THE PERMEABILITY OF MEMBRANES 115 



From what we have learnt in the preceding section, it is plain that what is 

 needed is a membrane with properties similar to those of Traube's copper ferro- 

 cyanide, but more perfectly semi-permeable. 



As we shall learn in more detail later, there is a remarkable similarity between 

 the properties of the cell membrane and those of the artificial one, although it is 

 not to be supposed that they have anything in common as regards their chemical 

 nature. Both are permeable to ammonium chloride, impermeable to ammonium 

 sulphate. It is usually stated that the cell membrane is impermeable to potassium 

 chloride, while the copper ferrocyanide membrane, as we have seen, is freely 

 permeable to it. But this statement needs qualification. Overton (1904, 

 pp. 188-209) has shown that the muscle cell is not completely impermeable to 

 potassium chloride and that, in fact, potassium salts fall into two groups, the 

 first, typified by the sulphate and phosphate, to which complete semi-per- 

 meability exists, and the second, typified by the chloride. It will be noted 

 that this behaviour is similar to that of the copper ferrocyanide membrane, 

 which, according to Walden (1892), is permeable to chlorides, bromides, iodides, 

 and thiocyanates, impermeable to sulphates, phosphates, and oxalates. The 

 muscle cell, however, is only very slowly permeable by potassium chloride. 

 Meigs (1913), moreover, finds that a celloidin membrane, impregnated with 

 calcium phosphate, has most of the properties of the cell membrane, as regards 

 permeability. It is impermeable to the chlorides of sodium, potassium, and 

 calcium, to cane-sugar and alanine, somewhat permeable to glycerol and urea, 

 freely permeable to alcohol. Although it seems scarcely likely that the cell 

 membrane is actually composed of calcium phosphate, it is important that an 

 artificial membrane of nearly perfect semi-permeability can be prepared. 

 Philippson (1913), again, shows that, if collodion membranes are impregnated 

 with an ethereal extract of muscle, they become almost impermeable to inorganic 

 acids, while retaining their permeability to organic acids, increasing in the 

 series, formic-acetic-lactic-butyric. This result is of interest as a further step 

 in the artificial production of membranes with properties similar to those of 

 the cell membrane. 



That a membrane of some kind is actually formed on the surface of contact 

 between protoplasm and water is shown by the observations of Kiihne and of 

 Pfeffer referred to below (page 128). If any substances are present in the cell 

 which lower surface energy, we know that they will be concentrated at the 

 surface, and from Ramsden's experiments (page 55) we are prepared to find 

 that a coherent membrane will probably be formed. It is not necessary, then, 

 that an actual visible skin should be present, although in certain cases it 

 appears to exist. Moreover, the kind of membrane contemplated in the state- 

 ment just made forms, or may be regarded as, an integral part of the living 

 protoplasm itself, and as long as this is living, will probably share its power 

 of change and adaptation in response to changes in the environment. This 

 point of view will require further treatment later. 



When we come to the constituent cells of higher organisms, which are 

 dependent for their food supply on substances in the blood or other liquid 

 bathing them, we are at once met with a difficulty, if we assume the existence 

 of such a semi-permeable membrane. If it prevents food-stuffs from being 

 washed out of the cell, it must also prevent them from getting in. 



This difficulty has caused certain investigators to deny altogether the 

 existence of a membrane impermeable to electrolytes and other crystalloids. 

 Martin Fischer and Gertrude Moore (1907, p. 342), for example, appear to 

 hold that imbibition by colloids is capable of explaining the phenomena for 

 which a semi-permeable membrane was postulated. 



In order to understand the nature of the evidence on this question, it is 

 necessary to forestall somewhat a part of the subject matter properly belonging to 

 the chapter on osmotic pressure. Suppose that we have a vesicle, say of copper 

 ferrocyanide, containing a solution of sugar, and that we immerse it in water. 

 Since the membrane is impermeable to sugar, but permeable to water, the sugar 

 molecules inside exert a pull on water molecules, which enter and distend the 



