There has been a debate about quantum effects in biological systems for several years now. With photosynthesis, scientists show for the first time that there are quantum effects in living systems [Y.-C. Cheng, G.S. Engel, G.R. Fleming, Chem. Phys. 341 (2007) 285]. This could helps us to build a better solar panels, energy storage or even quantum computers. We all probably learned about photosynthesis, how plants and some bacteria turn sunlight into chemical energy in school. It might seem, therefore, that we have understood many things about this, but scientists are still exploring new things about even the most basic stuff of photosynthesis. In this article I will try to give a brief description of how quantum mechanics is assisting the fundamental energetic process that drives all life on the Earth.
A mini excursion to Photosynthesis : The details of the Photosynthesis is not my aim to present here, the brief outline is given from molecular point of view below:
Both plants and photosynthetic bacteria capture solar energy in similar manner through the following steps:
- Photons of sunlight are absorbed by the Light harvesting Complex or Antenna complex which contains chlorophyll molecule and other light-absorbing “chromophores” in a large assembly of proteins and other molecular structures.
- When a photon strikes a chlorophyll in a plant, it`s energy knocks out an electron from chlorophyll. Here is where things gets little abstract, we usually think the Chlorophyll should be positively charged as it`s just an loss of electron. But we need to re-frame it little bit, think it of in this way, the negatively charged electron gets excited into higher electronic energy level so a positively charged hole is created where it was supposed to be i.e. in ground state, and the molecule remains neutral overall.This is called an exciton and & it can store energy it`s just like a battery where electron and hole act like two electric poles.
- In order to initiate the chemical reaction the exciton needs to be landed in the reaction center through an exciton transport chain of other molecules with nearly 100% rate, where electron transfer occurs and ultimately we get glucose and oxygen.
The mystery of high efficiency of excited state energy transfer/Exciton transfer:
Traditional Model: Transferring the exciton from antenna complex to reaction center was a long lasting puzzle to the scientific community . From LHC(Light Harvesting Center), Initially the energy transport process was visualized as that Light harvesting complex can transfer energy from chlorophyll or other neighboring chromophore through electronic interaction [as for eg: dipole -dipole interaction] of the chromophores in the electronic excited states until it gets into the reaction center. Basically it was a non coherent random hopping process from one chromophore to another directed by an overall energy gradient [just like an drunk person going downhill] But there are two issues with this model.
- The reaction center might be far away from the LHC.
- At the same time Chlorophylls are stack densely stacked together.
So it’s difficult for the exciton to chose one optimized path to reach the reaction center and it most likely to get lost than to perform photosynthesis. But, over the course of several billion years, nature has managed to make this energy transfer process extremely efficient. The efficiency of transmitting this energy through the cell can reach near 100 percent . For comparison, modern solar cells reach an efficiency of around 15 percent. Moral of the story, new model is required to explain this but before jumping into the another model it`s important to know the basic ideas of quantum mechanics.
Quantum mechanics in a nutshell:
Our familiar classical macroscopic world is made of two major components:
Between Sunlight and a cricket ball we don’t face any difficulty to answer which is wave/radiation or Particle. But our this common expectation does not match in microscopic/subatomic world, where an object like electron is a particle and wave at the same time [wave–particle duality]!! Well sounds quite bizarre,right? Welcome to the ghostly world of Quantum mechanics.
This is the heart of Quantum theory is the wave particle duality which says a quantum object[like electron, photon etc.] is a particle and a wave at the same time, Wave character[wave length] and particle aspect[momentum] is connected by this equation.
But what decides the boundary between classical and quantum world? Lets figure it out.
Applicability of Quantum mechanics:
The mass factor: QM is important where the wave character is high/prominent, so natural intuition tells lower mass particle [eg: electron] should be treated quantum mechanically, not a 75kg human being !
Temperature : Quantum effect is dominant at lower temperature . For a particle [say free particle] at thermal equilibrium it’s total energy is kinetic energy and we can write,
P2/2m = KBT, or p=(2m KBT)1/2
So,if T decreases then associated matter wave λ increases, and quantum nature becomes important. That’s why cold atoms and molecules show quantum characteristics often. So, one should not even think about observing quantum nature for large molecules and at higher temperature, but plants love to be exceptional in this respect, the following sections will describe the same.
To read about chemical reactions at very low temperature you can read my blog about this
Quantum model of energy transport:
In our familiar macroscopic world if something is in one spot it can’t be in any other spot at the same time. But in quantum mechanics things aren’t that straight forward, one of the fundamental features of the quantum mechanics is superposition, which tells that a particle can simultaneously exist in many different places with different probabilities.
