IN VIVO FUNCTION OF SUBCr.LLXTLAK PARTICLES 9 



biochemical cytology ( 35 ). The chiet characteristic ot the newer cytology is the 

 vast body of biochemical data upon which it is based. The last decade has 

 witnessed an ever-accelerating expansion in our knowledge ot metabolic pathways. 

 As new reactions are imcovercd their distributions among isolated subcellular 

 fractions are plotted, thus providing an important step towards grasping the 

 interrelationships among these biochemical events. It is fitting that the technic]ue 

 of differential centrifugation was introduced by two cytologists, R. R. Bensley and 

 N. H. Hoerr, and then pursued most vigorously by biochemists, A. Claude, 

 W. C. Schneider and G. Hogeboom, C. de Duve and J. Berthet, O. Lindberg and 

 L. Ernster, and many others. The beautiful work described in subseciuent chap- 

 ters of this volume illustrates the usefulness of such isolated fractions for the 

 biochemist. 



We have already seen how microscopy, phase contrast and particularly elec- 

 tron microscopy, can help assess the state of preservation of cell organelles 

 throughout this process and how it can indicate the degree of purity of isolated 

 fractions. It helps also to emphasize uncertainties, such as the fate of microvilli 

 and Ciolgi apparatus of the liver cell, and possible losses which may occur as the 

 endoplasmic reticulum is fragmented. It has long been known that a con- 

 siderable amount of material is lost from nuclei during their isolation in aqueous 

 media. In this connection, it might be worth commenting that the 'supernatant 

 fluid' data are the most difficult, of all the fraction data, to interpret in terms of 

 the living cell. For example, it would be of utmost significance, in terms of in 

 VIVO function if not in terms of biochemical sequences, to know that the amino 

 acid-activating enzymes discussed by Dr. Stephenson later in this volume (49) 

 are truly in a soluble phase bathing the ergastoplasm and mitochondria. But I 

 believe we cannot categorically assert this to be the case. There remains a possi- 

 bility that these, like other enzymes of the 'supernatant fluid,' may have leached 

 from the nucleus, endoplasmic reticulum, or some other organelle. 



There is, thus, uncertainty aloout some localizations indicated in figure 8, even 

 for the liver cell. However, there can be little question that the mitochondria 

 are the chief sites of oxidative phosphorylation in the cell. Dr. AUfrey, later in 

 this volume (2), discusses an adenosine triphosphate (ATP) synthesis by iso- 

 lated nuclear fractions of thymus; but it remains to be seen to what extent the 

 findings regarding thymus and other lymphoid tissues can be generalized to 

 tissues like liver. 



The work of Chance and colleagues has demonstrated that isolated mitochon- 

 drial suspensions show the same sequence of events in electron transport as do 

 suspensions of living yeast and tumor ascites cells. Chance comments on this 

 important achievement in these words: "It is, of course, of considerable reas- 

 surance to biochemists that the isolated material does not involve a serious 

 artifact" (5). 



