MadSci Network: Neuroscience

Re: Why are the organic componds ,esters, have a sweet smell?

Date: Thu May 12 00:09:38 2005
Posted By: Sean Hunt, Secondary School Teacher
Area of science: Neuroscience
ID: 1112547307.Ns

Dear Gary,

This is a biochemistry question which is not yet fully answered. Your 
question is at the cutting edge of research!
Both smell and taste rely on protein receptors (in the nose) recognising 
The lining at the roof of the nasal cavity has a small area of mucous 
membrane where specialised nerve cell endings are positioned. The cells 
of these receptor nerves have several hair-like extensions that dangle 
down to the surface of the mucous membrane where they are exposed to 
molecules in the inhaled air.
The sensitivity and the selectivity of smell are astonishing. Humans can 
tell the difference between several thousand smells. For some smells just 
four or five molecules of the material are enough to be noticed.
Although scientists have yet to work out the details, it is likely that 
the membranes of receptor nerve cells carry proteins with receptor sites 
able to recognise specific smelly molecules. Binding of the molecule to 
the receptor site results in a message being transmitted to the brain.
Esters are organic compounds arising from a combination of a carboxylic 
acid and an alcohol, giving the R-CO-O-R linkage. Low-molecular weight 
esters give fruits their characteristic odours and flavours. It is 
interesting that almost all the low-weight esters smell pleasant, while 
their acid counterparts are acrid and repellent. This is simply an 
artifact of our evolution; not surprisingly, a selective advantage was 
gained by organisms which turned up their noses at rotting foods but ate 
fresh ones. The coevolution of flowering plants (angiosperms) and humans 
is an interesting story in its own right.  
   The relation of the smell brain (rhinencephalon) to intelligence is an 
old story. It is argued that one reason for mammals developing their 
reasoning powers was that our verminous ancestors, small rats rummaging 
around for dinosaur eggs, were primarily nocturnal. Along with the 
development of sensitive olfactory organs - tiny molecular analysers - 
they had to interpret the smells. This is a lot harder on a neurological 
level than the task the brains of the dinosaurs were charged with, which 
amounted to "chase and eat." The development of the cerebrum directly 
parallels that of the smell brain. To this day, smells remain extremely 
strong triggers of emotions, more so than sights or sounds.

Best wishes,


Thanks to: Dr T.V Padma, Unilever and Pharmcentral.

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