CARBOHYDRATES 63 



amounts have also been isolated irom liver and pancreas. Ruminants 

 probably utilize a portion of the combined xylose of the plants they 

 consume after the polysaccharides containing it are hydrolyzed by the 

 bacteria in the rumen. 



D-Glucose 



This sugar (also known as dextrose, starch sugar, or corn sugar) is 

 the most abundant of the hexoses. It is found in free form in ripe 

 fruits, sweet corn, and honey, where it is associated with fructose. 

 Glucose occurs in many of the large carbohydrates, including the most 

 abinidant examples. It is sweet in taste and is readily metabolized by 

 microorganisms of many kinds and by all higher plants and animals. 

 In the latter, glucose is the usual form of transport for carbohydrates, 

 many others being converted to it before circulation in the blood. 

 Cells put the atoms of glucose to a great variety of uses in the course 

 of their normal biochemical operations. Some of these functions will 

 be outlined later. 



D-Fructose 



This sugar is a ketohexose and the only really abundant ketose in 

 nature. It is often called levulose or fruit sugar and is widely dis- 

 tributed with D-glucose. It occurs in several of the larger carbohy- 

 drates. In the free form fructose is found with the pyranose ring, but 

 in its derivatives the furanose system is common. 



D-Galactose 



This aldohexose is found largely in combined forms which yield it 

 on hydrolysis. The free sugar is neither so sweet nor so water soluble 

 as glucose or fructose though it is a diastereoisomer of the former. 

 D-Galactose occurs in combination with D-glucose in the disaccharide 

 lactose. 



D-Mannose 



This sugar is another diastereoisomer of D-glucose. It is found 

 principally as a component of mannans, a group of hexosans occurring 

 in fruits like the berries of the mountain ash tree. This sugar is also 

 abundant in vegetable ivory, the endosperm of the nut of a low-grow- 

 ing palm of tropical America. The ivory is used in making ornaments 

 and buttons. The name mannose is derived from manna, a term ap- 

 plied to any one of the fruits or plants rich in this carbohydrate. 



