18 Applied Biophysics 



of lipid. This instrument has only recently been developed, and 

 the essential principle involved consists in the comparison of the 

 relative intensities of light reflected from the cells and built-up 

 step films of barium stearate of known thickness deposited on a 

 similar substrate to that used for the erythrocytes, which are 

 examined in the form of the dried hemolyzed "ghosts." 



Gorter and Grendel ^"'* reported that the fat-soluble lipid is 

 sufficient to form a bimolecular layer, 50 A. in thickness, cover- 

 ing the surface of the envelope. Danielli and Davson ^^ and 

 Danielli and Harvey ^- have proposed a more stable form of 

 membrane which consists of a lipid layer several molecules in 

 thickness stabilized by the adsorption of protein on the internal 

 and external surfaces which are in contact with the more aqueous 

 environment. It cannot be denied that this "paucimolecular 

 theory," ^^ which is a modification of Overton's concept of a 

 homogeneous lipid layer, serves to rationalize a large body of 

 existing permeability data. 



Rather critical evidence has been presented recently by Par- 

 part and Dziemian ^^ which suggests that a considerable propor- 

 tion of the lipids in the erythrocyte envelope is firmly bound to 

 the structural fabric of the ghost in the form of fat-insoluble 

 lipo-protein "complexes." The molecular ratio of the fat-soluble 

 fractions, comprising the phospholipids, cephalin and lecithin, 

 and the sterol, cholesterol, is more related to permeability than 

 the total lipid contents of the erythrocytes in different mammals. 

 The cephalin fraction is relatively uniform in the different cells, 

 but there is a much greater divergence between the molecular 

 ratios of lecithin and cholesterol. These results have some bear- 

 ing on the structural features of the envelope, for the perme- 

 ability to fat-soluble substances shows little variation in the 

 species examined, but a higher proportion of lecithin and 

 cholesterol is present in the cells which are more permeable 

 to lipid-insoluble substances. It would appear that the cephalin 

 has a structural role in the organization of the erythrocyte 

 membrane, while lecithin and cholesterol are involved in the 

 more labile diffusion processes. In support of this, we may cite 

 the evidence offered by Chargaflf and coworkers,*- ^' ^' ^ who 



