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Quantum mechanics

Bringing the quantum to life

02 Apr 2015
Taken from the April 2015 issue of Physics World

Life on the Edge: the Coming of Age of Quantum Biology Jim Al-Khalili and Johnjoe McFadden 2014 Bantam Press £20.00hb 368pp

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Ever since Erwin Schrödinger’s classic 1944 book What is Life?, physicists have been eagerly looking for applications of quantum mechanics to biological systems. By suggesting that the laws of thermodynamics are not well suited to explain highly reliable biological processes (such as heredity) that depend on a relatively small number of particles, Schrödinger opened the door for a search for quantum effects in biology. Have we made any progress in the last 71 years towards understanding how quantum physics can explain the mysteries of life? The answer, Jim Al-Khalili and Johnjoe McFadden argue in their latest book, is an enthusiastic “yes”.

In Life on the Edge: the Coming of Age of Quantum Biology, the authors – a theoretical nuclear physicist and a molecular geneticist – take us on a head-spinning tour of the many biological effects whose mechanisms are starting to be explained by quantum mechanics. Some of the examples they provide have solid experimental backing. For example, it is now well known that enzymes – the ubiquitous proteins that speed up vital biochemical reactions – rely on quantum tunnelling of electrons and protons to enhance reaction rates. Quantum mechanics may also help to explain the extraordinary efficiency of one of the most important biochemical reactions on the planet: the harvesting of light energy by photosynthetic complexes in plants. After a chlorophyll molecule absorbs a photon, an electron–hole pair is created, and this pair must travel some distance along a series of biomolecules in order to reach the site where photosynthetic oxidation reactions take place. It may accomplish this not by undergoing a slow, classical random walk, but by relying on a much more efficient quantum walk. In effect, it could travel to the reaction site by all possible paths.

Some of the finest writing in the book, though, relates to applications of quantum mechanics to biology that are still not fully understood. The ability to distinguish different smells may, for example, have a quantum connection, and the quest to elucidate why different odorant molecules smell the way they do is laid out in an exciting “whodunit” fashion. The key mechanism, it seems, may be the resonant frequencies of the molecular bonds within odorants, but how does the body sense this? One leading theory is that each receptor molecule in the nose becomes activated if it binds to an odorant molecule with the “right” vibrational modes – that is, modes that can exchange energy with electrons tunnelling between different sites on the receptor molecule.

Equally fascinating is the historical narrative of how scientists have come to understand the mechanisms that many animals (most notably migrating birds) use to orient themselves based on the Earth’s magnetic field. The most convincing explanation, the authors suggest, is related to chemical reactions that take place in a protein located in the eyes of the animal. These reactions rely on quantum tunnelling of electrons, and the rate at which they occur is extremely sensitive to weak magnetic fields – thus giving the animal a chemically based quantum compass.

About a third of the book is devoted to much more speculative applications of quantum physics to major unsolved problems in the biological sciences. Can the quantum tunnelling of protons across DNA base pairs spontaneously induce genetic mutations? Could life on Earth have emerged in a comparatively short timescale because the particles that made up primordial biomolecules were quantum entangled, thus allowing them to perform an efficient quantum search for a self-replicating configuration? Is consciousness partly explained by the effect of macroscopic electromagnetic fields on ions that quantum tunnel through membrane channels in neurons? These proposals are, in most cases, little more than speculations by the authors, and are rightly labelled by them as such. Nevertheless, these speculations are presented in a clear, entertaining form, and they give the authors a handy excuse for providing interesting overviews of some of the most fascinating problems in biology.

Physicists are used to associating quantum effects with low-temperature, isolated systems in very well-controlled laboratory conditions. For many, then, the most fascinating question in the book is how quantum coherence is able to persist long enough in hot, messy biological systems to make quantum effects relevant there. To address this question, the authors present a carefully amassed body of evidence that biological noise actually plays a role in enhancing the timescale of quantum coherence in biological systems, rather than reducing it. This suggestion explains the title of the book: life exists on an edge, dependent on conditions that are carefully tuned (presumably by evolution) to allow quantum mechanical effects to influence biological processes. But it also raises questions of a different sort. Might a better understanding of the conditions in which biological quantum systems operate lead us to novel technologies? Could we, for example, create photosynthesis-mimicking power generators with efficiencies that go much beyond the classic Carnot limit? Could it even be the key to the creation of artificial life forms?

The breadth of the science and scholarship presented in the book is outstanding. Perhaps to minimize reader fatigue, the authors make abundant use of nature documentary-style anecdotes; this does, however, slow the pace of the book, and busier readers might have preferred it if the authors had left them out. Some readers could probably also have done without lengthy descriptions of introductory biology and physics concepts, especially when the details of a few of the biological quantum effects are occasionally somewhat glossed over.

On the other hand, the book is remarkable for the way it incorporates information from scientific literature published just a few months ago – contributing to the impression that quantum biology is an exciting, emerging field. By the end of the book, most readers will be yearning for a sequel in a few years, when some of the biological questions left open in this book will have been answered and new, exciting ones will be awaiting explanations – explanations that, if the authors are right, will most likely involve quantum mechanics.

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