122 The Nature of Biological Diversity 



terms of fibrils, particles, and membranes of diverse organizations and 

 dimensions. 



More surprising perhaps than the variations in form revealed by 

 subcellular explorations is the similarity displayed by structures 

 formerly thought to represent diverse units of the cell. Thus it is that 

 the modern cell biologist can relate structures formerly not appar- 

 ently related. Generalizations encompassing the whole of metazoan 

 cell structure, and including protozoans in some instances, may now 

 be formulated, thus making much easier the task of classifying the 

 new fine-structure information. Simultaneously, there is provided 

 through comparative cytology a clearer understanding of the relation- 

 ships and functions of several of the newly defined systems. 



It will be the purpose of this brief chapter to illustrate the diversity 

 in form displayed by some of the subcellular components of cells, and 

 to point out the pervading similarities and structural associations 

 which relate these components despite their apparent differences. 



Diversity in mitochondria 



Variation in form at the subcellular level could, of course, be illus- 

 trated by comparing representatives of a number of intracellular 

 structures. These might be fibrous, granular, or membranous in their 

 basic structure. But possibly none is better for the purpose than mito- 

 chondria. These, as was well known from light microscopy, are uni- 

 versally occurring organelles of the cell. They contain enzymes for 

 the complex series of reactions involved in oxidative phosphorylation 

 in the production of energy-rich ATP. The electron microscope ob- 

 servations of Palade (1953) and of Sjostrand (1953) about ten years 

 ago on these organelles established the existence of a fairly compli- 

 cated fine structure, which is uniform among the population in a 

 single cell type but varies in different types (Figs. 1A to 1C). It was 

 apparent, however, from these early studies that a basic architecture 

 was common to all mitochondria (Palade, 1956a). Thus each of these 

 organelles in its fully differentiated form possesses an external limit- 

 ing membrane; inside this, and generally separated from it by space 

 of about 8 millimicrons, there is an internal membrane which through 

 various infoldings forms shelves, or cristae, as they came to be called. 

 Observations made subsequent to this have not materially changed 

 the picture. What has been revealed is a wide diversity of patterns 

 displayed by the foldings or proliferations of the internal of the two 

 membranes. These achieve a startling complexity in some cells, such 

 as to make structural analysis very difficult (Pappas and Brandt, 



