Photosynthesis is a biochemical cycle that produces oxygen and a simple sugar, glucose. In present research, scientists are learning to make photosynthesis a process that can displace commercial chemical production and as a replacement to fossil fuels. It is in photosynthetic processes that research is done to improve the chemical apparatus used by plants and bacteria—making the processes environmentally safer than commercial syntheses or energy production.
What is the Chemistry Behind Photosynthesis?
Many of us in the U.S. learn about photosynthesis in High School science classes as one fundamental chemical process. The process is portrayed by a single equation. Plants and bacteria produce oxygen (O2), water (H2O), and a simple sugar- glucose (C6H12O6) :
6CO2 + 12H2O + UV-Light —> C6H12O6 + 6O2 + 6H2O.
However, the one equation does not tell a complete story. Photosynthesis involves two fundamental processes that include hundreds of individual reactions –light activated reactions and reactions occurring with no light activation. The two processes with numerous equations are known as biochemical cycles — the cycles of photosynthesis can be represented by the following chart:
Photosynthesis utilizes hundreds of distinct chemical reactions. Diagram of photosynthesis in the chloroplast of a leaf. Image from Brookhaven National Lab https://www.bnl.gov/chemistry/ap/images/Home_01_HR.jpg
How To Modify Photosynthesis?
While the question becomes- what reactions are most conducive towards modification? No plant- nor bacteria-based photosynthetic reaction is modified easily.
As researchers sought ways to modify photosynthesis, they did not find suitable mimicking reactions to take the place of natural photosynthetic reactions. Attempts modifying photosynthesis result with inefficient substitutes that cannot compete with plants, themselves.
One example is to replace biological apparatus of the Calvin cycle with a synthetic catalysts. The Calvin cycle produces a simple sugar- glucose, from carbon dioxide. Thus,
CO2 + H2O –> C6H12O6.
While the reaction, as written, is not easy to fathom, biology performs the process simply. The plant or bacterium uses molecules called enzymes to push the carbon dioxide molecule to become a glucose molecule. Enzymes are far larger than the molecules they catalyze. In this particular case, the enzyme surrounds the carbon dioxide molecule while the hydrogens and oxygens are added in one step.
Proposed analogous synthetic reactions use a metal catalyst to add 12 hydrogen atoms and 6 oxygen atoms in separate steps to the carbon dioxide to make the simple sugar- glucose, C6H12O6. While researcher’s results showed the metal catalyst as ineffective, molecules, that can better mimic enzymes, are required.
Quoting from a publication of the American Chemical Society in 2017–researchers from Lawrence Berkeley National Laboratory at the University of California, Berkeley can be quoted “… it would be unreasonably hopeful to imagine we could currently capture all the performance capabilities of biological CO2 reduction…” Chemical processes of photosynthesis adapted to an almost static soil and mostly pure water over the course of billions of years– our current attempts pale in comparison. Knowing that sunlight shining on plants and other organisms is variable as well further confounds the issue.
The following table captures the essence of the argument of the previous three paragraphs:
Thus, researchers improve light harvesting actions of organisms. Of all aspects related to photosynthesis, organisms efficiently harvest only 3 percent light. The 3 percent number is the biggest reason to approach photosynthesis research to improve light harvesting
Modifications to Photosynthesis Understood from a ‘First Principles Approach’
When discerning ways to modify photosynthesis, scientists are left with one easy option. The improvement of light-gathering efficiency is addressed because the photosynthetic apparatus shuts out more light that it can handle. Given that plants and bacteria respond to increased light through the slow evolutionary processes that spawned their genesis, we proceed with evolution in mind. When increased light normally coincides with growth and carbon dioxide uptake, we take it one step at a time. Once light gathering efficiency is improved, scientists can take the next step: the discernment of plant photo-biochemistry and chemistry.
The present course of climate change has made research in this area a major concern. Of late, average yearly temperature changes appear to increase exponentially. When the year 2050 arrives, we may not possess the luxury of accepting fossil fuels as our source chemistry dependence–if we are still around to do so.
ADDITIONAL READING & REFERENCES
GARY F. MOORE and GARY W. BRUDVIG. Annual Reviews in Condensed Matter Physics. 2010, Energy Conversion in Photosynthesis: A Paradigm for Solar Fuel Production.
ICHIRO TERASHIMA, et. al. Plant Physiology. 2011,Leaf Functional Anatomy in Relation to Photosynthesis.
PEIDONG YANG and JEAN-MARIE TARASCON. Nature Materials, 2012, Towards Systems Materials Engineering.
CHONG LIU, et. al. Science. 2016, Water splitting–biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis.
NIKOLAY KORNIENKO, et. al. Proceedings of the National Academy of Sciences. 2016, Spectroscopic elucidation of energy transfer in hybrid inorganic–biological organisms for solar-to chemical production.
J. BLOEMEN, et. al. Acta Horticulturae. 2013, Understanding Plant Responses to Drought: How Important is Woody Tissue Photosynthesis?
C. LIU, et. al. Science. 2016, Water Splitting-Biosynthetic System with CO2 Reduction Efficiencies Exceeding Photosynthesis.
STUART A. WEST, et. al. Proceedings of the Royal Society, B. 2002, Sanctions and mutualism stability: why do rhizobia fix nitrogen?
KELSEY K. SAKIMOTO, et. al. Accounts of Chemical Research. 2017, Cyborgian Material Design for Solar Fuel Production: The Emerging Photosynthetic Biohybrid Systems.