CARBOHYDRATES 155 



I. Non-nitrogenous Organic Substances. 



The most important members of this group are the carbo- 

 hydrates, fats, and acids enumerated below. 



I. Carbohydrates.— These compounds form the largest part 

 of the body of all plants and contain carbon, hydrogen and 

 oxygen, the elements hydrogen and oxygen being present in the 

 same proportion as they exist in water. The chief carbohydrates 

 are the sugars, starch, inulin, celluloses and pentosans. 



a. Sugars. — Almost all the sugars possess a more or less 

 sweet taste, and are generally met with dissolved in the cell-sap. 

 The commoner representatives are glucose, fructose, cane-sugar 

 and maltose. 



(i) Glucose, dextrose or grape-sugar (CgHjjOg), occurs in most 

 fruits, and especially in grapes whose cell-sap may contain from 

 20 to 30 per cent. ; ripe apples contain on an average 7 to 10 

 per cent. ; cherries 9 to 10 per cent., and plums 3 to 5 per cent 

 of this sugar. 



(ii) Fructose, fruit-sugar, or levulose (CgHigOg) is found also in 

 ripe fruits associated with grape-sugar. 



Both dextrose and levulose reduce Fehling's solution, and are 

 directly fermentable by yeast. 



Ex. 80. — Dissolve 35 grams of copper sulphate in 500 c.c. of water, label 

 this solution A : then dissolve 160 grams of caustic potash and 173 grams of 

 sodium potassium tartrate in 500 c. c. of water and label the solution B. By 

 mixing equal quantities of A and B, Fehling's solution is produced. (The 

 solution A and B should be kept separate and only mixed when needed as 

 the mixture does not keep long. ) 



Squeeze a few drops of grape juice into a test tube containing 10 c.c. of the 

 Fehling's solution : heat over a Bunsen flame and note the reddish precipitate 

 of cuprous oxide (Cu'iO). 



Test the juice of ripe plums and other fruits in the same way. 



(iii) Cane-sugar or saccharose (C12H22O11) occurs dissolved in 

 the cell-sap of the stems and roots of many plants and especially in 

 the sugar-cane, mangel and sugar-beet, from which it is extracted 

 on a commercial scale. 



