STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 67 



fractions is arbitrary and merely reflects the forces used in the separation; all 

 molecules in both fractions may be considered to be particulate. 



8. On the Origin of the Mitochondria and Microsomes 



The finding of Claude (1944) that microsomelike particles rich in RNA 

 could be isolated from lysed mitochondria led to two suggestions: (a) that 

 the microsomes isolated from homogenates are artifacts of mitochondrial 

 disintegration, and (b) that the microsomes are normally produced in this 

 way. Hypothesis (a) was clearly disproved in the ensuing years, which also 

 revealed that isotopic amino acids and nucleotides are incorporated into 

 microsomal proteins and RNA, respectively, at a greater rate than their 

 appearance in mitochondria. Accordingly, as noted earher, the hypothesis 

 has been proposed that mitochondria are produced by the aggregation of the 

 smaller particulates. This is not contradicted by the metabolic evidence and 

 indeed tends to be supported, as noted earher by the cytological evidence of 

 Harvey (1946) and of LiUie, who, in 1906, followed the development of 

 microscopically visible particulates. It may be added further that Lilhe 

 beheved that microsomes originated in the nucleus, a hypothesis which has 

 also been suggested on the basis of observations of nucleolar function, of the 

 contmuity of the external nuclear membrane with the endoplasmic reticulum, 

 and the relative rates of RNA synthesis in various parts of the ceU. On the 

 other hand, it has also been suggested that these particles are self-duphcating, 

 as are viruses, and may possess a measure of autonomy with respect to their 

 multiplication within the cell. 



9. Properties of Chloroplasts 



Intensified investigation of the detailed mechanism of photosynthesis and 

 of the role of intracellular structures in cellular activity has resulted in many 

 studies on the isolation and properties of chloroplasts and smaller fragments 

 of these organelles. Chloroplasts may be defined as cytoplasmic bodies con- 

 taining the light-absorbing pigments, chlorophyll and carotenoids, essential 

 to photosynthesis. It is supposed that various plant viruses arise in the 

 variegation of plastids and are multiplied within plastids or chloroplasts. In 

 many instances the pathology of virus infections in plants is readily revealed 

 in aberrations of pigment and chloroplast production. Efforts to demonstrate 

 this as a primary consequence of the viral multiphcation are not yet convinc- 

 ing, although electron microscopy of sectioned chloroplasts of virus-infected 

 leaves has revealed virus particles in close proximity to the chloroplast 

 structures (Leyon, 1953). 



Within a given plant there appears to be a relative constancy of the 

 number of plastids per cell. They divide to increase their number if the cell 

 has too few; some may degenerate if many are present. Cells possessing large 



