You might think that given how ubiquitous and essential photosynthesis is, we’d have long ago discovered how it works. Instead, major parts of the process remain a mystery. New research suggests one of these stages has considerable parallels with exciton condensates, something physicists have had to go to great lengths to produce in the lab.
Professor David Mazziotti of the University of Chicago heads a lab that is using computer modeling to try to understand the way atoms and molecules interact in important chemical processes. Few such reactions are as vital and common as photosynthesis, where plants and algae use the energy from sunlight to make sugars and starches.
The process is begun by photons knocking loose electrons in leaves, allowing both the electron, and the “hole” where the charge used to be, to move through the chromophore (chlorophyll molecule), carrying solar energy. Although that much has been known for a long time, Mazziotti and colleagues report that collections of electrons and holes and holes don’ always move like individuals.
An electron and its hole are collectively known as an exciton, and when considered together have quantum properties different from each on its own. An exciton is a boson, for example, while the electron and hole are each fermions. Modeling the behavior of numerous excitons, rather than each individually, the researchers realized how much their behavior resembled Bose-Einstein condensate, sometimes known as the “fifth state of matter” after the traditional solid, liquid, gas, and plasma.
Bose-Einstein condensates allow large collections of atoms to display the sort of mind-bending quantum behavior usually only seen at the sub-atomic level. Not only can they do entirely without such otherwise universal phenomena as friction, but they can also engage in weird quantum activities like combining wave and particle behavior.
To make Bose-Einstein condensates scientists need to cool ordered materials to temperatures barely above absolute zero, but plants are doing something similar outside your window right now (if it’s daylight). "Photosynthetic light harvesting is taking place in a system that is at room temperature and what's more, its structure is disordered – very unlike the pristine crystallized materials and cold temperatures that you use to make exciton condensates," first author graduate student Anna Schouten said in a statement.
The discovery was not made earlier partly because plant excitons are short-lived, usually recombining quickly. As well as low temperatures, exciton recombination can be delayed with powerful magnetic fields, but of course, plants don’t have these either.
“As far as we know [photosynthesis and exciton condensates] have never been connected before, so we found this very compelling and exciting,” said Mazziotti.
Perhaps even more strangely, a chromophore-worth of excitons does not become condensate-like together. Instead, patches the authors call "islands" form. These islands are no irrelevant curiosity, however.
A leafy collection of excitons; “May lack some of the properties associated with macroscopic exciton condensation,” the paper notes, but “it is likely to retain many of the advantages, including efficient energy transfer.” If so, it would make photosynthesis more efficient, contributing to the richness and abundance of life. Indeed, under ideal conditions, exciton condensation may double the rate of energy transfer compared to what would otherwise be possible.
Even super-computers struggle to model the complexity of atomic and subatomic behavior during photosynthesis, so models get simplified even more than in many other scientific scenarios. However, Mazziotti cautions exciton collective behavior is one thing that should not be left out. “We think local correlation of electrons are essential to capturing how nature actually works,” he said.
The study is open access in PRX Energy