STRUCTURE AND ACTIVITIES OF CELLS 



In many, nitrogen is an added constituent and sulfur, phosphorus, or iodine 

 may be present in some compounds. There are three great groups of the 

 organic compounds present in relatively large amounts in cells. These are 

 the carbohydrates, lipids, and proteins. Organic compounds which do not 

 fall in these three groups are present in small amounts. These difTerent 

 categories of chemical compounds will be briefly described in this section 

 in order that the nature of food and the changes which occur in it within 

 the animal can be adequately discussed in the remainder of the chapter. 



Water constitutes 60 to 90 per cent of the cell contents and holds the 

 inorganic salts and many of the organic compounds in solution. The physical 

 characteristics of water that are responsible for its great importance in the 

 phenomena of life are shared by only a few other compounds — ammonia 

 (NH3), hydrogen peroxide (H2O2), and hydrogen fluoride (HF). With 

 sufficient knowledge of chemistry and zoology we could speculate concerning 

 hypothetical creatures with liquid ammonia as the solvent in their reacting 

 systems. Instead, we shall attempt to grasp the essentials of animal life as it 

 really exists. 



Two atoms of hydrogen and one of oxygen are joined by covalent bonds, 

 that is, by a sharing of electrons, to form a molecule of water. The two 

 atoms of hydrogen are located on the same side of the oxygen atom, about 

 105° from one another (Fig. 2.SA). In other words, the water molecule is 

 asymmetric or polarized, tending to behave as if positively charged on the 

 hydrogen side and negatively charged on the oxygen side. This results in 

 marked electrical attraction between the hydrogen side of one water molecule 

 and the oxygen side of another. Such intermolecular electrostatic attraction 

 between water molecules is known as hydrogen bonding and is responsible for 

 the relatively high melting and boiling points of water as compared with 

 other compounds of its chemical family. The importance of the relationship 

 between the temperature range of the liquid state of water (0 to 100°C.) and 

 that of the earth's surface is obvious. Hydrogen fluoride, another good 

 solvent, melts at about -92°C. and boils at +19°C. Where living organ- 

 isms can exist is basically conditioned by the melting and boiling points of 

 water, the solvent of the cell contents. Other physical qualities of water 

 make it exceptionally useful in the equalization of heat in organisms and in 

 connection with temperature regulation in warm-blooded animals (p. 125). 



The inorganic constituents of the cell occur either as salts or in combination 

 with certain of the organic molecules. When an acid and a base combine to 

 form a salt, ions carrying opposite electrical charges are brought together 

 and held by electrostatic attraction or what are known as ionic bonds. Salts 

 are not soluble in most solvents, but the polarized molecules of water tend 

 to neutralize the electrostatic attraction between the ions of a salt molecule. 

 Once separated, the ions become encapsulated by water molecules and cannot 

 recombine. Thus salts may be almost completely dissociated, or ionized, in 

 the cell and body fluids and forrn an electrolytic solution that can con- 

 duct an electrical current. Sodium (Na"*"), potassium (K"^), calcium (Ca'^'''), 



23 



