CARBOHYDRATES 455 



Galactose is an important sugar; as part of the lactose mole- 

 cule, it becomes part of the carbohydrate food of the nursing 

 animal. It is also found in brain and nerve tissue generally. 



The disaccharide sucrose is the sugar of our daily life. It 

 may be cane sugar, or beet sugar, as in France. Sucrose is 

 of very wide distribution in the plant kingdom but not, in the 

 same sense, in the animal kingdom, where it serves as food to be 

 broken down into simple sugars. It is interesting that honey is 

 almost wholly "invert sugar" (a mixture of glucose and fructose), 

 yet the bee collects cane sugar (sucrose) from the flowers. The 

 hydrolytic agent acting as catalyst in this reaction is probably 

 the formic acid secreted by the bees. 



The first synthesis of a sugar was done by Emil Fischer. 

 More recently, the French-Swiss chemist Pictet reports the 

 artificial synthesis of sucrose in his laboratories in Geneva, 



Maltose and cellobiose are less common plant disaccharides. 

 Lactose is found in milk and nowhere else, it is a constituent of 

 the milk of all mammals ; it is the sugar for the nursing young. 



Among the trisaccharides, rafiinose is the best known. While 

 its presence has been reported in certain invertebrates, it is 

 almost solely a plant sugar. 



In addition to their nutritive qualities, sugars come into play 

 in organisms in the generation of such pure physical forces as 

 osmotic pressure. The sugar content of plant vacuoles is the 

 chief factor involved in turgor. 



Because sugars are readily soluble in water, it is probably in 

 this form that most food is translocated, or "shipped," in plants 

 and animals. Digestion consists in the breaking down of higher 

 foods into a diffusible or assimilable form. Food in plants is 

 stored as starch, fats, or protein but shipped as sugar. When the 

 sap flows in the spring, it is sugar that is sent up to the young 

 opening leaves. 



Sugars play their chief role in organisms as the source of 

 energy released in respiration. They are reduced by oxidation 

 to their simplest member glucose, the energy stored in them 

 being thus liberated; carbon dioxide and water result: 



CeHisOe + 6O2 = 6CO2 + 6H2O 



Other substances are oxidized (notably the fats), but, as R. B. 

 Harvey states, from the standpoint of the quantity of energy 



