20 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



in form of acicular crystals ; it is strongly dextro-rotatory. It gives 

 Trommer's test ; but its reducing power, as measured by Fehling's 

 solution, is one-third less than that of dextrose. 



By prolonged boiling with water, or, more readily, by boiling with 

 a dilute mineral acid, or by means of an inverting ferment, such as 

 occurs in the intestinal juice, it is converted into dextrose. 



2 H 2 2O n +H 2 0=C 6 H 12 O 6 



[maltose] [dextrose] 



[dextrose] 



It undergoes readily the alcoholic fermentation. 



The three important physiological sugars (dextrose, lactose, and 

 maltose) may be distinguished from one another by their relative 

 reducing action on Fehling's solution (1-0 : 0'71 : 0'63), by their 

 rotatory power, or by the phenyl-hydrazine test described in Lesson 

 XIII. 



POLYSACCHABIDES 



Starch is widely diffused through the vegetable kingdom. It 

 occurs in nature in the form of microscopic grains, varying in size 

 and appearance, according to their source. Each consists of a 

 central spot (hihim) round which more or less concentric envelopes 



of starch proper or granulose alternate 

 with layers of cellulose. Cellulose has 

 very little digestive value, but starch is 

 "* a most important food. 



Starch is insoluble in cold water : it 

 forms an opalescent solution in boiling 

 water, which if concentrated gelatinises 



J/$P) ^Mi^Vx^^^l^Nl^ on coonn g- It s most characteristic re- 

 action is the blue colour it gives with 

 iodine solution. 



On heating starch with dilute mineral 

 FIG. 4.- section of pea showing starch acids dextrose is formed. BV the action 



and aleurone grains embedded in the . * " i 



protoplasm of the cells : a, aleurone ol diastatic terments, maltose is the cruel 



grains ; st t starch grains ; i, inter- .. ., 



cellular spaces. (Yeo, after Sachs.) end product. In both cases dextrin is 



an intermediate stage in the process. 



Before the formation of dextrin the starch solution loses its opal- 

 escence, a substance called soluble starch or amidulin being formed. 

 This, like native starch, gives a blue colour with iodine solution. 

 Although the molecular weight of starch is unknown, the formula 

 for soluble starch is probably (C 6 H 10 O 5 ) 200 . Equations that repre- 

 sent the formation of sugars and dextrins from this are very complex, 

 and are at present hypothetical. 



