Why things smell the way they do

Locks and keys

One widely accepted theory of why one molecule smells like it does is based on the work of a British scientist John Amoore, in the 1950s. He suggested that the receptors (in the lining of the nose) that send the signal from each aroma to the brain worked like a lock and key. The receptors were the locks and particular smells were the keys; people with anosmia had some of their locks missing. He reasoned that the combination of particular aroma/smell molecules with different shapes opened the 'nose locks' and sent an electrical signal to the brain, depending on the shape of the molecule.

Shaking and vibrating

However, in more recent times, another British scientist, Luca Turin, has offered a different explanation. His idea centres not on the shape of molecules but on how they shake and vibrate. Each type of molecule has a slightly different pattern of shaking and vibration, rather like different tuning forks. He believes that it is the unique vibration and shaking of each molecule that is detected by the cells in the nose and then interpreted by the brain.

The fact that smell, according to Turin's theory, invokes a spectrum (of different frequencies) like sight and sound might explain the strange and rare condition known as synaesthesia. This is where someone's senses are mixed up. Some musical composers claim to be able to hear in colours or smell sounds.

Gene count

The Nobel Prize winners Axel and Buck have discovered on explanation for our olfactory abilities. They have shown that the human genome (our genetic make up) contains a 'family' of some 1,000 different genes that produce a similar number of olfactory receptor proteins. This represents nearly 3% of our total gene count, which is estimated to be about 35,000, suggesting that smell to our ancestors was a very important sense indeed.

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