168 



THE CELL AND PROTOPLASM 



living organisms. Microscopic investiga- 

 tions of the slimy content of these cells, 

 which was named protoplasm, never became 

 a very popular fashion ; and although dur- 

 ing this past century enormous advances 

 have been made in biological fields, gen- 

 erally, the conception of structure in proto- 

 plasm has advanced very little. Now and 

 then during this time some hardy or in- 

 quisitive soul would make a contribution 

 towards a slightly better understanding of 

 some phase of this material. At one time, 

 perhaps a contribution would be made to 

 its chemical nature ; at another, to its micro- 

 scopical appearance; and at still other 

 times, descriptions were presented of its 

 colloidal behavior and of its various physi- 

 cal properties. Taken all in all, the prog- 

 ress has not been very great; but that is 

 not surprising when the difficulties are con- 

 sidered, and especially when it is recog- 

 nized that a comprehension of this proto- 

 plasmic material demands a knowledge of 

 matter which has only recently been de- 

 veloped to such an extent that exploratory 

 studies are now more than possible. I refer 

 here to our modern knowledge of atomic 

 and molecular entities as minute, three- 

 dimensional particles. 



Information concerning these minute 

 particles has been accumulating during the 

 past decade and has made possible the ex- 

 tension of the microscopic horizon down 

 beyond the ultramicroscopic, or colloidal 

 region, into the molecular and atomic 

 world. This work is so recent, however, 

 that one feels hesitant and overbold in at- 

 tempting to apply the information to liv- 

 ing material, especially since in doing so he 

 must deal with the least well known of 

 molecular substances, the proteins; and 

 also with the least kno^vn states of matter, 

 colloidal and molecular solutions. 



Among the many difficulties which stand 

 in the way, one of the most obvious is that 

 of an appreciation of comparative sizes 

 when passing from a molecular level up to 

 a microscopic level or in passing in the 

 opposite direction. The cell, it is well 

 known, may be only about 20 p in diameter. 

 Within it a nucleus occurs of perhaps half 

 that diameter ; if it is a plant cell, plastids 



may exist which are only 2 p in size, and 

 still smaller particles may be found down 

 to the limit of visibility, which is about half 

 a micron, or about 5000 Angstroms, The 

 latter are the units by which atoms and 

 molecules are usually measured; thus, a 

 glucose molecule is about 5 A in diameter. 

 In comparison with such molecules, one of 

 the smallest of the microscopically visible 

 particles is equivalent in size to a packet 

 consisting of about a billion glucose mole- 

 cules. Nevertheless, it is with these small 

 molecular entities that we must deal if we 

 are to comprehend the molecular machin- 

 ery involved in vital processes. 



I have purposely switched the viewpoint 

 from structure to activity and to molecular 

 machinery or mechanisms composed of 

 molecules in order to bring out the rela- 

 tion of structural levels to types of cellular 

 activity, which may also be considered as 

 occurring on different levels. The whole 

 cell may be thought of as a structure con- 

 sisting of a nucleus embedded in cytoplasm 

 which in turn is surrounded by an enclos- 

 ing membrane; perhaps its structural na- 

 ture may be more readily recognized when 

 mitosis is in progress. If a dynamic view- 

 point is taken, the cell during mitosis may 

 be looked upon as a machine or a mecha- 

 nism which is performing in a definite rou- 

 tine way at the level of microscopic vision. 



As a step downward in size, one is more 

 likely to form a conception of cell inclu- 

 sions as structures rather than to think of 

 the whole cell as a single structure. The 

 nucleus considered as an inclusion then 

 becomes a mechanism with many activities 

 ascribed to it as well as a structure; the 

 plastids likewise, but with fewer activi- 

 ties ; the mitochondria also become mechan- 

 isms, but with perhaps only one function ; 

 while the still smaller granules seem to 

 be present without any particular name or 

 duty to perform. 



At this point I wish to call attention to 

 and to emphasize the localization of activi- 

 ties within the cell. The chloroplast may 

 serve as an example. Just as the cell is par- 

 titioned into structures which have speci- 

 fied activities, so in the chloroplastid one 

 set of molecular mechanisms or a particular 



