Humans can discriminate tens of thousands of odors. While we may take our sense of smell for granted, it adds immeasurably to our quality of life: the aroma of freshly brewed coffee; the invigorating smell of an ocean breeze or a field of wildflowers; the fragrance of a lover or the natural smell of a baby. Our olfactory sense also warns us when milk turns rancid, when a baby’s diaper needs changing and when there’s a gas leak. In animals, the sense of smell is essential for detection of predators and other dangers, food sources and mates.
How this amazing sense works to discriminate odors is controversial. The mainstream mechanism vying for consideration is chemical. Often referred to as the shape theory of olfaction, it proposes that attractive and repulsive interactions between molecules come into play when an odorant interacts with its receptor in the nose – ultimately triggering perception of the smell. These molecular interactions reflect the chemical features of whatever you’re sniffing: molecular size, shape, and functional groups – combinations of atoms such as hydroxyl (OH) or carbonyl (C=O) that possess special chemical reactivity.
The alternative mechanism is called the vibrational theory of olfaction. It assumes that transfer of an electron occurs when odorants bind with their receptors. This process is thought to occur when olfactory receptors detect odorant molecular vibrations. The suggestion that a molecule’s smell is based on its vibration frequency is similar to how the sense of hearing functions. The vibration theory has been promoted by a popular book on the topic.
