Oxygenic photosynthesis drives both oxygen evolution and carbon fixation in the biosphere. For over 3 billion years two membrane complexes, photosystem I and photosystem II, have evolved into efficient light capturing complexes. We determined the structure of the photosystem I (PSI) complex from higher plants to a resolution of 2.8 angstrom. Plant  PSI is  one  of  the  most  intricate  membrane  complexes  in Nature.  It  is comprised  of  two  complexes, a  reaction  center  core and  the light-harvesting,  LHCI, antenna.  The structure includes 16 subunits and more than 200 prosthetic groups, the majority of which are light harvesting pigments.  The model reveals detailed interactions, providing mechanisms for excitation energy transfer and its modulation in Nature's most efficient photochemical machine.