PROTOPLASM AND COLLOIDS 



By L. V. HEILBRUNN 



ZOOLOGY LABORATORY, UNIVERSITY OP PENNSYLVANIA, PHILADELPHIA, PA. 



Faced with the problem of understand- 

 ing the mechanism of the living cell — of its 

 power to divide, to shorten, or to respond 

 to stimuli — our only hope of success lies in 

 our power to discover, insofar as possible, 

 the physico-chemical properties of the pro- 

 toplasmic material of which the cell is com- 

 posed. The task is not a simple one. Since 

 protoplasm is obviously colloidal, we must 

 attempt to apply the technique and the 

 point of view of the colloid chemist. But 

 whereas the student of inanimate colloids 

 can introduce his material into any con- 

 tainer of his choice, the student of the phys- 

 ical chemistry of protoplasm must devise 

 methods for making measurements within 

 the tiny confines of living cells. 



Then, too, the cell physiologist has to do 

 with a material far more complex chem- 

 ically than anything a conservative or rea- 

 sonable chemist would care to study. For 

 protoplasm is not only protein; it contains 

 lipids, carbohydrates, and salts as well. 

 Some of these substances are in true solu- 

 tion, others are colloidal; and in addition 

 there may be a very high concentration 

 of suspended material in the form of gran- 

 ules or fat droplets. Finally, the cell 

 physiologist has an additional worry; for 

 although a chemist can ordinarily treat a 

 colloid with any one of a variety of re- 

 agents, protoplasm is extremely sensitive. 

 It promptly dies if exposed to drastic re- 

 agents, and the dead protoplasm has vastly 

 different properties from the living. 



And yet, in spite of all these difficulties, 

 some methods have been devised for the 

 physico-chemical study of the protoplasmic 

 colloid, and at least certain elementary in- 

 formation has been obtained. I shall not 

 attempt to survey all the progress that has 

 been made in various parts of the world, 

 and I hope my listeners will pardon me if, 

 for the most part, I confine my remarks 

 to the work done in my own laboratory. 



Study of the colloidal properties of proto- 

 plasm should yield two types of informa- 

 tion. In the first place, it should provide 

 data concerning the true nature of the col- 

 loidal system or systems within the living 

 cell. And secondly, and I feel that this 

 is more important, the colloidal study of 

 protoplasm should interpret the behavior 

 of the cell in physico-chemical terms. It 

 should, for example, explain in so far as 

 possible the amazing sensitivity of proto- 

 plasm to the electric current, to mechanical 

 impact, and to ultraviolet radiation. It 

 should show why dilute solutions of ether 

 and other fat solvents are able to prevent 

 protoplasmic activity without seriously in- 

 juring the cell; why magnesium is like- 

 wise an anesthetic. Eventually, of course, 

 the cell physiologist must seek to interpret 

 all the normal and experimental behavior 

 of a cell in terms of its colloidal properties. 



Because of the greater interest of biolo- 

 gists in the behavior and activity of proto- 

 plasm I shall pass rather hurriedly over 

 that aspect of my subject which has to deal 

 with the physical make-up of living sub- 

 stance. Within the past 25 years numerous 

 studies have been made on protoplasmic 

 viscosity. It is no longer necessary to 

 hazard rough guesses as to the viscosity of 

 the protoplasmic fluid, for reasonably ac- 

 curate methods of measurement have been 

 devised. Li\ing cells are remarkably tol- 

 erant toward strong centrifugal force; so 

 that by subjecting the cells to such force, it 

 is possible to observe the movement of gran- 

 ules through the protoplasm. From the 

 speed of such movement, one can (with the 

 aid of Stokes' law) calculate the viscosity 

 of the medium through which the granules 

 move. Thus, it has been shown that the 

 protoplasm of the egg of the sea-urchin 

 (Arhacia) has a viscosity only several times 

 that of water. So, too, the interior proto- 

 plasm of the ameba is a fluid of low vis- 



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