182 



THE CELL AND PROTOPLASM 



now uniformly spaced, and the spaces be- 

 tween filled with the fine sand to represent 

 water molecules. The spaces are, say, 

 about 4 inches between grapefruits, which 

 are about 5 inches in diameter, and form 

 channels of sand which are continuous 

 throughout the room. 



If, however, we wish to illustrate the 

 cytoplasm as being completely hyaline in 

 appearance, that is, as containing no visible 

 particles, we may choose only the small 50 

 A size ; then instead of grapefruits we may 

 make use of some proportionally smaller 

 objects, such as radish seeds, to represent 

 them. The channels of sand which separate 

 the radish seeds in our 9-foot room are now 

 only about a millimeter instead of four 

 inches in width, and the number has in- 

 creased enormously. 



But in the cytoplasm it seems probable 

 that a great variety of particles must exist, 

 ranging in size from perliaps 50 A up to 

 nearly 5000 A — all invisible microscopically. 

 We may assume, for a slightly more detailed 

 picture, that each particle has its allotted 

 portion of water which varies with the di- 

 ameter of the particle; the small particles 

 are accompanied by proportionally small 

 water channels, the large particles by larger 

 channels. 



It is interesting now to consider some of 

 the possibilities which must follow upon 

 aggregation of these particles. When sev- 

 eral particles of varying sizes combine into 

 a larger particle, the allotted water must be 

 considered not only as accompanying them, 

 but also as being squeezed out from between 

 them to form a deeper layer around the 

 larger clump. A lively imagination may 

 now picture the formation of submicro- 

 scopic vacuoles and the clumps forming into 

 sponge-like larger clumps, which may be 

 still invisible — in some cases because they 

 are still submicroscopic in size, and in other 

 instances because the portion of the sponge 

 which is formed by the particles is too at- 

 tenuated, although large enough to be vis- 

 ible in water. Viscosity behavior, forma- 

 tion of gel and sol states localized in various 

 places in the cell, even the formation of 

 spindle fibers now come within range of 

 visualization. 



With this picture many activities and 

 processes begin to come into view more 

 clearly, although they are still surrounded 

 by fog. The introduction of the protein 

 chain makes the visualization of a structure 

 or a mechanism constructed of submicro- 

 scopic and molecular entities become nearly, 

 but not quite, possible. Other molecular 

 structures are essential, however, to the 

 functioning of the machine. 



Some of the organic molecules, like those 

 of the flavine and pyridine compounds, the 

 small succinic acid molecules, or the still 

 smaller molecules or atoms of some metallic 

 elements, are without doubt necessary to 

 bring about a condition of activity. The 

 cytoplasmic particle may be many times the 

 size of the small prosthetic groups which 

 become attached to it. Relatively, the size 

 is comparable to the large automobile and 

 the tiny spark plug; and the analogy may 

 be carried somewhat farther, for the protein 

 particle without the prosthetic group may 

 be about as active as the large automobile 

 without the spark plug. 



The manner of attachment of these 

 prosthetic groups is in many cases uncer- 

 tain ; however, they often require two points 

 of attachment. It is interesting to note 

 how nicely certain prosthetic groups fit on 

 to the protein side-chains. The distances 

 between the j^olar atomic groups which are 

 likely to act as points of attachment are in 

 several instances approximately the same in 

 both protein molecule and in the ]:)rosthetic 

 groups, that is about 7 A. (See Figs. 8, 9, 

 10, 11.) This distance between residues 

 along the protein chain may be also the same 

 as that when the same two residues occur 

 attached to adjacent hydrated back-bones. 



This interesting spatial correspondence 

 between the probable active regions of the 

 prosthetic groujis and the active ends of the 

 side-cbains is nicely illustrated by tliree- 

 dimensional models shown in the reproduc- 

 tions given in Figs. 8, 9, 10, and 11. By 

 first referring to Fig. 3, the distance from 

 center to center of adjacent residues on one 

 side of the chain is seen to be 7.0 A. In 

 Fig. 8 a small piece of the protein chain 

 is shown on the left (A) with two residues, 

 one of arginine and the other of tyrosine. 



