144 PRINCIPLES OF GENERAL PHYSIOLOGY 



equally concentrated solutions of the alkaloid on both sides of the membrane is 

 of no effect or a minimal one. The characteristic effect is manifested only while 

 the drug is in the act of passing through the membrane. 



As long as we remember that the cell membrane is a modifiable part of the 

 cell system, the various facts described above need not cause surprise. 



SUMMARY 



Since protoplasm has the properties of a liquid and can also be shown to 

 contain free, uncombined salts, there must be some means by which free diffusion 

 between the contents of a cell or organism and the surrounding medium is 

 controlled. 



There is every reason to suppose that the regulation of the passage of substances 

 between the inside and outside of a cell is effected by means of a film or membrane. 



The membrane of the cell must allow water to pass freely, but hold back 

 dissolved substances. Such a membrane is known as a semi-permeable one. 



Artificial membranes can be made of various degrees of permeability ; thus, 

 some will only hold back colloids, others will allow certain crystalloids to pass, 

 but not sugar, and so on. 



Different views are held as to that property of a membrane which makes it 

 permeable to some solutes and impermeable to others. Reasons are given in the 

 text for accepting, with some modifications, the original sieve theory of Traube, 

 according to which the passage of a solute through a particular membrane, depends 

 on the size of the pores in the membrane in relation to the molecular, or particulate, 

 dimensions of the solute. The hydration of solutes must be taken into account. 

 In a few cases, the question of solubility in the substance of the membrane appears 

 to play a part. 



The protoplasmic substance of the cell is capable of forming a new membrane 

 on a fresh surface. The substances present in the protoplasm which lower surface 

 energy, and there are a large number of them, will be concentrated at the interface 

 between protoplasm and external phase, and some of them may be coagulated. In 

 this way a membrane is formed. It is to be noted that the cell membrane is, 

 accordingly, an integral part of the cell system, and capable of modification with 

 changes in the composition of the cell contents. 



In this way a difficulty is overcome. If the cells are always impermeable to 

 such solutes as sugar, amino-acids, and salts, how is growth to take place or the 

 functions of the cell to be performed ? We must conclude that the permeability of 

 the membrane is not always the same ; a fact which is also demonstrated by 

 experiment. 



The difficulty alluded to has caused certain investigators to deny altogether the 

 existence of a semi-permeable l^^fcrane covering the cell protoplasm. Evidence 

 of various kinds is given in the text, which shows that, in the condition in which 

 cells are usually met with, they are actually impermeable to crystalloids. 



This evidence consists in the permanent change of volume which cells undergo 

 under the action of various dissolved crystalloids, in the difference between the 

 concentration and nature of crystalloids in the interior of the cell and in the outer 

 medium, and, lastly, in the resistance opposed by living cells to the passage of 

 electrical currents, notwithstanding the fact that they contain free electrolytes. 



Although this may be regarded as the usual state of cells at rest, their 

 permeability may be altered, without killing them, and therefore reversibly, by 

 the action of various substances on the membrane. Of these we may mention 

 electrolytes in particular. Narcotics and light are also found to have an 

 influence. 



The chemical nature of the membrane depends on the constituents of the 

 protoplasm which lower surface energy. As fatty or lipoid substances possess this 

 power in a marked degree, it is to be expected that the membrane will manifest 



