Photosynthesis May Rely On Quantum Effect

forgethistory sends us to PhysOrg for a summary of new research suggesting that the near instantaneous energy transfer achieved by photosynthesis may rely on quantum effects. From the article: "Through photosynthesis, green plants and cyanobacteria are able to transfer sunlight energy to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. Speed is the key — the transfer of the solar energy takes place almost instantaneously so little energy is wasted as heat. How photosynthesis achieves this near instantaneous energy transfer is a long-standing mystery that may have finally been solved."

Re:Nuclear Sense of Smell vindicated?

[porttikivi]

http://www.sciammind.com/article.cfm?articleID=885 622AA-E7F2-99DF-3859D89E5980A4B2 [sciammind.com] My first link Googled by "smell quantum".

Re:Nuclear Sense of Smell vindicated?

[Ihlosi]

like why cyanide smells like almonds.

Wasn't that because bitter almonds actually do contain cyanide ?

Re:Nuclear Sense of Smell vindicated?

[chhamilton]

It was actually featured in a Slashdot story not long ago:

http://science.slashdot.org/article.pl?sid=06/12/1 1/1952201 [slashdot.org]

Unfortunately, the original Nature article is now subscriber only (http://www.nature.com/news/2006/061204/full/06120 4-10.html [nature.com]). The guy behind the work is one Dr. Lucia Turin, and he has indeed achieved some commercial success through his company Flexitral [flexitral.com].

More to it than that

[Lockejaw]

When a photon strikes a chlorophyll, it adds its energy to an electron, allowing the electron to escape from its atom (previously known quantum mechanics). It was previously thought that the electron would then go bouncing around between chlorophyll molecules until it found a pheophytin molecule (slightly different chlorophyll). Once it hits that molecule, it activates an electron-transport chain (a similar process happens when burning glucose in a mitochondrion).
TFA suggests that the hopping uses quantum superposition to traverse the chlorophyll molecules more quickly. When the traversal reaches the pheophytin, the superposition collapses into that single state which found the pheophytin.

Speed helps, but that's only half the picture

[Atraxen]

The summary is slightly misleading, but this disconnect has big implications for the reader's understanding (imho)...

I can name plenty of chemical reactions that are complete on the femtosecond scale, and while speed helps, that's certainly not the whole picture. What matters is how mismatched the energy levels between the reactant and the product are. When transitioning between energy levels, either energy is transferred out of the system by nonradiative release (heat), luminescence, photofragmentation, or transfer to a chemical partner - this last case is what the article is referring to. Getting to an energy level which can react is going to result in a heat deposition for at least some photons because any photon of a higher energy than the reacting state must deposit some of that energy just to be able to react at all.

http://www.monos.leidenuniv.nl/smo/basics/images/j ablonski.gif [leidenuniv.nl] Unfortunately, this scheme doesn't show photofragmentation or energy transfer to another molecule, but I'm in a rush so it'll do.

The squiggly lines show possible heat depositions - the molecule starts in the ground state, absorbs a photon (the yellow up arrow), then relaxes to the excited state. This excited state then does whatever it's going to do. If 100% of the time under a set of conditions (i.e. a quantum yield of 1.00), the excited molecule follows a particular pathway we call that perfectly efficient. In the specific example of photosynthesis, this means that all of the absorbing chlorophylls transfer the energy along the photosynthetic pathway (I'm lumping all the subsequent processes together here). It does not mean that 100% of the energy got transferred along the way - there will always be some photon that deposits more energy than the reacting state has, meaning some energy will be converted to heat.

In short form (if you didn't feel like reading all this): efficiency in this case refers primarily to how often the molecule dumps its energy into photosynthesis instead of all to heat, luminescence, etc. It's not referring to the energy throughput, as some photons will always be an imperfect energy match, and the extra energy will end up as heat.

100% efficiency

[140Mandak262Jamuna]

Some half witted reporter's failed attempt in dumbing down a routine research paper.

Yeah, sure the energy transfer efficiency is 100% for every photon that participates in the reaction. But of all the photons falling on the leaf, hardly 2% of them participate in reactions. Some gets reflected, some gets absorbed without any reaction. Even solar cells have better energy conversion efficiency than plants. Really. As for quantum effects, almost all the photo reactions are quantum mechanics. They have to be. The film camera emulsion has greater percentage of photons participating in reaction than chlorophyll.

Re:Nuclear Sense of Smell vindicated?

[jfengel]

Yes, they do. GP had a bad example. But there are other cases of things with similar smell and very different shapes that are not well handled by the classic lock-and-key theory (e.g. enantiomers).

The resonance theory is a good and interesting alternative, despite serious difficulties understanding the mechanism.

100%

[fermion]

I must take issue with the 100% efficiency. Efficiency, as I know it, is a ability to convert stored energy into useful work. I know of no engine, artificial or natural, that can do this with 100%, which is of course prohibited by the known laws of thermodynamics. In particular, I have seen photosynthesis calculation that set the efficiency of photosynthesis as low at 3%. Even in the simplistic case, it appears that 50-70% of the energy in the process of photosynthesis.

Re:I don't understand

[Anonymous Coward]

Hmm... if I get the gist of the article right it's sort of like this. The traditional version has the excited electron bouncing from molecule to molecule until it finds a home, each "bounce" would have to cost that electron energy. Now, in a resonance structure the electrons are pretty much shared evenly between all atoms (think benzene), they are extended this accross molecules instead. So when the electron gets excited to a higher energy level it is popping into an unoccupied resonance structure (pi bonds, how I hate thee) where it is "seen" equally by all associated molecules. The resonance, however, does not hold as strongly at one molecule, the one that the electron terminates at. The traditional model is kind of like pin-ball, the quantum would be analagous to tossing a ball from your right hand to the left hand, the electron never sees all the stuff in between your hands. Very simplified.