568 



SCIENCE 



[N. S. Vol. XLVII. No. 1223 



membranes are living, as though that were 

 sufficient explanation. Axe not the mem- 

 branes that do obey the laws living, too? 

 Those membranes through which certain sub- 

 stances can pass from a lower to a higher con- 

 centration are very probably different from the 

 membranes used in experimental work. It is 

 only by assuming that the membrane plays an 

 active part in determining this one-sided 

 permeability that the thermodyamic laws are 

 brought into question. The sieve theory, even 

 when so modified as to render negative os- 

 mesis'^ available, does not seem convincing. 

 No gradation of molecules in the membrane, 

 either of concentration or kind, allowing the 

 solute to pass from one molecule to another, 

 could possibly explain the facts. Nor does the 

 theory of the funnel-like arrangement of lipin 

 molecules,^ ingenious though it be, allay one's 

 curiosity. However, there is no doubt that 

 most of the recent ideas on this question have 

 been correct in postulating structures as a ne- 

 cessity in any explanation of this elusive 

 (vitalistic) phenomenon. 



Wlierever selective permeability occurs there 

 is at least one layer of cells through which 

 the solute has to pass. It is inconceivable 

 that if all parts of these cells were exactly 

 alike there would be selective action. But 

 there seems to be no theoretical difficulty 

 provided certain differences exist. The two 

 membranes in contact with the two fluids need 

 to be different. The substance under con- 

 sideration easily passes the membrane on the 

 entering side of the cell, but passes either not 

 at all, or with great difficulty the membrane 

 on the exit side of the cell. In the cell a 

 chemical change occurs in the substance that 

 has entered. After this change it can pass the 

 second membrane easily, but passes only with 

 difficulty the membrane by which it gained 

 entrance. 



It is not necessary to assume a profound 

 change to make conditions favorable for this 

 differential solubility. But it is necessary 

 that the substance formed should exist in 



ir. E. Bartell, J. Am. Chem. Soc, 36, 646 

 (1914). 

 2T. B. Eobertson, Science, 45, 567 (1917). 



higher concentration than the original sub- 

 stance when the two are in equilibrium. A 

 change from an alkaline to a neutral or acid 

 medium, involving a change from a sodium 

 salt to the free compound or to, say, the 

 chloride would be sufficient, or a synthesis of 

 the entering compounds to more complex ones 

 would suffice to make the theory plausible. 



Because of our ignorance of just what hap- 

 pens in these membranes and cells, it can not 

 be stated with certainty that specific changes 

 do occur in a substance on passing through the 

 cells and membranes. But the conditions nec- 

 essary for such chemical and physical altera- 

 tions seem to exist. The probabilities in this 

 hypothesis can be more readily appreciated 

 by taking an example. 



The fats are hydrolyzed in the intestines to 

 the sodium soaps. These sodium soaps, but 

 not the fats, are able to pass the intestinal 

 wall membrane. On entering the cell space 

 inside the wall, the soaps are not free to pass 

 on to the lymph or blood, because of the com- 

 parative impermeability of the second mem- 

 brane to the soaps. "While between these two 

 membranes the soaps are synthesized to fats 

 by the aid of enzymes in the presence of glyc- 

 erol. Now these fats are able to pass through 

 the second membrane to the lymph or blood; 

 but the membrane on the intestinal side is 

 comparatively impermeable to the fats. It is 

 thus evident that there is reason for much 

 higher concentration of fat in the lymph than 

 in the intestines. 



Though this illustration was chosen because 

 it fits the theory, there is no inherent reason 

 why the same conditions should not apply, 

 say, to the absorption of the amino acids. 

 They may be absorbed by intestinal cells as 

 the sodium salts, then be changed to free 

 amino acids, and pass thus to the blood. It is 

 not necessary to assume that the compoimd 

 exists in the same form in the blood as in the 

 intestinal cell. It may change to any con- 

 ceivable compound after passing the second 

 membrane, may even change back to the one 

 that passed the first membrane. There is more 

 difficulty in accounting for the selective perme- 

 ability of membrane cells in the case of rather 



