During photosynthesis, energy harvested from the sun is used to move electrons through a protein-cofactor complex known as the photosynthetic reaction center (PRC). When the structure of the PRC was first revealed a decade ago, scientists were surprised to find two potential electron-transfer paths, even though electrons use only one, says Steven G. Boxer of Stanford University. He along with Dewey Holten and Christine Kirmaier of Washington University, St. Louis, and coworkers have now shown why electrons take one path (passing through cofactors shown in green) and not the other (red) (Biochemistry 2006, 45, 3845). Previously, scientists speculated that differences in the cofactors' electronic coupling or their energies might account for the electrons' preference for only one of the two roughly equidistant paths. Kirmaier's team demonstrates that energetics is paramount: The researchers first cut off electron transfer down the green branch by removing a cofactor. They then show that they can coax electrons into taking the red branch instead by adjusting the energies of its cofactors with a series of carefully chosen mutations.