PHYSIOLOGICAL CHEMISTKY 285 



Glucose readily combines with alcohols, acids, phenols, etc. , to form glucosides. 

 These are resolved into their constituent groups by hydrolysis with acid. To 

 understand their structure, the formula for glucose is best written with four of its 

 C atoms in a ring thus : 



H 



CH - (CHOH) 2 - CH - CHOH - CILjOH. 



When the H atom of the hydroxyl group of the C atom which exercises aldehydic 

 functions in the open chain formula for dextrose is replaced by some radicle a 

 glucoside results. Since this C atom is assymetric, two stereoisomeric glucosides 

 (a and ft) are possible. These show striking differences from one another in their 

 susceptibility to ferment action. 



Laevulose (C 6 H 12 6 ) is found along with dextrose in fruits and honey 

 and results from the hydrolysis of cane sugar (see Disaccharides). It 

 is very rarely found in animal tissues. It is crystallisable with great 

 difficulty, being usually obtained as a putty-like mass. It is laevo- 

 rotatory (()D= -93). 



Galactose (C 6 H 12 O 6 ) is a dextro-rotatory sugar produced, along with 

 dextrose, by hydrolysing lactose (see Disaccharides). Protagon, a 

 lipoid substance in brain tissue, yields galactose on hydrolysis. Certain 

 gums also yield it on hydrolysis. It differs but slightly from dextrose 

 in its reactions. Its presence can be detected by the fact that when 

 oxidised, as by boiling with nitric acid, it yields mucic acid (see p. 278) 

 which forms characteristic crystals. 



EXPERIMENT. Test for galactose. Add 3 c.c. pure HN0 3 (con.) to 10 c.c. of a 

 strong solution of lactose in a small evaporating dish. Boil gently over a free 

 flame for three minutes, and then gently lower the flame and allow to evaporate 

 till the volume is reduced to about 3 c.c. Transfer to a test tube, cool under the 

 tap, add 2 c.c. water, and allow to stand. Crystals of mucic acid separate out. 



II. DISACCHARIDES. 



Chemically, each molecule of a disaccharide consists of two molecules 

 of a monosaccharide less one molecule of water, 



Their structure can be demonstrated by hydrolysing them, ie. by 

 causing them to take up a molecule of water, in consequence of which 

 they split up. In disaccharides the two monosaccharide molecules are 

 linked together in the same manner as glucose and the other con- 

 stituent radicle in glucosides. 



The chief means of hydrolysing include boiling with dilute acid and 

 the action of certain ferments called invertases, which are contained in 



