PROTOZOA AS CELLS V 



scale as well as at the familiar level of light microscopy. Fibers, 

 folds, blisters, and scales embellish different species in characteris- 

 tic patterns. Often these differentiations involve the cell membrane 

 itself or are intimately layered below it, so that long-standing 

 questions as to whether pellicles or cuticles are living structures 

 can for some species be answered in the affirmative. Descriptions 

 of these widely varying characters will be given in the taxonomic 

 sections to follow. 



The one generalization we can make is that most if not all 

 protozoa have at the actual surface of the protoplasm a limiting 

 membrane that shows the same structure as has been reported 

 for other cell types. Such a plasma membrane is best described 

 as a three-ply sheet ; it appears in cross-section as two dark lines, 

 each about 2 m/x thick, separated by a light zone. The total 

 thickness of the membrane is about 7 to 8 m/x (Bennett, 1956; 

 Sjostrand, 1959; Robertson, 1960). Comparison of this structure 

 with the molecular arrangement proposed on other than morpho- 

 logic grounds has led to the assumption that the three-ply 

 construction represents a bimolecular leaflet of mixed lipid 

 molecules whose non-polar ends face each other and whose polar 

 surfaces are covered by thin layers of non-lipid material. The 

 varying nature of this material, probably protein in most cases, 

 is accountable for the highly individual properties of membranes 

 and for their chemical asymmetry, as well as for some of the 

 variations in apparent thickness or density of membranes seen in 

 electron micrographs. 



If the conclusions drawn by avid students of membrane 

 structure are correct (see Robertson, 1960), all protoplasmic 

 membranes, not just that at the cell surface, share this fundamental 

 molecular architecture ; the three-ply sheet is considered to be the 

 unit membrane. 



The importance of membranes within as well as around cells 

 is no news to cell physiologists, but their abundance on an 

 ultramicroscopic scale and their elaborate involvement in many 

 familiar organelles have been among the major discoveries of 

 electron microscopy (see Text-fig. 1). It is becoming clear that 

 membranes provide the framework upon which orderly sequences 

 of reactive compounds are applied. Complex chemical processes 

 can thus be handled by an assembly line of fixed enzymes ; the 



