fraction, which now possesses specific nucleotide end groupings. The 

 final step is believed to involve the transfer of this complex to the ribo- 

 nucleoprotein of the microsomes, followed by polymerization of the 

 amino acids to produce protein (Stephenson, ct cl.. 1959). 



Figure 3-27. Electron Micrograph of a Freeze-Dry Preparation of Micro- 

 somal Particles (Ribosomes) from Pea Seedlings. The polystyrene sphere is 

 2600 A in diameter and serves to show the relative size of the ribosomes. 

 (From Ts'o, P. O. P., 1958. "Structure of Microsomal Nucleoprotein Par- 

 ticles from Pea Seedlings," in "Microsomal Particles and Protein Synthesis," 

 R. B. Roberts (Ed.), 1st Symposium Biophys. Soc, Pergamon Press, Fig. 1. 

 Courtesy of Dr. P. O. P. Ts'o, California Institute of Technology.) 



Microsomes exhibit a small amount of respiratory activity involving 

 DPN- and TPN-linked cytochrome reductases as well as an extramito- 

 chondrial cytochrome (cytochrome b,-;) which can transfer electrons to 

 molecular oxygen. In most cases, there is no evidence that microsomal 

 respiration or electron transport is coupled to phosphorylation of ADP 

 to yield ATP; however, the microsomes of the bacterium, Azobacter, 

 appear to be capable of some oxidative phosphorylation (Brachet, 

 1957). Although most of the glycolytic reactions in carbohydrate break- 

 down presumably occur in the soluble fraction of the cytoplasm, the 

 hexokinase reaction catalyzing the phosphorylation of glucose to glucose- 

 6-phosphate appears to be confined, at least in some cells, to the micro- 

 somes and mitochondria (Lehninger, 1959). 



58 / CHAPTER 3 



