PROrOPtASAI 



31 



N 



r 



^ 



r 





w ^ 





.1 







J 



A B 



Figure 2.7. Diagram of a colloidal solution as a sol (.4) and a gel (B). The sol con- 

 tains water as the continuous phase in which the colloidal particles (dark rods) are dis- 

 persed. In the gel the colloidal particles have coalesced to form a continuous lacy network 

 in which the water droplets (light circles) are dispersed. 



if the system is subjected to large changes of temperature, acidity, alka- 

 linity or salt concentration, the colloidal solution is destroyed; the par- 

 ticles coagulate and settle out. 



Many of the properties of colloids are a result of the enormous 

 amount of surface area between the dissolved particles and the dissolv- 

 ing medium. For example, a cube 1 cm. on each edge has a total surface 

 area of 6 cm., but an equal volume of material divided into particles 

 0.01 micron on an edge has a total surface area of 6,000,000 square cm. 

 Many chemical reactions occur only at a surface, and for this reason a 

 colloidal system is a much better medium for chemical reactions than is 

 any other type of mixture. 



Many of the unique properties of protoplasm follow from the fact 

 that it is a colloidal system composed of protein molecules in water. The 

 protein molecules are too large to form a true solution in water and too 

 small to settle out. Protoplasm is constantly and rapidly changing from 

 sol to gel and back; one portion of the protoplasm of a cell may be a 

 sol while others are gels. The constant, rapid change from sol to gel is 

 one expression of the "aliveness" of protoplasm. Any extreme of tem- 

 perature, acidity, alkalinity, or the presence of certain chemicals will 

 cause an irreversible change to the gel or sol state and the protoplasm 

 is no longer alive. Protoplasin contains a large amount of water-80 per 

 cent of muscle is water, for example-yet, because the water is part of a 

 colloidal system, bound to the proteins present, muscle itself can be- 

 come quite solid during contraction. Muscle contraction, like many 

 other biological phenomena, involves a change from the sol state to the 

 gel. Shortly after death muscle undergoes rigor mortis, an irreversible 

 change to the gel state. 



Questions 



1. Discuss the characteristics of living things. Are any of these found in nonliving 

 systems? Can you think of any which should be added to the list? Any which do not 

 seem essential? 



