CHEMICAL SUMMARY 191 



glucose and glucose form maltose, glucose and galactose form lactose, 

 and glucose and fructose fonm saccharose or cane sugar. One gram of 

 a disaccharide on oxidation produces 3.95 calories of heat. 



Pentoses, with five carbon atoms, are found in many plant cells. 



Polysaccharides are -formed by the combination of simple sugar mole- 

 cules. Gums, pectens and mucilages are formed of monosaccharides, and 

 starch, dextrin, glycogen and cellulose are formed of glucose. The mo- 

 lecular weight of starch is estimated at between 1260 and 32400. Its 

 osmotic pressure is so small that it appears to be zero. 



By the oxidation of the CHO group of the aldehydes the OO'OH group 

 of the organic acids is obtained. The open chain acids are called fatty 

 acids and bear the same name as the aldehydes from which they are 

 formed. In the series, formic, acetic, propionic, butyric, valerianic and 

 caproic, each member has one more carbon atom than the one preceding. 

 The fatty acids most common in animal cells have even numbers of 

 C atoms. These acids are butyric, caproic, caprylic, capric, lauric, myris- 

 tic, palmitic and stearic. The solubilities of these acids in water decrease 

 as the number of carbon atoms increases, the higher members of the 

 series being insoluble. 



In the same way that unsaturated hydrocarbons differ from paraffines 

 in the presence of double or treble bonds between adjacent carbon atoms, 

 so the unsaturated fatty acids differ from those given above. The un- 

 saturated fatty acids, oleic, linoleic have respectively 1 and 2 double 

 bonds. Linolenic has 3 double bonds. These unsaturated acids may take 

 up oxygen and become saturated, and in this way they act as reducing 

 substances. 



The oxy-acids contain more oxygen than the normal acids. Propionic 

 acid is CH s CHiCOOH whereas oxypropionic, usually called lactic acid, 

 is CH s CHOHCOOH. 



Keto acids contain the ketone group, and also the COOH group. 



Fatty acids combine with bases to form salts, which in case of the 

 higher members of the series are called soaps. The soaps fonmed by 

 action of alkaline earths on the higher fatty acids aie insoluble in water 

 whereas the lower fatty acids form soluble soaps with these bases. Alkali 

 soaps hydrolize in water, forming a fine emulsion of fatty acid that makes 

 the solution opalescent. 



Fatty acids combine with ammonia, but the compound may be of either 

 of two forms. With ammonium hydrate, which is a true ibase, they form 

 ammoniacal soaps, but with dry ammonia and a dehydrating agent they 

 ■form acid amides containing one less molecule of water. In ammoniacal 

 soaps ,N is pentivalent in the group C0ONH 4 , whereas in amides nitrogen 

 is trivalent in the group 'CON'H 2 . 



The esters of fatty acids with alcohols are called fats. The triatomic 

 alcohol glycerol is the most common but the monatomic alcohol cholesterol 

 and other alcohols are also found. 



S'ince glycerol is a triatomic alcohol, it can combine with one, two or 

 three fatty acid molecules, forming mono-, di-, and triglycerides, of which 

 the last is the most common. Fat globules found in cells are usually 

 mixtures of triglycerides and fatty acids. In the analysis of fats, the 

 acid number denotes the content of free fatty acids, the saponification 

 number, or combining power with hot KOH, is an index of the average 

 of molecular weight _ of each 'fatty acid divided 'by its valence, and the 

 iodine number is an index of the content in unsaturated fatty acids. The 

 percentage of volatile fatty acids is usually determined, but the separation 

 of all of the fatty acids involves much labor. 



The oxidation of 1 g. fat may yield as much as 9.5 calories of heat. 



Mono or diglycerides may combine with other substances by means of 

 the remaining alcohol groups. Such compounds are included under the 

 loose term, lipoids, but have been called by Leathes phospholipines, 



