Why is the efficiency of light absorption in photosynthesis so high?

Hey! I’m Daniel.

I am studying photosynthesis with Dr. Sato. We want to develop a model of the transfer of molecular excitation energy in between molecules during this process. My daily work consists of studying quantum physics and chemistry and doing a little bit of computer programming. I love this project because I am interested especially in theoretical physics research.

Photosynthesis is the process used by plants and other organisms to convert light to energy that may be used to support their activities. It starts with the absorption of a particle, bundle of light – a “photon” – by certain molecules in cells. We call the absorption of this energy an “excitation” of the molecule. This energy is then transferred to other molecules until it arrives at a “reaction centre”, which is a complex of several proteins, pigments and other molecules, where the energy is converted to the energy contained in the separation of two electric charges.

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There is a lot of evidence that can support the idea that the transfer of energy between molecules could involve a phenomena  like this: imagine that the excitation energy of one molecule, which interacts with another molecule in the surrounding protein matrix in the cell, becomes delocalized over the excited molecule and the neighboring one in this way: a measurement of the energy state (configuration) E of any of the two molecules may yield the following results: (E = not excited) or (E = excited), so that the result is uncertain. This type of state of an object that seems to be a mixture/superposition of two or more states occurs often in the microscopic world, as it is described by physics today using a theory called quantum mechanics. As far as we know, there is no way of predicting the outcome of the measurement of the energy, in this case. It’s natural to wonder “But what is the state of the object before the measurement?” See, it simply does not seem to be in a definite state. “Look, mate, it surely must have a CERTAIN amount of energy though!” Well, then go ahead and measure it. Say you get E = excited, and you may say “Ha! I told you it had a certain energy”. Now measure it again on a molecule in the same circumstances. You may get “E = not excited”. Hum… interesting! Now we give up. We can only describe the statistical features of the state by taking many measurements on it and noticing probabilities of certain outcomes. Careful! There’s absolutely no mystery here, whatsoever! Just data. 🙂 However, these superpositions seem to decrease rapidly as the two molecules interact with the environment, a process that is called “quantum decoherence”, and that could explain the emergence of a state in which the energy is definitely in the next molecule, and so on.

 

 

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The Schrödinger equation of quantum mechanics!

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Maybe dogs do photosynthesis too. Huuum… interesting.

So, I am creating a model of the transfer of energy between molecules using quantum mechanics, to compare our calculations of features of this transfer with the result of spectroscopy measurements of photosynthetic complexes. This could help us to describe the causes of these superpositions of energy states, and determine whether this quantum process of excitation, superposition and rapid decoherence is the mechanism that allows the high absorption efficiencies in photosynthesis. Thanks a lot for your time! 😀

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