280 COLLOIDS IN BIOLOGY AND MEDICINE 



protoplasm. Formerly from an interpretation of the experimental 

 facts as pure osmosis, the pellicle was considered semipermeable, 

 that is, permeable for water but impermeable for everything else. 

 This view is theoretically untenable since the cell requires numerous 

 substances and there must also be an exit for excreta. As a matter 

 of fact, the studies of recent years have shown that the plasma 

 pellicle is by no means as impermeable for many substances as was 

 assumed. The attempt has been made to decide its chemical and 

 physical condition from the nature of the substances which pass 

 through it. 



The statements on page 281 show that there is no uniform opinion 

 as to whether the plasma pellicle consists of albuminous or lipoid 

 substance or a mixture of both. In my opinion it differs in each 

 instance depending on the contents of the cell. However, colloid 

 research offers at least a foundation for a conception of the plasma 

 pellicle. 



In animal cells, with few exceptions, we can discover no membrane, 

 yet many of their properties indicate that they also possess some sort 

 of a pellicle. Colloid chemistry gives us a basis for the explanation of 

 such phenomena. The conditions are most simple when the cells are 

 surrounded by air. We know from page 33 that colloids concentrate 

 and unite into a firm skin at an interface, fluid/air. The process is 

 much more complicated in the case of cells in a fluid or semifluid 

 medium. Let us recall the following experiment: If ether is shaken 

 with water containing albumin or albumose, there will form a foam 

 consisting of drops of ether surrounded by albumin or albumose films. 

 Certain other colloids and fluids permit the formation of fluid foams, 

 which have unmistakable similarities to agglomerations of cells. 



Although this analogy may at first sight seem entirely superficial, 

 we must remember that the interface between two immiscible 

 fluids and between a fluid containing solid or semisolid bodies (gel), 

 possesses other properties than does the interior (see pp. 14-17, 

 et seq.). The surface of a cell must have special properties; such sub- 

 stances as shall lower the surface tension must collect there. These 

 substances are probably lipoids (lecithin and cholesterin) which have 

 been demonstrated in every cell, animal as well as vegetable. The 

 thickness of the transition layer varies, according to different ob- 

 servers of different substances, from 1 to 25 w, whereas the thick- 

 ness of the material coherent pellicle (lecithin, etc.) does not need to 

 be thicker than from 0.3 to 7 /z/z. These are minimal figures which 

 show that a membrane may be entirely invisible with the microscope 

 and yet fulfil all the conditions of a true membrane as far as the 

 transfer of material is concerned. 



