The photosynthetic pigments in purple bacteria are located on extensive infoldings of the outer membrane that sometimes look like little hollow bags or vesicles, sometimes are interconnected in hollow tubules, and more often appear as dense, stacked layers of unit membrane.

These photosynthetic membrane structures have their counterparts in blue-green algae and in the chloroplasts of eucaryotes. Green bacteria carry their photosynthetic pigments in quite different cigarshaped vesicles, which are just under the outer membrane but are not connected with it.

The light reactions of photosynthesis occur in these membrane folds or vesicles, and the dark reactions take place in the cytoplasm. The outer membrane frequently has larger infoldings called mesosomes, which seem to be involved in cell division. These and the photosynthetic apparatus are the nearest things that bacteria have to organs.