CELLULAR TOPOCHEMISTRY 277 



As for glycerides, which do not possess a hydrophilic group, they can only 

 attach themselves to lipophilic side-chains and then, as shown in Fig. 88, 

 only by the interposition of a phosphatide or similar molecule. The sterols 

 have a polar structure analogous to that of the phosphatides. But the phos- 

 phatides are more reactive for their hydrophilic end bears an ionized 

 phosphate group (negatively charged) and a positive ammonium group. 

 By contrast the sterols can only form esters. 



From the point of view of enzymology, cytoplasm is a not highly organ- 

 ized multi-enzyme system containing notably all the enzymes of the 

 glycolysis system and which consequently is qualified for the formation of 

 ATP energy-rich bonds. In addition to the conversion of sugars into pyru- 

 vic acid, cytoplasm can saponify lipides and split proteins into amino acids. 

 In a cell living anaerobically, 80-90% of the glycolysis occurs in the 

 cytoplasm. 



(b) The Outer Region of the Cell 



In all cells it is possible to distinguish an inner region and an outer 

 region. In certain cases, such as the amoeba, the region of clear ectoplasm 

 is plainly differentiated from the granular endoplasm. 



The ectoplasm is itself surrounded by a thin layer of polysaccharide and 

 lipide-containing material called the plasmalemma. This portion is elastic 

 and contractile. The difference between the endoplasm and ectoplasm is a 

 result of differences in the shapes of the constituent macromolecules. 



We may represent the cellular "membrane", from the interior to the 

 exterior, as formed, firstly, of a highly hydrated protein structure consisting 

 of leaflets parallel to the surface, this explains its contractility and elasticity. 

 Beyond this membrane are situated one or two layers of complex lipides, 

 followed by a layer containing polysaccharides. 



The presence of enzymes in the outer region of the cell has been demon- 

 strated, chiefly with the aid of indirect methods. Thus, a portion of sub- 

 strate can be introduced to see if the requisite enzyme is present ; this has 

 been done for certain esterases, phosphatases, etc. 



(c) Microsomes 



There is always present in cytoplasm an important basophilic material 

 which forms the endoplasmic reticulum of vesicules and canaliculi. At 

 least part of this material appears to consist of the particles which we call 

 microsomes. The discovery of microsomes (50-150 ma in diameter) is due 

 to Claude, who isolated them from a mush of cells by prolonged centri- 

 fugation at 20,000 rev/min. The microsomes contain a large part of the 

 nitrogen of the cell and a comparatively (more than the mitochondria) large 

 fraction of the phospholipides and RNA. It was at one time considered that 

 the microsomes consisted of mitochondrial fragments, but this view is no 



