Photosynthesis, the process by which plants make food (sugar) and oxygen using carbon dioxide and water, seems simple enough. However, it is comprised of many smaller steps which researchers have recently shown involve quantum mechanics.
In his recent article published in Nature, Professor Greg Scholes of U of T’s Faculty of Chemistry discusses how quantum mechanical processes are present in the photosynthesis that occurs in cryptophyte algae.
Cryptophyte algae cells reside in marine and fresh water environments. Each cell is comprised of chloroplasts which have photosynthetic complexes that harvest and store light energy. Energy moves within and between proteins, however it needs to be transferred quickly in order to be used. The proteins which harvest light are called “antennae.” They absorb photons from light and the algae’s pigment molecules (these determine what colour of light the algae absorbs) enter an excited state. This excitation energy is directed to reaction centers, resulting in an energy transfer across the cell membrane that triggers a series of biochemical events for chemical energy generation.
Antennae direct and concentrate light energy so that electronic excitations can occur within a certain time period. Furthermore, antennae allow cells to contain fewer reaction centers, and those containing different pigments (and absorb different colours of light) can be attached to the same reaction centre. The pigments in light-harvesting processes are strategically spaced: close enough to allow expedient energy transfer but far enough away to prevent overlapping of pigment molecules. In addition, multiple pathways for energy delivery are connected to the reaction centre.
Scholes’ paper suggests the efficiency of energy transfer between proteins is influenced by quantum mechanics. “Using quantum mechanics you can steer the energy and direct it because now you’re moving the energy more like a wave than a particle; when two waves come back together they formulate pathways and direct the energy. By putting information in using a laser, the different energies of the molecules in the protein absorb light and quantum mechanics and an image is produced that shows the way the different molecules absorb light.”
Scholes’ research began nine years ago when his first PhD student concluded the initial experiment that produced initial maps of the ways different molecules in the algae absorb light.
It took two years to set up the following experiment which showed how quantum mechanics governs photosynthesis. These two years involved building the mechanism that could measure laser pulses and developing a measuring technique.
Though computer programs drove the experiments, post-doctoral student Elisabeta Collina and PhD student Cathy Y. Wong sometimes needed to be there long hours to overlook the procedure. “Wong is finishing mapping out what goes on in the proteins,” says Scholes. “She has optimized the required techniques and now the molecular interactions and the proteins’ functions can be understood at a greater level of detail.”
Although cryptophyte algae cells were originally used out of convenience, Scholes says that they continue to be of use during research due to their unique characteristics. “The cryptophyte algae is comprised of a few different species of varying colour. They can completely change the colour of the light they absorb by slightly varying the molecules. On a practical level, this experiment could help you understand how you might design an organic plastic to work better and certain limitations that haven’t been addressed could now be addressed; furthermore, algae could be grown in big reactors and used to generate fuel.”
The experiment is also relevant on a theoretical level. Scholes explains, “For many years, people have been fascinated with whether this fundamental theory of physics, quantum mechanics, is actually used in living systems. People have speculated that quantum mechanical processes are present in the brain but these individuals have been ridiculed. This experiment, however, shows that there’s an example of quantum mechanics being used in an organic system, which makes us question what we knew about certain theories.”
Furthermore, this experiment has opened up new opportunities to show how quantum mechanics fits into evolution: “Is quantum mechanics a fluke or is it something that was optimized by evolution?” asks Scholes.
This question can only be answered by the involvement of geneticists, ecologists, and other scientists, but Scholes’ discovery is an excellent starting point for further research on the importance quantum mechanics plays in our world.
