THE LIVING CELL 39 



the cytoplasm must be appreciably permeable to salts, sugars, 

 amino acids and such crystalloidal substances, although under ex- 

 perimental conditions the degree of this permeability appears to be- 

 lower than one would expect. It is possible that movement from 

 cell to cell of substances in solution also occurs through the cyto- 

 plasmic connections already mentioned. 



Experiments show that some classes of substances penetrate the 

 cell very quickly, as is the case for example with alcohol (poisonous 

 in higher concentrations), urea and other organic compounds. There 

 has been much speculation as to why some substances are able to 

 pass the cell-membrane more easily than others. There is some 

 evidence that membranes, both natural and artificial, act as molecular 

 sieves, allowing smaller molecules to pass through, but holding up 

 larger ones ; though this theory only accounts for some of the facts. 



If a cell is killed the cytoplasmic membrane immediately becomes 

 very much more permeable to dissolved substances, its organisation 

 having been destroyed. Thus, while Beet cells are in life semi- 

 permeable to the red pigment of the sap, if the cells are killed, by 

 heat or by exposure to chloroform, the red pigment quickly begins to 

 diffuse out of the cells into the external liquid in which they may 

 be lying. 



In some cases the entrance of dissolved substances into a cell 

 proceeds according to the laws of ordinary diffusion, continuing until 

 the concentrations inside and outside the cell are equal in respect of a 

 particular substance. This holds for various organic compounds. If 

 the substance is being constantly used up in one way or another inside 

 the cell, or is passing into neighbouring cells, equality of concentration 

 may never be reached, and entrance will continue indefinitely. A 

 similar consideration applies to the entrance of inorganic salts, and of 

 the ions produced by their dissociation : but with these entrance may 

 continue even after the state of equal concentration has been reached. 

 This is a fact of great importance in connection with the absorption 

 of mineral salts by the root-hairs from the very dilute soil-solution 

 (Chapter VII.). It is clear that here forces over and above those 

 productive of ordinary diffusion are operative, though their nature 

 is still problematical. 



