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How do you smell?

A sense of smell

Jean-Baptiste Grenouille was born without a scent but with a superhuman sense of smell. In his quest to understand the world and himself he tried to capture the perfect scent eventually driving him to murder. Jean-Baptiste isn't real but a character from the book turned film Perfume, but this story just shows how emotional and strong the sense of smell is.

What is smell?

How does one define a smell? A perfume catalogue is full of descriptions such as "balsamic", "fatty", "gourmand" or "woody". But these words are imprecise. Is there a way to approach smell scientifically, and just how do you smell?

It turns out how well you can define a smell is partly due to language. In the English-speaking world, the language for describing smells isn't that rich.

"There's a group of people in Malaysia where they have a very, very elaborate vocabulary for smell, and they can describe smells in the same way we can describe colours and they are much better at identifying smells," says Simon Gane, a researcher at the Royal National Throat, Nose and Ear Hospital.

For example they had a specific word to describe "a bloody smell that attracts tigers."

But you've placed more importance on sight too. "There's been work looking at tracing the importance of smell by looking at how many mistakes accumulate in the smell genome since we developed three-colour vision and you can see that our ancestors started not caring about smells much when they got three-colour vision. These mistakes in the smell genome have accumulated far quicker than if there was evolutionary pressure to keep the sense," says Simon.

The nose

When you smell, tiny molecules made up of tens of atoms drift through the air up the nose, travel through a thin layer of mucus and are detected by a set of about 400 receptors high up in the nose. These receptors are expressed on nerve cells which have a direct link to the olfactory bulb which is in turn linked to a primitive part of the brain associated with emotions and memory, giving us strong memories linked with smells.

But figuring out the way the receptors operate is still an unresolved problem in biophysics.

One theory is that smell operates under a lock-and-key method. "You have a specific-shaped molecule fit into a specific-shaped receptor. And that would fire off the receptor," says Jenny Brookes, a post-doc fellow at University College London. Such mechanisms are well established for processes that involve enzymes, but there are some differences between smell and enzymes.

"They can't really be that similar because there isn't an actually chemical reaction happening with smell," says Jenny.

By triggering a combination of the 400 smell receptors our brain interprets that signal as a smell. But the lock-and-key method has a few question marks hanging over it. Similar-shaped and -sized molecules would give different smells, and it lacked a method for predicting what molecules would smell like. There must be something missing.

Quantum detector

The addition to the 'lock and key' theory is based on the way molecules wiggle or vibrate. "Imagine a molecule as balls on springs, and the way the molecule vibrates depends on the mass of the balls," says Simon.

We also need a little bit of quantum mechanics. Electrons can pass through barriers they shouldn't be able to with the quantum-mechanical phenomenon of tunnelling. This counterintuitive property arises out of the maths of treating a particle as wave.

These two aspects are combined in a 'swipecard' model which proposes that the nose works as an electron spectroscope.

"You and I have approximately 100–150 amps of electrons flowing through ourselves," says Luca Turin, a researcher at Ulm University, Germany, "You can imagine diverting a few microamps of that to power a spectroscope would not be a particularly big deal."

According to the theory, inside the receptor electrons are tunnelling all the time, from a donor to an acceptor. "When there is no odorant or molecule there at all there's still a finite probability that an electron will tunnel. It's like a background rate of electrons. Although this part is quite contentious because we don't know what exactly would be the electron donor or acceptor," says Jenny.

But when the odorant molecule sits in the receptor, the vibrations affect the background stream of electrons tunnelling by offering an alternative path for the electrons to tunnel through. This is a bit like a boat blocking one branch of a stream causing most of the water to use another route. Electrons are more likely to use the new path so the receptor detects this increase in the rate of electrons passing through the new path and fires off a signal to the brain.

"The key thing about the vibrations is that when the molecule has a certain frequency, one that will match an energy intrinsic to that receptor then that will open a gateway for an electron to tunnel even more preferentially then when it wouldn't be there." says Jenny.

There have been several tests on this theory. One experiment published in 2013 used a process to swap hydrogen with deuterium, (a hydrogen atom with an extra neutron) in smell molecules. This resulted in molecules that had the same shape as before, but the vibrations were very different due to the extra mass. And this difference was noticeable in the smell.

While it's evidence for the vibrational theory, it's yet to be widely accepted.

"I think the state of play is that people have given up on shape, but are not quite embracing vibration," says Luca.

"The smoking gun would be electrons being transferred," says Luca. But this is a very difficult task to prove and scientists studying smells are quite niche. "The field of smell is very peculiar, it's intellectually interesting but medically irrelevant. There are no interesting diseases of smell, and it's a very small field," he says.

"I suspect we'll find another receptor system akin to smell, and that in fact uses molecular electronics to work, and smell would just be a subset of that," says Luca.

So if it does turns out that smell exploits electron tunnelling, you can be uplifted by the fact that your nose uses complex physics and is actually a very sensitive quantum device.

Further reading

Quantum biology: Do weird physics effects abound in nature?

What Does Cancer Smell Like?

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