Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides were incorporated in dimyristoylphosphatidylcholine (DMPC) liposomes. The first and second electron transfer rates (k A B (1) and k A B (2), respectively) between the first and the second quinone electron acceptors have been measured as a function of temperature, across the phase transition of DMPC (23 o C). The Eyring plots of k A B (1) display straight lines. In contrast, the Eyring plots for k A B (2) in proteoliposomes show a break at about 23.5 o C. This physical discrimination between the two electron transfer reactions demonstrates that the stiffness of the lipid environment of the RCs and/or the protein-protein interactions influence the parameters governing k A B (2), but not the gating process limiting k A B (1).