The Science of Smell

Sniff, sniff! What’s that smell? Whether it’s a home-cooked meal or a pair of dirty sneakers, the power of smell is an amazing thing. But how does smell work chemically? In a recurring section of this blog, we’ll soon be featuring a molecule of the week, many of which are associated with a particular smell, but how does the structure and shape of a particular molecule translate into all the wonderful (and not so wonderful) smells that surround us.

The human sense of smell (olfactory sense) is capable of distinguishing over 10,000 different odor molecules¹! Our keen sense of smell is enabled by over 900 different genes within our DNA, approximately 40% of which code for different olfactory receptors. Hopefully you read Spencer’s post last week (if not scroll down), because these receptors are actually members of the Class A Rhodopsin-like family of G protein-coupled receptors (GPCRs).

All around you are thousands of molecules in their gaseous form buzzing through the air. These easily evaporating, or volatile compounds, readily go into the gas phase and are inhaled as you breathe in. As these molecules pass through your nose, they come pass over a small postage stamp sized area in your nose that contains millions of olfactory receptor neurons, an area known as the olfactory epithelium. Each of these neurons has miniscule projections, called cilia, which reach out into the air. These cilia are the olfactory receptors, which are proteins specifically designed to bind to the volatile compounds that are found in the air. Each receptor has the ability to bind a range of odor molecules but with varying strengths². Once bound, the protein undergoes a structural change, binding and activating an olfactory-type G protein within the cell. This then triggers the conversion of adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP) causing the opening of ion channels in the cell wall. As Ca²⁺ and K⁺ enter into the cell, an electrical signal is created which shoots down the neuron and is subsequently received and translated in the brain into a smell³. 

Depending on what combinations of molecules bind to the array of receptors in your nose, you smell a different odor! Due to the hundreds of receptors and their varying affinities for different molecules, we have the ability to distinguish thousands of combinations of molecules from the smell of freshly cut grass, to fruits, to freshly baked cookies!

References:

1. http://www.senseofsmell.org/smell101-detail.php?id=1&lesson=How%20Does%20the%20Sense%20of%20Smell%20Work?
2. http://www.ncbi.nlm.nih.gov/pubmed/15630933
3. http://en.wikipedia.org/wiki/Olfactory_receptor#cite_note-pmid15630933-6