BACTERIAL PHOTOSYNTHESIS 195 



for the release of the hpids. Since the carotenoids, upon (\\traction, 

 are accompanied Idv a proportionate amount of the phospliohpid 

 (Newton and Newton, 1957), these pigments can serve as indicators 

 of the leaching of lipids in the solvents used in preparing specimens 

 for electron microscopy. Using such indications, we observed that 

 the image of the chromatophore was not appreciably different if 

 obvious leaching had occurred or if leaching had been minimized 

 bv using the solvents at the lowest practical temperature and poly- 

 merizing the monomeric methacrylate at dry ice temperature. 



It is a common practice to fix tissues rather briefly because ex- 

 posure beyond an ill-defined optimum period in the customary 

 buffered osmium tetroxide fixatives produces a pronounced deterior- 

 ation of structure. In view of this practical limitation on fixation 

 time, the loss of lipid could have been due to inadequate osmication 

 or to an inherent lack of reactivity. These possibilities were tested 

 by experiments (1) with lyophilized chromatophores, (2) with the 

 proteinaceous residue obtained by extracting the lipids in a sequence 

 of boiling solvents (ethanol, 24 hours; ether, 24 hours) in a Soxhlet 

 apparatus, and (3) with the lipid films produced by evaporation of 

 the combined extracts. The rate and extent of osmication of each 

 of the three systems was determined by the change in dry weight as 

 a function of time. Numerous samples containing about 10 mg of 

 material were exposed to the fixative in a pre-equilibrated moist 

 chamber which contained a large volume of 1 per cent osmium 

 tetroxide. At intervals, samples were removed and reweighed after 

 dessication to constant weight. 



The lipid films incorporated osmium rapidly and became black; 

 in 1 hour the weight increased by 40 per cent. At this point, the 

 bulk of the film was insolubilized, but about 10 per cent of the mass 

 could be extracted by ether or methacrylate monomer; 20 per cent 

 could be removed by ethanol. The incorporation of osmium was 

 finished in 8 hours with a net weight increase of 50 per cent. If we 

 use 900 as the average molecular weight of the lipids and assume 

 that the osmium tetroxide is reduced to the metal (at. wt. 190.8), 

 then at saturation each lipid molecule is related to two or three 

 atoms of osmium. 



Under comparable conditions, the defatted protein actually in- 

 corporated osmium more rapidly than the lipid films, but the net 

 uptake was much less. The weight increment was 18 per cent in 1 

 hour. The final value was 20 per cent. The osmium content at 



