Mesosomes are extensions of the plasmamembrane within the bacterial cell (cytoplasm), involving complex whorls of convoluted membranes. They are seen in chemoautotrophic bacteria such as Nitrosomonas and in photosynthetic bacteria such as Rhodopseudomonas where they are the sites of the photosynthetic pigments. Mesosomes are also involved in. cross wall (septum) formation during cell division.