The lock and key theory of odor (systemic effect theory) suggest that odor acts very much like a specific neurotransmitter, a drug or an enzyme. In this paradigm, an odorant has a specific effect on behavior or emotion – one odor for one emotion or one odor for at most a few emotions.
The stereochemical theory of odor suggests that a molecule that fits into an olfactory receptor can fire nerve cells, ultimately producing a particular odor perception.
Lucretius, one of the early Greek atomists, suggested that substances having odor gave off a vapor of tiny ‘atoms’, all the same shape and size, and that these atoms gave rise to the perception of odor when they entered pores on the nose.
Five basic odors were associated with different molecular shapes. Football shaped molecules fit in to a "camphoraceous" receptor, and smell like mothballs. Necklace-shaped molecules have a musky odor because they fit into a "musky" receptor.
While, wedged-shaped molecules have a pepperminty odor, tadpole-shaped molecules smell like flowers, and long thin ether molecules are, well, ethereal.
A substance must have certain physical characteristics to have the property of odor:
*It must be volatile enough to give off vapor that can reach nostrils
*It must be water soluble, so that it can pass the layer of moisture (mucus)
*It must have lipid solubility to allow it to penetrate the lipid layers that form the surface membranes
Theory of odor
