SUGARS 39 



solution — such as Fehling's solution; — and warmed. 

 Some of the oxygen is taken from the cupric salt, and 

 red cuprous oxide is formed. This is the ordinary test 

 for sugars that behave in this way — the " reducing 

 sugars " as they are called. Maltose is also a reducing 

 sugar, though to a less extent than glucose. On the 

 other hand, sucrose (cane sugar) does not reduce 

 Fehling's solution at all. 



In the living cell glucose very readily takes up oxygen 

 and breaks up its molecule into carbon dioxide and 

 water. This is the basis of the process of respiration, 

 which is of fundamental importance in the economy of 

 living organisms : — 



CfiHi^Oe + 6O2 = 6CO2 + 6H2O 



glucose oxygen carbon water 

 dioxide 



Sucrose or saccharose (cane sugar), CjgHggOi^, is 

 the most important sugar commercially. It is the 

 main product of the sugar cane and the sugar beet, 

 and is the principal sugar we use as food. It is very 

 much sweeter than glucose. The importance of sugar 

 as a food for animals depends on the characters already 

 mentioned, solubility and high potential energy. It is 

 thus easily absorbed through the wall of the alimentary 

 canal into the blood, and the readiness with which it 

 is oxidised places large amounts of energy at the dis- 

 posal of the muscles, where the energy appears in the 

 kinetic form, as motion and heat. Sugar is, in fact, 

 used in this way by all living cells, but most energetically 

 by the muscles, which are the great energy spenders 

 of the animal body. 



Sucrose is not a reducing sugar, and, in the living 

 cell, is converted into glucose and laevulose before being 

 oxidised. This process of the conversion of sucrose 



