248 CELL HEREDITY 



not easy to distinguish such a low nucleic acid content from contamina- 

 tion by ribosomes, so one can only conclude that, if any RNA is present 

 in mitochondria, there surely is not much. Further investigations will 

 require the use of special techniques designed to follow the behavior of 

 small numbers of molecules. 



Chloroplasts, the organelles in which photosynthesis occurs in plant 

 cells, have much in common with mitochondria: (1) they are both mem- 

 branous structures existing free in the cytoplasm; (2) their membranes 

 contain many insoluble components requiring precise spatial orientation 

 for their functioning; (3) they contain small amounts of nucleic acid, if 

 any; and (4) their mode of origin is not established. Many lines of evi- 

 dence suggest that chloroplasts like mitochondria arise from pre-existing 

 units of the same kind, not by de novo origin from undifferentiated cyto- 

 plasm. 



Chloroplasts are surrounded by an envelope consisting of two smooth 

 continuous membranes separated by a distinct space. The internal or- 

 ganization appears complex at first glance, and different from species 

 to species. On careful examination, however, a basic plan common to all 

 chloroplasts can be discerned. The structural unit is the disc, which 

 can be visualized as a much flattened vesicle or as a pair of plate-shaped 

 membranes joined at their ends to form a closed structure. In some 

 algae, the chloroplasts consist of single stacks of 10 or 12 of these discs. 

 In higher plants, chloroplasts contain many stacks of discs, each stack 

 called a granum, oriented more or less precisely by thin membranes 

 which run between the grana. 



The discs are embedded in a matrix which contains the soluble en- 

 zymes involved in the conversion of photosynthetically reduced CO2 to 

 sugars and starch. The discs themselves enclose an inner space of un- 

 known function and content, but presumably essential for photosyn- 

 thetic activity. The macromolecular organization of the disc membranes 

 has been a favorite subject of speculation for many years, but as yet 

 we know only that it must be a multicomponent system, containing many 

 insoluble enzymes of photosynthesis as well as chlorophylls, carotenoids, 

 and phospholipids. It is a reasonable guess from structural considera- 

 tions that the molecular composition of the disc membrane is asymmetri- 

 cal in cross section, with chlorophyll present in a single layer on one 

 side of the membrane. This idea fits well with some current hypotheses 

 of the nature of the photochemical steps in photosynthesis. 



Although the disc is probably a functional unit, it definitely is not 

 a hereditary unit, since dark-grown strains of algae which contain no 

 chlorophyll also contain no disc membranes. Nonetheless, such strains 

 retain their ability to form normal chloroplasts when provided with light 



