Introducing quantum biology :
The link between quantum physics and life has long been debated, perhaps most famously by Erwin Schrödinger in his 1942 book “What is Life?”. However, research into quantum biology is still very much in its infancy – this is despite the fact that applications of quantum theory are readily used in many different fields, ranging from explaining chemical reactions and how the sun works, to MRI machines and computers. One of the main reasons for this is that the quantum world is inherently counter-intuitive and bears little resemblance to the intuitively sensible macroscopic Newtonian universe. Indeed, while some attempts to integrate quantum theories into a “Grand Unified Theory” of everything have met with some success, there are still big holes in our understanding. For this reason, quantum physics has largely remained the domain of physicists and theoretical mathematicians, which has been compounded by the fact that studying the “quantum world” generally requires highly controlled conditions as quantum effects rapidly disappear when quantum systems interact with their environments (called “decoherence”), and therefore generally require the use of expensive equipment. In contrast, biological research is generally cheaper and easier to do and has been incredibly successful in explaining the top-level biochemistry of life, but as any chemist will tell you, to truly understand the underlying chemistry does require quantum physics to explain how chemistry actually works.
Quantum biology is preoccupied with the study of living systems from the simplest redox reaction to the complex signalling network that constitutes the nervous system. It has been said that the brain is not simply a computer system, and as the Nobel prize winning scientist, Roger Penrose has proposed somewhat controversially, it may utilise quantum principles to enable it to process information and generate awareness (Hameroff and Penrose 2014). Quantum theories of the mind have led to a whole new field of science – “quantum neurophysics” (Tarlaci and Pregnolato 2016), which mirrors the idea that life is anchored in the quantum world (Al-Khalili and McFadden 2014). This suggests that life has always exploited quantum effects. Interestingly, it is now becoming clear that bacteria can transfer electrons both between the same species and with other species in a form of symbiosis via “bacterial nanowires”. In effect, these are biological conductors; they can transfer energy. Significantly, this “conductance” appears to have been solved in at least two ways by nature: one is more similar to classical metallic conductance based on free electron theory, while the other seems to depend on quantum effects – such as “tunnelling” (Lovley and Malvankar 2015). It is thus possible that if life is using quantum principles today, they were probably important in its origins. As another Nobel Prize winning scientist was purported to have said, Albert Szent-Györgyi, “life is nothing but an electron looking for a place to rest” (Lane 2015).
A quantum world :
Quantum theory began as an attempt to explain the deviation of material phenomena from their classical description. The ultraviolet catastrophe and the photoelectric effect demanded a new interpretation of the microscopic world. Quantum literally means “how much” but is today used to describe a minimum unit of energy or matter. His investigation of blackbody radiation led Planck to postulate a “quantum of action”, a minimum energy change that became known as Planck’s constant, or h, which equals 6.6 x 10-34 J.s (Joule seconds). The implications of this were profound, not least of which was a new model of atomic structure, which allowed for explanations as to how atoms maintain their shape. Quantum theory also gave rise to the introduction of spin, an intrinsic property that satisfies Pauli’s exclusion principle and describes the interaction of fundamental particles with a magnetic field.
One of the more counterintuitive ideas proposed in the new theory was that electromagnetic radiation – light – which was accepted to behave as a wave, might also demonstrate discrete, ‘quantised’ behaviour. A particle of light became known as a photon. The mathematical formalisation of quantum theory led to Schrodinger’s famous wave equation as well as Heisenberg’s Uncertainty Principal, which limits the precision of simultaneous measurements. While Planck and Einstein, among others, described how light behaves as a particle, de Broglie showed that just as a photon can behave both as a wave and a particle, all particles could have a “wave function” ascribed to them – matter-waves. This was pivotal, as electrons could thus also behave as waves, displaying wave effects such as diffraction and interference and leading to fascinating quantum effects such as “tunnelling” and “superposition”. The phenomenon of “tunnelling” explains how objects can permeate classically forbidden energy barriers because they can be described as probability waves. The likelihood of this tunnelling can be predicted by the Schrödinger equation and depends on energy and mass, and the width of the barrier. This makes tunnelling likely for very small particles like electrons and protons, but extremely unlikely for large objects such as humans.
Wave-particle duality also allows for “quantum coherence” such that an electron, proton, atom, or a group of atoms, exist in “quantum superposition” – in effect, existing as a collection of all possible states. However, when particles are observed, they appear in one particular state and thus display the classical properties we associate with the everyday world. To exist in a non-classical quantum state, quantum systems need to be isolated from external interference from the environment; as soon as this system interacts with it, it becomes “decoherent” and they would appear to behave as particles rather than probability waves; effectively they are being “observed” (a sort of Schrödinger’s cat condition). The more particles involved, the quicker the quantum state collapses – as maintaining a coherent state becomes increasingly difficult with increasing size due to interaction with the environment. This is why a tennis ball, although it can be technically assigned a wave length, is always observed as a tennis ball; its de Broglie wavelength is many orders of magnitude smaller than that of an electron and its wave effects are negligible. It is also why a cat does not exist in superposition; it is intimately coupled to its environment. The important message here is that microscopically, coherence is possible, not only for single entities, such as electrons, but also for larger groups of atoms – indicating that they can behave as one entity. But this state is rapidly lost via interaction with the wider environment; and is why we view the macroscopic world as classical. During interactions of quantum systems with their environments, energy and information is exchanged. These quantities are traditionally studied in thermodynamics. For this reason thermodynamic principles are useful in the investigation of quantum interactions. Perhaps the most extraordinary quantum phenomenon is “entanglement”, or as Einstein put it, “spooky action at a distance” – which describes the ability of two entangled particles to “know” the state of the other when one is observed, regardless of distance and instantaneously. This is known as “non-locality”, as encompassed by Bell’s theorem and is a profound departure from classical physics. Despite the fact that entanglement defies intuitive understanding, there is strong experimental evidence in its favour (Schmied et al. 2016). From the quantum point of view, once entangled, two particles have to be regarded as the same entity, irrespective of distance. Thus entanglement is not only key to understanding reality, but is key to many current and future technologies, including quantum computing (Gribbin 2013).
