Molecular Structure in Protoplasm 43 



occur which could not take place if these positions in space were 

 altered. In other words, the usefulness to the protoplasm of one 

 large molecule may be dependent upon the arrangement of a dozen 

 or more molecules immediately adjacent to it in the particle. 

 Organization comparable to this on a visible level may be observed 

 in the orderly behavior of chromosomes at mitosis; but organization 

 on a molecular level is not so obvious, although rather specific 

 arrangements of molecular structures seem to be essential in the 

 synthesis of glucose, and definite arrangements of enzymes are 

 involved in respiration. 



To comprehend organization of molecules within particles of 

 visible size, and even of particles below visibility, seems at first 

 glance to be quite impossible because the number of component 

 molecules is so great. Fortunately, however, these molecules possess 

 many features in common, and many individuals of one type may 

 occur; as a result of this the particle as a whole, consisting of a 

 thousand such molecules, may appear in some aspects as a struc- 

 tural repetition pattern. Comprehension of one of these structural 

 units is a long step toward the comprehension of the whole. 



We have just mentioned the possibility of a definite structural 

 interrelation between a dozen or more molecules within a single 

 particle as necessary for a particular activity. A molecular group 

 of this sort may be considered as a mechanism when by virtue of its 

 spatial configuration it is enabled to carry on a particular set of 

 activities, to serve a specific purpose in the cell. It is conceivable 

 that small mechanisms may become aggregated into still larger 

 composite particles. Thus several individual, molecular mechanisms 

 may become component parts of a single larger, more complex 

 machine. It is further conceivable that these micellar mechanisms 

 may in turn become aggregated into much larger submicroscopic 

 particles and that each of these may become a mechanism of great 

 complexity adapted to a still different sort of activtiy. In each case 

 the component building blocks, whether molecules or small aggre- 

 gates, lose their obvious individual identity when they become 

 attached into a larger particle. 



Why then stop at a particular size? Why, indeed? A microscopic- 

 ally visible plastid is most certainly a mechanism since it may 

 function in building starch grains. It is a mechanism on a visible 

 dimensional level and may be thought of as built of millions of 

 smaller particles, but the individuality of these particles has become 

 lost in the formation of the larger structure. Or on a still higher 



