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How we smell: competing theories

Surprisingly, the mechanics of how we smell things and recognise odours still aren’t fully understood.

How we smell: competing theories

US scientists Richard Axel and Linda Buck won the 2004 Nobel prize for their groundbreaking work in this field.

They discovered that 1000 different genes – that’s about three per cent of all our genes – code for smell receptors in our noses. But not everyone subscribes to their lock and key theory…

How we smell

 

Shape theory

This represents the current orthodox thinking in the world of smell recognition – and it has done for some 60 years. Here, part of an odour molecule – the ‘key’ – docks with a receptor in the upper part of our nose – the ‘lock’. This chemical interaction is converted into an electrical signal that travels to the olfactory bulb in the brain.

rosesSeveral receptors corresponding to one scent send signals to one location in the bulb. Then information from several of these areas is relayed to other parts of the brain where the information is combined – forming a pattern. Nobel laureates Richard Axel and Linda Buck have been the main champions of the shape theory, uncovering how the receptors in the nose link with the brain.

Molecule vibration theory

The shape theory sounds reasonable – many things in biology, including our sense of taste, work with a ‘lock’ and a ‘key’. But there’s a problem. Why do molecules with virtually identical shapes, such as Cis-3-hexene-1-ol (cut grass) and Cis-3-hexene-1-thiol (rotten eggs), smell different?

FishThe problems with shape theory are being explored by the charismatic multi-disciplinary scientist, Luca Turin. “The lock and key picture doesn’t explain all the cases. Something else is happening,” he says.

All molecules vibrate at a very specific frequency and Turin believes it’s the molecule’s vibration, not its shape, that we’re detecting. He says there is evidence that molecules with similar vibrational frequencies smell the same.

 

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Issue: 
The Science of Fear
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