46 



LIFE: ITS BEGINNINGS AND NATURE 



7 microns in diameter. Smaller objects, 

 such as the particulate matter in protoplasm, 

 are measured in millimicrons. It is possible 

 to measure particulate matter . with the 

 same degree of accuracy that can be at- 

 tained in the macroscopic world. 



To demonstrate the relative sizes of ob- 

 jects we may use a cell as the starting point. 

 A large cell of your body, a cell lining 

 your mouth, for example, would occupy a 

 space about the size of a needle point 

 (Fig. 2-17). If this were magnified ten 

 times, you would see it quite easily with 

 the naked eye but the parts would not be 

 very well defined. Magnifying it another 

 ten times (lOOX) brings the nucleus and 

 cytoplasm into full view; even the nuclear 

 structure can be made out. Another tenfold 

 increase (lOOOX) shows the chromosomes 

 in outline, but not in detail. A magnifica- 

 tion of 100 to 3000 is the range in which the 

 microscopist works with a light microscope. 

 Any further increase in size must be viewed 

 through the electron microscope, which 

 operates much the same as the light micro- 

 scope except electrons are used instead of 

 light waves for a source of illumination. Of 

 course, these cannot be seen directly with 

 the eye because our eyes are sensitive only 

 to light rays and not to electrons, but such 

 objects can be photographed (Fig. 2-9). 

 Furthermore, the treatment of the material 

 is so drastic that living things cannot be 

 studied under an electron microscope. In 

 order to study any material it must be 

 sliced into extremely thin sections (less 

 than 1 micron ) . With this instrument, mag- 

 nifications can go up to 100,000 diameters, 

 which will reveal viruses and the larger 

 molecules such as nucleoprotein molecules. 

 The electron microscope allows us to see 

 things in the molecular state. Beyond this, 

 we must rely on methods familiar to the 

 physicist to demonstrate the size and shape 

 of particulate matter, methods that are be- 

 yond the scope of this book. Physicists are 

 able to measure the size, shape, and be- 

 havior of molecules and atoms, and are now 



working on the nature of the components 

 of the atoms themselves. For the present 

 discussion it is only necessary for us to think 

 in terms of the size of particles at the molec- 

 ular level, because protoplasm is molecu- 

 lar. 



Colloids and crystalloids 



If a solid is ground to particles the size 

 of dust, and these placed in water, they 

 will form a murky fluid and after a time 

 will settle to the bottom of the container. If 

 the particles are ground still finer they will 

 reach a size when they remain in suspen- 

 sion and do not settle out even after a long 

 time. These particles are then in the col- 

 loidal state. Therefore, whether or not a 

 substance exists as a colloid is merely a 

 matter of size. Physicists have set an arbi- 

 trary figure for colloids; they state that par- 

 ticles ranging in size from 0.1 to 0.001 

 micron are in the colloidal state. A colloidal 

 system can often be observed with the 

 naked eye. For example, if egg albumin, 

 which is composed of large protein mole- 

 cules, is placed in water the solution has 

 an opalescent appearance. Light rays will 

 strike the suspended particles and be scat- 

 tered, rather than pass directly through as 

 would be the case if the particles were 

 smaller. We do not see the individual parti- 

 cles, only the effect produced by scattered 

 light. However, if the particles are larger 

 than 0.1 micron in diameter, the light will 

 be blocked altogether and the system will 

 appear opaque, as it does in milk, for ex- 

 ample. The larger particulate matter in milk 

 will, of course, separate out (cream on the 

 surface ) and is therefore not colloidal. 



The large size of colloidal particles also 

 prevents them from passing through an ani- 

 mal membrane. If egg albumin is placed 

 in a loop of frog skin and submerged in 

 water, very little, if any, of the albumin 

 will be found in the water even after hours 

 have elapsed. Furthermore, colloidal parti- 

 cles move slowly when compared to smaller 

 particles such as atoms or ions. This might 



