CHAPTER V 



THE PERMEABILITY OF MEMBRANES AND THE 

 PROPERTIES OF THE SURFACE OF CELLS 



AN amoeba, after having taken in a vegetable cell, proceeds to digest the 

 substances contained therein. The products, in order to serve as food, must 

 diffuse from the digestive vacuole into the other parts of the protoplasm. But, if 

 they were able to diffuse out from this protoplasm into the water around, they 

 would be lost to the organism. There is good reason to believe, therefore, that 

 there must be some layer or film on the outer surface of an amoeba through which 

 dissolved non-colloidal substances, such as sugar and amino-acids, cannot pass. 



Evidence was given in our first chapter to show that living protoplasm must 

 have the properties of a liquid. This fact also points to the necessity of some kind 

 of an envelope, otherwise the organism would stand great risk of colloidal 

 dispersion through the water. 



The nature of this limiting membrane, with respect to the substances which it 

 allows to pass through, and those which are kept back, is of much importance. 



THE PROPERTIES OF MEMBRANES IN GENERAL 



It is obvious that a membrane, being merely a thin sheet or film, may be 

 composed of almost any substance. But, for our purpose, it is useful to classify 

 membranes according to their behaviour towards water, and towards substances 

 dissolved in it. In the first place, there are such things as glass or mica, which 

 allow neither water nor substances dissolved in it to pass through. Such may be 

 called impermeable and have a comparatively small importance. There are also 

 some materials which are impermeable to water, but allow certain other liquids or 

 gases to pass through; for example, india-rubber is impermeable > to water, but 

 allows pyridine to pass through. A metal, palladium, may be regarded as 

 impermeable to water under ordinary circumstances, but allows hydrogen to pass 

 through. Such cases are of interest in certain problems. 



The most important membranes for the physiologist are those which allow water! 

 to pass through, but hold back dissolved substances. There are various degrees in 

 this respect ; some membranes, such as parchment paper, gelatine, etc., will not 

 allow colloids to pass, but are freely permeable for crystalloids. Copper ferro- 

 cyanide, on the other hand, holds back the majority of both colloids and 

 crystalloids, but allows water to pass. A membrane which does not permit any ? 

 dissolved substance to pass, while permeable to water, is known as semi-permeable^ 

 Such a membrane has not been prepared in the laboratory, although the copper! 

 ferrocyanide of Traube approximates to it very closely. When we wish to speak 

 of a membrane which allows water to pass, but not a particular given substance, 

 we say that it is semi-permeable as regards that substance. 



Membranes may also be looked at from another point of view, that of their 

 structure. This may be of the nature of a sieve, so that different membranes have 

 different sizes of holes. Or a membrane may allow certain substances to pass 

 through it because of their solubility in the substance of which the membrane is 

 composed. Or, thirdly, they may possibly form reversible chemical compounds 

 with the substance to which they are permeable. The two last cases need not 

 delay us long at this stage. As a case of a membrane which is permeable by a 

 substance, because of the solubility of this substance in the membrane, we may take 



