Plants Know Physics Better Than We Do

Dec 13 2013

Quantum physics is odd. Really odd. Particles can be in two places at once, behavior can depend on whether or not you’re looking (think the Weeping Angels from Doctor Who), and the ever popular “spooky action at a distance” – quantum entanglement – allows two objects to be connected no matter how far apart they are.

The reason these behaviors don’t show up in our daily lives is that they are delicate and largely irrelevant for objects as large as you and me. Typically, physicists need to go to very low temperatures and length scales of tens of nanometers (that’s a billionth of a meter!) before these behaviors manifest. So you might think quantum physics is interesting, but not too important overall. However it turns out that Nature may have learned to harness quantum effects first, by helping plants to collect light efficiently!

Everyone knows that plants need water, light, and soil to survive. Plants convert light into energy through a process called photosynthesis. The idea is simple: incoming light impacts an antenna within a cell, and excites some electrons; these electrons are then channeled to another cell apparatus called the reaction center where some chemical magic happens and then voila, you’ve got energy. The quantumy part happens in the antenna, and the crucial idea here is that a particle can in some sense be in two places at once.

In quantum physics, everything is governed by probability. An object isn’t actually in one place, there is some probability that it will be found here and some probability that it will be found somewhere else. The probability distribution is mathematically described by something called the wavefunction, and this is one of the most crucial concepts in quantum physics. Now while this effect doesn’t matter for you or I, we’re too big, it matters quite a bit for electrons where the wavefunction can extend over many nanometers.

The antenna in photosynthesis is actually composed of a large number of molecules separated from each other by only a few nanometers. It’s within these molecules that electrons are excited by light, but then there needs to be some way for the electrons to get to the reaction center. One way this could happen is if the electron randomly hops from one molecule to another, gradually making its way towards the reaction center. Alternatively, since each molecule is very close to its neighbor, the electron wavefunction can actually extend across a huge number of molecules. In a sense the electron, a single particle, extends over the entire antenna at once. This allows the electron to find the most efficient path towards the reaction center which saves energy and boosts efficiency!

Scientists have seen this type of massive quantum electron spread in green sulphur bacteria at room temperature, which is truly remarkable. So the next time you’re outside, take a close look at a leaf. You’re seeing quantum physics in action!

This guest post was written by Dr. Patrick Vora, an NRC postdoc currently at the US Naval Research Laboratory focusing on quantum optics. Image via Wikipedia.

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