Quantum biology and energy efficiency

Our physical body is made of approximately 7 octillion atoms (7*1027). This of course vary given your current body weight and is approximated for an adult of 70 kg. Most of those are hydrogen (4 octillion) and oxygen (1.8 octillion) atoms, followed by 7x1026 carbon atoms. These are the founding blocks of the compounds that make each of us: water, proteins, fats… Still, we think of our body as a typical macroscopic object, and even if you are fascinated with quantum mechanics and learn how its laws govern the behavior of the atoms, most likely the idea of it influencing biological systems or even, as some propose, the very nature of conciseness, is more on the science fiction side.

However, research into fields of biology and quantum physics lead to the emergence of a new discipline, quantum biology, with the idea that the explanation of some biological concepts and processes requires more than classical physics laws. In the end, all these atoms, and molecules they form, are quantum mechanical systems. The main debate is if these systems in such warm and crowded surroundings as a human body (non-equilibrium conditions in physics words) are capable of preserving some more exotic quantum states, e.g. maintain entanglement without decoherence. This is not a small problem and is one of the main obstacles in quantum computation nowadays - maintaining more and more atoms in an entangled state and preventing the collapse of this state prior to the actual computation (read more in our earlier post).

Quantum entanglement in photosynthetic light-harvesting complexes (such as the protein structure of a green anoxygenic bacteria) was reported over 10 years ago, demonstrating entanglement in the complex non-equilibrium chemical and biological processes necessary for life. This work claimed to established a manifestation of this characteristically quantum mechanical phenomenon in biologically functional structures. Quantum physics was also used to explain the very large efficiency of the photosynthesis process:

„Life is a fundamentally non-equilibrium process. Living organisms need to stay out of equilibrium in order to survive, prosper and reproduce, and that requires a continuous supply of free energy. (When living organisms come to equilibrium with their environment, we call them dead.) The primary source of this free energy is the sun. Photosynthesis is the phenomenon whereby the chlorophyll pigments of plants capture sunlight and convert that energy in to a chemical form for later use. It is the basic step of harnessing solar energy on which most living organisms depend, either directly or indirectly through a food chain. In the first step of photosynthesis, the energy of the captured photon is used to dissociate water (in to H + and OH−) and create charge separation across a membrane. This is essentially an electrical step, and the resultant ions subsequently drive chemical processes for the synthesis of glucose. It is established that the efficiency of this energy conversion exceeds 95% (some quote even 99%). One can easily estimate the efficiency by counting the number of photons captured and the number of glucose molecules synthesised, since energies of the radiation and the chemical bonds are known. „

Apoorva D. Patel in Efficient Energy Transport in Photosynthesis: Roles of Coherence and Entanglement, arXiv

Hildner et al. observed coherence—prolonged persistence of a quantum mechanical phase relationship—at the single-molecule level in light-harvesting complexes from purple bacteria. The results bolster conclusions from past ensemble measurements that coherence plays a pivotal role in photosynthetic energy transfer. Yet, this is far from a scientifically accepted fact, since measurements that are seen as evidence to these claims by ones are interpreted as experimental signals that do not involve coherence by others, as nicely observed in a retrospective by Philip Ball.

In a meantime, quantum biology pushes the boundaries further to the study of cognition and artificial intelligence, and some recent articles consider another very energy-efficient biological system-our brain. In his work, T.N.Palmer considers the work of Summhammer et al. on the motion of K+ ions in voltage-gated ion channels in the neuronal membrane wall, in which they note that it is difficult to explain the high rates of ion flow using classical physics. Palmer takes this one step further in hypothesizing that the brain will use a quantum process over a classical process when there is an energetic advantage to do so. Very interesting considerations of Counterfactuality, Free Will, and Why is Quantum Physics so Unintuitive can be found in his article, where he concluded that “ through its evolution over many millions of years, the miniaturisation of the neuronal pathways in the brain has resulted in an exceptionally energy efficient organ. The disadvantages of such miniaturisation (e.g. relatively slow reaction times in the presence of predators) has been offset by two advantages: an ability to be creative and an ability to be aware of the world around us. It is argued that those advantages arise from two specific manifestations of energy efficiency: the role of stochastic noise and the role of quantum parallelism respectively" .

Experimental evidence to many questions raised in quantum biology is far from being definite. However, it is definitively expected, as noted by Cao et al. that „there is a deep understanding to be gained in tackling the emergence of the essentially classical world of biology from its quantized molecular origins.“, and every atom in our body can play a role in this. To which new knowledge this will take us remains to be seen.