Ideas from the stranger side of physics could explain some long-standing mysteries of biology
EVER felt a little incoherent? Or maybe you've been in two minds about something, or even in a bit of delicate state. Well, here's your excuse: perhaps you are in thrall to the strange rules of quantum mechanics.
We tend to think that the interaction between quantum physics and biology stops with Schrödinger's cat. Not that Erwin Schrödinger intended his unfortunate feline - suspended thanks to quantum rules in a simultaneous state of being both dead and alive - to be anything more than a metaphor. Indeed, when he wrote his 1944 book What is Life?, he speculated that living organisms would do everything they could to block out the fuzziness of quantum physics.
But is that the case? Might particles that occupy two states at once, that interact seemingly inexplicably over distances and exhibit other quantum misbehaviours actually make many essential life processes tick? Accept this notion, say its proponents, and we could exploit it to design better drugs, high-efficiency solar cells and super-fast quantum computers. There's something we need to understand before we do, though: how did the quantum get into biology in the first place?
On one level, you might think, we shouldn't be surprised that life has a quantum edge. After all, biology is based on chemistry, and chemistry is all about the doings of atomic electrons - and electrons are quantum-mechanical beasts at heart. That's true, says Jennifer Brookes, who researches biological quantum effects at Harvard University. "Of course everything is ultimately quantum because electron interactions are quantised."
On another level, it is gobsmacking. In theory, quantum states are delicate beasts, easily disturbed and destroyed by interaction with their surroundings. So far, physicists have managed to produce and manipulate them only in highly controlled environments at temperatures close to absolute zero, and then only for fractions of a second. Finding quantum effects in the big, wet and warm world of biology is like having to take them into account in a grand engineering project, says Brookes. "How useful is it to know what electrons are doing when you're trying to build an aeroplane?" she asks.
Might this received wisdom be wrong? Take smell, Brookes's area of interest. For decades, the line has been that a chemical's scent is determined by molecular shape. Olfactory receptors in the nose are like locks opened only with the right key; when that key docks, it triggers nerve signals that the brain interprets as a particular smell.
Is that plausible? We have around 400 differently shaped smell receptors, but can recognise around 100,000 smells, implying some nifty computation to combine signals from different receptors and process them into distinct smells. Then again, that's just the sort of thing our brains are good at. A more damning criticism is that some chemicals smell similar but look very different, while others have the same shape but smell different. The organic compounds vanillin and isovanillin, for example, smell differently but are two similarly shaped arrangements of the same molecule.
There is an alternative explanation. Around 70 years ago, even before the lock-and-key mechanism was suggested, the distinguished British chemist Malcolm Dyson suggested that, just as the brain constructs colours from different vibrational frequencies of light radiation, it interprets the characteristic frequencies at which certain molecules vibrate as a catalogue of smells.
The idea languished in obscurity until 1996, when Luca Turin, a biophysicist then at University College London, proposed a mechanism that might make vibrational sensing work: electron tunnelling. This phenomenon results from the basic fuzziness of quantum mechanics, and is a staple of devices from microchips to microscopes. When an electron is confined in an atom, it does not have an exactly defined energy but has a spread of possible energies. That means there is a certain probability that it will simply burrow through the energy barrier that would normally prevent it escaping the atom.
Turin's idea is that when an odorous molecule lodges in the pocket of a receptor, an electron can burrow right through that molecule from one side to the other, unleashing a cascade of signals on the other side that the brain interprets as a smell. That can only happen if there is an exact match between the electron's quantised energy level and the odorant's natural vibrational frequency. "The electron can only move when all the conditions are met," Turin says. The advantage, though, is that it creates a smell without the need for an exact shape fit.
It was a controversial notion. In 2007 Brookes, then also working at University College London, and colleagues showed that the mechanism is physically plausible: the timescales are consistent with the speed with which the brain responds to smell, and the signals generated are large enough for the brain to process (Physical Review Letters, vol 98, p 038101). And in January this year Turin, now at the Alexander Fleming Biomedical Sciences Research Centre in Vari, Greece, and his colleagues delivered what looks like evidence for vibrational sensing. They showed that fruit flies can distinguish between two types of acetophenone, a common base for perfumes, when one contains normal hydrogen and the other contains heavier deuterium. Both forms have the same shape, but vibrate at different frequencies (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1012293108). That sensitivity can only mean electron tunnelling, says Andrew Horsfield of Imperial College London, a co-author on Brookes's paper: in classical models of electron flow the electron would not be sensitive to the vibrational frequency. "You can't explain it without the quantum aspect."
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