During the photosynthetic process, highly organized membranal assemblies convert light into biochemical energy with high efficiency. We have used whole-mount cryo-electron tomography to study the intracellular architecture of the photosynthetic membranes of the anaerobic purple photosynthetic bacterium Rhodopseudomonas viridis, as well,as the organization of the photosynthetic units within the membranes. Three-dimensional reconstruction demonstrates a continuity of the plasma membrane with the photosynthetic membranes that form tunnel-like structures with an average diameter of 31 nm +/- 8 nm at the connection sites. The spacing between the photosynthetic membranes at their cytoplasmic faces was found to be 11 nm, thus enforcing a highly close packaging of the photosynthetic membranes. Analysis of successive tomographic slices allowed for derivation of the spacing between adjacent photosynthetic core complexes from a single-layered photosynthetic membrane, in situ. This analysis suggests that most, if not all, photosynthetic membranes in R. viridis are characterized by a similar two-dimensional hexagonal lattice organization. (C) 2007 Elsevier Inc. All rights reserved.