Brown Marmorated Stink BugThe Physics of Smell
                                    Copyright of Bigstock                action, reaction, and olfaction synthesized...               

Introduction         Shape Theory          Electron Tunneling             Vibration Theory           Bibliography




Disclaimer: unless            makes intuitive sense to you, it may be                     a good idea to brush up on electron tunneling before reading this article...



                       The Vibration Theory

   Shape theory makes sense intuitively and has been the historic model for olfaction, but current research points to a slightly more complex model. Vastly different-shaped molecules sometimes have the same odor, while very similarly shaped compounds sometimes smell quite different from each other. Shape theory cannot explain these differences: by the theory, if two molecules have the same shape, the must have similar smells; likewise if they have different structures, they should also have different smells. But some animals can distinguish between different isotopes of the same molecule (1). In these cases, the molecular composition and structure are the same, but different isotopes have slightly different masses.
    The answer to these problems may lie in the molecule's vibrational patterns. In place of shape theory's 'lock and key' analogy, the vibration theory proposes a 'swipe card' method: as the molecule vibrates olfactory receptors, electrons will jump between different areas in the receptor by means of quantum tunneling (1). If two molecules had very different shapes but the same vibrational patterns, we would expect them to have similar smells. Likewise if two molecules have similar structures but different vibrational patterns, such as the changed mass of isotopes, then the two molecules probably have different odors.This theory is still in the developmental stages, but it seems to hold water and appears to be a more accurate mechanism for olfaction.                                                                                                                                                                                                                                Nancy Rica Schiff

Electron Tunneling in Your Nose




                                                                                                                                      




        First, a molecule makes contact with the olfactory receptor (illustrated by the five                cylinders). In this case, the molecule is camphoraceous (smelling of the campor tree).













                                                                                                                                                     





    Next, the molecule docks with and deforms the olfactory receptor, allowing the electron to tunnel to the donor site.





















    In the most important step, the molecule's vibrational patterns mediate an electron tunneling event from the donor site to the acceptor site. It is this step that accounts for different smells in similarly-shaped molecules: if the two molecules have similar vibrational patterns, then they will encourage similar electron tunnelings and therefore have similar smells.

















               Finally, the electron is expelled from the acceptor site and passes through a receptor,                 triggering an olfactory response which is then processed into a smell.









    All diagrams from Jennifer Brookes (3) and modified by the author