GENERAL ZOOLOGY 



and magnesium (Mg"*""*") are indispensable cations in living systems, and 

 phosphate (HPO^"^), bicarbonate (HCOg"), chloride (CF), and iodine 

 (I~) anions are also necessary for normal function. In addition, traces of iron 

 (Fe"'"''"^, Fe''"''), cobalt (Co'^"''), and copper (Cu"*""*") cations, as well as of a few 

 other metals, are found. Reference will be made to specific ions during the 

 discussion of the functions of cells. 



Carbohydrates are compounds containing carbon, hydrogen, and oxygen. 

 Most of the carbohydrates can be classified as (1) monosaccharides, the simple 

 sugars; (2) disaccharides, the compound sugars; and (3) polysaccharides, 

 glycogen and the starches. Among the simple sugars are the five-carbon 

 pentoses (CgHjoOs)^ which are rare, and the very common hexoses 

 (CgHjgOe) ^^ch ^^ glucose (Fig. 2.8C). The fact that the atoms of hydrogen 

 and oxygen occur in the ratio of two to one in these sugars, as in water, is 

 the reason for giving the name carbohydrate to the entire group. Two 

 hexoses linked together form a disaccharide (Cj2H2 20,i); maltose is formed 

 by the linking of two molecules of glucose (Fig. 2.8Z)), and sucrose is a com- 

 bination of one molecule of glucose and one of fructose. Polysaccharides are 

 built up of many molecules of a single simple sugar joined, as in the forma- 

 tion of a disaccharide, by the so-called glycosidic linkage (Fig. 2.85); glucose 

 is usually the simple sugar involved. The sugars are soluble in water, but 

 most polysaccharides are not. Carbohydrates are easily oxidized in the cell 

 and are its chief source of the energy utilized for its work. 



The lipids are a heterogeneous group of organic compounds containing 

 chiefly carbon, hydrogen, and oxygen. These compounds are not soluble in 

 water and are classified as (1) the simple lipids, including the fats and 

 waxes; (2) the compound lipids; and (3) the steroids. Simple lipids are 

 esters; an ester is a combination of an alcohol and an acid. Specifically, 

 a chemically pure fat, such as palmitin, olein, and stearin, is an ester of 

 glycerol and three molecules of a single fatty acid (Fig. 2.8£'). Natural fats, 

 such as butter, lard, and tallow, are mixtures of chemically pure fats. In the 

 cell, fats are readily oxidized, with the liberation of energy, much of which is 

 transformed into heat. Waxes, which are also simple lipids, commonly occur 

 as secretions of insects and mammals; some oils, such as that of the sperm 

 whale, contain waxes. They are esters of alcohols other than glycerol, to- 

 gether with free alcohols and fatty acids. Among the compound lipids, the 

 phospholipids, such as lecithin which is common in egg yolk, contain phos- 

 phoric acid and a nitrogenous base, such as choline, in addition to glycerol 

 and fatty acids (Fig. 2.^F). Another group of compound lipids known as the 

 cerebrosides, because they are commonly found in nervous tissue, are combi- 

 nations of a fatty acid, a nitrogen-containing alcohol, and a simple sugar. 

 Steroids are crystalline, alcoholic compounds characterized by the presence 

 of the complex phenanthrene ring system (Fig. 2.8//). This group contains 

 cholesterol which is of widespread occurrence, vitamin D, the sex hor- 

 mones, and the hormones of the adrenal cortex, all of great physiological 

 importance. 



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