Nothing has ever made me look at the world around me and think, “and all this actually works?” like organic chemistry class has. Nature is so incredibly complex, it’s difficult to contemplate how everything, right down to the atomic level, functions harmoniously to make life work. What I find most intriguing are the little tidbits of explanation that make me go, “oooooh, that’s why”. Followed, of course, by a tiny lightbulb illuminating above my head. One of my favorite tidbits has to do with our sense of smell.
Like our ability to see, it’s easy to take our ability to smell for granted. Before last spring, I’d never thought about why or how we’re able to smell the roses. As it turns out, up inside our noses (lovely image), there are like 50 million teensy tiny receptor cells. These are basically cells with locks on them, and only a certain kind of key (a specific molecule) will allow them to tell your brain that your smelling something. Not only will that key unlock your sense of smell, but it also allows for identification of the smell and instant recollection of memories associated with it (that way your brain can tell you to get inside when it’s about to rain).
The first step in the creation of aroma is the dispersion of potentially odorous molecules into the air. This happens when things evaporate or are otherwise made into tiny, airborne particles (like how asphyxiating cedar dust is thrown all over as the chainsaw devours a tree trunk). If you get close enough, or there’s enough of a molecule in the air (many parts per million, or ppm), some of those molecules will get into your nose. Once inside your nose, they molecules will drift up to your olfactory epithelium, where all those cells with receptors live. If you have a receptor for the molecule that’s in your nose, you’ll be able to smell it. Some things are “odorless,” like methane from a gas stove (which is why they have to add a smelly compound to it for safety). There simply isn’t a receptor in the human nose that matches the shape of the methane molecule, so we can’t smell it. It’s kind of like that old saying about trees falling down in the forest with no one around to hear. Just because you can’t smell it, doesn’t mean it’s not floating around in the air (and this is where I tell you to buy a carbon monoxide detector for the second time).
What’s really amazing about our sense of smell is how precise the receptors are. There are a lot of molecules that look very similar, but are actually detected as unique by our nose, eyes, hands, etc. It probably makes sense that huge molecule, with like 60 carbon atoms on it, might smell different than a little molecule, with 5 carbon atoms. But, our noses are even better than that. We can actually distinguish between two molecules with the exact same number and types of atoms that are “chiral”. A “chiral” molecule means that there are two version of the same molecule, but one of them is like your left hand and one of them is like your right hand. They look a whole lot alike, and can even do many of the same things, but when you put one on top of the other, they just don’t quite match up (superimpose).
As you can see, these two molecules look a whole lot alike. But your nose can tell your brain which is the tangy caraway (left) and which one is refreshing spearmint (right). Super cool. Both versions are called Carvone. These types of molecules are called enantiomers, and they play a lot of really important biological roles. For example, natural adrenaline that your brain makes fits into an enzyme that gives you that jolt of energy. Unnatural adrenaline (man-made) is the enantiomer of the same adrenaline molecule found in nature, and it lacks that jolt.
There’s a ton of chemistry involved in sensory perception, from the shapes and sizes of molecules to the actual atoms that make up the compound. Some molecules that lie in flat lines smell bad because they sort of sharply poke the receptor cells, and Sulfur is responsible for the classic rotten egg smell.
I hope this is at least half as mind-blowing to you as it is to me. I mean, seriously. My eyes can’t see the difference between a caraway molecule and a spearmint molecule, but somehow my nose knows.
Sources: Organic Chemistry, Seventh Edition by L.G. Wade, Jr. (Prentice Hall), and Dixie State University of Utah powerpoint: http://cactus.dixie.edu/smblack/chem1010/lecture_notes/2D_chemistry_of_smell.pdf