Photosynthesis and Quantum Biology

When researchers study quantum mechanics, the go-to environmental state of the system being studied is usually artificial. Such conditions are usually very cold, and very contained, ensuring that no outside influences can disturb the incredibly sensitive quantum nature of the object in question.

So, when the word ‘quantum’ is mixed with the word ‘biology’, physicists often scratch their heads.

Why is this?

Biological, organic systems, like ourselves, or any living thing for that matter, are usually very warm, wet and noisy. This is not the ideal place for a quantum process to occur. But researchers across the world have challenged this notion, stating that quantum processes do occur in organic creatures, we just haven’t been looking hard enough.

Take Greg Engel, of the University of Chicago for example. One of the pioneers in the search for the answers behind quantum biology, Engel explained that the idea is becoming widely accepted as more information is discovered on the subject.

“the general notion that the language and mathematics of quantum information, including coherence, can be used to understand photosynthetic dynamics in ultrafast spectroscopy experiments seems to be growing in acceptance.” Said Engel to Philip Ball of Physics World.

But what are we talking about exactly?

Let’s use photosynthesis as an example. When light is taken in by a photosynthetic organism, that is to say, absorbed by chlorophyl molecules, chromophores (pigments), and proteins (collectively known as a photosystem), the light excites the chromophores, and energy is passed along towards the reaction center. A process once thought to be incoherent and merely following an energy gradient (high to low), it is now believed that the process is actually coherent after all, due to quantum coherence.

I know, its hard to take in. Just give it a moment.

Think of the chromophores as stepping stones towards the reaction center. There are many paths that the energy provided by the photon can take, but the process is entirely too efficient as studies suggest (Almost 100% efficiency!), meaning the shortest path is always being taken.

But how could it know which path to take?

If quantum coherence is at play, then all possible paths are ‘taken’ at once, but the one that is most efficient is chosen.

It’s mind-boggling, but this is the nature of the quantum world.

If you’ve ever heard of Schrodinger’s cat, then you understand what I’m talking about. A thought experiment proposed by Edwin Schrodinger, Schrodinger’s cat essentially posits the question ‘what happens to a cat in a box if it is given an equal chance of death or life?’ The cat, to the observer, would be in a state of being both dead and alive, until the box is opened. Only then would its true state be known.

Though incredibly simplified, the main jist is that particles that are in a quantum state are known to be in a superposition; that is to say, both on or off, dead or alive, at the same time.

So, in the case of photosynthesis, perhaps all possible chromophore paths are being taken at once. Perhaps, they are in a superposition of sorts.

To test the theory, researchers like Engel, and a physical chemist by the name of Graham Fleming of the University of California at Berkeley, among others, looked for tell-tale signs of quantum coherence in the Fenna–Matthews–Olson (FMO) pigment–protein complex of the thermophilic bacterium Chlorobium tepidum. By analyzing the synchronization of excitons, or the wave-like excitations between chromophores, they hoped to find what are called ‘quantum beats’, which would occur when the exciton paths interfered.

Amazingly, they did find the ‘quantum beats’, but as other researchers proposed thereafter, what they thought were quantum beats may have just been vibrations. David Jonas of the University of Colorado at Boulder and his colleagues suggested that the so called ‘quantum beats’ were actually molecular vibrations, a phenomena referred to as Raman scattering.

Dwayne Miller of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, came to a similar conclusion after testing the theory himself, stating, “The beats are comparable in amplitude, frequency and decay rate to trivial Raman vibrations of the electronic ground state excited in the process. It is Raman that they saw, not long-lived electronic coherence.”

To confirm this theory, photochemist Gregory Scholes of Princeton University and photosynthesis expert Robert Blankenship of Washington University in St. Louis, Missouri produced custom made mutant versions of the FMO complex. Their particular complex variants have differing electronic states than the original, natural FMO complexes, for the purpose of determining whether excitons are involved in the discovered quantum beats. If they are in fact involved, then the beats should have a different frequency, but they did not. This means that the detected vibrations came solely from the aforementioned electronic ground state.

Miller goes on to explain that nature, in the absence of quantum coherence, chose to use decoherence rather than fight against it. Because the dissipation of energy occurs so quickly after decoherence, it can find its way down the most energy-efficient path, being ‘led’ by the variance in the electronic properties of the molecular environment.

“It’s a bit like laying paving stones down a hillside to direct hikers to stay on the trail and not explore the full landscape,” explained Miller.