KERATIN AND MOLECULAR BIOLOGY 



45 



Cytoplasmic Structures 

 Particulates 



(a) Mitochondria. These oval or elongate objects (diameter <~ \fx) 

 were recognized by light microscopists and characterized by various 

 staining reactions. Electron microscopically they appear as a small 

 vesicle enclosed by a double membrane, the inner membrane being cast 

 into characteristic folds. This internal structure may now be considered 

 as definitive (Fig. 22a). Mitochondria are extremely common in most 

 cells have been shown by biochemists (p. 115) to be the site of numerous 

 enzyme systems associated with cell metabolism. 



Fig. 22. Intracellular membrane bounded organelles; (a) mitochondria; 

 (b) "particle covered" vesicles and membranes (see Plate 10A); (c) 

 " pleats " of surface (plasma) membrane (y-membrane pairs); (d) and 

 (e) parallel sheets of phospholipid-protein complexes; (e) " Golgi " 

 membranes. 



(b) Smaller (0-5-0-2/x) membrane enclosed particles. A variety of small 

 bodies named microbodies, ultramitochondria, small vesicles, etc., scarcely 

 visible in the light microscope, are recognized as distinct on grounds of 

 size, absence of an internal system of membranes (of the mitochondrial 

 type) and the texture of their contents. Their nature is obscure and they 

 are certainly heterogeneous. Some may be virus inclusions, others sacs 

 of special enzymes (lysosomes), small secretory granules, etc. Some 

 vesicles contain phospholipids recognized by characteristic clusters of 

 concentric shells of membranes (see below). 