Quantum effects in biology :
Unlike quantum mechanics itself, which has been repeatedly verified by experimental observation, quantum biology is still predominantly theoretical, although evidence does appear to be accruing. All biological systems are fundamentally quantum, being made out of atoms which are best described by quantum theory. However, quantum phenomena such as tunnelling, coherence and entanglement in a biological context are often referred to as non-trivial quantum effects. The possibility that electrons could move along enzymes after excitation to a higher energy level, a bit like they might do in metals, was first suggested by Szent-Györgyi in 1941 (Szent-Györgyi 1941), but it was DeVault and Chance in 1966 who proposed it could be due to quantum tunnelling (DeVault and Chance 1966). Since then, the study of both electron and proton tunnelling in enzymes has become a well-established field of research. Quantum tunnelling has also been proposed as a novel mechanism for both olfaction and neurotransmission. Certainly, it continues to be suggested that components of living systems could be using quantum principles, for instance, by absorbing light energy and transferring it across a series of molecules (Tamulis and Grigalavicius 2014). It is possible that quantum tunnelling and coherent energy and charge transfer were important for the beginnings of life, especially in relation to photosynthesis, and could have facilitated more efficient light-harvesting and transfer of energy and electrons (Tamulis and Grigalavicius 2014; Trixler 2013). Indeed, it seems that many biomolecules may have been selected for during evolution with respect to their “quantum criticality”, or their potential to behave somewhere between an insulator and a conductor (Vattay 2015).
Quantum effects are conventionally described as properties that are preserved at very low temperatures. One of the factors that can influence quantum coherence is the temperature of an environment, as this indicates the energy of its constituent particles, and thus its potential to interact with a quantum system. For many years, this led to the conclusion that life was simply too “warm and wet” for coherence to occur. However, there is some indication that it may well be possible that life has actually tuned itself to use thermal vibrations to “pump” coherence, rather than disrupt it, which results in a phenomenon known as “quantum beating”. Critically, it appears that this effect has been detected in light harvesting complexes. In essence, light can be harvested very efficiently and its energy transferred using wavelike resonance. Hence, there could be a “Goldilocks zone” where this efficiency is optimised. In effect, just the right amount of “noise” can result in enhanced “coherence” and “tunnelling” due to stimulating particular vibrational modes in proteins – so called exciton-vibrational coupling (vibronic coupling) (Engel et al. 2007; Fassioli et al. 2014; Lim et al. 2015; Weber et al. 1995). It is thus interesting that emerging mathematical models suggest that quantum coherence can be maintained for significant periods of time in complex biological systems, potentially orders of magnitude longer than in simple quantum systems at room temperature, by hovering in the “Poised Realm” between the pure quantum and incoherent classical worlds (Vattay, Kauffman, and Niiranen 2014).
Coherent energy transfer is not limited to photosynthetic complexes. Tubulin proteins, which constitute the cytoskeleton of a cell, contain chromophores such as tryptophan, whose free pi electrons may facilitate coherent energy transfer; (Craddock et al. 2014; Craddock, Priel, and Tuszynski 2014). There is evidence that long-range electron tunnelling could occur in many proteins. This may or may not be enhanced by aromatic molecules that are common in oxidoreductases, which are key components of respiratory chains (Winkler and Gray 2014). Indeed, it has been suggested for some time that electron tunnelling may play an important role in how mitochondria produce energy (Hayashi and Stuchebrukhov 2011; Moser et al. 2006), which could ensure a tight coupling between electron flow and protonation via a process known as “redox tuning” (de Vries et al. 2015). There are also theories that strong electromagnetic fields generated by mitochondria could generate “water order”, and thus protect against decoherence (Pokorny, Pokorny, and Kobilkova 2013).
In addition to the proposed possible role of quantum effects in bioenergetics suggested above, it has also been suggested that quantum effects could be important in bird navigation (Zhang 2015). While this is supported by behavioural evidence, there does appear to be experimental evidence for significant quantum effects in photosynthesis (Engel et al. 2007), and possibly, in enzyme function involving proton tunnelling, which is investigated by using kinetic isotope effects (KIE) comparing hydrogen, deuterium and tritium transfer (Layfield and Hammes-Schiffer 2014). Data has also been published suggesting that lysozymes appear to demonstrate a “Fröhlich condensate” – in effect, this is where energy in the form of terahertz (THz) photons, rather than simply heating up the protein and being dissipated as heat, actually induces a form of quantum molecular resonance similar to a Bose-Einstein condensate (Lundholm et al. 2015). Fröhlich’s ideas, which he first proposed in 1968 (Frohlich 1968), have been pivotal in providing a theoretical backbone to the concept of quantum effects in biology.
References that we have cited on The Guy Foundation website are available on our Useful reading page.