66 GENERAL BIOCHEMISTRY 



Lactose 



This carbohydrate, also called milk sugar, occurs in the milk of 

 mammals to the extent of about 6 per cent in human milk and 4.8 per 

 cent in cow's milk. Glucose is transported in the blood to the mam- 

 mary gland, and lactose is synthesized there from the glucose. Since 

 lactose is a disaccharide of both galactose and glucose, the galactose 

 presumably must be formed by an inversion of the configuration of 

 carbon atom four of glucose. 



Lactose is a reducing sugar and exhibits mutarotation. It is less 

 soluble and much less sweet than sucrose. The a diastereoisomer is 

 less soluble than the [^ form. Sometimes the a isomer crystallizes in ice 

 cream, making the product seem sandy in texture. The more soluble 

 ^ isomer is used in diets for infants. a-Lactose may be represented 

 by the following: 



CHoOH H OH 



H OH CH,OH 



l-ff-u-galactosido-J-a-D-^lufose 



Pentosans 



These polysaccharides are made up of pentose units. Their struc- 

 tures are largely unknown, but two general types are recognized. 

 Xylans yielding D-xylose on hydrolysis occur in vegetables, leaves, 

 lichens, straw, corn cobs, oat hulls, wood. Some waste materials in 

 this category are now employed industrially where the xylans are con- 

 verted to furfural for use in organic synthesis and in oil refining. 

 Arabans are found in the gummy exudates of trees and shrubs like 

 cherry and mesquite. In fact these carbohydrates are found in many 

 plant gums and juices and yield L-arabinose on hydrolysis. 



Starch 



Hexosans are the polysaccharides made up of hexose units and are 

 widespread in both plant and animal kingdoms. All living forms 

 probably possess one or more kinds. Since the group is important, it 

 will be discussed in terms of certain members. 



