Five facts about smell - Speaker Deck

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FIVE FACTS ABOUT SMELL BY @ALICEBARTLETT

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1. Humans have three chemical senses.

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1. Taste 2. Smell 3. Trigeminal

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2. 1.2% of people do not have a sense of smell*. [Gilbert and Wysocki 1987]

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*I am one of those people. Hello.

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3. Some smells are easily detectable to humans even at very low concentrations. Nobody knows why.

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**SUB-LIST TIME**

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FIVE SMELLY SMELLS: ! 1. GRAPEFRUIT JUICE 2. CORK ODOUR 3. BUTTER 4. PEPPERONI 5. STRAWBERRY [Ohloff 1990]

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FIVE SMELLY SMELLS: ! Nobody knows why the human nose is better at detecting these compounds but it really is.

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Comparison of detection thresholds in water: ! Limonene: 10ppb ! Menthene-8-thiol: 0.00001ppb (grapefruit) Demole E., P Enggist and G. Ohloff, Helv. Chem. Acta, 65, 1785-1794 (1982) (limonene)Fazzalari, F A., editor, Compilation of Odor and Taste Threshold Data, ASTM Data Series DS 48A (1978)

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TODAY YOU LEARNT: ! Grapefruits are 1 million times smellier than lemons*. *kinda

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4. There are seven primary smells

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These classifications are more psychological than chemical but are very handy for talking about smells.

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**SUB-LIST TIME**

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SEVEN PRIMARY ODOURS: ! 1. Musky (moth balls) 2. Floral (roses) 3. Pepperminty 4. Ethereal (detergent) 5. Pungent (vinegar) 6. Putrid (bad eggs) [Amoore et al. 1964]

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5. We don’t really know how humans detect smell.

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The facts:

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You have between five and six million smell receptors

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You have 347 different types of olfactory receptor.

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One olfactory receptor will detect several (similar) molecules and react with different intensities to them.

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Your olfactory receptors seem to react in a combinatorial way. [Buck et al. 1999]

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The problem

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We don’t know how this reaction works. There are two theories and neither have been disproven

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“Shape Theory” states that molecules fit into receptors like a lock and key.

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This makes sense intuitively and is the historically preferred theory

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BUT

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Whilst shape theory works for most compounds, there are some exceptions:

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Some compounds with vastly different shapes smell the same

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And some compounds with very similar shapes smell totally different

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This model has also not been able to predict any smells from looking at the shape, which you would expect it to be able to.

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“Vibration Theory” states that it is not the shape but the vibration pattern of the molecule that determines it’s odour

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This would explain why two compounds with different shapes can smell the same.

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But there are still challenges.

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Whilst some compounds with the same vibration energy smell the same, some smell different!