photosynthesis – a quantum coherent system (part 2 / 2)

Welcome to the Part 2 of the series, Photosynthesis – A Quantum Coherent System. In the first part we visited the Plant Cell and understood what does a plant cell looks like and where exactly the process of photosynthesis takes place. We also looked at how a photon is used to excite an electron and transport a “battery like thing” called the exciton to the reaction center to create an even more stable battery called “NADPH“. If you have no clue as to what I am saying here and would like to spare 5 minutes for a quick read, do redirect yourselves to: Photosynthesis – A Quantum Coherent System (Part 1 / 2).

in this post….

In the second and the final part of this series, lets have a look at some evidence that proves the vital role played by quantum coherences within the photosynthetic complex. Our journey will be as follows:

  • CLASSICAL PHYSICS EXPLAINS EXCITON TRANSFER

    This section will bring us up to speed on how energy transfer within photosynthetic complexes was theorized as per semiclassical physics and why it didn’t make much sense.

  • apparatus that measured quantum coherence

    Let’s see how physicists used 2DES and Fourier Transforms to measure quantum coherence within these molecules.

  • The observations

    What were the observations, and what did these observations mean.

  • references

    List of sources and references I used to write this post.

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CLASSICAL PHYSICS EXPLAiNs exciton transfer

Classical physics’ explanation of the energy transfer between chlorophyll molecules is fairly straight forward. The exciton from one molecule hops on to the other molecule until the reaction center is reached, and the path taken is completely random.

One of the paths taken by exciton towards the reaction center. There can be multiple paths possible.

Great explanation, but there’s a catch. Exciton’s lifespan lasts for a few picoseconds (1/12th of a second). If the exciton is not transported to the reaction center in time the energy will be lost and the process would be a failure. And according to the drunkard’s walk theory, if engaged in a random walk towards the destination, the distance covered will be the square root of the time elapsed. Hence one will cover 2 meters in 4 minutes, 3 meters in 9 and so on. Very inefficient !! But the leaf is able to transfer this exciton to the reaction center almost every time with 100% accuracy. How ?

This led to a lot of scratching heads. Physicists started hunting for more possible explanations, which landed them on quantum simulations. Since the actual transfer between molecules is not actually a particle but light energy that is released when the electron snaps back into its atoms, the possibility of a wave-like motion between molecules was proposed, and this explained perfectly how near 100% accuracy is achieved.

So instead of hopping from one molecule to another, a wave that propagates in all direction through the chlorophyll forest, thereby reaching the reaction center.

Not an accurate representation, but you get the gist of it!

apparatus that measured quantum coherence

Okay, we are at the most technical part of the series. Due to the complexity of the molecules and their ultra fast chemical reactions, it is impossible to use simple tools to just see what’s going on. But the theory behind it is very simple:

Optical pulses are incident on the sample (the photosynthetic molecules) and any changes within these pulses are detected later on. Then a comparison between the incident pulse and the detected pulse (in the frequency domain) is performed to determine the behavior of the system.

In order to fully understand what does the above statement means and how everything works, I recommend you to understand two very important things:

Fourier Transform

The below explanation by Grant Sanderson will bring you up to speed in understanding Fourier Transform. Remember FT is used to convert an observation from time domain to frequency domain.

Fourier Transform by Grant Sanderson on his channel 3Blue1Brown

2-Dimensional Electronic Spectroscopy

A prerequisite for this is Fourier Transform, so please do watch it if you don’t know what it is. Below is an explanation by Kristin L.M, Lewis and Jennifer Ogilivie from University of Michigan on how 2DES works in the context of Photosynthesis itself and shows the entire apparatus.

Probing Photosynthetic Energy and Charge Transfer with Two-Dimensional Electronic Spectroscopy on American Chemical Society

observations

The final part of this series, if you have reached here, you should be proud. The journey from a leaf to some wave forms was not easy.

So lets see what was seen as an outcome of those 2DES experiments that led physicists to believe the existence of Quantum coherence behind all the excitation transfer.

Before we move ahead. Please note the following

  • All and any graphical representations are taken from hard worked research papers from science journals.
  • With every image I will provide their DOI link so that you can download them for yourself if needed.
  • These representations are not my work.

Okay, since the excitation lasts for only picoseconds, it is necessary to use femtosecond spectroscopy, where a femtosecond is 1/15th of a second. In the experiment, we send in a pulse at a specific frequency say F1 and time t1. This is called the coherence pulse, or the pulse responsible to generate coherence. Second pulse at time t2 is sent to generate excitation within the system and a third pulse is sent at time t3 that causes rephasing of the system, in turn generating a detection signal. The incident pulse at time t1 and the detection pulse at time t3 is then plotted on a 2D spectrum graph.

The below graph shows the results from the paper: Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems (DOI: 10.1038/nature05678)

The above representation shows cross peaks represented at white arrows, and diagonal peaks represented by black arrows. The interesting fact about these peaks is that they appear beating (meaning their amplitude reduces and increases over time) these are called the quantum beats and these peaks lasts for around 660fs contrasting the general assumptions that coherences within photosynthetic process gets destroyed after a short period of time (around 200fs).

The above shows the oscillating waves while photosynthesis in progress within the molecules that are caused due to electronic excitation. The black line on the blue peaks is covering the exciton’s peak amplitudes.

The signals detected are converted into their frequency domain using Fourier Transform and then their power spectrum is plotted to understand the behavior of different excitons in the sample under experiment. This comparison is shown in the above figure.

Hence these above wave-like energy transfer of excitation through the chlorophyll molecules denotes that this happens through a perfectly maintained quantum coherent systems that lasts for longer than assumed. Which shows that the salad in our plate was an outcome of a very hard working quantum coherent system hiding away in broad day light.