Of the five senses which keep us in touch with the world, most of us are particularly aware of eyesight and hearing. Of course we are very thankful for these gifts. One sense that we tend to take for granted however, is the sense of smell. This sense does not seem very complicated or amazing. Nevertheless a little research reveals that our sense of smell is not only exquisitely designed, but it is also poorly understood by biologists. Of all our senses, that of smell seems to be the most complicated.
When we consider the other senses, we discover that taste involves four basic kinds of receptor (salt, bitter, sweet and sour) on the surface of the tongue. All taste sensations are combinations of messages from these four receptors. Colour vision similarly involves three kinds of receptor: specifically for green, red and blue light. All visual images come from messages to the brain sent from these three colour receptors as well as from a receptor for light itself. The ear, on the other hand, is said to be the most sensitive human organ. The hair cells in the inner ear are all much alike whether they are designed to detect bass tones or treble tones or anything in between. The sense of smell on the other hand, is quite a different proposition. Imagine a sense which involves 350 entirely different kinds of receptor. It is evident that smell is more interesting than we might have expected.
Biologists expect that the number of odours which an organism can detect, is proportional to the number of relevant genes. In people, about 350 different genes code for 350 different receptors. This is a very large cluster of related protein coding genes, the largest block of genes discovered so far in the human genome. This is an interesting fact when one considers all of the complicated functions of the human body. If the number of genes discovered in human DNA totals about 22,000 (as many experts now believe), then the proportion of genes coding for smell receptors is about 1.5% of that total.
The reason that we need so many receptors is because of the wide variety of chemically different odour-causing molecules in the air. The receptor molecules in the nose are located on tiny projections emerging from nerve cells. These projections are located in the mucous membranes high up in the nose. When an odour molecule collides with an appropriate receptor, the two fit together like lock and key. The receptor protein then initiates a chain of chemical reactions in the nerve cell’s membrane so that the electrical condition in the nerve cell changes. As a result, the nerve cell sends an electrical impulse toward the brain. The stimulation of various combinations of the 350 different kinds of receptor in the nose, results in the perception of at least 10,000 different odours. Each receptor responds to just one part of a molecule’s structure. Thus, if there are several reactive sites on the surface of one molecule, several different receptors may be stimulated at the same time by this one type of molecule. The blending in the brain of the different messages, leads to the sensation of a specific odour.
Some smells are mixtures of large numbers of air borne molecules. That lovely aroma of coffee, for example, contains about 500 different kinds of molecule. Although we understand these basics, the chemistry of our sense of smell is nevertheless far from clear. Some molecules with very different composition, nevertheless smell much the same. Moreover, some molecules that are extremely alike, nevertheless elicit entirely different sensations of smell. Mirror images of an organic molecule called carvone, for example, smell either like cumin or peppermint, depending upon which arrangement the component atoms assume.
A recent article in the online journal Public Library of Science Biology (May 2004) was entitled Unsolved Mystery: The Human Sense of Smell: Are We Better Than We Think? (p. 572-575). The popular perception, so author Gordon Shepherd declares, is that the human sense of smell is much less effective than that of some animals such as dogs, cats and rodents. Well maybe we should think again! Although humans have only 350 functional olfactory receptor genes, compared to much higher numbers for other mammals, it turns out that humans perform extremely well in odour detection tests. For example, when tested for the lowest amount of a chemical which they can detect, people performed better than dogs in some tests and much better than rats in others. Moreover, humans outperformed even the most sensitive machines (such as the gas chromatograph) designed to detect airborne chemicals.
Thus the author concludes, “Humans are not poor smellers… but rather are relatively good, perhaps even excellent smellers” (p. 573). The author ponders how it is that people have such excellent noses when they have so “few” detector molecules compared to various animals. The popular evolutionary interpretation is that people lost their sense of smell as they gained in brain power and the ability to walk upright. Obviously the scientists need to reconsider. We now know that people smell very well with far fewer kinds of receptor than animals require. The reason people are able to do this, apparently, lies in the much more sophisticated interpretive capability of the human brain. For any individual odour, the brain calculates how many different kinds of receptor are stimulated and what is the relative proportion of these stimulated receptors. Scientists have also recently discovered that smell perception involves many more areas of the brain than previously thought.
While humans possess fewer genes for smell, and thus fewer receptor molecules, they nevertheless smell extremely well, as well or better than animals with far more genes. It is evident that scientists who try to draw conclusions about organisms based on comparisons of their chemical components, may be in for a surprise. Dr. Shepherd therefore remarks: “The mystery being addressed here is a caution… against any belief that behavior can be related directly to genomes, proteomes, or any other type of ‘- ome'” (p. 575). (The genome is the genetic information in the DNA, and proteome is the complete list of proteins in an organism.) None of these measures adequately determines what an organism is like and what its capabilities are.
There is far more to the wonderful design of our bodies than we can even understand. Now that we realize how complicated the design of the odour detection system in our bodies really is, we will be doubly thankful for the wonderful gift of smell.
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