The Remarkable Process of Photosynthesis

Our historical perspective on photosynthesis has evolved greatly since the 18th century. With the advent of innovative scientific technology, we are capable of perceiving the near-perfect efficiency many photosynthetic life forms possess when converting light to electrical energy (Caffarri, et al., 2014). Researchers posit that by applying our current understanding of quantum mechanics to its role in photosynthesis, it is possible to harvest the Sun’s energy for all of humankind (The Royal Society, 2016).

The overall efficiency of photosynthesis, the process of converting sunlight to biomass energy, is relatively low. At most, photosynthetic organisms are relatively inefficient, as limited energy is absorbed and not all absorbed energy is converted into carbohydrate (Marais, et al., 2018). However, the primary light-harvesting period of photosynthesis proceeds with near-perfect quantum efficiency, illustrating that under ideal conditions, nearly every photon absorbed is transferred to the photosynthetic reaction centres (Marais, et al., 2018).

This level of production is inconceivable in classical mechanics, and instead is indicative of phenomena observed in quantum physics, specifically, quantum superposition. At the core of photosynthetic quantum superposition is the theory that when light hits a molecule that can undergo two distinct hydrogen transfer reactions, both the left and right reaction will occur in a quantum superposition (Scholes, et al., 2017). Classical physics, however, states each reaction will proceed one step at a time (Scholes, et al., 2017), disproving the perceived efficiency of photosynthesis.

By defining a particle only in terms of its likelihood to be found in various positions, researchers have determined that an excited photon may occupy multiple spaces at once (Ball, 2018). This revelation of photon dynamics aptly explains the efficiency of photosynthesis; instead of an incoherent “hopping” progression towards the reaction centres, electrons inspect different routes and almost instantaneously identify the most efficient path to Photosystem I or II (Ball, 2018), a process illustrated in Figure 1.

In coordination with superposition, quantum coherence also acts to aid these excited photons—also known as excitons—to instantly determine the most efficient pathway to the reaction centre (Hildner, et al., 2013). Quantum coherence, the existence of excitons at various energy levels, explains how electrons contact the reaction centre so efficiently (Hildner, et al., 2013). Physicists at the University of Glasgow extracted molecules of the light-harvesting system within plants to examine the photons’ response in hopes of understanding the mechanisms of electron travel within energy pathways. They discovered that when directed onto the complex, a fluorescence was emitted, as the excitons within the system moved from high-energy levels to lower energy levels (Hildner, et al., 2013). Remarkably, the fluorescence oscillated between high and low states, demonstrating that despite the system’s variability, the excitons were still able to instantly discover the most efficient route to the reaction centre (Hildner, et al., 2013). 

These results are corroborated by O’Reilly and Olaya-Castro (2014) who identified a combination of coherent motions and noisy vibrations within chlorophyll. By integrating these two motions, the chlorophylls become synchronized and an efficient energy transfer is detected, as energy is equally distributed around the antenna (The Royal Society, 2016).

Using modern biomolecular techniques and the information gathered from these experiments, we can advance our understanding of the quantum mechanisms on Earth. Our knowledge surrounding the confirmation of light-harvesting complexes allows us to analyze how quantum effects influence the efficiency of light-harvesting in the natural world and can potentially strengthen artificial light-harvesting machinery. By synthesizing these understandings, we can create more effective solar cells and inspire the advancement of sustainable solar power for people around the world.

Works Cited:

Ball, P., 2018. Is Photosynthesis Quantum-ish? Physics World, 31(4), pp.44–48.

Caffarri, S., Tibiletti, T., Jennings, R.C. and Santabarbara, S., 2014. A Comparison Between Plant Photosystem I and Photosystem II Architecture and Functioning. Current Protein & Peptide Science, 15(4), pp.296–331.

Hildner, R., Brinks, D., Nieder, J.B., Cogdell, R.J. and van Hulst, N.F., 2013. Quantum Coherent Energy Transfer over Varying Pathways in Single Light-Harvesting Complexes. Science AAAS, 340(6139), pp.1448–51.

Marais, A., Adams, B., Ringsmuth, A.K., Ferretti, M., J. Michael, G., Hendrikx, R., Schuld, M., Smith, S.L., Sinayskiy, I., Kruger, T.P.J., Petruccoine, F. and van Grondelle, R., 2018. The Future of Quantum Biology. Journal of the Royal Society Interface, 15, pp.1-14.

O’Reilly, E.J. and Olaya-Castro, A., 2014. Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature. Nature Communications, 5(1), pp.1-10.

Rammler, T., Wackenhurt, F., zur Oven-Krockhaus, S., Rapp, J., Forchhammer, K., Harter, K. and Meixner, A.J., 2019. Quantum Coherence in the Photosynthesis Apparatus of Living Cyanobacteria. bioRxiv, pp.1-13.

Scholes, G.D., Fleming, G.R., Chen, L.X., Aspuru-Guzik, A., Buchleitner, A., Coker, D.F., Engel, G.S., van Grondelle, R., Ishizaki, A., Jonas, D.M., Lundeen, J.S., McCusker, J.K., Mukamel, S., Ogilvie, J.P., Olaya-Castro, A., Ratner, M.A., Spano, F.C., Whaley, K.B. and Zhu, X., 2017. Utilizing Coherence to Enhance Function in Chemical and Biophysical Systems. Nature, 543(7647), pp.647–656.

The Royal Society. 2016. Quantum physics of plants. [video online] Available at: <https://www.youtube.com/watch?v=vBpsHAxsxAg&ab_channel=TheRoyalSociety> [Accessed 24 Jan. 2021].