62 PHYSIOLOGY 



(3) PolysaccJtarides, 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 

 formation as represented by the following equation : 



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

 (Examples, starch, dextrin, &c.) 



THE MONOSACCHARIDES 



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 l--fructose behind. For 

 the preparation of d-fruetose 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 sub- 

 stances, 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. 



(6) 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, a^nd 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, mannite, and dulcite C 6 H 14 O 6 . 



(d) Another important general reaction of the monosaccharides depending on the 

 COH or the CO group is the reaction with phenyl hydra/inr. On warming a solu- 

 tion of sugar with a solution of phenyl hydrazine in acetic acid, the following reactions 

 take place. The first reaction results in the production of a hydrazone : 



CH 2 OH(CHOH) 3 CHOHCHO + H-jN.NH.CeHg = 

 CH 2 OH(CHOH) 3 CHOH.CH : N.NH.C 6 H 5 -f H 2 O. 



The hydrazone then reacts with another molecule of phenyl hydrazinc with the pro- 

 duction of an osazone : 



