Using flies to sniff out a new theory of smell

Our sense of smell is really quite incredible. Every time we take in a breath or taste food, countless molecules swarm into our nasal passages. As they move up the nasal tract, these visitors arrive at a patch of cells on which there are over 10,000 different kinds of docking stations. These cells are odor receptors, and each of them can register a different odor. Together they make up a chemical detector that is much more sensitive and versatile that anything we can come close to building.

In a paper published in the journal PNAS in February, the authors demonstrate through a series of ingenious experiments that smell can be sensitive enough to pick up on tiny differences in atomic vibrations.

The conventional theory of smell works somewhat like a lock and a key. The molecules are the key, and they ‘lock in’ to receptors that fit their exact shape and size. This is the shape theory of smell, and the basic idea had been suggested in the 1st century BCE by the Epicurean philosopher Lucretius. The idea has since garnered substantial evidence with the discovery of odor receptors, leading to the 2004 Nobel Prize in Medicine for working out the overall picture of how smell works.

An alternative hypothesis is the vibration theory. This proposes that smell works not by detecting the shape of molecules, but by measuring how the atoms in a molecule are vibrating.

Molecules are groups of atoms that are held together by chemical bonds. These bonds are somewhat elastic, causing the atoms in the molecules to constantly jiggle about. This is analogous to what would happen if you were to connect balls together with springs (something that physicists love to do). But the analogy breaks down at this microscopic scale, and one needs to resort to the laws of quantum mechanics to understand what is happening. It turns out that, similar to the balls and springs, molecules have certain ways in which they prefer to jiggle. They can stretch, rock, wag and twist around.

So, which is it? Does smell work via shape or vibration? The authors set out to address this question with flies.

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Plastic flowerpots and the mighty alphabet of the universe

Take a look at this toy. Depending on where you stand on the aesthetic spectrum you may find it rather tacky, but there’s also something very endearing about it. It is sold on pavements and at traffic lights in metros across India, the latest in a line of quirky street-side toy fads that have in the past included items such as a birdcage shaped head tickler. People have been sufficiently entranced by its charms to make this little flowerpot a mass-produced phenomeon. In Delhi and Bombay it’s ubiquitous – I’ve seen it on office desks, window sills, toilet water tanks, and car dashboards are starting to look incomplete without one.

It grabs your attention with its garishly bright colors and shiny plastic, making no pretenses of looking ‘natural’. But it moves in a surprisingly organic fashion. The leaves sway to and fro, with the flower nodding along in cheery unison. It sways in the breeze, but even if the air is completely still it keeps rocking to its own solar-powered beat.

The American physicist Richard Feynman was once interviewed on BBC’s science program Horizon, where he made a case for science on aesthetic grounds. He was responding to a certain criticism of science – that it misses out on some essential aspect of our human experience by reducing the world to mechanics. Feynman retorted that when we consider a flower, our appreciation of its beauty may be informed by our aesthetic experiences, but the beauty is basically accessible to all of us who can perceive it. However, by asking more pointed questions about the flower – what is it for? how does it work? what was the world like before flowers?  – we can gain a deeper appreciation. An understanding of science only adds to our appreciation of beauty, it does not detract.