62 PHYSIOLOGY 



(3) Polysaccharides, composed of three or more molecules of a mono- 

 saccharide. The number of molecules which are associated in the com- 

 pounds of this group may be very large. We can regard their general forma- 

 tion as represented by the following equation : 



nC 6 H 12 6 - nH 2 = (C 6 H 10 5 ) n . 

 (Examples, starch, dextrin, &c.) 



THE MO NOS ACCRA RIDES 



Only four hexoses out of the large number which have been synthetised 

 are assimilable by the animal body. These are mannose, glucose, galactose, 

 and fructose, the three former being aldoses, while the last is a ketose. All 

 of them are derivatives of d-glucose. They may be synthetised in several 

 ways. The most interesting, because it probably represents the mechanism 

 of synthesis of hexoses in plants, is the formation from formaldehyde. In 

 alkaline solutions formaldehyde polymerises with the formation of a mixture 

 of hexoses known as acrose. From this mixture a -acrose can be isolated by 

 the formation of its osazone and the reconversion of this osazone into sugar. 

 It is found to be identical with i-fructose. If a solution of this i-fructose be 

 treated with yeast, d-fructose is fermented, leaving 1-fructose behind. For 

 the preparation of d-fructose it is necessary to convert the inactive sugar into 

 the corresponding acid, mannonic acid. This with strychnine or morphia 

 forms salts which can be separated into the d- and 1- groups by fractional 

 crystallisation. From the d- modification d- mannose can be obtained, and 

 this by conversion into the osazone and reconversion into a sugar gives 

 d-fructose. 



All the monosaccharides, however many carbon atoms they contain, 

 present certain general reactions determined by their chemical composition. 



(a) Like ordinary aldehydes and ketones, the sugars act as strongly 

 reducing substances, and, like aldehydes, reduce ammoniacal solution of 

 silver to metallic silver, and many of the higher oxides of metals to lower 

 oxides. On this behaviour is founded the commonest of all the tests for the 

 presence of reducing sugar Trommer's test. 



(b) On oxidising a monosaccharide the COH group becomes converted 

 to COOH. Thus, glucose on oxidation gives gluconic acid : 



COH(CHOH) 4 CH 2 OH + = COOH(CHOH) 4 CH 2 OH. 



On further oxidation the end group CH 2 OH also is affected, and we obtain 

 a dibasic acid. Thus glucose gives saccharic acid. 



(c) By means of nascent hydrogen the monosaccharides can be reduced 

 to the corresponding polyatomic alcohol. Thus the three hexoses, glucose, 

 fructose, and galactose, give the corresponding three alcohols, sorbite, man- 

 nite, and dulcite C 6 H 14 6 . 



(d) Another important general reaction of the monosaccharides depending 

 on the COH or the CO group is the reaction with phenyl hydrazine. On 

 warming a solution of sugar with a solution of phenyl hydrazine in acetic 



