photosynthesis – A Quantum coherent system (Part 1 / 2)

Plants are earth’s oxygen tanks. Our planet’s life depends on these humble green creatures. Like any other living being, they (plants) too work very hard to stay alive. As a result of their hard work we get variety of required nutrients in the form of fruits, vegetables, whole grains, legumes and even protein (because the source of our protein also depends on plants in one way or the other).

Photosynthesis is a process wherein plants collect raw resources from the ambient surroundings and converts them into energy to survive and produce delicious produce for other animals to consume. But did you know, all this complex conversion and generation happens in a perfectly balanced quantum coherent state.

I am writing this post to bring you an introduction of how the discovery of a quantum coherence within photosynthesis took place and what were the findings. Due to the complexity of this topic, I have decided to split this post in two parts, Photosynthesis – A Quantum Coherent System (Part 1 / 2) and Photosynthesis – A Quantum Coherent System (Part 2 / 2).

The first post brings you to speed with photosynthesis, explaining briefly the photosynthetic process. While the second part deals simply with the experiments and evidence measured proving the existence of coherence during the photosynthetic process. So if you think you’d like to jump on the second part: Photosynthesis – A Quantum Coherent System (Part 2 / 2), feel free to do so.

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plants – the producers

In order to survive and in turn to provide critical elements of life, plants themselves need basic resources in the first place. They need carbon, hydrogen and oxygen atoms to create sugars to power themselves. The carbon atoms are received from carbon dioxide (CO₂) from the air, hydrogen atoms from water (H₂O) and oxygen is supplied by both CO₂ and H₂O molecules. Photons from sunlight is used to power these green engines to extract individual atoms from the molecules and convert them into necessary sugars (glucose – C₆H₁₂O₆) and oxygen.

All of these happens within a plant cell. Hence we need to get inside it and understand how it provides a perfect environment for the existence of the quantum beat. (More about quantum beats later).

The plant cell

Plant cells are the basic units of plant’s life. These cells are termed as eukaryotic cells having a nucleus and special structures called organelles. Each organelle is responsible to perform specialized functions. The organelle of interest of this post is the Chloroplast. However, if you would like to know more and interesting information about cells and their even greater role, do give this great post a read: Symbiogenesis – the hidden model of evolution.

Chloroplasts are where the central action of photosynthesis takes place. They contain stacks of green coin-like objects called Thylakoids, that contains molecules called chlorophyll giving plants their green color. These thylakoids absorbs the photon from the sunlight that fuels it up to bolt carbon, hydrogen and oxygen atoms together to produce essential sugars, resulting into the process of photosynthesis.

If we take a deeper look at the thylakoids we will find the chlorophyll molecule (shown in the images below). On further analysis of this molecule, we can see a tree like structure branching out. In a single leaf there are billions of these molecules.

The most interesting and apparently the basic structure of chlorophyll molecule is the porphyrin ring. This is the ring surrounding the one magnesium atom with nitrogen atoms. Lets have a look at what is so special about this Mg atom.

The exciton

Having the atomic number 12, magnesium is the 8th most abundant element found on the earth’s crust and comes 3rd amongst the elements found in water.

What is special about this atom is that the electrons in its outermost orbit is loosely bound to the rest of the atom. The absorbed photon by the chlorophyll molecule easily knocks out this electron into its surrounding carbon atoms. This knock out effect creates a “battery like thing” with negative and positive charges at its poles. This is because when an electron is released it leaves behind a positive charge called a hole. This battery like thing is called an Exciton. The following image gives a better idea.

Okay so far so good. We have travelled all the way from a leaf to an exciton. Next is to take a look at this exciton and understand what is its role in all of this. Just think about it, exciton has two opposing charges in place and we all know unlike charges will attract each other, which is likely to happen. But, there is another point, the battery. Since there are two opposing charges, a battery is formed meaning there is some energy stored between the hole and the electron. When the electron snaps back in the atom (because of the attractive force between them as explained before) this energy is released in the form of either heat or light. If heat is released the photosynthetic action did not take place and the incident photon energy was lost, but, if light is emitted (which happens most of the times) another electron from the adjacent magnesium atom is released creating another exciton, and then the energy released from that exciton creates another one.

This process continues creating an image that the exciton is travelling to reach a destination. This destination is called the reaction center. Reaction center is where the charge separation takes place. This is where the electron is finally removed off the atom and is used to create a more stable battery called NADPH.

Even though it is interesting we will not discuss about NADPH right now. But the scope is the transport of that electron to the reaction center. Since there are so many densely coupled chlorophyll molecules within the leaf, the exciton can take any random path to reach the reaction center. The difficulty rises when we bring in time. Electron excitation process is extremely fast (in picoseconds) which requires the transportation to be extremely accurate (and it is) in order to reach its reaction center. The exciton can take any random path but each path will take different time (depending on its distance from the RC). Maybe this transfer is not simply hopping from one molecule to another but more like a wave in all directions. If it would be a wave then reaching the reaction center will always be possible, which actually happens 100% of the time. This is the scope of Part 2 of this post.

Lets take a look at this wave like behavior in the part 2 of this post: Photosynthesis – A Quantum Coherent System (Part 2 / 2)

references

1. Life on the Edge, By Jim Al-Khalili and Johnjoe McFadden

1. Engel, G., Calhoun, T., Read, E. et al. Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature 446, 782–786 (2007). https://doi.org/10.1038/nature05678
2. From coherent to vibronic light harvesting in photosynthesis, Chanelle C Jumper, Shahnawaz Rafiq, Siwei Wang and Gregory D Scholes, https://doi.org/10.1016/j.cbpa.2018.07.023