MOLECULAR STRUCTUKIO IN r'ROTOPLASM 



181 



structure of cliains are uot so hypothetical 

 as one might be inclined to think from first 

 impressions. The ultramieroscope undoubt- 

 edly reveals submicroscopic particles which 

 approach in size the dimensions of those 

 shown by the ultracentrifuge to exist in 

 protein solutions. The X-ray diffraction 

 patterns may be associated with layered 

 particles in which the chains fit the dimen- 

 sions of atomic structures, which in turn 

 are based on chemical analyses of the pro- 

 teins and of amino acids. Diffusion meth- 

 ods, osmotic pressure methods, and ultra- 

 centrifuge methods agree pretty well on the 

 sizes of protein particles in dilute solution. 

 Taken all in all it has seemed to us that 

 further consideration of layered particles is 

 warranted. 



The distribution of size classes of the 

 particles below visibility in the cytoplasm 

 is practically unknown. From the increase 

 in numbers with diminution in size in the 

 visible range and the ultra-violet range, and 

 also from consideration of the enormous 

 numbers of minute particles seen in the 

 ultramieroscope, one may form a rough esti- 

 mate. For our present purposes we assume 

 that the number varies inversely with the 

 size. 



By making use of this assumption one 

 may gain a crude picture of the spaces be- 

 tween the particles; that is, of the fluid 

 channels in the cytoplasm. At least this 

 gives one a tangible basis for further 

 studies. It must be kept in mind, however, 

 that we are considering cytoplasm with 

 about 85 per cent water and 10 per cent 

 protein, and that this does not include ^'acu- 

 oles of any appreciable size. When this 

 amount of protein in hyclrated condition is 

 hypothetically cut up into minute chunks 

 of, say, 50 A or 500 A and mixed with the 

 remainder of the water, one may compute 

 the size of the channels between them. 

 Thus for 50 A particles, only, uniformly 

 distributed in the remainder of the water, 

 the space between particles is about 35 A to 

 40 A; for 50.0 A particles, it is about 400 

 A from one to the next; and for 5000 A 

 particles, it is about 4000 A. This means 

 that the amount of water allotted to each 



particle is a zone whose depth is somewhat 

 smaller than the radius of the particle. The 

 significance of this becomes evident when 

 one attempts to imagine how molecules 

 about the size of a cane-sugar molecule can 

 diffuse thi-ough water clianiiels which are 

 only slightly wider than the molecule itself, 

 as in those between the 50 A particles. 

 There would be sufficient room, however, for 

 ready diff'usion where the particles are 500 

 A and larger. 



These channel dimensions (although uni- 

 form distribution of particles is assumed) 

 must be approximately those in cytoplasm, 

 unless it contains localized density varia- 

 tions. That these variations occur seems 

 very probable, and if so the influence of 

 the large and the narrow channels in this 

 hyaline cytoplasm may perhaps be made 

 evident by the restrictions placed upon dif- 

 fusion of larger foreign molecules and by 

 the consequent reactions. But this leads us 

 farther afield than we care to go at this 

 time. 



These dimensional relationships of par- 

 ticles and channels are not easily visualized 

 when they refer to a microscopic cell, but 

 perhaps a homely comparison may be help- 

 ful. Imagine a 10-micron cubical cell en- 

 larged to a scale in which 1 micron becomes 

 about 10 inches. The cell becomes a nine 

 foot room, and the particles, which are just 

 at the range of microscopic visibility, about 

 5000 A in diameter, become the size of 

 grapefruits; while the water molecules be- 

 come about the size of fine grains of sand, 

 such as will pass through a 200 mesh sieve. 



We now wish to fill this nine foot room 

 with cytoplasm magnified to a correspond- 

 ing scale in order to obtain a tangible first 

 approximation to the dimensional inter- 

 relations of the cytoplasmic components. 

 Cytoplasm usually has a density of 1.03 

 (Leontjev 1935) and a water content of 

 85 to 90 per cent. When all of the mate- 

 rials in the water are aggregated into par- 

 ticles of 5000 A and uniformly distributed, 

 the spaces between them will be nearly 

 equal to the diameter of the particles. 

 These particles enlarged to the size of grape- 

 fruits to fit the scale of a 9-foot room are 



