40 ORGANIC SUBSTANCES I CHEMICAL CHARACTERS 



into glucose and laevulose is an example of hydrolysis, 

 i.e. the splitting of a molecule by reaction with water. 



C 12 H 22 O n + H 2 = C 6 H 12 6 + C 6 H 12 6 



sucrose water glucose laevulose 



Various sugars are found mixed in different proportions 

 in living cells and are constantly being converted from 

 one form into another. 



The Polysaccharides. 



These are another important class of carbohydrates. 

 They derive their name from the fact that they are 

 formed by the putting together of many molecules of 

 sugar to form one molecule of polysaccharide, a process 

 known as condensation. The most important poly- 

 saccharides of plants are the starches and the celluloses. 

 There are many different kinds of each, but for our 

 purpose we may treat them in each case as one. 



Starch is formed as solid grains in the protoplasm 

 of^the living cells of plants. The substance of each 

 grain is laid down in concentric layers round a centre 

 called the hilum of the starch grain. Starch is formed 

 by the union of many molecules of glucose with the 

 elimination of one molecule of water from each : 



nC 6 H 12 6 _ (C 6 H 10 5 ) M + nH 2 



The value of n has not been exactly determined, but 

 it is probably somewhere about 100. Thus the starch 

 molecule is far more complex than the glucose mole- 

 cule. Starch has a very characteristic blue reaction 

 with watery solution of iodine in potassium iodide. 

 Most starches are quite insoluble in cold water ; they 

 are hydrolysed in the presence of dilute mineral acids 

 into various simpler carbohydrates, and eventually 



