Our nose uses quantum physics to smell at least according to our best theory of how
smell works.
Now how our noses work was always a bit of a mystery to be but I recently learned how
they worked and so I thought I would make a video about it.
When you sniff, odour molecules get sucked into your nose and then get captured by a
layer of mucus, and are then taken to a postage stamp sized area in the top of your nasal
cavity called the olfactory epithelium.
This contains bundles of neurons containing special receptor sites which detect the molecules
and send signals to the brain.
It has been difficult to know exactly how receptors work because nobody has worked out
a way of looking at them while they are in a living nose.
You can take them out of a nose and look at them, but they loose their structure when
they are not supported by the cell membrane that they sit in, it would be like taking
a jellyfish out of the sea which just collapses into a mush.
We know that the sensation of smell is caused by odour molecules being caught by the receptor
sites which causes those neurons to fire.
But, because we can’t directly look at the receptors, we have to infer how they work
from indirect evidence.
Now there’s two main theories about how smell receptors work: shape and vibration.
The shape theory says that the smell receptors have specific shapes that fit the odour molecules,
kind of like a key fitting into a lock.
However this theory doesn’t quite work because we know that we don’t have one kind of receptor
for every type of odour molecule.
We have about 300 different kinds of receptors, but we can detect about 10,000 different smells,
so there is something more complicated going on.
The latest theory is that each receptor is built to fit just one section of a molecule,
so any molecule that has that part has a similar smell, so any molecule that has a sulphur
hydrogen bond will smell like rotten eggs.
This matches with the evidence because you can get many different shaped molecules that
end up having the same smell because they have got a similar molecular group.
However this theory can’t explain everything.
There’s certain molecules that are made of the exact same groups but are just arranged
in a different way, but they smell very different to each other.
For example, vanillin, which smells like vanilla, has all the same molecular groups just in
a different order to isovanillin, which has a very nasty sickly medicinal smell.
So there is an alternative theory of smell receptors, the vibration theory.
In the vibration model of smell, the smell receptor can tell the difference between different
molecules based on how they vibrate.
Each chemical bond has a certain resonant frequency that it naturally vibrates at.
Kind of like how an open guitar string always resonates at the same frequency and so always
gives you the same pitch.
So different molecules have a different signature set of vibrational frequencies which depend
on what atoms they are made from and how they are all connected.
In the past, scientists have harnessed this property to figure out the chemical composition
of molecules using light, in a process called Raman spectroscopy.
When you shine laser light through a bunch of molecules some of the light is absorbed
to make these molecular bonds vibrate, and then light with different energy, and hence,
a different frequency is emitted.
You can then look at the frequencies of this new light, which we see as their colour, and
work out what the molecules are made of.
And it’s worth mentioning that when it comes to light each specific colour relates a specific
frequency which has a specific energy, there is a one to one to one mapping between them.
This has been a great way to detect different kinds of molecule, so perhaps our smell receptors
are doing something like this.
But for many years the vibration theory of smell was not very popular because there is
no way our noses can do Raman spectroscopy as, unfortunately, our noses don’t emit
lasers beams, and we can’t be using natural light because we can smell in the dark.
But there is another method to detect the vibrations of molecules that uses electrons
instead of light which comes from the realm of quantum physics.
It’s a form of quantum tunnelling.
Quantum tunnelling is the phenomena where quantum particles like electrons can travel
to places that normal ‘classical’ particles can’t.
They can jump through walls by disappearing from one side and immediately appearing on
the other.
This is one of the counter intuitive behaviours of quantum particles and is a consequence
of quantum particles behaving as spread out waves.
In a specific situation electron tunnelling can actually be used to find the resonant
frequencies of molecules.
If we take two metals and separate them by a small barrier and then apply a voltage to
make an electron get pushed to one side.
Normally in classical physics the electron can’t get across this barrier, but if the
gap is very small it can quantum tunnel to the other side.
But there is an additional condition.
An electron in a metal has a certain energy, and it can only tunnel to the other side if
there is an empty hole which has got the same energy.
But if the hole on the other side is at a lower energy the electron can’t tunnel because
there is no where for the spare energy to go.
But if we introduce a molecule into the gap something interesting happens.
If the energy difference between the electron and the hole is just the same as the energy
needed to vibrate one of the resonances of the molecule, then the electron is allowed
to tunnel across, and it drops its extra energy into vibrating the molecule when it goes across.
Scientists have built machines that use this property to probe molecules, the technique
is called inelastic electron tunnelling spectroscopy.
You can put in different molecules, change the energy difference between the electron
and the hole and see where the electron tunnels, which tells you about the resonances of the
molecule and so tells you what it’s made from.
So perhaps our nose is doing the same thing.
Perhaps our smell receptors are behaving like that metal and the gap, which is waiting for
an odour molecule to come in, which allows an electron to pass across the receptor and
trigger the nerve.
To find out scientists did some very clever experiments.
This theory makes a very specific prediction, that the odour of a molecule depends on the
frequencies it vibrates at.
So if you could change the frequencies of a molecule you’d also change it’s smell.
So that’s exactly what they did.
They took a molecule, but replaced all of the hydrogen atoms with a heavier from of
hydrogen, called deuterium, which instead of just having one proton in the nucleus the’ve
got a proton and a neutron.
Deuterium has all the same chemical properties as hydrogen, but is much heavier.
The scientists got a bunch of the normal molecules, and the heavier ‘duterated’ molecules
and got different subjects to sniff them to see if they could tell the difference, humans,
fruit flies and white fish.
They found strong evidence from these creatures that these two different forms of the molecule
actually smelled different!
This is really cool because it is really good evidence that our noses use quantum physics
to smell the world, which I find utterly amazing!
Quantum physics shouldn’t play much of a role in our warm complicated bodies, so it’s
crazy that it does.
But as successful as the vibration model is, it can’t explain everything.
You may have heard of chiral molecules which are molecules that are made of all the same
stuff but arranged as mirror images of each other.
For example carvone has a left handed and right handed form.
Because these are made of the same atoms and bonds they have all the same vibrations, so
will look identical to vibration tests like Raman spectroscopy and inelastic electron
tunnelling.
So according to the vibration theory they should smell the same, but they don’t.
One form smells like caraway or dill and the other smells like spearmint.
So what can we conclude from all of this?
It seems like you need both the shape model and the vibration model to explain how we
smell.
Perhaps our receptors first check the shape of the molecules, and then if they get past
that barrier they are checked for their resonance vibrations through quantum tunnelling.
But we won’t know exactly how this works until we can figure our a way of directly
observing the receptor sites in action.
But isn’t it crazy to think that these experiences of smell we have of a flower, or coffee or
freshly baked bread, or a newborn baby, this experience of the world came to your brain
through something as strange as quantum tunnelling.
So quantum physics which seems for many people to be so strange and abstract, well, its going
on right up your nose.
Just like all my other videos I’ve made a poster for this one you can either buy it
or get it for free, check in the description to see where to get those.
Also, um, if I have made any mistakes in this video, which does happen I have put that in
the description too, so if you spot anything check there and if it is not there then leave
a comment and I’ll address that.
Other than that, I’m going to be making a tonne more videos now under the banner of
Domain of Science so I’ll see you on the next one.