38 KERATIN AND KERATINIZATION 



other constituents. Measurements of the surface tension of free cells 

 show this to be very low (0-1 dyn/cm) which would suggest that the 

 protein is present as an adsorbed surface layer. The chemical analysis 

 (Davson and Danielli, 1952) also shows that proteins are present. 



These experimental findings suggest the model (Harvey and Danielli, 

 1938; Danielli, 1942; and Stoeckenius, 1959) shown in Fig. 18 (a) which 

 pictures the membrane as a continuous lipid-like layer covered on each 





\AAAA/vVAVW\ P 

 99999999999999999 



55666666666555555 

 AA/VWWWWV P 



FlG. 18. Two interpretations of the nature of the plasma membrane. 

 Both agree that the membrane consists of a lipid layer covered with 

 protein layers as indicated at (a) the Danielli-Harvey model; (b) 

 shows a somewhat more detailed interpretation in which the lipid is 

 represented as a bimolecular leaflet of lipid molecules covered with 

 monolayers of non-lipid protein. 



face by a layer of protein. Its thickness cannot be determined precisely by 

 permeability or impedance measurements, but an order of less than 100 A 

 is indicated. Direct measurements, using a special device, the leptoscope, 

 made by Waugh (1954) confirm this order of magnitude. 



Structures of these dimensions are within the range of the electron 

 microscope and, as mentioned above, our knowledge of membrane 

 structure has been greatly augmented recently by the use of this instru- 

 ment. Sections cut through the plasma membranes of a variety of fixed 

 cells have shown that its thickness is of the order of 70 A (± 10 A) and 

 that it frequently reveals a fine structure consisting of two dense outer 

 surfaces enclosing a less dense inner layer (Plate 4A) (Mercer 1959, 

 Robertson, 1959). This finding may be compatible with the structures of 

 Fig. 18 if we assume that the protein layers, P (and perhaps part of the 



