72 



CARBOHYDRATES AND THEIR 



[CH. 



In connexion with the occurrence of various sugars in leaves it is 

 of interest to note that glucose, fructose and mannose can pass over 

 into one another in alkaline aqueous solution. This has been explained 

 by their conversion into the enolic (unsaturated) form common to all 

 three hexoses : 



CHO 



I 

 HCOH 



I 

 HOCH 



I 

 HCOH 



I 

 HCOH 



CH2(0H) 



Glucose 



CHO 



I 

 HOCH 



I 

 HOCH 



I 

 HCOH 



I 

 HCOH 



I 

 CH2(OH) 



Mannose 



CH.,(OH) 



I 

 CO 



I 

 HOCH 



I 

 HCOH 



I 

 HCOH 



I 

 CH2(0H) 



Fructose 



CH(OH) 



ii 

 COH 



I 

 HOCH 



1 

 HCOH 



I 

 HCOH 



CH2(0H) 

 Enolic form 



E.vpt. 75. To show the prcsiince of both hexoses and sucrose in tlie leaf (Davis, 

 Daish and Sawyer, 15). Take about 5 gms. of fresh leaf of either the Beet or Mangold. 

 (Leaves of the Garden Nasturtium {Tropaeolum majus) and Wild Chervil {Chaero- 

 phyllum sylvestre) may also be used.) Tear them into small pieces and drop them 

 into boiling 90-98 % alcohol in a flask on a water-bath. In this way the enzymes of the 

 leaf are killed, and no changes will occur in the carbohydrates present. After boiling for 

 a short time, the alcohol is filtered oft' and the extraction repeated. Evaporate the 

 filtrate to dryness in an evaporating dish on a water-bath. The filtrate will contain 

 chlorophyll and various pigments, sugars, glucosides, aromatic compounds and other 

 substances according to the plant used. Then add about 20 c.c. of water and at in- 

 tervals a few drops of basic lead acetate until it ceases to form a precipitate. By this 

 means all hexoses combined with aromatic substances as glucosides (see p. 142) are 

 precipitated as insoluble lead salts. The precipitate is filtered off and the lead in the 

 fitrate removed by sodium carbonate, avoiding excess. Filter tigain and the filtrate 

 will contain the sugars. Boil the latter and add Fehling's solution drop by drop till 

 reduction ceases. Filter off" the copper oxide and then boil the solution with dilute 

 sulphuric acid for a few minutes and make neutral to phenolphthalein. Reduction 

 will occur on bidding more Fehling and boiling, owing to the inversion of the cane- 

 sugar present. 



Expt. 76. To show the presence of hexoses in the leaf by means of the formation of 

 glucosazone. Leaves of Beta^ Vhaerophyllum st/lvestre, or Tropaeolum may be used. 

 Extract as in the previous experiment and precipitate the glucosides with the 

 minimal amount of basic lead acetate. Test foi- osazDue in the filtrate as in 

 Expt. 41 {d). 



Expt. 77. To obtain starch from green leaves. Weigh out 25 gms. of leaflets of the 

 Pea [Pisum sativum). The leaves should have been picked in the evening after a 

 sunny day, and it does not matter if the cut leaves are left overnight. Dip the leaf- 

 lets for a moment into boiling water, remove excess of water and drop them int() 

 200 c.c. of 96-98 % alcohol and boil till the chlorophyll is extracted : then filter. 



