The Chemical and Physical Structure of Protoplasm - 91 



Coarse, Colloidal, and Crystalloidal Di- 

 mensions. Anything that is visible, either 

 with the naked eye or with a compound 

 microscope, is designated as coarse; and above 

 this borderline of visibility, vast space exists 

 to accommodate the familiar objects of our 

 universe. But even below this borderline, 

 there is ample room for great variability in 

 the size of submicroscopic particles. 



Particles that are too small to be resolved 

 by any type of light-focusing microscope are 

 subdivided into two categories: all larger 

 particles — like protein and polysaccharide 

 molecules — fall into the colloidal size range; 

 and all smaller molecules — like water, 

 salts, and monosaccharides — lie in the 

 crystalloidal range. No real line divides col- 

 loidal from crystalloidal particles, and the 

 various particles of protoplasm are finely 

 and continuously intergraded in size. Arbi- 

 trarily, however, a line is set at 1 millimicron 

 (m^), which is equivalent to 10 Angstrom 

 units (Fig. 2-6), or 0.001 micron. Accord- 

 ingly, particles having diameters extending 

 from 100 millimicrons (the limit of resolu- 

 tion of an ordinary compound microscope) 

 down to 1 millimicron are said to be col- 

 loidal; whereas particles with diameters of 

 less than 1 millimicron are designated as 

 crystalloidal. 



On a practical basis, the appearance of a 

 fluid indicates whether it is a colloidal or 

 crystalloidal system. If the dispersed particles 

 of a fluid are very small, that is, of crystalloid 

 dimensions, the fluid as a whole is clear and 

 transparent — like a sugar or salt solution. 

 The dispersed molecules or ions of such a 

 solution are not large enough to interfere 

 with the light as it passes through. But if a 

 fluid contains particles that are large enough 

 to fall in the colloid range, the system has a 

 cloudy or translucent appearance — like a 

 starch or albumin solution. In this case the 

 dispersed particles (large organic molecules) 

 are not large enough to block the passage of 

 light waves completely, but they are large 

 enough to scatter, or diffract, the light. Fi- 

 nally, a coarse system — like milk — appears 



opaque. In this case, the dispersed globules 

 of butter fat are aggregated masses of mole- 

 cules, and such particles are large enough to 

 reflect light. Consequently, when milk is ex- 

 amined with a microscope, the coarse dis- 

 persed particles are individually visible. 



Although colloidal particles are not large 

 enough to be seen with a standard micro- 

 scope, most of them can be resolved by the 

 electron microscope and many can be de- 

 tected by means of the darkfield microscope 

 (Fig. 2-10). The latter instrument resembles 

 an ordinary microscope, except that it re- 

 quires a more intense beam of light; and this 

 beam is passed horizontally, rather than ver- 

 tically, through the material under examina- 

 tion. Under the darkfield, or ultramicro- 

 scope, the colloidal particles of the proto- 

 plasm appear as a myriad of bright specks 

 zigzagging in random directions against a 

 black and empty background. Each particle 

 may not be large enough to reflect a definite 

 pattern of light; but the particles are suffi- 

 ciently large to scatter some of the light 

 vertically toward the ocular. Thus the ultra- 

 microscope shows very little as regards the 

 precise color, shape, or size of colloidal par- 

 ticles. It merely permits these particles to be 

 identified and enumerated, and it reveals the 

 nature of their movements. 



Another practical method of distinguishing 

 between colloid and crystalloid particles is 

 to test their capacity to penetrate a mem- 

 brane such as cellophane or parchment. The 

 pores or channels through a cellophane mem- 

 brane appear to be just small enough to pre- 

 vent the passage of colloid particles, but these 

 pores are large enough to permit crystalloidal 

 particles to pass through. Thus if a closed cel- 

 lophane bag, filled with an aqueous solution 

 containing glucose and starch, is immersed 

 in water, after a short time glucose — but not 

 starch, will begin to escape from the bag into 

 the surrounding water. Such permeability 

 considerations are very important in deter- 

 mining the exchange of substances between 

 cells and their surrounding media. Generally 

 speaking, colloidal particles are not able to 



