose increased by a factor of 20.)
Now here comes my favorite part:
(4) A specific combination of receptor-interactions is needed for a stimulus, so a given receptor can play a role in various smells. Thus not surprisingly, for about 400 receptors, there are about 10 000 or so different odors that we recognize. For example, in the diagram, hexanol’s interactions with receptors 2 and 6 leads to a sweet, herbal smell. The same receptors along with receptor 5, when interacting with heptanol, lead to a smell that’s sweet and violet-like. Returning to the rose, each of three main fragrant compounds is responsible for a key combination. Our perception is based on a mosaic of at least those three receptor-combinations along with a bouquet of all the other fragrant components of the rose.
A rose and especially a citrus blossom are far more interesting than the most expensive perfumes. Flowers do not spray compounds into a mist; often the molecules are secreted from the surface of petals and other parts; at times they start as liquid and evaporate when air collides with them. It takes thousands of flowers to yield a few hundred grams of the essential oils used in commercial products. In cheaper perfumes only a few of the fragrant compounds are synthesized. Instead, a single flower is sufficient to excite our senses. It synthesizes its perfume on location and does not have to deal with preservation issues.
There is more odor science to to be revealed on the matter, to be sure. The year after the prize was awarded, after using vaporized odorants delivered either through the nose(orthonasal) or the mouth(retronasal), investigators measured brain responses with fMRI. They were not the same. This explains the Jekyll and Hyde signatures of the durian fruit and Limberger cheese. Both smell bad when they are not in your mouth; durian smells like rotten onions and Limberger is produced from the same bacteria that make sweaty socks reek. But while these foods are being ingested, the unpleasant smells are not perceived and replaced by a pleasant blend of taste and alternate aromas.