  Photosynthesis transfers energy efficiently through a series of antenna
complexes to the reaction center where charge separation occurs. Energy
transfer in vivo is primarily monitored by measuring fluorescence signals from
the small fraction of excitations that fail to result in charge separation.
Here, we use two-dimensional electronic spectroscopy to follow the entire
energy transfer process in a thriving culture of the purple bacteria,
Rhodobacter sphaeroides. By removing contributions from scattered light, we
extract the dynamics of energy transfer through the dense network of antenna
complexes and into the reaction center. Simulations demonstrate that these
dynamics constrain the membrane organization to be small pools of core antenna
complexes that rapidly trap energy absorbed by surrounding peripheral antenna
complexes. The rapid trapping and limited back transfer of these excitations
lead to transfer efficiencies of 83% and a small functional light-harvesting
unit.
