Photosynthesis is the process by which light is transformed into energy within plant cells. We learn about it in school and, if you're like me, that's about the last time you thought about it in any great detail.
If prompted - like now - you may remember that this process takes place in organelles within the cells called chloroplasts, and that it involves a chemical called chlorophyll. And at this point, you are (if you're like me) at the limit of your photosynthetic knowledge, and you assume that the biologists, botanists, and biochemists have figured out the specifics of this process.
It turns out that things are still a little mysterious in this area, but maybe biophysics can shed some light on photosynthesis. (Apologies for the unintentional, but very serendipitous, pun.)
In a way, the involvement of physicists in understanding this process shouldn't be surprising. The transfer of energy is directly at the center of the answer to the question "What is physics?" and this is one of the most fundamental energy transfers that take place within the Earth, and certainly one of the first energy transfers that had to take place for the development and evolution of life, since the energy that we use to exist ultimately comes from the heat energy that the Earth receives from the sun.
This first got reported back in May of 2009, when researchers published findings that showed biological systems related to photosynthesis could maintain quantum entanglement even at room temperatures. This was a somewhat startling observation, because quantum systems are notoriously "touchy" and tend to require a great deal of isolation to retain their quantum properties and avoid the decoherence, resulting in a collapse of the quantum wavefunction. Typically, this sort of isolation also requires supercooling the system to tremendously low temperatures, so the idea that a biological system at room temperatures could maintain the entanglement has a lot of significant implications. However, just the demonstration that the biological system can exhibit quantum entanglement doesn't actually demonstrate that quantum effects are actually at play in practice, so it was at best suggestive of quantum effects, but ultimately circumstantial and inconclusive, requiring further experiments and analysis.
Fast forward to a new study published just last week, in which physicist at University College London (UCL) have identified processes within photosynthesis energy transfer that, they claim, can only be explained using quantum mechanics. Classical methods just don't quite work to explain the vibrational modes that take place during the energy transfer. In the words of study co-author Alexandra Olaya-Castro:
"Energy transfer in light-harvesting macromolecules is assisted by specific vibrational motions of the chromophores. We found that the properties of some of the chromophore vibrations that assist energy transfer during photosynthesis can never be described with classical laws, and moreover, this non-classical behaviour enhances the efficiency of the energy transfer."
The key quantum behavior comes in analyzing the probability of the system existing in different states. In classical probability, the probability of finding a chromophore in a certain state is always positive (or 0, of course), but cannot be negative. Their analysis, however, demonstrated that the probability distribution contained some values that were negative ... and this sort of behavior can only be explained and understood within quantum mechanical systems. The lead author Edward O'Reilly explains:
"The negative values in these probability distributions are a manifestation of a truly quantum feature, that is, the coherent exchange of a single quantum of energy. When this happens electronic and vibrational degrees of freedom are jointly and transiently in a superposition of quantum states, a feature that can never be predicted with classical physics."
In addition to the pure science geek joy of finding new examples of quantum behavior (which is enough for folks like me), this sort of research is far from abstract and could have direct technological applications. A more complete scientific understanding of the physics inherent in photosynthesis has potentially revolutionary implications in developing advanced solar cell technology, which could transform solar energy into other forms of energy at a much higher degree of efficiency than existing technologies, and shift us away from consumable resources toward viable renewable alternatives.
- ArXiv - "Quantum entanglement in photosynthetic light harvesting complexes," May 23, 2009
- Technology Review - "First Evidence of Entanglement in Photosynthesis," May 28, 2009
- The Foresight Institute - "Quantum entanglement in photosynthesis?", May 28, 2009
- Scientific American - "Quantum Entanglement, Photosynthesis, and Better Solar Cells," Sept. 1, 2009
- i09 - "Are Plants Really Using Quantum Entanglement in Photosynthesis?", Jan. 20, 2010
- UCL press release - "Quantum mechanics explains efficiency of photosynthesis," Jan. 9, 2014
- Nature Communications - "Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature," Jan. 9, 2014 (Follow link, then click to download PDF or other version.)
- i09 - "New evidence that plants get their energy using quantum entanglement," Jan. 10, 2014
Image source: Getty Images/Bob Stefko