Scientists have long understood how photosynthesis works – but only up to a point. The very fine details are hard to discern. Everything takes place at the molecular and even the atomic scale, involving proteins that are hard to see, and the coming and going of photons and electrons. Photosynthesis relies on processes that are governed by quantum physics – the laws of nature at its most granular.
But now scientists in China and Japan on Thursday published a report in the journal Science that describes a more robust method of examining the microscopic light-harvesting structures in a plant.
Experts not associated with this research describe it as an incremental advance in understanding how plants, algae and cyanobacteria convert light to chemical energy. In theory this kind of research, with its close examination of natural photosynthesis, could have implications for engineers who work on solar energy and researchers who try to mimic nature with designs known as "artificial photosynthesis."
The scrutiny of the natural processes could also inspire new techniques of genetic engineering in plants themselves. Remarkable though it may be, photosynthesis -- which first appeared on Earth as early as 3.5 billion years ago -- is inefficient at exploiting the energy from the sun. Thus scientists would like to figure out how to make staple crops, such as corn, more efficient in their photosynthetic processes.
In the new paper, "Structural basis for energy transfer pathways in the plant PSI-LHCI supercomplex," Xiaochun Qin and colleagues report that they have succeeded in developing a better technique for sampling and crystallizing the proteins involved in one of the two main "photosystems" in a pea plant. They studied those proteins using X-ray crystallography.
Earlier efforts in studying this photosystem had already identified a network of molecules, including chlorophylls, carotenoids and phylloquinones, but the resolution of the imagery was insufficient to detect the precise structure of this molecular complex. The new technique essentially has brought the picture into sharper focus. What the scientists see is a system in which "antenna" proteins capture light and feed them into a kind of molecular reactor.
“The efficiency of the process is determined by the three-dimensional structures of the proteins and the cofactors (pigments, lipids etc.) that perform the process. A slight change in the three-dimensional structure of the proteins will significantly affect the efficiency,” said the study’s co-author, Jian-Ren Shen, Director of the Photosynthesis Research Center at Japan’s Okayama University, in an e-mail to The Post.
"The principles utilized in the natural photosynthesis will help design artificial solar energy utilization systems with a higher efficiency," Shen wrote.
The secrets of nature might also be used by humans to boost agricultural yields, said Robert Blankenship, a professor of biology and chemistry at Washington University in St. Louis, and author of the textbook "Molecular Mechanisms of Photosynthesis."
"If you could improve the efficiency of agriculture by a factor of two or three, that would be another green revolution," Blankenship said.
Blankenship, who was not involved in the newly published research, said that artificial photosynthesis "will take some of the principles that are found in the natural system and implement them in an artificial device. I like to use the analogy of birds and airplanes.... The airplane doesn’t work the same way the bird does. But if there hadn’t been birds and insects and creatures that could fly, people would never have imagined that it was possible to build something that could fly.”
Harry Atwater, director of the Joint Center for Artificial Photosynthesis at Caltech, said scientists still have a ways to go before they understand exactly what plants are doing: "Even though nature has had millions of years to develop and optimize photosynthesis, there are still outstanding and unknown questions about how photosynthesis operates. It's truly a scientific challenge."
One difficulty with fully understanding the process is that, unlike the parts of a solar panel, which are rigid and designed to last, the crucial proteins in the photosystem of a plant are dynamic, and don't last very long before they fall apart. The plant then regenerates these structures. Everything's changing constantly -- because the plant, unlike the solar panel, is alive.
“These are molecular machines, essentially," said Michele Vittadello, assistant professor of chemistry at Medgar Evers College of the City University of New York. "These are molecules that get irradiated by light, and light interacts with them, and while it is interacting with them, there are modifications in the structure that occur, there are chemical bonds that are broken and are being formed. It’s a dynamic thing. Even if you do X-ray crystallography and you get a snapshot of a structure, this is just a photograph in a movie, it's not the whole picture.”