The photosynthetic protein complexes in plants are located in the chloroplast thylakoid membranes. These membranes have an ultrastructure that consists of tightly stackedÔgranaÕ regions interconnectedbyunstackedmembrane structure of isolated granamembranes has been studiedhere by cryo-electron microscopy. The data reveals an unusual arrangement of the photosynthetic protein complexes, staggered over two tightly stacked planes.