86 



SUBCELLULAR PARTICLES 



full of tightly packed cristae, stacked in a regular fashion like logs on a pile. 

 The double membrane structure of both the outer envelope and the cristae is 

 clearly seen on close inspection of the electron micrographs. Mitochondria isolated 

 from beef heart muscle can be sharply resolved into two fractions: the heavy or 

 more rapidly sedimenting fraction and the light or more slowly sedimenting frac- 

 tion (19). For present purposes we shall consider only the more intact particle, 

 viz., the heavy mitochondrial particle, the electron micrograph of which is seen 

 in figure 2. You will note first that the arrangement of the cristae is not as regular 

 as in the mitochondrion hi situ and also that the distinction between outer en- 

 velope and cristae is not quite so sharp. When the particles of the heavy mito- 

 chondrial fraction are frozen and thawed they have the appearance shown in 

 figure 3. There has clearly been extensive loss of material. The cristae are rela- 

 tively few in number and very irregularly arranged, and the outer envelope is 

 rather diffuse. But the double membrane structure of both the envelope and 

 cristae is well defined. When such damaged particles are sonicated, all mito- 

 chondrial form disappears, and fragments with double membrane structures are 

 found, as shown in figure 4. These are unmistakably derivatives of the cristae and 

 outer envelope. The heavy mitochondrial particles both before and after freezing 

 carry out oxidative phosphorylation. But only the particles with mitochondrial 

 form carry out the citric acid cycle (34). By appropriate methods mitochondria 

 can be comminuted into smaller and smaller fragments which still retain a well 

 defined outer membrane surrounding the internal cristae. Such particles can 

 carry out the citric acid cycle. It is only when the outer envelope is completely 

 stripped that the capacity for citric cycle oxidations is abolished. 



BUiPf 



W 



Fin. 2. Isolated mitochondria — heavy fraction. X 40,000. 



