Whether for finding food, avoiding predators, or choosing a mate, the sense of smell is critical for the existence of almost all creatures. We humans are able to distinguish over 10,000 different odor molecules, and utilizes our sense of smell for a multitude of activities from enjoying the aroma of freshly brewed coffee to deciding whom not to sit next to on the bus. The earliest and most frequently used naturally occurring fragrance originated from botanical extracts.

The chemical composition of floral scents has been extensively investigated for centuries because of the commercial value of floral volatiles in perfumery. Floral scents have been incorporated in fragrance formulations from earliest settlements of man. Plants did not naturally evolve to produce their scents for the benefit of man; however, it is clear that humans have found an aesthetic value in certain types of floral scents, and their promise of floral scents may have contributed to man’s decision to cultivate and propagate specific plant species. While there’s variation in human taste, most people prefer the scents of bee-pollinated and especially, moth-pollinated flowers, which they often describe as “sweet-smelling”. Petals are the main source of scent in most plants, however some plants have developed highly specialized anatomical structures termed “scent glands” for fragrance production; in other plants, the non-specialized floral epidermal cells are recruited for fragrance production and emission.

Some volatile compounds found in floral scents have important functions in vegetative processes as well. They may function as attractants for the natural predator of herbivores or as airborne signals that activate diseases resistance via the expression of defense related genes in neighboring plants and the healthy tissue of infected plants. They may also serve as repellants against herbivores.

Fragrance Chemistry

Most fragrance compounds belong to three major groups: phenyl-propanoids (including benzenoids), fatty acids derivatives and terpenoids which are often greatly modified (oxidized, esterified, methylated, etc).

Science of Scents

Every time we inhale, currents of air swirl up through the nostrils, over the bony turbines, to a sheet about the size of a postage stamp that contains millions of olfactory receptor neurons. This is the olfactory epithelium. Each of the millions olfactory neurons has minuscule filaments (cilia) extending from its knob. This knob is located at the lip of the olfactory neuron and the cilia project from the knob directly into the atmosphere. This is the only part of the brain that projects into the atmosphere. The cilia contain olfactory receptors, and specialized proteins that bind low molecular weight molecules (odorants). The knob transfers information that’d broadly distributed in the olfactory epithelium into a highly organized information map that is in essence a map of the information provided by the different olfactory receptors. Research has shown that different odorants are represented by distinct spatial activity patterns in the glomerular layer of the olfactory bulb that suggests a combinatorial mechanism for olfactory coding wherein the responses of olfactory receptors to odorants produce spatial patterns of olfactory bulb activity that are characteristic for a given odorant or blend of odorants, e.g. perfume. Thus it appears that these spatial patterns of activity create the information that leads to recognition of odor quality and intensity and discrimination between odors. This information is processed at higher levels of the olfactory system and in the brain giving rise to the perception of smell.

For a molecule to trigger an odor sensation in humans, it must have at least a certain degree of volatility to reach the olfactory epithelium located in the upper nasal cavity. The human olfactory system can recognize and discriminate a vast variety olfaction molecules, on the order of thousands, due to the expression of an extremely large gene family of odorant receptors. A single olfactory neuron is believed to express a single odor receptor gene. These receptors, localized to the olfactory neuron cell surface, activate G-proteins and initiate a cAMP-mediated signal transduction cascade, leading ultimately to odor sensing. Each receptor has a pocket (binding site) that is just the right shape to bind either a specific molecule or a group of structurally similar molecules. The interaction of the right molecule with the right receptor causes the receptor to change its shape( structural conformation). This conformational change gives rise to an electrical signal that goes first to the olfactory bulb and then to the areas of the brain that convert the electrical signal to a smell.

Therefore, olfaction, it seems, uses hundreds of receptors that each recognizes multiple, but related chemical compounds and in this way we are able to discriminate between thousands of odors over wide concentration ranges.

The major receptor for smell is the olfactory cleft. In the figure above you see only one but there is one for each nostril. The passages below the olfactory cleft merge at the nasopharynx. This is directly above the soft palate under which the mouth connects to the throat.
As may be easily seen in the diagram, whatever contacts the tongue (where we taste) will also stimulate smell. One site for mixing both smell and taste is as a substance enters the mouth and is (at that point) also directly onder the nose. As a substance is chewed and swallowed it also must pass through the nasopharnyx, which is directly below the smell receptors.
This explains why most people simultaneously smell and taste. When they have nasal conjestion they experience a lesser sense of taste. In reality, taste stays the same and the normal augmentation of taste with smell is diminished.

Three Physical Characteristics of things we can smell:

1.   It must be volatile - that is, it must easily evaporate at normal temperatures and atmospheric pressures so that molecules of the substance can be carried through the air into the person's nose

2.   It must be somewhat water soluable - because the molecules of the substance must pass through the mucus that coats the inner surface of the nasal cavity and reach the olfactory cells

3.   It must be lipid soluable - because the olfactory hairs are composed primarily of lipids and the surface of the olfactory cells are also lipid containing.

What may be the most remarkable thing about the sense of smell is the amount of the substance necessary to create a noticable smell. There is a substance called methyl mercaptan which is noticable in concentrations of 1/25,000,000,000 milligram per milliliter of air. This is added to natural gas so that leaks from gas pipes will be noticed.

Dogs are well known for having an acute sense of smell. Arabian war horses are mares or geldings (not stallions) because a stallion smelling a mare in heat suddenly loses all interest in war, even if the mare is more than a mile away. Such animals are said to be macrosmatic, a name for very high olfactory acuity.

Human beings, on the other hand are microsmatic and have little or no olfactory sensitivity as compared with other species. While some people may be macrosmatic when compared to other people, they quite insensitive compared to many other animals. Even with this species deficit, we are able to distinguish and enjoy (or detest) a remarkable number of scents.