514 



SCIENCE 



[N. S. Vol. XLII. No. 1085 



explanations are independent of any pos- 

 sible formation of an intermediate com- 

 pound between enzyme and substrate, after 

 adsorption has taken place. 



There is a further way in which adsorp- 

 tion plays a part in the chemical processes 

 of cells, including those under the influence 

 of catalysts. It is a familiar fact that the 

 concentration of water plays a large part in 

 the position of equilibrium attained in re- 

 versible reactions of hydrolysis and syn- 

 thesis. A synthetic process is brought 

 about by diminution of the effective con- 

 centration of water. There are, doubtless, 

 means of doing this in the elaborate mech- 

 anisms of cell life, and, in all probability, it 

 is by adsorption on surfaces, which are able 

 to change their "afSnity" for water. 



I pass on to consider briefly some other 

 cases in which the phenomena at phase 

 boundaries require attention. 



Let us turn our gaze from the interior 

 of the cell to the outer surface, at which it 

 is in contact with the surrounding medium. 

 From the nature of adsorption there can be 

 no doubt that, if the cell or the surround- 

 ing liquid contains substances which de- 

 crease surface energy of any form, these 

 constituents will be concentrated at the 

 interface. There are many such substances 

 to be found in cells, some of lipoid nature, 

 some proteins, and so on. Further, the ex- 

 periments of Ramsden have shown that a 

 large number of substances are deposited 

 in surface films in a more or less rigid or 

 solidified form. We are thus led to inquire 

 whether these phenomena do not account 

 for the existence of the cell membrane, 

 about which so much discussion has taken 

 place. We find experimentally that there 

 are facts which show that this membrane, 

 under ordinary resting conditions, is im- 

 permeable to most crystalloids, including 

 inorganic salts, acids and bases. There is 

 no other explanation of the fact that the 



salts present in cells are not only in differ- 

 ent concentration inside from that outside, 

 but that there may be absence of certain 

 salts from one which are present in the 

 other, as, for example, sodium in the plasma 

 of the rabbit not in the corpuscles. More- 

 over, the experiments of Hoeber have shown 

 that electrolytes are free in the cells, so that 

 they are not prevented from diffusion by 

 being fixed in any way. The mere assump- 

 tion of a membrane impermeable to colloids 

 only will not account for the facts, since, 

 as I have shown in another place, this would 

 only explain differences of concentration, 

 but not of composition. The surface con- 

 centration of cell constituents readily ac- 

 counts for the changes of permeability oc- 

 curring in functional activity, since it de- 

 pends on the nature of the cell protoplasm, 

 and chemical changes of many and vari- 

 ous kinds occur in this system. If such be 

 the nature of the cell membrane, it is evi- 

 dent that we are not justified in expecting 

 to find it composed of lipoid or of protein 

 alone. It must have a very complex com- 

 position, varying with the physiological 

 state of the cell. Indeed, complex artificial 

 membranes have been prepared having 

 properties very similar to that of the cell. 



This view that the membrane is formed 

 by surface condensation of constituents of 

 the cell readily accounts for the changes of 

 permeability occurring in functional activ- 

 ity, since its composition depends on that 

 of the cell protoplasm, and chemical changes 

 of various kinds take place in this system, 

 as it is scarcely necessary to remind you. 

 In fact, the cell membrane is not to be re- 

 garded as an independent entity, but as a 

 working partner, as it were, in the business 

 of the life of the cell. In the state of 

 excitation, for example, there is satisfactory 

 evidence that the cell membrane loses its 

 character of semipermeability to electro- 

 lytes, etc. This statement has been shown 



