Wednesday, 18 April 2012

Is quantum mechanics relevant for biological systems?

It is commonly believed that you don’t need quantum mechanics to properly describe how biological systems work. They are simply too large, too hot and too messy for any fragile quantum effects to matter. But over the past few years, some scientists have come to realize that this might not necessarily be the case. Our ability to recognize odours might involve phonon-assisted quantum tunnelling. The way some birds navigate during flight might include perceiving subtle differences in magnetic fields in a quantum coherent way. The stability of DNA double helix structure might possibly depend on quantum entanglement between neighbouring nucleic bases. We know of at least one example where the presence of quantum effect is indisputable: the coherent transport of light energy in photosynthetic systems. I will attempt to describe that in a later article.

For now, it seems we are quite uncertain as to how significant quantum effects are in the phenomena I mentioned, so perhaps the first question to address is, how do we even know it is possible for quantum effects to survive in organic systems?

There are two main arguments made against the relevance of quantum mechanics in biology: (1) the issue of disparate energy scales and (2) decoherence. 

The first one says that the energy scales needed for quantum effects to show up are very different from the energy scale of biological systems. In terms of an inequality, we write hf < kT, where h is Planck’s constant (the signature of quantum physics), f is the frequency, k is Boltzmann’s constant and T is the temperature. The inequality states that energies typical of quantum processes are much smaller than the thermal energies you observe in biochemical processes. This means we expect thermodynamic effects would easily overwhelm any quantum effects. Since you don’t even need to know that atoms exist if you are just doing thermodynamics, you won’t need quantum mechanics.

As it turns out, this is not a terribly convincing argument because it holds only for systems in thermal equilibrium, that is, when it is actually reasonable to assign a fixed temperature to the system. But the only time biological systems are in such static, unchanging equilibrium is when they are dead. Even when organisms try to regulate bodily functions, the equilibrium involved is a dynamic one. 

The more reasonable argument involves decoherence, roughly the process by which quantum systems lose their quantum properties due to interactions with the environment. It explains why large objects typically don’t exhibit quantum effects even if they are made up of atoms and electrons: the larger the system is, the more likely it interacts with the environment and washes out any quantum properties. There is a simple way of knowing whether decoherence matters in a system or not. It is shown in the diagram below.

Suppose we assign a number J to how strongly particles in a system interact with each other and we assign another number k to how strongly the system as a whole interacts with its surrounding environment. Then we can just examine the value of the ratio J/k. If J/k is large, it means that the system interacts weakly with its environment and so the delicate quantum effects within the system are more likely to survive. If J/k is small then the interaction of the system with its surroundings dominates and effectively removes any quantum behaviour in the system. Most biological processes have a small J/k value. But some, like photosynthesis, carry a J/k value that is about 1 in size. This value is both not too large and not too small and we do not know for certain if quantum effects disappear at this level. Research on the light-harvesting properties of pigments in plants and bacteria have indicated that quantum effects indeed play a role here. This presents us with the intriguing prospect of finding quantum effects in other biological systems.    

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