92 - The Cell 



penetrate the plasma membrane, and crys- 

 talloidal particles may or may not penetrate, 

 depending upon a variety of other factors 

 (Chap. 6). 



Size and the Motility of Particles: Brown- 

 ian Movement. The absorption of heat by 

 any kind of matter accelerates and intensi- 

 fies the random movements of the component 

 molecules, ions, and other particles. In fluid 

 bodies like protoplasm, these thermal move- 

 ments tend to scatter the particles evenly 

 throughout the system. But the speed at 

 which each particle moves, impelled by ther- 

 mal energy at a given temperature, is gov- 

 erned largely by its size. When the diameter 

 is doubled, the rate of movement is quar- 

 tered. In other words, the rate of thermal 

 movement of a particle is inversely propor- 

 tional to the square of its diameter. Accord- 

 ingly the random movement of colloidal par- 

 ticles, such as protein molecules, is very small 

 compared to the movement of crystalloid 

 particles, such as inorganic molecules and 

 ions. The actual motion of the colloidal par- 

 ticles as seen in the ultramicroscope, and the 

 movement of small microscopically visible 

 particles such as bacteria and starch grains 

 are not directly due to the thermal energy of 

 these relatively large particles. Instead, they 

 represent a phenomenon called Brownian 

 movement. Brownian movement is exhibited 

 by all small microscopic bodies as well as by 

 ultramicroscopic particles generally. It arises 

 from the bombardment of the larger particles 

 by the surrounding multitude of smaller par- 

 ticles. In the protoplasm, the smaller par- 

 ticles responsible lor this bombardment are 

 chiefly water molecules. The bombardment 

 is unequal from moment to moment, being 

 heavier first on one side and then on another. 

 Accordingly, the displacement of particles 

 by Brownian movement is very irregular and 

 unpredictable. 



Solutions; Molecular Polarity. In an ideal 

 solution, all the molecules or ions of the 

 solute become individually separated and 

 evenly dispersed throughout the dispersion 

 medium, which is called the solvent. In ac- 



tual solutions, however, the situation is com- 

 plicated by the fact that the solute molecules 

 or ions become associated with the molecules 

 of the solvent; and the effective size of the 

 dispersed particles in a solution tends to be 

 greater than expectations based on the molec- 

 ular and ionic volumes of the dissolved sub- 

 stances. Accordingly, in protoplasm and other 

 aqueous solutions, the solute particles are 

 associated with a greater or lesser number of 

 water molecules; or in other words, each 

 solute particle is said to be hydrated. 



The forces that attract water to the mole- 

 cules or ions of the solute are mainly elec- 

 trical. In many molecules, the electrical 

 charges (protons and electrons) do not have 

 a symmetrical placement in the molecule as 

 a whole. In the case of water, for example, 

 the whole molecule is an uncharged body, 

 but the negative charges are more concen- 

 trated toward one end, and the positive 

 charges toward the other. This endows water 

 molecules with an electrical polaritv, and in 

 fact, water is designated as a moderately 

 polar compound. Water is attracted to other 

 molecules and ions in a solution with greater 

 or lesser force, depending upon whether these 

 other particles are strongly or weakly polar 

 in their electrical configuration. Generally 

 speaking, water, salts, proteins, and simple 

 carbohydrates are distinctly polar, whereas 

 fats and polysaccharides are relatively non- 

 polar compounds. 



Among the components of protoplasm, 

 those that dissolve in water include: most 

 inorganic substances; sugars, amino acids, 

 and other simple organic compounds; many 

 proteins; and to a slight extent, the phos- 

 pholipids and polysaccharides. Other lipids 

 are virtually insoluble in water, although 

 they dissolve mutually in each other. The 

 water-soluble components in the protoplasm 

 are all hydrated to a greater or lesser degree, 

 and the salts, amino acids, proteins, and 

 phospholipids are also more or less strongly 

 ionized. These considerations have great im- 

 portance in determining chemical and physi- 

 cal activities in the cell. 



