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The importance of smell

The captivating scent of cakes and the compelling aroma of freshly brewed coffee attract you to a bakery in the morning. A male moth is fluttering around, frenetically following the scent plume released by his female.

What do these two phenomena have in common? Much more than we suspect, when we look at the molecular level. Imagine if we had a very powerful microscope enabling us to detect details at the levels below one millionth of a millimeter. What we would discover is that in both cases volatile molecules floating in the air are responsible for both events, when they collide with your nose or the antennae of the insect.

We are dealing in both cases with molecular recognition, which means deciphering the information encoded in molecular structures.

Such messages are conveyed by the beautiful and unique architecture of molecules, just like a concept is encrypted in the elegant combination of strokes making a Chinese character.

To decode and interpret such chemical messages, specific proteins sitting at the membrane of olfactory neurons in our nose or in the antenna of an insect check on the identity of each volatile molecule arriving, like sentinels at a city gate, and report to the interior. Thus, chemical information, translated into electric signals, travels to the brain and affects behavior choices, as well as mood. All this is often done without us paying much attention to smells in our environment.

The antennae of insects house thousands of tiny chemosensitive hairs, where binding proteins and receptors recognize volatile molecules (odorants and pheromones) and translate the chemical messages encoded in their structure into electric impulses that are sent to the brain. The human nose, despite its completely different anatomy, performs analogous functions using a similar biochemical machinery to detect and recognize environmental smells. Image Credit: Paolo Pelosi, used with permission.
The antennae of insects house thousands of tiny chemosensitive hairs, where binding proteins and receptors recognize volatile molecules (odorants and pheromones) and translate the chemical messages encoded in their structure into electric impulses that are sent to the brain. The human nose, despite its completely different anatomy, performs analogous functions using a similar biochemical machinery to detect and recognize environmental smells. Image Credit: Paolo Pelosi, used with permission.

It is the sense of smell that guides our food choices, that wets our appetite and unconsciously, but inexorably drives us to our favorite coffee shop. And it is smell again that makes us choosy about food and justifies the absurd price of an aged bottle of claret.

Smell again can detect warning messages from that bottle of milk left too long in the fridge or give away the pretended freshness of fish.

Without the sense of smell, we would miss a lot of interesting experiences, emotions, and pleasure; our life would lack a whole dimension. However, from a physiological point of view, it would be a perfectly normal life.

But this is not the case for most animals. From mammals to insects, fish and snakes, olfaction is essential for finding and recognizing the partner of the same species, detecting a prey and escaping a predator, or selecting the good food and avoiding poisonous plants.

A male insect with impaired sense of smell would not be able to recognize the love message sent by the female of its own species and would end its life without passing its genes to the next generation. A similar short life without the chance of reproduction would be the fate of a gazelle unable to smell the approaching lion or that of a mouse which could not distinguish good food from poison.

The chemical language of molecules, used by most animal species to exchange information, can be as complex as any language we use to communicate with each other. Breaking the olfactory code has the flavor and excitement of deciphering an ancient scripture and reveals a key to understand and eavesdrop the desperate call of a moth calling her partners or the gossip going on in a beehive about the best foraging sites or the health of the queen. Konrad Lorenz indicated the study of behavior as the ring of King Solomon, enabling us to understand the language of animals. Often such behavior, particularly in insects, is mediated by odors and pheromones, which represent the basic units of their language.

The structure of an odorant-binding protein (OBP) of the backdiamond moth (Plutella xylostella) encapsulating a molecule of the specific pheromone. OBPs are located at the interface between external environment and olfactory receptors and represent the first biochemical tools to interact with molecules of smells and pheromones. Thanks to their extremely compact and robust structure, OBPs can be used in biosensors for instrumental monitoring of environmental odours.
The structure of an odorant-binding protein (OBP) of the backdiamond moth (Plutella xylostella) encapsulating a molecule of the specific pheromone. OBPs are located at the interface between external environment and olfactory receptors and represent the first biochemical tools to interact with molecules of smells and pheromones. Thanks to their extremely compact and robust structure, OBPs can be used in biosensors for instrumental monitoring of environmental odors. Image Credit: Paolo Pelosi, used with permission.

We are just beginning to decipher this alphabet (or these alphabets, because humans, cats, and mosquitoes all speak different idioms), but understanding the complex rules of grammar and syntax of the chemical language is a task still far from being reached.

We might think that knowledge sometimes produces cold. The more we understand Nature, the more it loses interest and attraction. This is not my view. Lifting the veil of mystery that surrounded for a long time our perception of odors probably dissolved that aura of magic and dream associated with perfumes and flavors, but at the same time revealed new, exciting, and unpredicted aspects of the complex phenomena behind our sense of smell. More than once have I experienced the excitement of opening a door and discovering a new unexpected scenario with more doors waiting to be opened.

Featured image credit: Cup of coffee and coffee beans by Unsplash. CC0 Public Domain via Pixabay.

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