PROTOPLASMIC STRUCTURE loi 



the external surface of the cell is not pecuhar in this 

 respect, and that other protoplasmic interfaces (those 

 within the cell) have similar properties. Film-fomiation, 

 variations of electrical polarization, and dependent 

 phenomena are thus to be regarded as constant features 

 of the processes at such interfaces; e.g., at the surfaces 

 of contractile fibrils, neuraxones, vacuoles, nuclei, 

 alveoH, and other protoplasmic structures. Secondary 

 effects peculiar to living protoplasm and dependent on 

 metabohsm may be superposed on these simple physical 

 effects. 



An important difference between the structure of 

 protoplasm and that of a typical artificial emulsion, 

 like oil in water, is that the two phases separated by a 

 protoplasmic film are not necessarily aqueous and non- 

 aqueous, respectively, but may both be aqueous. Any 

 suspension of living cells, such as blood, illustrates this 

 condition; both the interior of the corpuscle and the 

 serum are complex aqueous solutions, and the film 

 bounding the corpuscle is too thin to be optically detect- 

 able as a separate structure. Yet it can readily be 

 shown by osmotic methods that a semi-permeable 

 membrane is there present. Similarly within the limits 

 of a single cell various structurally distinct regions, 

 sometimes optically distinguishable, sometimes not, are 

 to be regarded as separated by thin films. The details 

 of this film structure will naturally vary in different 

 types of cell. 



The surface layer of the entire cell, the plasma 

 membrane, is the modified surface-film which separates 

 the internal protoplasm from its surrounding medium 

 and through which the necessary interchange of material 



