Cell Constitution 



43 



been observed in the EM or have been iso- 

 lated by fractionation, they have been dem- 

 onstrated to be aggregates of qviite small 

 particles. 



It has been found that when fragmented 

 cell material is centrifuged, three easily sep- 

 arable fz'actions can be obtained: the "large 

 granule" fraction, the "small granule" frac- 

 tion and the supernate. We shall consider 

 them in that order. 



LARGE GRANULE FRACTION 



Mitochondria. Observed in intact cells, 

 mitochondria appear as granules, elongated 

 threads or as intermediate forms depending 

 on the type of cell and the conditions obtain- 

 ing at the time of observation. In size they 

 vary from 0.5 jx to about 1 /a, when granular, 

 to several micra when filamentous. They may 

 manifest active movement, changing from 

 elongated threads to globules, as observed in 

 phase contrast (Hughes and Preston, '49). 

 Filaments may curl into loops, rotating the 

 while. They may also bifurcate and again 

 fuse longitudinally. This incessant movement 

 suggests both that their substance is semi- 

 fluid in nature and that contact between cyto- 

 plasmic substrate and enzymes contained in 

 the mitochondria may be facilitated thereby. 



According to Claude ('50), mitochondria 

 are probably capable of self-duplication. 

 They grow by elongation but apparently di- 

 vide transversely. Granules about 0.1^ in 

 diameter were observed within the mito- 

 chondria. However, it is not certain that 

 these pre-exist in vivo. EM observations in- 

 dicate that at least certain mitochondria are 

 enclosed in a formed membrane (Miihle- 

 thaler et al., '50; Palade, 52b). This was 

 presumed to be the case from the way in 

 which isolated mitochondria swell or shrink 

 as a function of the osmotic pressure of the 

 suspending medium. In the sarcosomes of 

 insect muscle it is stated that, after osmotic 

 lysis, a residual limiting membrane may be 

 observed analogous to the "ghosts" which re- 

 main after lysis of red cells (Williams and 

 Watanabe, '51, '52). These sarcosomes mani- 

 fest most of the chemical, enzymatic and 

 staining properties of mitochondria. Their 

 presence in abundance in insect muscle is 

 correlated with the need for immediate en- 

 ergy supply to this active tissue. 



Birefringence has been observed in mito- 

 chondria in living cells by Giroud, Monne 

 and others. The analysis is only fragmentary 

 but may be considered consistent with the 

 view that oriented protein and lipid mole- 



cules are present. A more detailed study of 

 mitochondria with improved methods of po- 

 larization microscopy may prove rewarding. 



The chemical composition and enzymatic 

 properties have been studied in mitochon- 

 dria isolated by differential centrifugation. 

 Among the constituents are protein (ca. 50 

 per cent), lipids (25 to 30 per cent, con- 

 sisting chiefly of phospholipids), nucleotides, 

 flavins, and relatively small amounts of ri- 

 bose nucleic acid (UNA) (Claude, '49). 

 Mitochondria may account for 15 to 20 per 

 cent of the cell mass and, according to 

 Schneider and Hogeboom ('51), they may 

 represent as much as 26 per cent of the 

 total nitrogen and 33 per cent of the total 

 protein of the mouse liver. 



The enzymes demonstrated to be present 

 in the mitochondrial fraction include cyto- 

 chrome oxidase, succinoxidase, cytochrome 

 c, members of the Kxebs cycle, D-amino 

 acid oxidase, diphosphopyridine nucleotide 

 (DPN) -cytochrome c reductase, triphospho- 

 pyridine nucleotide (TPN) -cytochrome c 

 reductase, acid phosphatase and ATP-ase. 

 This fairly complete complement of the oxi- 

 dizing mechanism clearly indicates the res- 

 piratory function of mitochondria, and their 

 ability to carry out aerobic phosphorylation 

 and possibly to supply energy for other syn- 

 thetic processes including peptide and pro- 

 tein synthesis. The mitochondria appear to 

 constitute the "power plant" of the cell. This 

 discovery represents one of the great mile- 

 stones in modern biochemistry and analytical 

 cytology. 



Mitochondria have been studied in some 

 detail with the EM (see Palade, '52b). An 

 internal layered structure, whose perfection 

 depends importantly on the method of prepa- 

 ration, has been observed. Possibly these 

 lipid-protein layers are the locus of the 

 organized enzymes known to be present in 

 mitochondria. 



Secretion Granules and Other Members of 

 the "Large Granule" Group. Characteristic 

 particulates are found in the large granule 

 fraction of secreting cells, such as pancreas 

 and liver; they are thought to derive from 

 zymogen and other secretion granules. In 

 the pancreas they may be about as abund- 

 ant as the mitochondrial components. Claude 

 ('50) suggests that secretion granules may 

 arise by a progressive transformation of 

 mitochondria, a view expressed long 

 ago by classical cytologists on morphological 

 grounds. 



Other members of the large granule frac- 

 tion have been assigned special names on 



