STRUCTURAL UNITS IN CELLULAR PHYSIOLOGY 



By J. D. BERNAL 



BIKKBECK COLLEGE, UNIVERSITY OF LOXDOX, LONDON, ENGLAND 



The approach to the study of biolo^iical 

 structures has been along two converging- 

 lines. Through the gradual improvement 

 of the microscope the structures of tissues, 

 and later of cells themselves, have been 

 elucidated down to the limits of optical 

 resolution. The second approacli has been 

 through chemistry, the finding of the ulti- 

 mate molecular constituents of solid and 

 fluid tissues, the analysis of more and more 

 complex compounds, and the stud.y of re- 

 actions between them. These two lines 

 have yet to make contact and the gap be- 

 tween them is still large and particularly 

 significant. The simplest way to express 

 it is in terms of actual scale ; even the 

 ultraviolet microscope cannot determine 

 the structure of anything much smaller 

 than 2000 A or 0.2 p, and pure chemical 

 analysis cannot determine the structure of 

 a molecule much larger than 20 A or 2mij. 

 It is the gap between these dimensions 

 that the new physico-chemical methods 

 have to fill. It is a very important gap 

 because in it are found both the larger 

 molecules — those of the proteins that are 

 fundamental for biological processes — and 

 the micellar structures which determine 

 the visible microstructure of cells and 

 tissues. The first approach to this study 

 was that of colloid chemistry and physics. 

 B}^ colloid studies, particularly those of 

 the ultracentrifuge and of electrophoresis, 

 we have obtained a general picture of the 

 behavior of matter aggregated in units 

 which lie in this range of dimensions. But 

 until the development of more recent meth- 

 ods this picture was indirect and lacked 

 the sharpness of either the microscope or 

 the chemical evidence. X-rays with their 

 shorter wave length enable us to detect 

 and measure any regularities which occur 

 in the region between 2000 A and 2 A, and 

 from the knowledge of these regularities 

 to build up a picture of the structure 



itself. This method still lacks the direct- 

 ness of the microscope and the direct ap- 

 proach which may be achieved through the 

 development of the electron microscope, 

 but it gives us the best picture we now have 

 of the intimate structure of living mate- 

 rials. Recent studies with X-rays, partic- 

 ularly on proteins and viruses, combined 

 with knowledge derived from colloid stud- 

 ies, give us at any rate some clue to the 

 underlying structure of many of the ap- 

 pearances of living cells which are pre- 

 sented in the microscope. 



It must be realized that these explana- 

 tions are necessarily^ extremely tentative 

 at this stage. They do not represent di- 

 rect analyses of actual living cells, but in- 

 ferences derived from the structure of 

 more easily studied preparations particu- 

 larly of fibers, as represented by the great 

 work of Astbury, and of crystalline pro- 

 teins and viruses. The suggestions put 

 forward here nuist be considered therefore 

 as purely speculative, intended to guide 

 the procedure of research rather than the 

 claim to be verifiable explanations of the 

 facts. 



The earlier work on cell structure has 

 dealt with the more inert parts of the cells, 

 particularly cell walls, whether it be cellu- 

 lose or keratin, and the structures derived 

 from them. We have through the work of 

 Sponsler, Astbury and Preston a fairly 

 accurate picture of the underlying molecu- 

 lar microstructure of such cell w^alls and 

 even some hint as to their mode of deposi- 

 tion. It is possible to go a little further 

 and throw^ light on the intermediate inter- 

 cellular structures, such as those of muscle 

 fibers. The myosin of muscle fiber is, as 

 Astbury has shown, essentially a con- 

 tracted keratin-like fiber molecule wiiich 

 only differs from the cell-wall keratins by 

 the fact that its degree of contraction can 

 be varied by its environment, a property 



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