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Last Updated: Wednesday, 8 October, 2003, 12:21 GMT 13:21 UK
Photosynthesis puzzle solved
By Dr David Whitehouse
BBC News Online science editor

A complete molecular-scale picture of photosynthesis - how plants convert sunlight to chemical energy - has been obtained, offering new insights into animal metabolism as well.

Blue-green bacteria
Cyanobacteria from a hot spring provided the key
Photosynthesis is the most important chemical reaction on Earth. It is responsible for virtually all energy available for life in the biosphere.

Biologists have determined the structure of the cytochrome, a protein complex that governs photosynthesis in a blue-green bacterium.

According to Professor William Cramer of Purdue University in the US, the discovery is a great leap forward in the understanding of photosynthesis.

"Where we once could see merely the tip of the iceberg, we can now perceive the entire mechanism of photosynthesis," he says.

The researchers say their work does not have any immediate applications, but it does provide an insight not only about a chemical process crucial to all life, but also about how cells handle and distribute energy.

Mobility, membranes and molecules

The key to the discovery was being able to crystallise cytochrome molecules, so that they could have their structure determined by an X-ray probe.

"Before we found a way to crystallise the cytochrome, we had a general picture of the photosynthetic process, but possessed only a fraction of a percent of the information we now have.

Photosynthesis molecular map
Part of the complexity of photosynthesis
"Now that we can examine these proteins closely with X-ray crystallography, it could lead to knowledge about how all cells exchange energy with their environment."

The molecular layout of the cytochrome gives some indication of the complex motion of electrons and protons across the bacterium's cell membrane, the boundary between the cell and its environment.

"Plant cell membranes are like the two ends of a battery," says Professor Janet Smith of Purdue University. "They are positive on the inside and negative on the outside, and they are charged up when solar energy excites electrons from hydrogen within the cell."

"The electrons travel up into the cell membrane via proteins that conduct them just like wires releasing the energy a plant needs to stay alive.

"While this general picture has been common knowledge to scientists for decades, the complex motions of electrons and protons in the membrane have not."

From a hot spring

The cell that provided the proteins for the team's work was a cyanobacterium, a single-celled organism commonly found in hot springs.

The particular cyanobacterium used in these studies was isolated by Swiss researchers at a hot spring in Iceland.

While animals do not employ photosynthesis, their cells do make use of similar proteins for respiration. The similarities could lead to a better understanding of our own metabolic processes.

"What we see when we examine these proteins is the nature of their partial similarity," says Cramer.

"The differences can now be explored more easily."

The research was published in the journal Science.

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