Riddle me this?

Woden

A non-science type person asked a science person friend of mine a simple question. Why do things smell, what is it about something that makes it smell good or bad.

He asked me and i didn't know and have never received a sufficient answer since thinking about the original question and developing it somewhat?

So why do things smell?

What is it about certain molecules with certain functional groups have a bad smell. for example amines are known for there terrible odour?

Or why is it that for example the alkyl chain in a molecule is increased the smell changes for example

ethanoic acid (the acid in vinegar) has a vinegary smell but if you change it to butanoic acid (the additon of 2 carbons and 5 hydrogen's iirc) you get what i think is the smell of smelly socks?

Whats going on here biologically and chemically anyone?

GuanYin

Its really a question of neuroscience and I'm not sure the exact details, but from what I know.....

Smells are the result of airborne chemical structures (and occasionally bacteria) that enter the nose and reach the olfactory mucosa, which is a layer of epithelial cells. The chemical collects here and is transferred to a nerve interface at the olfactory bulb.

Depending on the chemical and its interaction at this point, electrical signals are generated (this is basically down to electrophysiological events at the cell level - ie. changes in how ions, suchs as sodium, calcium, potassium etc are regulated in the cell) and transferred across olfactory nerves to the cerebral cortex of the brain. HEre specific odours are recognised or recorded by signal. The signal triggers the limbic system which controls the neurochemical responsible in emotion and association (ie. memory) which changes how you feel and think about the smell.

Woden

Originally posted by syke

Depending on the chemical and its interaction at this point

This seems to be the key sentence here to me. just like a molecule has a certain colour due to the emitted/reflected (whatever) wavelengths that interact with the sensors in the eye (then the brain does stuff )

so a more thorough knowledge of smell could possible be determined from a study of how the chemicals interact at the olfactory bulb?

GuanYin

Originally posted by Dataisgod
This seems to be the key sentence here to me. just like a molecule has a certain colour due to the emitted/reflected (whatever) wavelengths that interact with the sensors in the eye (then the brain does stuff )

so a more thorough knowledge of smell could possible be determined from a study of how the chemicals interact at the olfactory bulb?

Well the key point I was making was: "this is basically down to electrophysiological events at the cell level - ie. changes in how ions, suchs as sodium, calcium, potassium etc are regulated in the cell"

Cells have an electrical gradient across them as ions flow in and out of cells. Typically the key ions are Calcium (Ca2+), Sodium (Na+) and Potassium (K+). Chemical interacting with a cell membrane can cause increase/decrease in the current across the cell and this can lead to any number of related events, such as stimulation of nerve cells (or indeed the event can happen at the nerves themsleves).

It turns out my guess was close enough. Quick search of Pubmed revealed Czesnik et al. European Journal of Neuroscience , Volume 17 Issue 1 Page 113 - January 2003

You may need a subscription to read the full journal.

Woden

ok i'm with you, cheers syke

ucd has access to the full article

Capt'n Midnight

Part of it may have to do with the shape of the molecule as well as it's functional groups.. AFAIK they would have to bind to membrane proteins - not sure if they need to be transported into the cell or if they can trigger a response from the outside.

Smells are not as easy to classify as taste - there is the old hold your nose and try this trick - a lot of the sense of taste , or the subtlty of it is created by the smell of food.

yeah the acids of higher alcohols really stink - real silage/manure smells..

but for really bad smells you need sulphur - ethyl mercaptan , etc. hydrogen sulphide (stink bombs) is more toxic than cyanide but the smell is so strong people usually move away from it (chemotaxis )

BTW that smell you get off arcing electical contacts is Ozone.

Woden

i've always wondered what ozone smelled like, i know its produced in lightning storms and the like, however i'd need someone round me to say heh get that smell its ozone and i go oh yeah

yeah i'd say the sterics of the interaction are important alright which would explain the effect of chaining the alkyl chain length either in carboxylic acids or even amines

Capt'n Midnight

Don't forget the chain does affect the properties of the active group at the end...

Acetic Acid CH3COOH is Very Strong vinegar - looks like ice, melts at room temperature and really gets up your nose..

Tri-Chloro acetic acid CCl3COOH - it burns holes in things and is really nasty on organ macromolecules - eg: skin

Woden

sure the effect of numerous halogen substition like than can increase the acids strength by about a 1000 fold.

which is another affect to take into account i suppose is the effect on smell of the substition of H's with other atoms. Orignially i was more just thinking along the lines of the chain length