Chromatophore material from the bacterium Rhodopseudomonas spheroides was freed of ribosomes by centrifugation in 27 per cent RbC1 and then separated into "heavy" and "light" fractions by centrifugation through a sucrose gradient. The fractions differed from one another in the following ways. (a) The isopycnic density of the heavy fraction was between 1.15 and 1.18 gm/ml and that of the light fraction was 1.14 gm/ml. (b) The heavy fraction was able to bind ribosomes; the light fraction was not. (c) The light fraction was homogeneous in the ultracentrifuge and had a sedimentation constant, extrapolated to infinite dilution, of 153 s20,w. The heavy fraction was grossly heterogeneous. (d) Both the amount of bacteriochlorophyll relative to protein and the ratio of bacteriochlorophyll to carotenoids were greater in the light fraction. (e) The spectra of the two fractions in the near infra-red were different. Comparisons of the chromatophore fractions from cells with different amounts of bacteriochlorophyll showed that the specific bacteriochlorophyll contents of the two fractions did not change to the same extent as did that of the whole cells. The amount of heavy fraction from pigmented cells was roughly independent of the cellular pigment content and was about equal to that from pigment-free cells. The amount of light fraction depended on the pigment content of the cells; no light fraction was obtained from cells devoid of bacteriochlorophyll. The cytochrome complements of both fractions underwent quantitative as well as qualitative changes with varying growth conditions. The size of the photosynthetic unit in R. spheroides appeared to increase as the total cellular bacteriochlorophyll content increased; however, the number of units per light fraction particle remained constant.

By varying thc light intensity and tcmpcrature during growth it is possible to obtain cultures of Rhodospirillum molischianum in which the spccific bacteriochlorophyll contents differ by as much as fivcfold. We used such cultures to compare the changes in the electron microscopic appearance of the cells with thc changes in thc amount and bacteriochlorophyll content of chromatophorc matcrial isolated from cell extracts. Thc cells contained a variable number of internal membranes which arc invaginations of the cell mcmbrane. The shape, size, number, and arrangement of the infoldings varied as the specific bacteriochlorophyll content of the cells changed. In cells with little bactcriochlorophyll, the invaginations were mostly tubular. In cells with largcr amounts of bacteriochlorophyll, the invaginations were disc-shaped and the discs were apprcsscd togcther in stacks of 2 to 10 discs each. Variations in the number of discs per stack could bc accounted for by a simplc statistical model. The average area pcr disc increased with increasing bactcriochlorophyll content. Quantitative estimations of the relative volumcs occupicd by mcmbranes in cells with four different bacteriochlorophyll contents showed that thc amount of internal membranc alone had no direct relationship with the bacteriochlorophyll content of the cells; however, the total amount of membrane (cell membrane plus internal membrane) was directly proportional to the bactcriochlorophyll content. The specific bacteriochlorophyll content of isolated chromatophore material was proportional to the bacteriochlorophyll contcnt of whole cells; the total amount of chromatophore material was independent of the bactcriochlorophyll content of whole cells. Several possible explanations of this paradoxical discrepancy between the electron microscope obscrvations and the analytical results are discussed.