When Physics meets Biology : Quantum Biology!
In this article, we will briefly explore the emerging field of Quantum Biology. An area of research that is relatively still “speculative”, yet some recent experimental evidence sparked more interest in it.
What do we exactly mean by “Quantum Biology”?
When we say quantum mechanics you might simply think of “quantization”; a fundamental aspect of the quantum theory that describes how electron energies are restricted to discrete, rather than continuous, values. This notion of quantization underpins our understanding of chemical bonding and eventually the molecular structure of any matter whether inanimate or living. So if biological systems are eventually made up of atoms and subatomic particles that are known to be part of the quantum world, what’s new about Quantum Biology?
It would only be interesting if other non-trivial quantum effects were proven to play a role in biological processes, and surprisingly, there is growing evidence that they do.
The most significant examples are quantum tunnelling playing a role in enzyme catalysis, quantum entanglement in birds’ migration, and quantum coherence in photosynthesis. In this article, we will focus on the latter example.
Figure 1: Quantum effects may potentially explain different biological phenomena. (Credits: Waring S , 2018)
What is so interesting about photosynthesis?
Photosynthesis, in very basic terms, is the process by which plants turn solar energy into chemical energy, and consequently utilizes this energy in many ways. We are all familiar with the big picture of photosynthesis, that is, plants taking in Carbon dioxide, water and producing Oxygen and Glucose. A process that occurs in the presence of both “Chlorophyll” and light.
Figure 2: Photosynthesis equation
Now, this may seem simple, but one of the scientists’ most enjoyable habits is to stop at certain parts, zoom in, and ask questions. Consistent and logical explanations of the phenomena we observe are what we are constantly aiming at. But, it’s the missing piece of the puzzle that drives the scientific progress. When it comes to studying the world on a very small microscopic level, there are plenty of these missing pieces.
The molecular mechanism of the process of photosynthesis
Now, let’s go back to the part we will zoom in on when it comes to the process of photosynthesis; the part involving both, “light” and “Chlorophyll”. How exactly do plants utilize light to complete one of their most crucial functions?
The “Chloroplast”, the part of the plant cell responsible for photosynthesis, contains smaller units that function as photosynthetic systems. These photosynthetic systems contain a subunit that we refer to as, “Light-harvesting complexes”.
They capture light and transfer it to “Reaction centers” that ultimately execute the energy conversion we mentioned earlier. The illustration below demonstrates how the light’s energy is transferred across the photosynthetic systems, and it highlights three important steps.
Figure 3: Reactions taking place within the thylakoid, the light-harvesting complex circled in red. (Credits: Image modified by Khan Academy from "The Light-Dependent Reactions of Photosynthesis: Figure 8," by OpenStax College, Biology)
Figure 4: A detailed diagram of the energy transfer process across the light-harvesting complex. (Credits: Image modified by Khan Academy from "The Light-Dependent Reactions of Photosynthesis: Figure 7," by OpenStax College, Biology)
First, the chlorophyll and carotenoid molecules located at the top of the light-harvesting complex, absorb “photons” (units of light).
This photon is a source of energy, more precisely, an “excitation energy”. So, the absorption of a photon knocks out an electron from the molecule and turns the latter into an “exciton”. The second step is when the “exciton” travels down the other molecules to reach the “Reaction Center”.
The third step occurs at the reaction center where the energy conversion is achieved. So we can break down the process into three steps;
- Energy capturing
- Energy transfer
- Energy conversion.
The efficiency of Energy transfer across the photosynthetic system It is the second step of the process we mentioned above that is of interest to scientists. For the whole process of capturing and utilizing light to be efficient, we need as many photons absorbed by the light-harvesting complex to ultimately reach the reaction center and undergo conversion. For an exciton to move around randomly until it gets to the reaction center, there is the possibility that the energy will disseminate into the system.
In this sense, the system’s efficiency we talk about is all about “Energy
Transfer”. If we were to figure out that the said system is highly-efficient, we‘d sure want to know about the mechanism of energy transfer across it.
