Quantum photosynthesis

Biologists have traditionally left quantum theory to physicists. But the complicated interactions between matter and energy predicted by quantum mechanics appears to play a role in photosynthesis, according to a study published this week in Nature -- affecting how energy from the sun makes its way to a cell's reaction centers before being converted to chemical energy that powers cellular functions.

Cryptophye algae from the ocean
(species Rhodomonas)
Image: Dr. Tihana Mirkovic,
University of Toronto

"The main surprise was that you could actually see" these quantum effects influencing real world biology, said biophysicist linkurl:Rienk van Grondelle;http://www.nat.vu.nl/%7Erienk/ of VU University in Amsterdam, who did not participate in the work, and "that you could observe this phenomenon underlying how [photosynthesis] was working." Quantum mechanics is a theory that describes the behavior of subatomic particles such as photons and electrons. But scientists have long believed that predictions made by the theory would only be evident in an idealized world that lacks environmental noise of molecules moving around and bumping into one another. People thought that "at room temperature, the noisy environment would kill this kind of quantum interaction," said van Grondelle, who wrote an accompanying review in Nature. But examining the light-harvesting systems of two species of photosynthetic algae, physical chemist linkurl:Gregory Scholes;http://www.chem.utoronto.ca/staff/SCHOLES/scholes_home.html of the University of Toronto and his colleagues observed that energy introduced to the system acted in a distinctly quantum manner, even at ambient temperatures. In these algae, bilin pigments, like other light-harvesting antenna molecules, absorb solar photons, which excite their electrons. The resulting excitation energy then moves to complexes of proteins called reaction centers, where it is converted to chemical energy by a series of biochemical events. While classical energy transfer theory predicts that the energy "hops, hops, hops" from one molecule to the next in a kind of "random walk," Scholes explained, quantum theory predicts that energy flows through the system in a much more spread-out, directed fashion. "Think about it as the energy moving [through the system] like a wave rather than like a ball bouncing from one molecule to another," said physical chemist linkurl:Graham Fleming;http://www.cchem.berkeley.edu/grfgrp/ of the University of California, Berkeley, who was not involved in the research. Instead of traveling a single pathway -- one molecule to the next -- the wave-like energy can actually take three different pathways simultaneously, Scholes said. This wave-like motion provides the energy with a "memory" of where it's been that eliminates some of the randomness of how it moves through the cell, explained van Grondelle. "[It] can still follow many paths," he said, "[but] it will be certainly more directed" than the random walk of classic energy transfer. To examine whether quantum effects played a role in photosynthetic algae, the researchers excited the electrons in a pair of bilin molecules with two short laser pulses, mimicking the natural process normally initiated by the sun, albeit at a much higher intensity. Adding a third pulse just a fraction of a second after setting the energy flow in motion elicited a "photon echo" -- a beam of light emitted from the system that served as a snapshot of how the energy was distributed at that particular moment. Piecing together these snapshots, the researchers could see how the energy moved over time, and identified distinct oscillations in the echo, indicating that the principles of quantum mechanics were at play. "The fact that [the photon echo had] weaker and stronger parts to it periodically is a signature of the quantum effects," Fleming said. Otherwise, "it would just go down smoothly until it was zero." Similar quantum effects have also been documented in a widely-studied light-harvesting organism, purple bacteria; this new study, though, is the first to document these effects in the normal function of photosynthetic eukaryotes that convert carbon dioxide to oxygen. Exactly how these quantum interactions affect the process of photosynthesis remains to be seen, said Scholes. The fact that energy movement may not be completely random "doesn't necessarily mean the system is more efficient," he said. "It's more subtle than that." In fact, if different waves interfered with one another in a certain way, it "could [actually] make the system less efficient," he noted. But in some cases, such as when the sun is too bright, for example, a lower efficiency system may actually benefit the organism, he added. "What this means for moving the energy through the biological system is one of these deep questions we're still exploring." Also in question is how widespread these quantum effects are in nature, Fleming said. The fact that photosynthesis is an extremely fast process might be "crucial" for quantum principles to have a noticeable effect, he explained. "In real terms, of course, these quantum effects don't last very long at all," but in these light-harvesting systems, where electrons are being excited and energy is transferred in just a fraction of a second, "something of physiological significance happens even faster," he said. "That's not true in most of the rest of biology."
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[30th October 2000]