14 GENERAL BIOCHEMISTRY 



system. However, it is very complex when considered from the dy- 

 namic standpoint. Thousands of diverse activities may be ascribed to 

 protoplasm. Since protoplasm is a colloidal system (emulsion), we 

 will learn in Chapter 2 that many of the characteristic properties of 

 protoplasm can be best explained in terms of physics and physical 

 chemistry. This colloidal system contains proteins, lipides, and other 

 clVmical substances which possess the unique property of attracting 

 and holding large amounts of water within the protoplasmic structure. 

 The presence of water gives protoplasm its characteristic jelly-like 

 properties, causing it at times to behave as a semiliquid and at other 

 times as a semisolid. Consequently the behavior of cell membranes, 

 the permeability of cells, and the metabolic changes within the cells 

 must depend, in great measure, on the condition of the protoplasm 

 at any given moment. 



The turgidity of cells depends upon interrelationships existing be- 

 tween water and other constituents. Consequently the uptake (imbibi- 

 tion) of water by cells, the viscosity of protoplasm, and the elasticity of 

 the cell membrane require very delicate balance and control, details 

 of which still baffle scientific workers. 



Naturally a host of questions suggest themselves regarding this 

 protoplasm: what is its function in the cell and what are the chemical 

 activities of the cell itself; how does it receive its raw materials; what 

 changes are produced and how are these chemical changes stimulated 

 and controlled? Many of these questions can be answered by means of 

 chemical research, and it is through the study of problems of this type 

 that we are able to obtain a mental picture of the various phases of 

 growth and development in plants and animals. However, before we 

 can go far in the study of these questions we must ascertain, so far as 

 possible, the chemical composition of the cell and the nature and struc- 

 ture of the constituents, as well as their chemical and physical prop- 

 erties. 



Importance of Water 



It is impossible to overstress the importance of water in living 

 processes. Water is the solvent and dispersion medium for all proto- 

 plasmic constituents. Water not only acts as a transportation medium 

 for cell nutrients throughout the living organism, but also serves as the 

 medium in which reacting substances undergo fundamental changes. 

 It is necessary only to cite the ascent of sap in trees and the circulation 

 of blood in animals to emphasize the importance of water for the 



