I have mentioned here before that I have no sense of smell. Born that way. Taste, yes -- sweet, sour, salty and bitter -- but not a whiff of odor. I can tell if there is sugar in the coffee, but can't tell unsugared coffee from hot water.
Apparently, there are about 10,000 different odors that humans can detect and remember. Ten thousand! Linda Buck and Richard Axel won the 2004 Nobel Prize for Physiology or Medicine for discovering a family of more than 1000 genes -- three percent of of our genetic inheritance -- that code for the same number of different olfactory receptors. I quote just a few sentences from the Nobel committee's press release:
When an odorant receptor is activated by an odorous substance, an electric signal is triggered in the olfactory receptor cell and sent to the brain via nerve processes. Each odorant receptor first activates a G protein, to which it is coupled. The G protein in turn stimulates the formation of cAMP (cyclic AMP). This messenger molecule activates ion channels, which are opened and the cell is activated. Axel and Buck showed that the large family of odorant receptors belongs to the G protein-coupled receptors (GPCR). All the odorant receptors are related proteins but differ in certain details, explaining why they are triggered by different odorous molecules. Each receptor consists of a chain of amino acids that is anchored into the cell membrane and traverses it seven times. The chain creates a binding pocket where the odorant can attach. When that happens, the shape of the receptor protein is altered, leading to G protein activation.I'm not exactly sure what all this means, but it's from this elaborate molecular commerce and electrochemical signaling that we construct a world of smell.
The schematic above is also from the Nobel press release. Look at all those greedy little "hands" on the olfactory receptor cells reaching out to grab whatever volatile compounds come wafting by. Presumably, my nose is grabbing too, but somewhere between the receptors and the brain my circuits are kaput. Sometimes I try to image what smell is like. But of course I can't. What we don't experience, we can't imagine.
Which is why so much of our thinking is metaphorical. The solar system is a clockwork. Electrons are little billiard balls, or waterlike waves. The bacterial flagellum is a propeller. The cosmos is created and sustained by a person. And so we weave a universe out of what we know.