u 



CARBOHYDRATES 



[CH. 



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 



I 

 CH2OH 



Glucose 



CHO 



I 

 HOCH 



i 

 HOCH 



I 

 HCOH 



HCOH 



I 

 CH2OH 



Mannose 



CH2(OH) 



I 

 CO 



I 



HOCH 



I 

 HCOH 



I 

 HCOH 



I 

 CH2OH 



Fructose 



CH(OH) 



II 

 COH 



I 

 HOCH 



I 

 HCOH 



I 

 HCOH 



I 

 CH2OH 



Enolic form 



Expt. 74. Formation of laevulose and mannose from glvxiose hy alkalies. Into a 

 small flask put 50 c.c. of a 5% solution of glucose and add 5 c.c, of a 10% solution 

 of potash. Cork the flask and leave it in an incubator at 35° C. for 24 hours. Cool 

 and neutralize the alkali with a few drops of acetic acid. Test a few drops for laevulose 

 with Seliwanoff's reaction (see p. 53). Then add 5 c.c. of a solution of phenylhydrazine 

 acetate (5 gms. of phenylhydrazine dissolved in 5 c.c. of glacial acetic acid) and shake 

 well. After a few seconds, the solution becomes turbid and a precipitate of mannose- 

 hydrazone is formed (see p. 52). Examine under the microscope and note the charac- 

 teristic crystalline spheroids. 



It is not known whether the pentoses are formed de novo in carbon, 

 assimilation or whether they arise from the hexoses. A relationship of 

 interest in this connexion is that between the pentoses and the hexoses 

 actually occurring in plants, as will be seen by comparing the formula of 

 c?-xylose with that of c?-glucose and the formula of Z-arabinose with that 

 of c?-galactose. An additional interest lies in the fact that galactose and 

 arabinose occur so frequently together in gums, while other polysac- 

 charides give glucose and xylose on hydrolysis. 



Expt. 75. To show the presence of both hexoses and sucrose in the Zea/ (Davis, Daish 

 and Sawyer, 17). Take about 5 gms. of fresh leaf of either the Beet or Mangold. 

 i» [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 off 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 intervals 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. 157) are precipitated as insoluble lead salts. The precipitate is 

 filtered off and the lead in the filtrate removed by 1 % sodium carbonate, avoiding 



