Quantum Capture: Photosynthesis tries many paths at once

By Davide Castelvecchi
From Science News, April 14th, 2007; Vol.171 #15 (p. 229)

Quantum physics plays a larger role than scientists had expected in plants’ capture of light. New findings could explain life’s uncannily efficient use of solar energy, researchers say.

In organisms ranging from blue algae to giant sequoias, complicated assemblies of molecules of the pigment chlorophyll absorb sunlight’s photons and channel their energy to enable the plants to turn water and carbon dioxide into oxygen and sugars.

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.

The absorption of a photon causes a chlorophyll molecule to enter an excited state, in which one or more of its electrons hop to a higher energy level. The traditional view was that chlorophyll molecules within a complex swap excitations until that energy finds its way to a reaction center, where it initiates a chemical reaction. But at each exchange between molecules, the excitation might dissipate as waste heat, so scientists didn’t understand how the process could be so efficient.

Instead of bouncing from one molecule to another, excitations move like waves do, reports a team of chemists at the University of California, Berkeley and the Lawrence Berkeley National Laboratory. In a new experiment, Greg Engel and his colleagues found that groups of chlorophyll molecules spend a surprisingly long time in a so-called superposition of states—a quantum phenomenon in which many molecules share excitation energy and so are simultaneously excited and relaxed. The mixtures of different states can show wavelike behavior. For example, they can cancel each other or add up, like waves on a pond do.

In the experiment, the team froze chlorophyll complexes from blue algae and shot them with sequences of ultrashort laser pulses, each lasting just 40 femtoseconds, or millionths of a billionth of a second. Three pulses excited the molecules, and a fourth pulse detected interference patterns.

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.

The team’s results appear in the April 12 Nature.

The recent Berkeley experiment overturns 50 years of thinking about photosynthesis, says Rienk van Grondelle, a biophysicist at the Free University of Amsterdam. Previously, scientists thought that the energy wanders randomly. “Here, it moves in a very specific manner,” van Grondelle says.

Blue algae have relatively simple molecular machinery, van Grondelle notes. He says that the researchers’ next challenge will be to perform a similar experiment on the more intricate chlorophyll complexes of plants.

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