BACTERIAL PHOTOSYNTHESIS 197 



the final \aliie: nevertheless, 70 per cent of the hpid could be ex- 

 tracted in ethanol. The incorporation of osmium continued grad- 

 ually and required 3 days for completion. These data show that the 

 rate of osmication of the lipids is greatlv reduced when the lipids 

 are integrated in the chromatophore. If, as we propose, the lipids 

 are coated with protein (see below), then it might be expected that 

 osmication of the lipid would not proceed readih" until the protein 

 had been saturated. Be that as it mav, it is reasonably clear that 

 in our usual preparations the lipid does not contain much osmium 

 and will not ha\e a high electron densit\ . 



The role of osmicated protein in image formation is shown un- 

 equi\ocall\- b\" electron micrographs of hophilized bacteria from 

 which the lipids were removed by Soxhlet extraction before fixation. 

 The organisms are distorted bv this violent treatment, but the bulk 

 of the electron-dense material is still present (Fig. 9). Taken col- 

 lectixely, these observations support the \iew that osmicated protein 

 is imaged as the cortex of the chromatophore in our preparations. 



It should be noted, howe\ er, that even if the lipid were totallv 

 osmicated it might not be re\ealed. Stoeckenius (1959) has studied 

 thin sections of myelin forms prepared from phospholipid and fixed 

 in osmium tetroxide. The periodicit\- of the lamellae agrees with 

 the 40 A-spacing expected for bimolecular leaflets, but the dense 

 line is only 18-20 A thick. If the phospholipids in the chromatophore 

 are arranged in a monolaver (see below), the electron-dense line 

 might be only 10 A in diameter, which is well within the limits of 

 our measurement errors. 



Hypothesis of the Ultrastructure of the Chromatophore 



All of the protein can be accommodated in the region which is 

 imaged as the cortex of the chromatophore. The \olume of this zone 



is (1.3 X 10')A^ There are about 1.3 X lO"^" 



y (leO.V-lOOA^ 



grams of protein or, alternati\ely, some 67,000 axerage amino acid 

 residues (see above). The volume required bv the protein is about 

 10' .V when typical (cf. Frey-W'yssling, 1953) \ alues are chosen 

 for either the partial specific \olume (0.78 cc g) or the volume 

 (16lA'^) per residue. In \ iew of the agreement of such volume 

 considerations with the phvsicochemical and electron micro- 

 scope data, it is reasonable to infer that the bulk of the protein is 



