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Photosystem I -- An Intermediate Step -- May Not Be Necessary For Plants

ITHACA, N.Y. -- Plant biologists have long held the view that photosynthesis -- the process by which cells in green plants convert the energy of sunlight into chemical energy and use carbon dioxide to produce sugars -- needs two intermediate light-dependent reactions for successful energy conversion: Photosystem II and Photosystem I.

But according to a new study reported today (July 19), that dogma may have gone for a walk, and scientists may have to rethink some aspects of how photosynthesis works. "Scientists have thought they understood photosynthesis for a long time, and this revelation flies in the face of what we know," said Thomas G. Owens, Cornell University associate professor of plant biology. "What does this all mean? No one knows yet."

The Z scheme is the classic example of how scientists describe the light-dependent reactions of photosynthesis. When photons -- particles of light -- enter a plant cell, their energy is used to power two sequential light-dependent reactions. In Photosystem II (PSII), the light energy is used to split water, producing oxygen and electrons. In Photosystem I (PSI), these electrons are used to produce compounds that ultimately react with carbon dioxide to produce carbohydrates, lipids, proteins and nucleic acids -- the fundamental components of life.

But Owens and researchers from the Oak Ridge National Laboratory in Tennessee found that in some cases photosynthesis can occur in the absence of PSI, they report in the journal Science (July 19, 1996). The report, "Oxygenic Photoautotrophic Growth Without Photosystem I," was authored by James W. Lee, C.V. Tevault and Elias Greenbaum, all of the Oak Ridge National Laboratory, and Cornell's Owens.

The scientists from Oak Ridge used artificially developed mutants of a green alga to find this new life process.

"It is a very important discovery; I didn't believe any of this stuff when it was first reported. I thought it was impossible, but cooperation between
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Contact: Blaine Friedlander
bpf2@cornell.edu
607-255-3290
Cornell University News Service
19-Jul-1996


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