system
Given the following post, respond as if you're commenting on Reddit:
human
How does smell work?

At an interested engineer, totally not a biologist level, I can smell things.  When I try to understand how smell works, though, I get confused.  I've heard a couple of answers which I'm not convinced by, so thought I'd get some clarity.

* Resonating bonds between atoms in molecules.  This sounds semi plausible, but it doesn't account for handedness, where (AIUI) for instance a left hand identical arrangement of atoms smells of lemon and the same atoms arranged right-handed smell of mint.

* That suggests shape is the driver, which seems compelling, but then why do brand new molecules we never evolved for have a smell?  Not perfume blends, that's obvious, but molecularly.  If we don't have a receptor for a molecule shape, how could we smell it, and how could we evolve a receptor for something that doesn't exist?

* Smell buds, and every aroma is a unique combination of the nose equivalent of bitter/sweet/salt/acid/umami.  This seems pretty unconvincing to me as with shape alone - how could novel molecules smell?

* Nose as a diffraction column.  Doesn't seem intuitive, but unique graphs for aromas at least includes the possibility of novel molecules having a smell, but we return to the handedness question.

What am I missing?  Am I overthinking it all, missing something obvious?  I mean it seems like something we should know, but I'm not seeing consensus.

Thanks for your ideas about this, it's troubled me a while!
gpt
**Shape is the driver.** This is a core concept of biology. Biology is just physics due to shape due to chemistry.

There are tons of chemicals we can’t smell.

The ones we can smell are easy enough to explain. There is not simply one receptor per smell just as there is not just one taste receptor per compound in your food. Each smell is a composite signal of all the signals from different receptors that got triggered to specific degrees and your brain turns it all into a perception. You have receptors with specific shapes, and many different molecules will have a close-enough fit for that receptor to trigger it to some degree. Some of these signals may be time-modulated, as a receptor with a molecule that fits half the time will send half the overall signal as better-fitting molecule might.

You only need a toolkit of a certain number of receptors to generate unique signals for an arbitrary number of chemicals. Each combination of receptor-response “fingerprint” is unique; each scent need not have its own unique receptor.

Your immune system also works or you’d be too dead to type this, which suggests that a limited toolkit is quite sufficient for recognizing arbitrarily many new shapes. Instead, your infant body prepared a limited set of shapes ahead of time, and they’ve worked to keep you alive since.