5.06.2007

Chlorophyll Tricks

A recent little article in a recent Science News (Week of April 14, 2007; Vol. 171, No. 15, p. 229, subscription required) has a mind-bending piece about how plants harvest sunlight. Here's the central mystery:
The efficiency of photosynthesis, as this process is called, has long astounded scientists. Virtually every photon absorbed by chlorophyll initiates a photosynthetic reaction. Plants use up to 90 percent of the light that strikes them, whereas commercial solar panels use less than 30 percent.
It has been difficult for scientists to figure out how the chlorophyll molecules can hand off energy to one another to get to an 'excitation center,' where it is turned into work. Why doesn't the energy seem to dissipate? The mechanism proposed is that quantum superposition is the key:
The complexes stayed in a superposition of states for more than 600 femtoseconds after receiving the pulses. During that interval, "the system is exploring all areas at once without having to visit each place individually," Engel says. The paths that transfer energy to the reaction center are energetically favored over those that turn it into waste heat, he proposes.
If this is so, it's really quite amazing. The potential for improving artificial ways to harvest sunlight are obvious, but it raises for me even more fundamental questions. Where else is quantum mechanics playing a role? Roger Penrose has suggested that micro-structures in neurons might allow quantum superposition as well. He speculates that this may be key to intelligence. I had always thought of that as a pretty far-out idea, but Penrose may turn out to be right after all. Here's an article on the subject.

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