What Are the Quantum Mechanics of Photosynthesis?
Quantum mechanics meets biology. Sounds like something out of a horror movie. The ultimate creation of difficult concepts merged into a truly amazing construct that on the surface seems impenetrable to our investigations…right? Turns out, it’s the frontier of science that we are indeed making headway on. The most promising door into this realm of quantum biology rests with a rather familiar process turned a new: photosynthesis.
Let’s briefly review the process of photosynthesis as a refresher. Plants have chloroplasts which contain chlorophyll, a chemical that takes photonic energy and transforms it into chemical changes. The chlorophyll molecules is located in “a large assembly of proteins and other molecular structures” that makes up the photosystem. Linking the photosystem to the rest of the chloroplasts is a thylakoid cell membrane, containing an enzyme that encourages electric flow once a reaction occurs. By taking carbon dioxide and water, the photosystem transform this into glucose with oxygen as an additional product. The oxygen is released back into the environment where lifeforms intake it and release carbon dioxide that starts the process all over again (Ball).
The molecules responsible for the light-to-energy conversion are chromophores otherwise known as chlorophyll and they rely on dipole coupling. This is when two molecules do not share their electrons evenly but instead have an unbalanced charge difference between them. It is this difference that allows electrons to flow to the positively charged side, generating electricity in the process. These diploes exist in the chlorophyll and with the light being converted into energy the electrons are free to flow along the membranes and allow the necessary chemical reactions the plant needs to break down the CO2 (Choi).
The quantum part comes from the dipoles experiencing entanglement, or that particles can change each other’s state without any physical contact. A classical example would be having two cards of different colors flipped upside down. If I draw one color, I know the color of the other without doing anything to it. With chlorophyll, factors like surrounding molecules and orientation can influence this entanglement with other particles in the system. Sounds simple enough, but how can we detect that it’s happening? (Ibid)
We need to be tricky. Using traditional optical technology to try and image the chromophores (which are on the nanometer scale) isn’t feasible for actions on an atomic scale. Therefore we need to use an indirect method for imaging the system. Enter electron scanning tunneling microscopes, a clever way around this issue. We use an electron to measure the interactions of the atomic situation in question, and quantumly we can have many different states happening at once. Once the electrons interacts with the environment, the quantum state collapses as electrons tunnel to the site. But some are lost in the process, generating light on a scale we can use with the electrons to find an image (Ibid).
With the chromophores, scientists needed to enhance this image to note changes in the production of the molecules. They added a purple dye in the form on zinc phthalocyanine which under the microscope emitted red light when alone. But ass another chromophore near it (about 3 nanometers), the color changed. Note that no physical interaction occurred between them yet their outputs changed, showing that the entanglement is a strong possibility (Ibid).
Surely this isn’t the only quantum application scientists are exploring, right? Of course. Photosynthesis has always been known for its high efficiency. Too high, according to most models that exist. The energy transferred from the chlorophyll in the chloroplasts follows the thylakoid cell membranes, which has enzymes which encourage energy flow but are also separated in space, preventing charges from linking the chemicals together but instead encourage electron flow to the reaction sites where the chemical changes occur. This process should inherently have some loss of efficiency like all processes but the conversion rate is nuts. It was as if somehow the plant was taking the best routes possible for the energy conversion, but how could it control that? If the possible paths were available all at once, like in a superposition, then the most efficient state could collapse and occur. This quantum coherence model is attractive because of its beauty, but what evidence exists for this claim (Ball)?
Yes. In 2007, Graham Fleming (University of California at Berkley) picked up on a quantum principle of “synchronization of the wavelike electronic excitations – known as excitons” that could be occurring in the chlorophyll. Instead of a classical energy dump along the membrane, the wavy nature of the energy could imply that the coherence of the patterns was achieved. A result of this synch would be quantum beats, similar to interference patterns seen with waves, when similar frequencies would stack up. These beats are like a key to finding the best route possible because instead of taking paths which result in destructive interference, the beats are the queue to take. Fleming along with other researchers looked for these beats in Chlorobium tepidum, a thermophilic bacterium that has a photosynthetic process in it via the Fenna-Matthews-Olsen pigment-protein-complex that operates the energy transference via seven chromophores. Why this particular protein structure? Because it’s been heavily researched and therefore is well-understood, plus it’s easy to manipulate. By using a photon-echo spectroscopy method which sends pulses from a laser to see how the excition reacts. By changing the length of the pulse, the team was able to eventually see the beats. Further work with near-room temperature conditions was done in 2010 with the same system and the beats were spotted. Additional research by Gregory Scholes (University of Toronto in Canada) and Elisabetta Collini looked at photosynthetic crytophyte algae and found beats there at a duration sufficiently long ( 10-13 seconds) to allow the beat to initiate the coherence (Ball, Andrews, University, Panitchayangkoon).
But not all buy the results from the study. Some think the team mixed up the signal they spotted with Raman vibrations. These result from photons being absorbed then re-emitted at a lower energy level, exciting the molecule to vibrate in a fashion that could be mistaken for a quantum beat. To test this, Engal developed a synthetic version of the process that would show the expected Raman scattering and the expected quantum beats, under the right conditions which ensure no overlap between the two is possible and yet the coherence will still be reached an ensure the beat is achieved. They found their beats and no signs of the Raman scattering but when Dwayne Miller (Max Planck Institute) tried the same experiment in 2014 with a more refined set-up, the oscillations in the vibrations were not large enough to be of a quantum beat origin but instead could have arisen from a molecule vibrating. Mathematical work by Michael Thorwart (University of Hamburg) in 2011 showed how the protein used in the study could not achieve the coherence at a sustainable level necessary for the energy transference it was claimed to allow. His model did correctly predict the results seen by Miller instead. Other studies of altered proteins also show a molecular reason instead of a quantum one (Ball, Panitchayangkoon).
If the coupling seen isn’t quantum, is it still enough to account for the efficiency seen? Nope, according to Miller. Instead, he claims it’s the opposite of the situation – decoherence – that makes the process so smooth. Nature has locked in the path of the energy transference and over time refined the method to be more and more efficient to the point where randomness is reduced as biological evolutions progress. But this isn’t the end of this road. A follow-up study by Thomas la Cour Jansen (University of Groningen) used the same protein as Fleming and Miller but looked at two of the molecules being hit with a photon designed to encourage superposition. While the findings on the quantum beats matched Miller, Jansen found that the energies shared between the molecules were superimposed. Quantum effects do seem to manifest themselves, we just have to refine the mechanisms that they exist by in biology (Ball, University).
Andrews, Bill. “Physicists See Quantum Effects in Photosynthesis.” Blogs.discovermagazine.com. Kalmbach Media, 21 May 2018. Web. 21 Dec. 2018.
Ball, Philip. “Is photosynthesis quantum-ish?” physicsworld.com. 10 Apr. 2018. Web. 20 Dec. 2018.
Choi, Charles Q. “Scientists Capture ‘Spooky Action’ in Photosynthesis.” 30 Mar. 2016. Web. 19 Dec. 2018.
Masterson, Andrew. “Quantum photosynthesis.” Cosmosmagazine.com. Cosmos, 23 May 2018. Web. 21 Dec. 2018.
Panitchayangkoon, Gitt et al. “Long-lived quantum coherence in photosynthetic complexes at physiological temperature.” arXiv: 1001.5108.
University of Groningen. “Quantum effects observed in photosynthesis.” Sciencedaily.com. Science Daily, 21 May 2018. Web. 21 Dec. 2018.
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