The Physics Behind Recognising Smell

Our nose is very powerful. An adult human can distinguish between 10,000 different smells. Smelling helps us in a lot of ways. We use our sense of smell to distinguish between different things and it is very important for our survival.

A girl smelling a rose

Our human nose does not only help us to smell but also to taste. Our taste buds can detect only fives tastes; sweet, sour, salty, bitter and savory. All the other tastes are detected by our sense of smell. Now that we have figured out how important smelling is, let’s find out how we smell.

How do we smell?

When we take a breath, we suck up odor molecules with air into our nostrils over bony ridges called turbinates. 95% of our nasal cavity filters the air before reaching our lungs. Then, the air travels through the olfactory epithelium on the roof of the nasal cavity.

Olfactory epithelium has a layer of millions of olfactory receptor neurons which are special neurons that sense smell. The odor molecules in the air stimulate and get stuck in the mucus covering the olfactory epithelium. Then, they get dissolved and bound to the olfactory cells which send signals through the olfactory tract to the olfactory bulb.

The olfactory bulb lies beneath the front of our brain. The signals from the bulb tells our brain about the smell. Our brain has 40 million different olfactory receptor neurons. All these neurons help to distinguish between smells.

Recognising Different Smells

A rose, chocolate, banana, everything have their own odor. Different chemicals have different smells. We have discussed above that when our nose detects a smell, it sends signals to our brain. But how are different odors of different chemicals distinguished? How do we recognise the specific odor of a specific chemical?

To explain this, back in 1950s, a conventional theory came up which describes it on the basis of the molecular shapes. The theory says that the particular shape of the odor molecules allows them to fit into the specific receptors in our nose which triggers the sensation of the unique odor of those molecules. It’s like a lock and a key. Just like only the right key fits into its lock, the right odor molecule will fit into its receptor. This theory itself is called the lock and key mechanism.

So, based on this theory, when we smell a rose, the odor molecule of the rose locks into its specific receptor. The receptor sends signals to our brain which triggers the sensation of the smell of rose. But this theory also comes up with a problem.

The molecules, benzaldehyde and cyanide, both have the same smell of almonds. But each of the molecules has a different shape.

Benzaldehyde and Hydrogen Cyanide

According to the lock and key mechanism, only right odor molecule fits into its receptor. So why do these two molecules with different shapes have a same smell? This theory fails to answer this question. But we do have an answer which is given by the Quantum Biology.

The Quantum Theory of Smell

This quantum theory says that our nose is not smelling the molecules, it is listening to them. This theory is based on the vibration of the chemical bonds of the molecules.

Atoms are held together in a molecule by different chemical bonds. Quantum theory says that these bonds are like vibrating strings. The receptor molecules contain quantum particles; electrons. When these electrons jump from one atom to another, they vibrate the chemical bonds of the odor molecule in a certain frequency. This specific vibration causes the sensation of smell. So, according to this theory, our sense of smell is similar to our sense of hearing.

Chemical bonds of different molecules vibrate in different frequencies hence giving different smells. This theory also solves the problem we discussed earlier about the benzaldehyde and cyanide molecules. Even though these two molecules have different shapes, their chemical bonds vibrate in a same frequency which causes them to have a same smell.

Relation with Lock and Key Mechanism

This quantum theory comes up in harmony with the lock and key mechanism. At first, the odor molecule fits into its receptor, then the vibration of chemical bonds causes the sensation of smell. So, from this theory, if we can change the way the chemical bonds of a certain molecule vibrate, then we can change the smell of that molecule.

Hence, if we replace hydrogen atoms in a molecule with different form of hydrogen, deuterium atoms, then the shape of the molecule will remain same but since the deuterium atom is twice as heavy as hydrogen atom, the chemical bonds should vibrate slowly than that of the original molecule. And, since the vibration is changed, it should have a different smell.

[Also See: Bohr’s Atomic Model]

But does this theory really work? Quantum biologists came up with an experiment to test this theory. They use fruit flies for the comparison of smells. The flies were trained to avoid the modified version of fruity molecules. Then they had to pass through a maze and chose between two smells; original fruity smell and modified fruity smell. The flies can definitely smell the difference. They always prefer the original fruity smell. So they moved towards the original smell. This experiment shows that this quantum theory really works.

References: Spark, TED-Ed, Live Science