Quantum life: The weirdness inside us
03 October 2011 by Michael Brooks
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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 smellMovie Camera, 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 compound benzaldehyde, for example, comes in two almost identical molecular arrangements, vanillin and isovanillin, that have very distinctive smells.
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."
Smell is not the only thing that proponents of quantum biology think it might explain: there's also the mechanism that powers the entire animal kingdom. We all run on adenosine triphosphate, or ATP, a chemical made in cells' mitochondria by moving electrons through a chain of intermediate molecules. When we attempt to calculate how speedily this happens, we hit a problem. "In nature the process is much faster than it should be," says Vlatko Vedral, a quantum physicist at the University of Oxford.
Vedral thinks this is because it depends on the quality of "superposition" which allows the sort of quantum-mechanical wave that describes electrons to be in two places at once. He reckons quantum omnipresence might speed the electrons' passage through the reaction chain. "If you could show superposition is there and it's somehow also important for the electron flow, that would be very interesting," he says.
Vedral's first calculations support the idea, but he says it is too early to make any claims. It is hard to estimate all the parameters involved in electron transport, and it is possible that the classical calculations just used the wrong numbers. "And as yet we have no experimental proof," he says. Such proof might be quite close by - in how plants and some bacteria get their energy. It seems photosynthesis might be very much a quantum game.
Direct evidence that this is so came in 2007, when a group led by Graham Fleming at the University of California, Berkeley, took a close look at photosynthesis in the green sulphur bacterium Chlorobium tepidum. They detected "beating" signals characteristic of quantum wave interference in the photosynthesising centres of bacteria cooled to 77 kelvin (Nature, vol 446, p 782). In January last year, a group led by Gregory Scholes of the University of Toronto, Canada, showed a similar effect at room temperature in light-harvesting proteins from two marine algae (Nature, vol 463, p 644).
This is a trick we might like to learn from. Although photosynthesis is not particularly efficient overall, the initial stage of converting incoming photons into the energy of electrons within a photosynthesising organism's light-gathering pigment molecules is extremely effective. When sunlight is weak, plants are able to translate more than 90 per cent of photons into an energy-carrying electron; in strong sunlight plants have to dump about half the energy to avoid overheating.
Scholes's explanation for this is that when sunlight hits electrons, they are kicked into a quantum superposition that allows them to be in two places at once. That effectively "wires" light-gathering molecules to the reaction centre where the photosynthesis takes place for a few hundred femtoseconds. During that time, an electron can, according to quantum rules, take all paths between the two places simultaneously. Probing the process more closely causes the superposition to collapse - and reveals the electron to have taken the path that lost it the least energy.
Might we take a leaf out of biology's book? Scholes thinks so. "Every year there are thousands of papers published on energy transfer," he says. "It sounds harsh but we haven't learned a thing apart from the obvious." A better understanding of what is going on might also help us on the way to building a quantum computer that exploits coherent states to do myriad calculations at once. Efforts to do so have so far been stymied by our inability to maintain the required coherence for long - even at temperatures close to absolute zero and in isolated experimental set-ups where disturbances from the outside world are minimised.
This remains the central conundrum for the physicists studying quantum aspects of biology. If we can't do these things in our isolated labs, how can a leaf in your less-than-isolated garden do it? If only the European robin could do more than warble chirpily. Perhaps then it could tell us - and explain its own apparent quantum superpowers, too (see "Bird's eye view").
At the moment we have little more than educated guesses. One is that it is simply a wonder of evolution. Scholes thinks that proteins around algae's light-harvesting equipment might have evolved structures that shield disturbances from the environment and so allow processes within to exploit the magic of quantum physics to give them a selective advantage. Vedral thinks something similar, although why and how nature would do this, he says, is "completely unclear".
Turin shrugs his shoulders, too. "Life's 4 billion years of nanoscale R&D will have engineered many miracles," he says. We should learn to accept what we see and try to mimic it, he says - and not just in solar cells and quantum computers. While what makes a drug effective or ineffective is far from clear, for instance, we do know that the operation of things like neurotransmitters in our brains depends on redox reactions, which are all to do with electron flow. If those flows occur in weirder ways than we have hitherto imagined, that could open up a new path to design drugs to treat some of our most pernicious ailments.
Others think nature is leading us up the garden path. Is photosynthesis, for example, really made more efficient by exploiting quantum interference and superposition effects? "I think the jury is still out on this question," says Robert Blankenship of Washington University in St Louis, Missouri. "I think it is possible that, depending on the details of the system, it could just as easily decrease the efficiency." Simon Benjamin, a colleague of Vedral's at the University of Oxford, wonders how we can really put long-lived quantum states to work if indeed they do pop up in natural systems. "It's certainly too early to be making dramatic claims," he says.
All those stepping gingerly around this new field agree that caution is needed - yet there is a palpable sense of excitement. Max Planck first discovered quantum theory more than a century ago because of odd observations that could be explained in no other way. That led to the laser and the semiconductor and all the technological revolutions they have seeded. Quantum biology is at that early stage of inexplicable observations. Turin for one believes something big is emerging. "I can't help thinking we are seeing just a small part of a far, far bigger iceberg," he says.
Bird's eye view
Another instance of quantum effects in biology might be in how birds sense Earth's magnetic field (New Scientist, 27 November 2010, p 42). In 2004, Thorsten Ritz of the University of California, Irvine, showed how magnetic disturbances that would only show up on systems that could detect transitions between particular quantum-mechanical atomic spin states could disrupt the compass of the European robin, Erithacus rubecula.
Ritz suggested that birds come equipped with a sensor system containing spin states that flip in response to changes in Earth's magnetic field, producing signals that the bird's brain in some way detects. But how?
The first proposal was that some apparatus in the eye initiates a chemical response. But this would require a constant, fast flipping of spins to keep chemical information flowing, whereas the birds seemed to maintain delicate spin states for extraordinarily long times of up to 100 microseconds.
According to the late Marshall Stoneham of University College London and his colleagues, the problem might be overcome if the birds used something similar to a human visual peculiarity that detects light polarisation. Known as Haidinger's brush, this superimposes a faint, yellow bow-tie shape on our visual field, and is thought to result from the way blue light-absorbing lutein molecules are arranged in concentric circles within our eye. Stare at a blank piece of paper and a polarising filter or a blank document on a laptop screen and you can see it for yourself.
Stoneham calculated that a magnetic field could produce a similar distortion in a bird's visual field, the orientation of which would change with a change in magnetic field. Crucially, that would occur only if quantum states lasted long enough to affect many of the bird's light sensing molecules at the same time. Birds might see the result, Stoneham suggested, in a kind of a head-up display of the kind that is embedded in the windscreens of some luxury cars (arxiv.org/abs/1003.2628).
Michael Brooks is a consultant for New Scientist. His latest book is Free Radicals: The secret anarchy of science (Profile, 2011)