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A quantum sense of smell

24 Mar 2015

When you slice into an orange, it doesn’t take long for the sweet, sharp smell of citrus to fill the air. But what are you actually smelling? And how does your nose tell the difference between the orange’s tangy odour and the more subtle fragrance of an apple? In this podcast, Johnjoe McFadden and Jim Al-Khalili explain how quantum mechanics may be helping our noses distinguish between apples, oranges and a thousand other scents

On the face of it, Johnjoe McFadden and Jim Al-Khalili make unlikely collaborators. McFadden is a molecular geneticist who specializes in the study of tuberculosis. He thinks in pictures and concepts, and his laboratory at the University of Surrey in the UK is full of machines oscillating flasks and people monitoring colonies of bacteria. Al-Khalili, meanwhile, is a theoretical nuclear physicist. He thinks in mathematics and equations, and for the most part his work requires only a whiteboard and a computer.

What unites this apples-and-oranges pair of scientists is their interest in quantum biology – a new and growing field where practitioners seek to understand how quantum-mechanical processes affect biological systems. Biological systems such as the human nose.

In this podcast, you will hear McFadden and Al-Khalili discuss a possible quantum solution to a long-standing biological puzzle: how does the nose “know” the difference between scent molecules? One of the most intriguing theories, developed by the biophysicist Luca Turin, is that it might come down to a process called inelastic quantum tunnelling. As Al-Khalili explains in the podcast, inelastic quantum tunnelling occurs when an electron dumps a bit of excess energy in order to tunnel to an empty energy level in a nearby atom. Turin’s theory is that this type of tunnelling event is what triggers the firing of olfactory neurons in the nose, thus sending a signal to our brains that gives us the “experience” of smelling something. However, such tunnelling can only take place when a scent molecule is present and able to absorb the electron’s excess energy – and that will only happen if one of the chemical bonds in the scent molecule has the right vibrational frequency. So when we slice into an orange and take a sniff, our noses may actually be sensing the vibrations of chemical bonds in a molecule called limonene, which is responsible for most of the orange’s citrusy scent.

The nose isn’t the only biological system with a possible quantum connection, though. If this podcast whets your appetite for some more examples, you might want to check out McFadden and Al-Khalili’s new book Life on the Edge. The book is written for a popular-science audience, and at the end of the podcast, you’ll hear the pair discussing some of the challenges they faced in writing it.

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