STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 69 



therefore the center of the primary light absorption. Since these structures 

 within the chloroplast appear most often during the process of isolation, it 

 has been argued that grana are artifacts. This is not generally held, some 

 workers beheving that isolation and imbibition of water merely lower the 

 refractive index of the stroma so that the similarly refractive grana become 

 visible. 



Perhaps the most interesting evidence for the functional importance of 

 bodies of the size of grana is derived from the isolation, by means of differ- 

 ential centrifugation, of the cytoplasmic chromatophores of the blue-green 

 algae, Synechococcus cedorum, and the photosynthetic bacterium, Rhodo- 

 spirillmn ruhrum. Although most algae and higher plants contain chloro- 

 plasts, blue-green algae and photosynthetic bacteria lack these bodies. It had 

 been believed that their pigments were dispersed throughout the cell, 

 presumably in soluble form, but it is now known that the colored components, 

 including the phycobilins, exist within the chromatophores and chloroplasts. 

 The studies of Schachman el al. (1952) on hght-grown photosynthetic bacteria 

 revealed that these pigments are contained exclusively in large particles, 

 (about 50 mju. in diameter), about 5000 per cell. 



RJiodospirillum ruhrum can grow aerobicaUy in the dark and under these 

 circumstances is relatively unpigmented. Neither chromatophores nor un- 

 pigmented particles of comparable size are present in these cells. In the light 

 there is an extensive synthesis of pigment and particles, which then amount 

 to about 15 % of the nitrogen of the cell. The particles lack DNA and have 

 a smaD amount of RNA, although less than the small particle fraction of 

 the same cells. Such a system provides obvious advantages for the study of 

 pigment and particle synthesis. Deken-Grenson (1954) has also observed the 

 synthesis of grana in leucoplasts of etiolated leaves following the exposure 

 to light. 



In addition to the presence of protein, chlorophyll, carotenoids, and RNA, 

 chloroplasts contain many lipids, including neutral fat, sterols, waxes, and 

 phosphatides. Phosphatides can comprise one-third of the phosphorus of the 

 plastid and these may amount to 35 to 65 % of the hpid phosphorus of the 

 leaf. The fat-soluble vitamins, K and E, have also been found in chloroplasts. 

 Studies on the details of organization of various types of chloroplasts have 

 been described (Wolken and Schwartz, 1953; Wolken, 1956). 



III. The Distribution of Metabolic Function 



A. Methodological Notes 



Several approaches to enzyme localization are widely used at the present 

 time. Some of these shall be considered in a later section and involve the use 

 of more or less intact cells. For example, function may be contrasted between 



