CELLS AND TISSUES 49 



molecules in the gaseous state are so far apart that the intermolecular 

 forces are negligible and molecular movement is restricted only by 

 external barriers. Molecular movement in all three states of matter is 

 the result of the inherent heat energy of the molecules. By increasing 

 this molecular heat energy, one can change matter from one state to 

 another. W^hen ice is heated it becomes water, and when water is heated 

 it is converted to water vapor. 



If a drop of water is examined under the microscope, the motion of 

 its molecules is not evident. If a drop of India ink (which contains fine 

 carbon particles) is added, the carbon particles move continvially in 

 aimless zig-zag paths, for they are constantly being bumped by water 

 molecules and the recoil from this bump imparts the motion to the 

 carbon particle. The motion of such small particles is called Brownian 

 movement, after Robert Brown, an English botanist, who first observed 

 the motion of pollen grains in a drop of water. 



15. DifFusion 



Molecules in a liquid or gaseous state will diffuse, that is, move in 

 all directions until they are spread evenly throughout the space avail- 

 able. Diffusion may be defined as the movement of moleciUes from a 

 region of high concentration to one of lower concentration brought 

 about by their inherent heat energy. The rate of diffusion is a function 

 of the size of the molecule and the temperature. If a bit of sugar is 

 placed in a beaker of water, the sugar will dissolve and the individual 

 sugar molecules will diffuse and come to be distributed evenly through- 

 out the liquid (Fig. 3.9). Each molecule tends to move in a straight hue 

 until it collides with another molecule or the side of the container; then 

 it rebounds and moves in another direction. By this random movement 

 of molecules, the sugar eventually becomes evenly distributed through- 

 out the water in the beaker. This could be demonstrated by tasting 

 drops of liquid taken from different parts of the beaker. If a colored 

 dye is used in place of sugar, the process of diffusion can be observed 

 directly. The molecules of sugar or dye continue to move after they have 

 become evenly distributed throughout the liquid in the container; how- 

 ever, as fast as some molecules move from left to right, others move from 

 right to left, so that an equilibrium is maintained. 



Any number of substances will diffuse independently of each other. 

 If a lump of salt is placed in one part of a beaker of water and a lump of 

 sugar in another, the molecules of each will diffuse independently of the 

 other and each drop of water in the beaker will eventually have some 

 salt and some sugar molecules. 



The rate of movement of a single molecule is several hundred 

 meters per second, but each molecule can go only a fraction of a milli- 

 micron before it bumps into another molecule and rebounds. Thus the 

 progress of a molecule in a straight line is quite slow. Diffusion is quite 

 rapid over short distances but it takes a long time— days and even weeks 

 —for a substance to diffuse a distance measured in inches. This fact has 

 important biologic implications, for it places a sharp limit on the num- 



