The quantum weirdness powering biology

It allows particles to be in two places at once, pass through impenetrable barriers or maintain spooky connections with other particles – but can quantum physics really govern biology?

Quantum mechanics is generally considered to be the weirdest of sciences. But, it was thought, this is kind of OK because it was only needed to account for the behaviour of the tiniest objects – electrons, protons, photons etc – and not to the big stuff that we can see. But, one of the founders of quantum mechanics, Erwin Schrödinger, pointed out that things were not so simple. Schrödinger imagined sealing a cat inside a box. The cat’s fate was linked to the quantum world through a vial of poison that will only be released if a single radioactive atom decays. Quantum mechanics insists that the microscopic atom must exist in a peculiar state called superposition until it is observed, a state in which it has both decayed and not decayed at the same time. But because the cat’s survival depends on what the atom does, then, according to quantum mechanics, the very macroscopic cat must also exist as a superposition of a live and dead cat until somebody opens the box and observes it.

Of course nobody really believes that a cat can be simultaneously dead and alive. Schrödinger’s thought experiment was to point out that it isn’t so easy to compartmentalise the world into quantum and non-quantum regions. And although no sinister scientist has ever set up the cat experiment, in reality, the fate of lots of macroscopic objects will sometimes depend on the dynamics of individual particles or small numbers of particles that are subject to weird quantum laws, particularly when those macroscopic objects are alive.

In his book, What is Life published in 1944, Schrödinger claimed that some of life’s most fundamental building blocks must nevertheless, like unobserved radioactive atoms, be quantum entities able to perform counterintuitive tricks. Indeed, he went on to propose that life is different from the inanimate world precisely because it inhabits a borderland between the quantum and classical world: a region we may call the quantum edge.

Schrödinger was particularly interested in the question of heredity, and in 1944, a decade before Watson and Crick, the physical nature of the gene was still mysterious. However, it was known that genes are inherited with an extraordinary high degree of fidelity: less than one error in a billion. This was a puzzle, because one of the few other known facts about genes was that they were very small – far too small, Schrödinger insisted – for the accuracy of their copying to depend on the classical laws whose accuracy is dependent on the averaging of the disorderly motion of trillions of particles; what Schrödinger called ‘order-from-disorder’ rules. He proposed instead that genes must be composed of a ‘more complicated organic molecule’, one in which ‘every atom, and every group of atoms, plays an individual role’.

Schrödinger called these novel structures ‘aperiodic crystals’. He asserted that they must obey quantum rather than classical laws, and further suggested that gene mutations might be caused by quantum jumps within the crystals. He went on to propose that life’s defining characteristic might that its orderly behaviour is derived directly from the strict rules of quantum mechanics. He called this principle ‘order from order’.

Was he right? A decade later Watson and Crick unveiled the double helix. Genes turned out to be made from a single molecule of DNA, which of course is a kind of molecular string with nucleotide bases (the genetic letters) strung out like beads. That’s an aperiodic crystal in all but name. And, just as Schrödinger predicted, ‘every group of atoms’ does indeed play ‘an individual role’, with the position of even individual protons – a quantum property – determining each genetic letter. There can be few more prescient predictions in the entire history of science. The colour of your eyes, the shape of your nose, and aspects of your character, intelligence or propensity for disease are encoded at the quantum level.

And yet, by-and-large, the new science of molecular biology that followed Watson and Crick’s discovery remained wedded to classical concepts. This worked pretty well in the latter half of the 20th century, as molecular biologists and biochemists focussed on large-scale phenomena, such as metabolism, which is a product of very large numbers of particles operating under the order-from-disorder principle. But the attention of 21st century biology is now turning to the dynamics of ever-smaller systems – even individual atoms and molecules inside living cells – where quantum mechanics might play a role.

Recent experiments indicate that some of life’s most fundamental processes do indeed depend on weirdness welling up from the quantum undercurrent of reality. This is the new science of quantum biology.

One of the most intriguing candidates for quantum biology is bird navigation. Many birds and other animals are known to navigate around the globe during their annual migrations by detecting the earth’s magnetic field: they have a built-in compass. But how it works is a mystery – the earth’s magnetic field is very weak; so weak that it’s difficult to see how it could be detected inside an animal’s body. Further puzzles emerged in studies with the European robin, which demonstrated that its compass is light-dependent, and that, unlike a conventional compass, it detects the angle of magnetic field lines relative to the Earth’s surface, rather than its orientation. No one had any idea why a compass should be light-dependent or how such a compass could detect the angle of the Earth’s magnetic field.

Then in the 1970’s the German chemist, Klaus Schulten, discovered that some chemical reactions produced pairs of particles (free radicals) whose unpaired electrons remained connected via a peculiar quantum property called entanglement. This quantum phenomenon is so weird that even Einstein – who remember gave us black holes and warped space-time – couldn’t believe it, disparagingly calling it ‘spooky action at a distance’. It allows distant particles to remain instantaneously connected, no matter how far away they are: they can be flung to opposite ends the galaxy and yet remain connected. Despite Einstein’s scepticism, entanglement is real. Indeed, hundreds of experiments have detected its influence. Schulten discovered that entangled pairs of free radicals can be extraordinarily sensitive to both the strength and the orientation of magnetic fields. He went on to propose that the enigmatic avian compass was using quantum entangled free radicals. Hardly anyone took the idea seriously in the 1970’s but in 2000 he wrote an influential paper with his student, Thorsten Ritz, showing how light could be used to make a quantum entangled compass in a bird’s eye.

