PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 25 



7. Permeability of the Plasma Membrane for Various Sub- 

 stances and the Question of Its Composition and Structure. — 



Thus far in discussing the osmotic properties of cells we started 

 mth the concept that the plasma membrane separating the cell 

 sap from the surrounding solution represents an ideal semi- 

 permeable membrane, that it is easily permeable to water and 

 absolutely impermeable to substances dissolved in water, what- 

 ever may be their properties. This schematic representation 

 was essential for obtaining a 'clear picture of the general osmotic 

 properties of the cell. Actually, however, details are considerably 

 more comphcated; in nature, there do not exist ideal semi- 

 permeable membranes, as there do not exist ideal gases, hquids, 

 ideal solutions, etc. Each gas, hquid, or solution deviates more 

 or less from ideal physicochemical laws; and every semipermeable 

 membrane, being permeable to the solvent, is hkewise somewhat 

 permeable to the dissolved substances. Moreover, this per- 

 meability may differ for different kinds of substances. The 

 plasma membrane likewise does not entirely prevent the diffusion 

 of all dissolved substances ; otherwise, the absorption of nutritive 

 substances and their translocation from cell to cell would be 

 impossible. The capacity of protoplasm to allow substances to 

 penetrate through it is known as ''permeabihty," and the study 

 of this phenomenon is of the greatest importance for physiology. 

 The simultaneous realization of permeability and impermeability 

 of protoplasm is one of the most important contradictions in the 

 life of the cell. A complete explanation of these contradictions 

 will be possible only with a more consistent application of actual 

 experimental results than has formerly been practiced. 



De Vries, who first studied the phenomenon of plasmolysis, 

 noted that some substances, e.g., salts and sugars, produce a 

 stable plasmolysis remaining for hours; while others, e.g., glycerin 

 or urea, produce a temporary plasmolysis. After several hours, 

 the contracted protoplasm again distends and adheres to the 

 cell walls. De Vries explained this phenomenon correctly. The 

 substances slowly penetrate into the cell, their concentration 

 outside and inside the cell becoming equal; and the distending 

 influence of the substances contained in the cell sap again 

 becomes manifest. 



Further experiments have established that the number of 

 substances that enter readily into the cell is rather large. Many 