Understand it by this analogy: Suppose in your room let’s say you have three choices to do, reading books sitting in a chair, reading magazine lying in your bed and doing exercise on the floor. As you are a classical or macroscopic object, you can’t be at chair and bed or all three positions at the same time. Whereas if you are a quantum object and if the door is locked, you can be at all three places at the same time with different probabilities. Others might assign probabilities of seeing you,let’s s say 70% chance in bed, 20% chance in chair and 10% chance on floor. These probabilities can be viewed as spread out waves ,at every point in space. Another important point is this probability waves remain intact until it’s detected, as soon as it’s measured it collapses into one location, means if you are a quantum object then if someone opens the door, he will find you in either bed, on floor or at chair,but you were on every position inside the room when you are not seen!! Yes, that’s the reason behind calling Quantum mechanics weird.
Now the idea of being in many places at once can be extended to taking many paths at once, That is, if excitons could travel like waves, unlike particles, they could simultaneously try out all paths to the reaction center[similar to spreading of a wave in space], it doesn’t have to chose a particular route,.wastefulness of exciton can be eliminated and making photosynthesis a fast process. This is exactly what Physicists proposed decades ago in their revolutionary paper, the phenomena is called quantum coherence [ Panitchayangkoon et al. 12766–12770 ∣ PNAS ∣ July 20, 2010 ∣ vol. 107 ∣ no. 29 ].
The twist in the tail: Everything till now sounds impressive but this is not the end yet. The quantum superposition is lost when we do measurement in the system. In this context measurement means interaction of the quantum object with its environment. When there is interaction with the environment, the superposition or quantum coherence got destructed and classical properties [distinct wave or particle property] arises and the phenomenon is called decoherence. Decoherence is the reason scientists need to work in such specific conditions when they are dealing with quantum mechanical effects . For macroscopic particles, its coupling with the environment is large: many atoms or molecules are bouncing around,so much jostling due to the heat that coherence doesn’t exist long enough to be detected. The quantum effects can best be seen when the system is isolated, But, plants can maintain coherence ~ ps time scale at wet,warm and chaotic environment. This sounds crazy but it`s the reality. But how to get conformation of quantum coherence? See the next section
Indication of Coherence: General proof:
If |Ψ 1> and |Ψ 2> are the two eigen states with energy E1 and E2,where |ϕ1> and |ϕ2> are space part only, then the superposition state is,
For this state the probability density P oscillates [the Cosine term] with time as well as there is a cross |Ψ 1Ψ 2|term in the oscillations which are the indication of coherences .[here ωn= En/h].
In the decoherence picture,|Ψ 1|2 or |Ψ 2|2 represents probability of finding the particles around their respective positions, i.e. the side most humps, whereas the middle oscillatory part is the coherence.
Detection of coherence:
An experiment was performed by Panitchayangkoon et al. [12766–12770 ∣ PNAS ∣ July 20, 2010 ∣ vol. 107 ∣ no. 29] on Bacterial Light Harvesting protein Fenna-Mathews-Olson complex which has 7 exciton transport sites using a technique called 2D Fourier Transform Electron Spectroscopy,the research team was able to probe into the inner structure of the photosynthetic complex. They gave three successive LASER pulse with suitable frequencies which generated the light signals which then picked up by the detector.If there was really coherence [superposition]* among the excitons they should be able to see interference between the different pathways which manifests itself as oscillating beat pattern,in another word the exciton was not taking a single rout through the chlorophyll but was following multiple routs simultaneously.
The controversy regarding Role of Vibration: Numerous experiments were performed since then to confirm of quantum coherence , but the story is far from over. Although this quantum beat keep showing up there is still a debate on how to interpret it. Some experts [(Nature Chem. 6 196]in the field think that the beats are caused by molecular vibrations not coherence.Then there are others who thinks that vibrations in the reaction center of the photo system from spinach support quantum coherence between excitonic states, by strengthening the interactions between them, and promoting coherent energy transfer (Nature Phys. 10 676). Research is still going on to understand this in deeper level.
Future Direction: Maintaining the coherence among the qubits for a long time is a struggle with the creation of quantum computer. Scientists come up with the all kinds of clever ideas to protect the qubits from external influences like cooling them to near absolute zero temperature or attempting to keep them in complete isolation but so far nothing has worked efficiently to eliminate the decoherence . The underlying mechanism for long coherence time in photosynthesis can help us to build cool new quantum computer in the future. At the same time the interpretation of these quantum effects in photosynthesis may help in the development of light-harvesting devices with very high efficiency to cope the energy crisis.
Former JRF of Department of Chemistry
About the author: Hi, I’m Subarna. I love making sense of something difficult, breaking it down into understandable pieces, and then teaching it to others. I did my MSc from Rajabazar Science College,University of Calcutta. I Specialize in the field of Quantum and Computational chemistry. I keenly pursue playing cricket as a hobby supplemented with knowing ancient Indian culture and history.