In 2004, Ritz teamed up with a highly respected husband and wife team of ornithologists, Wolfgang and Roswitha Wiltschko, to test a prediction of the theory, that, if the bird’s magnetic compass was relying on entanglement, then it should be disrupted by high frequency radio waves. The team tested the theory on the European robin whose magnetic compass was indeed thrown off-course by high frequency radio waves, exactly as the theory predicted.

It appears that robins, and probably other birds and animals, use Einstein’s spooky action to navigate around the globe every year.

Navigation is important for birds and other animals but quantum biology is involved in much more fundamental biological processes, such as how enzymes work. Enzymes are the workhorses of life, speeding up chemical reactions so that processes that would otherwise take thousands of years happen inside living cells in milliseconds. How they achieve this speed-up – often more than a trillion-fold – has long been an enigma. But now, recent research by Judith Klinman at the University of California and Nigel Scrutton at the University of Manchester, among others, has shown that several of them employ a weird quantum trick called tunnelling. Basically, the enzyme encourages a process whereby electrons and protons vanish from one position in a biochemical and instantly rematerialise in another, without visiting any of the in-between places – a kind of molecular quantum teleportation.

Another exciting area of quantum biology is the process that makes nearly all the biomass on our planet, photosynthesis. This remarkable reaction is responsible for turning light, air, water and a few minerals into grass, trees, grain, apples, forests and, ultimately, the rest of us who eat either the plants or the plant-eaters. The initiating event is the capture of a photon of light by a chlorophyll molecule. Its light energy is rapidly converted into electrical energy, which is then transported to a biochemical factory called the reaction centre, where it is harnessed to fix carbon dioxide and turn it into plant matter. This energy transport process has long fascinated researchers because it can be so efficient – close to 100%. How is it possible that green leaves can transport energy so much better than our most sophisticated technologies?

Graham Fleming’s laboratory at University of California, Berkeley has been investigating this question for more than a decade using a technique called femtosecond spectroscopy. Essentially, the team shines very short bursts of laser light at the photosynthetic complex in order to discover the path of the photon as its makes its way to the reaction centre. Back in 2007, the team investigated a bacterial system called the FMO complex, in which the photon energy has to find its way through a cluster of chlorophyll molecules. The energy was thought to travel as a kind of electrical particle called an exciton that hopped from one chlorophyll molecule to another, much as Schrödinger’s cat might have hopped from one boulder to another across a busy stream. But this didn’t make complete sense. Lacking any navigational sense, most photon energy should hop aimlessly in the wrong direction, ending up in the metaphorical water. And yet, inside plants and bacteria that perform photosynthesis, nearly all packets of photon energy reach the reaction centre.

When the team shone the laser at the system, they observed a very peculiar light echo that came in beat-like waves. These ‘quantum beats’ were a sign that, instead of taking a single route through the system, the photon energy was using a phenomenon called quantum coherence to travel by all possible routes simultaneously. Imagine that, when confronted by the stream, the famous cat somehow divided itself into lots of identical quantum-coherent cats that hop across the chlorophyll boulders by every available route to find the quickest. Quantum beats have now been detected in many different photosystems, including those of regular plants such as spinach. The most important biochemical reaction in the biosphere is exploiting the quantum world to put our food on our table.

Finally, we come to the very mechanism of evolution. As mentioned above, Schrödinger suggested that mutations could involve a kind of quantum jump. In their classic DNA paper, Watson and Crick went on to propose that they may involve nucleotide bases switching between alternative structures, a process called tautomerisation that might involve quantum tunnelling. In 1999, we suggested that proton tunnelling might account for a peculiar kind of mutation – called adaptive mutation – that appeared to occur more frequently when they provided an advantage. After a long lull, we are currently attempting to find experimental evidence for proton tunnelling in DNA.

There is however a big puzzle at the heart of quantum biology. Physicists generally have to have to cool their experimental systems down to nearly absolute zero temperature and perform them in a near perfect vacuum to minimise the stormy molecular noise that would otherwise destroy delicate quantum effects. How quantum phenomena are maintained for biologically-relevant timescales inside warm, wet and molecularly noisy living cells remains something of a mystery. However, recent research offers a tantalising hint that, rather than avoiding molecular storms, life embraces them, rather like the captain of a ship who harnesses turbulent gusts and squalls to maintain his ship upright and on-course. As Schrödinger predicted, life navigates a narrow strait between the classical and quantum worlds: the quantum edge.

The author: 

Jim Al-Khalili is a professor of theoretical physics at the University of Surrey and a BBC broadcaster.

Johnjoe McFadden is professor of molecular genetics at the University of Surrey where his work focusses on investigating the genetics of disease-causing microbes.