You Use Quantum Physics to Smell

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.