Clues to Nature’s Power Plant

Imagine a solar power plant on the roof of your house that is as efficient at converting energy from the sun into electricity as crab grass is at taking over your lawn.

Those two processes are not as far apart as they might seem.

Both use photosynthesis, upon which all life on this planet depends. Plants use photosynthesis to produce the fuel for their growth, and in the process remove carbon dioxide from the atmosphere and make the oxygen we breathe. For years, scientists have sought to understand the process, and their latest breakthroughs could lead to a revolution in everything from the manufacturing of computers to the production of energy to higher crop yields.

Driving the research is this fundamental principle: If we could understand exactly how nature does it, we could build artificial “photosynthetic reaction centers” that would be far more efficient than the current generation of solar cells.

Italian scientist Giacomo Ciamician speculated early this century that artificial photosynthesis could someday fulfill most of our energy needs. Ciamician, one of the founders of modern photochemistry, had recognized that even the crudest of organisms can convert light from the sun to useful purposes with an astonishing level of efficiency. They do it by using millions of tiny photovoltaic cells.

In recent years, there has been some success in trying to harness the sun. Even high-tech windmills are a form of solar energy because they use the wind produced by heating and cooling of the atmosphere to turn turbines that generate electricity. And solar cells, or photovoltaic devices, are used on instruments ranging from communications satellites to digital calculators.

But even the best devices today are woefully inefficient compared with biological systems. What sets the natural world apart from the artificial devices produced in our factories is the fact that nature works on the smallest of scales--at the molecular level.

So to mimic the efficiency of nature, scientists must learn how to produce photovoltaic cells that work within a single molecule. Then they must learn how to extract the energy produced inside that molecule, and then how to convert it to a useful purpose. Which is exactly what they’re doing.

“A few years ago, this wasn’t possible,” says Devens Gust, professor of chemistry and director of the Center for the Study of Early Events in Photosynthesis at Arizona State University. “There’s more progress now because we have the tools to work with.”

The new tools range from better microscopes to faster computers that allow scientists to look inside biological systems to see how they use solar energy to meet their own needs. That’s enabling scientists to mimic nature, and there are many potential applications.

A team of scientists at Argonne National Laboratory, led by Michael R. Wasielewski, succeeded recently in building artificial photosynthetic devices no larger than a single molecule.

Someday, devices that small may be used as tiny switches in super-fast computers.

Gust, a photochemist, and a colleague, Thomas A. Moore, began trying to create artificial photosynthetic devices 15 years ago. Biological systems, from worms to humans to plants, use “packets” of light, called photons, to separate negative and positive charges across a thin membrane. That separation acts like the organism’s battery, storing the energy until it can be converted to more useful purposes.

For example, the organism uses the energy to produce what Gust calls “the gasoline of life,” adenosine triphosphate, to fuel its growth.

Gust says he and his colleagues have also created a photovoltaic device--patterned after nature but controlled through chemical bonds--that is the size of a single molecule, and he says it performs “about as well as those seen in nature.”

But much remains to be done.

“Lately, we’ve been trying to learn how to do something with the energy that’s stored in that charge-separated state” (the molecular battery), Gust says. “Right now, it’s just sitting there inside this one molecule. We’re trying to figure out how to get it out so it can be used.”

Someday, Ciamician’s vision may become reality. If a packet of molecular-sized, synthetic photovoltaic cells could work as well as those found in the simplest microorganisms, we would be well on our way toward harnessing an unlimited supply of energy from the sun.

And if that research leads to a better understanding of how nature achieves its own miracles, as most researchers believe it will, it might even help us improve the natural processes, leading to such things as higher agricultural yields from food crops and better protection from weeds. Like crab grass.

Lee Dye can be reached via e-mail at [email protected]