Diversity at the Subcellular Level and Its Significance 123 



1959). The central purpose of these variations seems to be to expose 

 increasing or decreasing areas of membranes to the structureless matrix 

 of the mitochondrion (Figs. 1A to 1C). Recently it has been found by 

 Fernandez-Moran, in collaboration with David E. Green, that the 

 surfaces facing the matrix are constructed of small 100 A particles. 

 Since biochemical studies of these particles show them to contain all 

 the functional enzymatic components of the electron transport chain, 

 they are regarded as the ultimate unit or "elementary particle"' of 

 mitochondrial function ( Fernandez-Moran. 1960 I . It will be valuable 

 to apply similar methods of observation to a variety of mitochondria 

 showing different levels of physiological activity. 



Variations in vytoplasitiie fine structure 



These structural variations in mitochondria — an expression of di- 

 versity in the subcellular level — are now well known to students of 

 cell fine structure and deserve this attention only because they serve 

 to illustrate how widely diverse morphologies can, upon analysis, be 

 shown to represent variations on a simpler basic structure. Mitochon- 

 dria, it must be admitted, present relatively minor problems of in- 

 terpretation, compared with the whole cytoplasm of the cell. However, 

 even the complexity and apparent diversity of this larger division 

 responds to analysis if one recognizes the existence of separate and 

 distinct systems which occur ubiquitously and can properly be com- 

 pared. 



Some measure of the diversity one encounters in the fine structure 

 of cytoplasm may be gained from a limited survey of representative 

 regions of a few diverse cell types. The range of variation shown is 

 not descriptive of the enormous range one can find in nature, hut it 

 is illustrative without being so extreme as to make comparisons and 

 correlations difficult. Figures 2 to 10 which follow are described and 

 compared in the accompanying legends. 