How do the excitons “figure” their ideal path towards the reaction center? The classical and “quantum-mechanical” perspectives
The classical model of how excitons reach the reaction center is by hopping randomly from one Chlorophyll molecule to the other while driven by an overall energy gradient. (Streaming down from a higher to lower energy level), but the problem is, this can’t explain the system’s high efficiency.
Scientists have wondered for a long time if the rules of quantum mechanics played a non-trivial role in biological systems. One of the relevant quantum concepts in our case is “Quantum Coherence”. Quantum coherence is the idea that an object (a particle for example) can have wave-like properties. Rather than having a defined position in space, a “Coherent Quantum Wave” can exist in many places simultaneously. If we were to treat the exctions in the process of photosynthesis as “Coherent Quantum Waves”, we can imagine them moving around from one molecule to the other by two or more routes at once. So they are simultaneously exploring multiple possible options and consequently taking the most efficient pathway towards the reaction center. (I already mentioned that a quantum-mechanical view comes with its counter-intuitiveness, right?) In 2007, Engel and Fleming, two prominent chemists, conducted an experiment using spectroscopy to provide evidence that photosynthesis in green sulfur bacteria made use of quantum coherence.
Reflections, aspects of the debate, and conclusion Quantum coherence in photosynthesis is relatively the most evidence-supported example of quantum effects contributing to a bio- phenomenon. The significance of whether the aforementioned theory is accurate or not is highlighted in more than one context. Firstly, there are the historical and philosophical speculations sparked as early as the rise of quantum mechanics itself. Quantum mechanics playing a non-trivial role in the underlying processes of biological phenomena, that is, providing a functional necessity to the bio-system, is a long-debated topic. It has already been addressed by Erwin Schrödinger himself, a pioneer in the field of quantum mechanics, in his famous book, “What is life?” These early speculations by either physicists or biologists stemmed from the belief that quantum mechanical effects on the molecular level could offer new paradigm-shifting insights into our understanding of the basis of life.
Figure 5: Ideas of Quantum Biology go back to the 1920s as illustrated by this timeline (Credits: Jim Al-Khalili, 2018)
Secondly, another aspect that is worth discussing relates to the fact that quantum effects could only be observed and studied in special conditions. Experimentalists in this field need to maintain the systems they deal with at temperatures close to zero and in a vacuum. The fact that bio-systems aren’t isolated from the surrounding environment and don’t function in the above-mentioned circumstances, questions the idea of quantum effects taking place in these systems. On the other hand, if bio-systems have their way of overcoming the “environmental-noise” that is known to disrupt the occurrence of quantum effects, it would be an interesting finding for researchers in the field of quantum computing, who work on constructing machines that utilize quantum coherence to process multiple routes of information transfer, simultaneously.
Lastly, these recently-suggested intricacies of how energy is transferred during photosynthesis can inspire the design of more efficient synthetic solar systems by deploying similar mechanisms to those found in nature.
To conclude, the exact mechanism of how photosynthesis occurs remains an area for continuous research that will constantly evoke more theories and models. Still, it’s a rather good example to reinforce how research endeavors inboth biological and physical sciences are complementary and contribute equally in answering some of the fundamental questions about nature.
References: 1- Waring S (2018) Quantum Biology: A Scientific Revolution in our Understanding of Biological Systems. Biol Syst Open Access 7: 185. doi:10.4172/2329-6577.1000185 2- McFadden J, Al-Khalili J. 2018 The origins of quantum biology.Proc. R. Soc. A 474: 20180674. http://dx.doi.org/10.1098/rspa.2018.0674 3- Tzu-Chi Yena, Yuan-Chung Cheng, Electronic coherence effects in photosynthetic light harvesting, Procedia Chemistry, Volume 3, Issue 1, 2011, Pages 211-221, ISSN 1876-6196, https://doi.org/10.1016/j.proche.2011.08.028.
4- Engel GS, Calhoun TR, Read EL, Ahn TK, Mancal T, Cheng YC, Blankenship RE, Fleming GR. Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature. 2007 Apr 12;446(7137):782-6. doi: 10.1038/nature05678. PMID: 17429397.