CHAPTER VII 

 THE CARBOHYDRATES 



THESE important foodstuffs occur abundantly in the vegetable 

 kingdom, and are also found in small quantities in the animal kingdom, 

 either in the free state or combined with proteins. As a group the 

 carbohydrates form one of the chief energy-producers or foods. 



As their name implies, they are composed of carbon, hydrogen, 

 and oxygen, the latter elements being present in the proportion found 

 in water (2:1). Their general formula is C m H 2ri O re . It was at one 

 time thought that carbohydrates were bodies containing six atoms 

 of carbon or some multiple thereof. This is now known not to be the 

 case ; indeed, the most recent classification of, the simplest members 

 of the group is according to the number of carbon atoms contained 

 in the molecule; those, for example, containing three atoms being 

 termed trioses, those with five atoms pentoses, those with six atoms 

 hexoses, and so on. It must not be thought that all bodies containing 

 C, H, 0, with the hydrogen and oxygen in the proportion of water, 

 are contained in the group of carbohydrates; for example, C 2 H 4 2 

 is the empirical formula for acetic acid, C 3 H 6 3 that of lactic acid. 



Nevertheless, the definition holds for the mcst part, and the best- 

 known members of the series do contain six molecules of carbon or 

 some multiple of six. These are generally grouped as follows: 



1. The monosaccharides C 6 H 12 6 



2. The disaccharides C 12 H 2 >0 U 



3. The polysaccharides (C 6 H 10 5 ) N 



SECTION I 



The Monosaccharides. The chief monosaccharides of physio- 

 logical importance are the hexoses, dextrose, levulose, and galactose. 

 Each has the empirical formula C 6 H 12 6 . Dextrose is the aldehyde of 

 the hexatomic alcohol sorbite; levulose is the ketoiie of the hexatomic 

 alcohol mannite; galactose is the aldehyde of another hexatomic 

 alcohol, dulcite. Each alcohol has the formula C r> H 8 (OH) r) . The 

 structural formula of these sugars may be represented as given on 

 p. 60. 



Levulose differs from the other two in being a ketone; dextrose 

 and galactose differ from each other and from their many isomers 

 in the position of the so-called asymmetric carbon atoms. All bodies 



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