MODEL OF A CELL MEMBRANE 



205 



Various optical attempts at measuring the thickness of the transition 

 layer have given values that are less than 0.2 X 10 -4 cm, i.e., unresolv- 

 able by a compound microscope (see Chapter VIII). That bacteria of 

 the genus Bacillus possess cell walls was recently verified with the aid of 

 the electron microscope. The photomicrograph (Fig. VIII-22) of 

 human tubercle bacilli made with an RCA electron microscope shows the 

 bacilli to be surrounded by what appears to be a membrane having a 

 thickness less than 0.1 X 10 -4 cm. 



Oil layer 

 monomolecular 



Fig. V-9. (a) A conventionalized picture of a drop of water covered with a 

 monomolecular layer of oil, suspended in air. (6) A drop of oil suspended in water 

 with oil molecules forming bounding phase. (By courtesy of W. D. Harkins and 

 Chemical Catalog Company. [Reinhold Publishing Corporation], New York.) 



These dimensions may be compared with those found by Langmuir 

 [1925] for palmitic acid molecules when these molecules were steeply 

 oriented to form a monomolecular surface film. Palmitic acid with its 

 chain of sixteen carbons to the molecule has a cross section of 20.5 sq A 

 and a length measured perpendicular to a supporting surface of about 

 24.2 A. The relative order of magnitudes indicates that the membrane 

 of the cell is at most about 100 molecules thick. 



If a closed membrane were one molecule thick, it could be pictured as 

 in Fig. V-9. This highly conventionalized picture, used by Harkins 

 [19251 in discussing the orientation of molecules in the surface of spheri- 

 cal liquid drops, shows a small spherical drop of water, Fig. V-9 (a), 

 covered with a monomolecular layer of oil suspended in air. Its surface 

 can be represented as composed of closely packed molecules with their 

 polar ends turned toward the water phase and the non-polar ends point- 



