Friday, March 17, 2017

Plants understand quantum mechanics better than humans do

I mentioned in class that enzymes exploit a feature of quantum mechanics called 'quantum tunnelling' in order to execute proton transfer more efficiently (see papers by Judith Klinman if you are interested, she has done lots of research on the subject and is one of the pioneers of the idea).
However, there is an even cooler example of quantum mechanics seen in plants which explains how they are so incredibly efficient (sometimes nearly 99% efficient) at using energy that they get from sun-light.

First and foremost, we must understand a bit about quantum mechanics.

A mechanical system is, loosely speaking, a system who's state evolves with respect to some parameter (usually time). So a simple mechanical system is, for example, a ball rolling down an inclined plane. If we want to describe the dynamics of this system, we have to ask the following question: what is the information about the system in its present state in order to predict how the system will evolve in the future?

In the case of 'classical' mechanical systems i.e. all intuitive/'real-world' systems such as the one described above, where we have only one object (the ball), the answer is quite simple: we need 3 numbers for the position (one for each of the x,y and z axis) and the same for the velocity in each axis. We also need to know the mass of the object for which we are trying to predict the dynamics of so that we can combine this with the velocity to get the momentum. This information gives us a definite state of the system (i.e. where the ball is right now, and what direction its momentum is pointing in) which, combined with the dynamical equation known as Newtons 2nd Law, gives us a deterministic system for which we can exactly predict the state of the ball for any time, arbitrarily far in the future or the past.

I have described this notion because firstly it will highlight the reasons why quantum mechanical systems are so different to 'classical'/normal ones, and secondly why quantum mechanics is so &$%£?!* cool!

In a quantum systems (generally described as systems that are around the nano-scale, and containing relatively few particles) there is a profound discrepancy between a systems state before and after it is observed (observed, in this context, largely just means 'interacted with by some other system' such as a measuring apparatus). Before a quantum system is measured, we do not have the luxury of knowing exactly what state the system is in. In fact, the best we can do is to have a probability distribution of all its possible states, the information that we get from this being how likely it is for the system to be found in a particular state when it is observed. What's even more strange, is that the probability distribution that we calculate seems to be a real thing, not just a convenient way of describing a system due to lack of information. What this means is that when a quantum system is not being directly observed, it is in what is called a superposition of all possible states, meaning that, for example, a particle is existing in theoretically an infinite number of places all at the same time. Then, when something decides to interact with the particle, it says 'I'd better choose a state' and arbitrarily chooses one, in concordance with the probability distribution.

Now, the biology...

As described in this video, plants receive energy from photons emitted by the sun, which hit the leaves and excite (give energy to) electrons in the chlorophyll molecules. These high energy electrons need to then bounce around from site to site to the reaction centre, where the energy from the electrons is stored for later use. However, the electrons must get there in around 1 nanosecond, otherwise too much interaction with the surrounding molecules will mean it looses its energy and the plant will die. What is more, travelling from receptor to reaction site is no trivial task - there are lots and lots (millions) of possible paths it can take and not even all of these lead to the reaction site. So, how does the electron know where to go? One answer could be that it just randomly bounces from site to site, much like an air molecule diffuses through a room by random interactions with surrounding air molecules, and eventually gets to the reaction centre. But this is certainly wrong; the immense number of possible paths that can be taken by the electron prevent it from plausibly choosing the right path even once, let alone millions of times.

So how then?

Quantum superposition of course! The excited electron exists temporarily in a state of quantum superposition (what was described above) whereby it travels every path at the same time. However, this is not sufficient to explain why the plant is so good at getting electrons to the right place - we said that when observed, a system in superposition will randomly choose a possible state in accordance with a probability distribution. This is where we see that plants have mastered quantum engineering far more that humans. The sites are arranged in such a way that, when the electron is in a superposition of all possible paths, the overwhelmingly probable path is the one that takes it right to the reaction centre! If you havn't realised by now, this is incredible stuff. It also shows that evolution acts not just on the macroscopic and microscopic range, but also in the real of quantum mechanics. There are some far more profound implications of this mechanism, though they cannot be properly understood without a minimum of knowledge about quantum mechanics.

I would recommend this book to those who are curious. It describes the presence of quantum mechanics in biological systems and is written so that even those who have no prior knowledge of either subject can understand. I can personally recommend this book very highly.

Quantum Biology is an emerging field and is absolutely incredible from both the perspective of a physicist looking at how quantum systems can be used to make remarkable mechanisms, and also from the perspective of a biologist looking into just how specialised organisms are. It can also potentially give us measurable statements about life's origins and how DNA/RNA can play an informational role in biological systems, among other things to be discovered.

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