v] HYDROLYZING ENZYMES 45 



A solution of arabinose which will give the pentose reactions can also 

 be obtained by hydrolysis of Cherry Gum. The gum oozes from the bark 

 of various species of Prunus, such as the Cherry (Primus Cerasus) and 

 the Bird Cherry (P. Padus). 



Expt. 39. Preparation of arabinose solution from Cherry Gum. The gum is heated, 

 on a water-bath in a round-bottomed flask fitted with an air condenser 1 , with 

 dilute sulphuric acid (1 pt. by wt. of gum : 7 pts. by wt. of 4 / sulphuric acid) for 

 about 5 hours. The solution is then neutralized with calcium carbonate and filtered. 

 Perform the tests a, 6 and c of Expt. 38 on the solution. A positive result is obtained 

 in each case. Since the solution contains other sugars as impurities, it cannot con- 

 clusively be used for tests c?, e and/. If a considerable quantity of gum is available, 

 crystallization of arabinose should be attempted by concentrating the aqueous sugar 

 solution, extracting this with 90 % alcohol and again concentrating in a desiccator 

 (see p. 53). If a very small quantity of gum only is available, the tests a, b and c 

 should be performed directly on a small piece of the gum in a test-tube. 



A purer preparation of arabinose, which may be used for all the tests 

 of Expt. 38, can be obtained by the hydrolysis of araban (see Expt. 48). 



Xylose. This sugar occurs very widely distributed in woody tissue as 

 the pentosan, xylan (see p. 53). A solution of xylose which will give the 

 pentose reactions can be obtained from the hydrolysis of straw. 



Expt. 40. Preparation of xylose solution from straw. Take about 50 gms. of straw, 

 which has b*een cut up into small pieces, and put it into a round- bottomed flask 

 fitted with an air condenser. Add sufficient 5 / sulphuric acid to cover the straw 

 and heat on a water-bath for 2-3 hrs. Filter off the solution, neutralize with 

 calcium carbonate and filter again. Make with the solution the tests a and c of 

 Expt. 38. The solution will also reduce Fehling's solution strongly, but this reduction 

 may be partly due to other sugars formed in the hydrolysis. 



The presence of xylan giving the pentose reactions can also be demonstrated in 

 straw, bran or sawdust by merely heating small quantities of these substances in a 

 test-tube with the above reagents (see Expt. 49). 



A purer solution of xylose can be obtained from the hydrolysis of 

 xylan (see Expt. 51). 



\Vherj xylose is oxidized with bromine, it yields xylonic acid which 

 has a characteristic cadmium salt. The formation of this salt is used as a 

 method for identifying the sugar (see Expt. 51). 



HEXOSES. 



Glucose. This substance, which is also known as grape-sugar, is very 

 common and very widely distributed in plants. It occurs in the tissues 

 of leaves, stems, roots, flowers and fruits. It is produced as a result of 



1 i.e. a wide piece of glass tubing about 3 ft. long passing through the cork. 



